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<title>A 3-Element Wire Yagi Design for 10.1373 MHz</title>
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<h1 style="text-align: center;">A 3-Element Wire Yagi Design for 10.1373 MHz</h1>
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<p style="text-align: center; font-weight: bold;"><a href="30m-3l.pdf">A 3-Element Wire Yagi Design for 10.1373 MHz</a></p>
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<p style="text-align: center;">This page exists to include the PDF in the topic index</p>
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<img src="../images/colorbar.gif" alt="hr" style="display: block; margin: auto;" width="540" height="4"><br>
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<p class="topics-footer">Topics: <a href='../topics.html#hf-lower'>HF Lower</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>Three-Element Yagi Models: Standards of Comparison</title>
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10">Three-Element Yagi Models:<br>
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Standards of Comparison<br>
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L. B. Cebik, W4RNL</h1>
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<p>I have had numerous requests for models of the 3-element Yagis that I use regularly as a standard of comparison for other models. To make the models more readily available for use or improvement, I have placed them at the end of this note. They are in the form of EZNEC antenna descriptions and contain all the information needed to create AO, NEC-Wires, NEC-Win, NEC4WIN, ELNEC, and EZNEC models. In fact, the antenna geometries are quite simple, consisting of 3 wires of 21 segments each. (Reduce the segments to 20 per element for versions of MININEC.) The original source of the models is a 20- meter Yagi included in an earlier version of AO and modified to simplify construction as a model.</p>
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<p>The antennas have not been modeled to build. A real Yagi would likely use elements of tapered diameter sections. The models use a constant diameter for each band: 0.5" for 10 meters; 0.75" for 15 meters; and 1.0" for 20 meters. In fact, all three antennas are essentially the same antenna, frequency scaled and then tweaked a bit to use standard size element diameters.</p>
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<p>Each model has a center design frequency, but the performance center (in this case, maximum front-to-back ratio) has been moved to provide close to band- edge to band-edge 2:1 SWR bandwidth relative to a feedpoint impedance of 25 ohms. The premise is that a well-designed beta match would only require shortening of the driven element to supply the needed capacitive reactance, while a gamma match would require driven element lengthening. In both cases, beam performance would be altered hardly at all.</p>
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<p>The beams have been modeled in free space. Performance and matching hold up well at heights from 1 wavelength upward. Adjustments would be necessary for some lower heights.</p>
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<p>The following tables show the modeled performance of the beams on each band in free space. Note that performance is very good for the boom lengths involved. However, one might well tweak slightly better performance from the beams with further optimization--better, but not too much better, unless one is aiming for either gain or front-to-back ratio without regard for other parameters. These models strove for a balance of gain, front-to-back ratio, and operating bandwidth as examples of what a decent 3-element Yagi might achieve. Figures are seriously exact from NEC-4 models.</p>
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<pre>Antenna: 3-element 20 meter Yagi: 1.0" diameter al. elements; boom =
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22.544'
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Frequency Gain (dBi) F-B (dB) Feed (R+/-jX) SWR (25 ohms)
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14.0 7.94 20.81 27.04 - 13.25 1.665
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14.1 8.03 27.30 26.50 - 6.35 1.288
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14.2 8.14 25.68 25.38 + 0.93 1.041
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14.3 8.27 19.82 23.81 + 8.78 1.439
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14.35 8.35 17.69 22.90 + 12.96 1.723
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Antenna: 3-element 15 meter Yagi: 0.75" diameter al. elements; boom =
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15.074'
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Frequency Gain (dBi) F-B (dB) Feed (R+/-jX) SWR (25 ohms)
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21.0 8.01 24.58 26.17 - 9.15 1.432
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21.1 8.08 28.57 25.60 - 4.49 1.196
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21.2 8.16 26.72 24.80 + 0.37 1.017
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21.3 8.25 21.84 23.80 + 5.48 1.258
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21.4 8.34 18.57 22.65 + 10.88 1.590
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21.45 8.39 17.24 22.04 + 13.70 1.802
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Antenna: 3-element 10 meter Yagi: 0.50" diameter al. elements; boom =
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11.212'
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Frequency Gain (dBi) F-B (dB) Feed (R+/-jX) SWR (25 ohms)
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28.0 7.87 17.35 27.04 - 18.07 1.985
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28.1 7.91 19.64 27.09 - 14.74 1.758
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28.2 7.95 22.42 26.98 - 11.37 1.554
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28.3 8.00 25.76 26.70 - 7.94 1.368
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28.4 8.05 28.44 26.27 - 4.43 1.197
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28.5 8.11 27.15 25.70 - 0.80 1.043
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28.6 8.17 23.87 25.02 + 2.95 1.125
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28.7 8.23 20.99 24.23 + 6.86 1.323
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28.8 8.30 18.65 23.36 + 10.94 1.574
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28.9 8.37 16.71 22.43 + 15.19 1.896
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29.0 8.44 15.06 21.45 + 19.63 2.309</pre>
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<center>
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<img src="3l-1.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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<p>The figure shows the free space azimuth pattern for the 20 meter model, which differs by little from the models for the other bands. Note that for each model, the highest front-to-back ratio is in the lower half of the operating band, while the gain climbs beyond the upper limit of the operating band defined by the 2:1 SWR ratio. The antennas can each be retuned for resonance at a higher gain, but at the expense of a lower front-to-back ratio and a lower feedpoint impedance. Further performance improvements might also be garnered through the use of a longer boom.</p>
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<p>Remember, however, that these are not antennas to be built. Significant adjustments to element lengths and even some slight revisions of spacing might be required with typical Yagi building materials. Those tapered- diameter element sections would complicate these models, which are only standards for comparison.</p>
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<center>
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<h4>Using the Standards</h4>
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</center>Modeling standards have a number of proper uses, so long as we remember that they do not define the limits of performance in any category.
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<p><b>1. Experimental Platforms</b>: the models are useful as models for a number of initial experiments in modeling. For example, we can get a fair idea of stacking performance for antennas of similar gain from a set of these models. Likewise, we can obtain initial ideas about antenna interaction as we place pairs of these antennas at various horizontal spacings and directional orientations. How the gain, front-to-back and feedpoint impedance vary from the baseline values can give direction to the most productive modeling we can do with more complex and slower running of models of actual antennas in their class.</p>
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<p><b>2. Comparison with Experimental Designs</b>: Antennas come in more sizes and shapes than pills in a pharmacy, and new shapes are emerging with great regularity (although most are spin-offs of rather ancient basic designs). Evaluating whether a new shape is worth building is a matter of judging whether the improvements to be made are likely to be worth the effort involved. Since a 3-element Yagi is a fairly standard amateur antenna, comparing the modeled performance of the new antenna against the modeled performance of the standard can aid one in the evaluation. The models need to be transferred to the proposed height over the proper type ground for the new antenna to make the comparison tighter, and both azimuth and elevation patterns require study. Additionally, if terrain modeling software is available, it should also be brought into play.</p>
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<p><b>3. Comparison with Models of Real Antennas</b>: Antennas we see in magazines may or may not have performance figures associated with them. Even where performance figures are given, their source and reliability may sometimes be open to question. Modeling these real antennas and comparing them to the standards can clarify source claims and provide a common setting for various other comparisons. However, these comparisons are meaningful only if due account is taken of the real antenna type. 2-element Yagis should not be expected to have the performance of 3-element Yagis. Likewise, 3-element trap beams should not be expect to be the equal of 3-element optimized monobanders. Within the limits of each type, appropriate accurate models of real antennas within a common modeling framework can take much of the guesswork out of comparisons.</p>
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<p>There are other uses for standards, but these are enough to provide the start toward their proper use. And if you get the urge to modify, improve, and actually build one of these antennas, so much the better. But be sure you do some investigation into good Yagi construction techniques (in other words, read Leeson's fine book, <i>Physical Design of Yagi Antennas</i>) before investing in a batch of aluminum tubing.</p>
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<pre>
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EZNEC/4 ver. A2.11
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3 el Yagi 1" elements 06-22-1997 08:49:34
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Frequency = 14.175 MHz.
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Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
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--------------- WIRES ---------------
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Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in)
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Segs
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1 -17.280, 0.000, 70.000 17.280, 0.000, 70.000 1.00E+00 21
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2 -16.500, 10.455, 70.000 16.500, 10.455, 70.000 1.00E+00 21
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3 -15.525, 22.544, 70.000 15.525, 22.544, 70.000 1.00E+00 21
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-------------- SOURCES --------------
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Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
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Seg. Actual (Specified)
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1 11 2 / 50.00 ( 2 / 50.00) 1.000 0.000 V
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No loads specified
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No transmission lines specified
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Ground type is Free Space
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-----------------------------------------------------------------------------
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EZNEC/4 ver. A2.11
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3 el Yagi 3/4" el 06-22-1997 08:49:51
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Frequency = 21.2 MHz.
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Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
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--------------- WIRES ---------------
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Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in)
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Segs
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1 -11.554, 0.000, 0.000 11.554, 0.000, 0.000 7.50E-01 21
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2 -11.032, 6.991, 0.000 11.032, 6.991, 0.000 7.50E-01 21
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3 -10.381, 15.074, 0.000 10.381, 15.074, 0.000 7.50E-01 21
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-------------- SOURCES --------------
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Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
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Seg. Actual (Specified)
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1 11 2 / 50.00 ( 2 / 50.00) 1.000 0.000 V
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No loads specified
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No transmission lines specified
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Ground type is Free Space
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-----------------------------------------------------------------------------
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EZNEC/4 ver. A2.11
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3 el Yagi 1/2" al elements 06-22-1997 08:53:38
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Frequency = 28.5 MHz.
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Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
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--------------- WIRES ---------------
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Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in)
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Segs
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1 -8.595, 0.000, 0.000 8.595, 0.000, 0.000 5.00E-01 21
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2 -8.207, 5.200, 0.000 8.207, 5.200, 0.000 5.00E-01 21
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3 -7.722, 11.212, 0.000 7.722, 11.212, 0.000 5.00E-01 21
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-------------- SOURCES --------------
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Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
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Seg. Actual (Specified)
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1 11 2 / 50.00 ( 2 / 50.00) 1.000 0.000 V
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No loads specified
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No transmission lines specified
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Ground type is Free Space
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----------------------------------------------------------------------------
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</pre><br>
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<br>
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<p><i>Updated 6-21-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
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<br></p>
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<p><a href="../radio.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
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<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<meta name="keywords" content="Yagi, HF Upper">
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<title>Build a 3-Element Yagi Index</title>
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<center>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10"> I Want to Build a 3-Element Yagi <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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</center>
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<center>
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<h3>L. B. Cebik, W4RNL</h3>
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</center>
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<center>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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</center><br>
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<br>
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<p>There are many sources of information on how to construct a 3-element Yagi. Some articles are even geared to the builder's first major construction effort with aluminum tubing and associated clamps and hardware. Most of these otherwise fine articles tend to offer a stock design for the 3-element Yagi. They end up giving the new Yagi-user very little information about the electrical design decisions involved in selecting the right 3-element Yagi for one's home station.</p>
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<p>This series of articles, which first appeared in <i>antenneX</i>, is not so much about construction as it is about electrical design. They focus on monoband Yagis so that the data can be clear and relatively precise. Multiband Yagis are advanced design exercises and best left, at the beginning, to the professional designers.</p>
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<p>There are a number of questions the prospective Yagi builder needs to ask himself or herself. Each of the articles attempts to answer one of them--not exhaustively, but at least thoroughly enough to permit you to make intelligent decisions.</p>
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<p>Here are the questions we shall tackle.</p>
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<ul>
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<li><b><a href="3lyg1.html">Part 1: How Big Shall I Make It?</a></b></li>
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<li><b><a href="3lyg2.html">Part 2: How Wide-Band Shall I Make It?</a></b></li>
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||||
<li><b><a href="3lyg3.html">Part 3: What Real Dimensions Shall I Use?</a></b></li>
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||||
<li><b><a href="3lyg4.html">Part 4: How Shall I Feed the Antenna?</a></b></li>
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</ul>
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<center>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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</center>
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<p><i>Updated 07-31-99. © L. B. Cebik, W4RNL. These items appeared in</i> AntenneX <i>between April and July, 1999. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<center>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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</center>
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<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Amateur Radio Page</a></p>
|
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<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<head>
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<meta charset="UTF-8">
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<meta name="keywords" content="Yagi, HF Upper">
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<title>Build a 3-Element Yagi Part 1: How Big Shall I Make It?</title>
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<link rel="stylesheet" href="../styles.css">
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</head>
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<body>
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||||
<center>
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||||
<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"> I Want to Build a 3-Element Yagi<br>
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Part 1: How Big Shall I Make It?</h1>
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</center>
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||||
<center>
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||||
<h3>L. B. Cebik, W4RNL</h3>
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</center>
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<center>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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</center><br>
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<br>
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<p>Those of us with more time than money get the urge to upgrade our antennas from dipoles to beams in the home workshop. The logical first step might be a 2-element Yagi. Often, we skip this step because we have heard or seen evidence that 2-element Yagis do not quite have the specifications we want. If we make use the common reflector-driver design, the gain is low and the front-to-back ratio is poor. If we opt for a driver-director design, we get more gain and better front-to-back ratio, but a very narrow beam width and a potentially very low feedpoint impedance. So we immediately jump to 3-element designs.</p>
|
||||
<p>Our next step is usually a semi-fatal one. We search the magazines and handbooks on our shelves for a design that looks like one we can build. We know what the author claims the beam will do, but we are not ever sure of how it does what it is supposed to do or whether our version is doing the same thing.</p>
|
||||
<p>So let's back up a bit and look at some 3-element Yagi basics. I shall focus on monoband beams, because they are much easier than multi-band beams to model and to build so that their performance matches the model. Even if you have your heart set on buying a tribander, perhaps these basics will help you better to understand both the antenna maker's claims and the performance you actually get.</p>
|
||||
<p>The basics I have in mind have little to do with the theory of parasitic element operation. <i>The ARRL Antenna Book</i> and some references I shall mention along the way do a good job at that level of explanation. Instead, I shall begin in the middle of things with some distinctions among Yagi designs. There are so many Yagi designs floating around today that they can be at first sight a bit bewildering.</p>
|
||||
<p><b>High Performance Designs</b></p>
|
||||
<p>The first decision we have to make is whether we wish to have a wide-band Yagi or a high-performance Yagi. Let's begin with the high-performance designs, since these have very great initial appeal. We shall return later to some wide-band designs.</p>
|
||||
<p>Once we have opted for a high-performance design, we have to figure out what we mean by that term. First, there is the matter of size. The element lengths will not vary by a large amount from one design to another (but they will vary significantly in terms of the performance). So, size turns out to be a question of boom length.</p>
|
||||
<p>James Lawson, W2PV, developed most fully the notion that the gain of a Yagi is highly dependent upon the boom length. His book, <i>Yagi Antenna Design</i>, has become one of the classic references for those interested in basic Yagi theory. Based on his work, many designers have optimized 3-element Yagis of various lengths for maximum performance.</p>
|
||||
<p>Maximum performance is a balanced mix of high gain, good front-to-back performance, and a satisfactory feedpoint impedance. Let's work backwards through the list.</p>
|
||||
<p>For almost any length boom, one can get a little more gain from a Yagi by so spacing and sizing the elements that the feedpoint impedance drops to about 10 Ohms or less. Most high performance designs you will encounter tend to opt for feedpoint impedances closer to 25 Ohms. The reasons are many, but one good reason is a matter of losses. Suppose that the total resistance of all the connections at the feedpoint, including those of a matching system for 50-Ohm coaxial cable, amount to 1 Ohm. (That is a quite high value and can be reduced by good construction practices.) With a 10-Ohm feedpoint impedance, about 9-10% of the power is going to heat. If the feedpoint impedance is 25 Ohms, then the loss is closer to 4%.</p>
|
||||
<p>To this consideration, we can add the fact that it is generally easier to design low-loss matching systems for a 25-to-50 Ohm conversion than for a 10-to-50 Ohm conversion. So 25 Ohms becomes a kind of <i>de facto</i> (but not absolute) limit to a Yagi's feedpoint impedance.</p>
|
||||
<p>For the higher feedpoint impedance, it is also somewhat easier to design a 3-element Yagi whose gain and front-to-back performance figures hold up across such HF bands as 20 and 15 meters--as well as the first MHz of 10 meters. As well, the designs can be replicated in the home shop with moderate building skills and are not so finicky as to require industrial laser measuring equipment in setting the dimensions.</p>
|
||||
<p>Within these constraints, then, lets look at three good designs for 3- element Yagis.</p>
|
||||
<center>
|
||||
<img src="3lyg-1.gif" width="573" height="254" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>As revealed in <b>Fig. 1</b>, the designs I have in mind use three different boom lengths. The long-boom design, shown for 28-29 MHz, is adapted from an optimized design by K6STI. It uses an 11.31' boom, which is nearly 1/3 wl long. The K9EUV design uses a 9.88' boom, and has the additional feature of using equal spacing between elements. (A fourth director element at the same spacing and the same length as the present director provides good 4- element Yagi performance.) The third design is a short boom Yagi, also by K6STI, that requires a 7.5' boom. For construction, you should round these boom-length numbers upward to the next whole foot to provide room for mounting plates and hardware. So we have 12, 10, and 8 foot boom 3-element Yagis.</p>
|
||||
<p><b>Caution</b>: Do <b>NOT</b> build these designs as given here. They all use uniform diameter elements. Converting them to the actual lengths needed by physical designs that employ tubing that decreases in diameter will net us different dimensions. But that is a subject for a future episode.</p>
|
||||
<p>The 10-meter designs all use 0.5" aluminum tubing. We can scale them for any band in the following way. Take the ratio of the new design frequency (for example 14.175 or 21.22 MHz) to the present design frequency (28.5 MHz) and invert it (since lower frequencies will require longer, fatter elements). Now multiply the element lengths, the element spacings, and the element diameters by this figure.</p>
|
||||
<p>As a sample, here is a collection of dimensions for the three basic designs for 10, 15, and 20 meter versions of the antenna.</p>
|
||||
<pre>Long-Boom
|
||||
Frequency (MHz) 28.5 21.22 14.175
|
||||
Element Lengths (feet)
|
||||
Reflector 17.90 23.11 34.56
|
||||
Driver 16.41 22.06 33.00
|
||||
Director 15.44 20.76 31.05
|
||||
Spacing (feet)
|
||||
Ref-Driver 5.2 6.99 10.46
|
||||
Driver-Dir 6.01 8.08 12.09
|
||||
Element dia. (inches) 0.5 0.75 1.0
|
||||
|
||||
Medium-Boom
|
||||
Frequency (MHz) 28.5 21.22 14.175
|
||||
Element Lengths (feet)
|
||||
Reflector 17.27 23.20 34.72
|
||||
Driver 16.63 22.34 33.43
|
||||
Director 15.62 20.99 31.41
|
||||
Spacing (feet)
|
||||
Ref-Driver 4.94 6.63 9.92
|
||||
Driver-Dir 4.94 6.63 9.92
|
||||
Element dia. (inches) 0.5 0.75 1.0
|
||||
|
||||
Short-Boom
|
||||
Frequency (MHz) 28.5 21.22 14.175
|
||||
Element Lengths (feet)
|
||||
Reflector 17.65 23.73 35.50
|
||||
Driver 16.15 21.71 32.47
|
||||
Director 15.41 20.71 30.98
|
||||
Spacing (feet)
|
||||
Ref-Driver 3.00 4.03 6.03
|
||||
Driver-Dir 4.50 6.05 9.05
|
||||
Element dia. (inches) 0.5 0.75 1.0</pre>
|
||||
<p>Clearly, I have violated my own scaling scheme by using the closest standard value of aluminum tubing as the element diameter. Hence, some very small adjustments are required in the element lengths and spacings, but generally, these are too small to make an operational difference. Nonetheless, some of the difference shows up numerically in the design frequency performance figures for the three designs. Here the table divides the designs by bands.</p>
|
||||
<pre>20-Meters: 14.175 MHz
|
||||
Antenna Design Free Space Front-to- Feed Impedance
|
||||
Gain dBi Back dB R +/- jX Ohms
|
||||
Long-Boom 8.11 27.31 25.71 - j 0.93
|
||||
Medium-Boom 7.78 35.50 27.02 + j 0.50
|
||||
Short-Boom 7.13 41.35 27.46 + j 0.01
|
||||
|
||||
15-Meters: 21.22 MHz
|
||||
Antenna Design Free Space Front-to- Feed Impedance
|
||||
Gain dBi Back dB R +/- jX Ohms
|
||||
Long-Boom 8.16 26.12 24.80 + j 0.37
|
||||
Medium-Boom 7.81 34.88 26.06 + j 0.94
|
||||
Short-Boom 7.17 61.67 26.56 - j 0.00
|
||||
|
||||
10-Meters: 28.5 MHz
|
||||
Antenna Design Free Space Front-to- Feed Impedance
|
||||
Gain dBi Back dB R +/- jX Ohms
|
||||
Long-Boom 8.11 27.15 25.70 - j 0.80
|
||||
Medium-Boom 7.80 34.77 26.51 + j 1.69
|
||||
Short-Boom 7.12 41.66 27.45 - j 0.01</pre>
|
||||
<p>These tables of predicted performance from NEC models show a number of things, including how to be misled. First, the numbers--even though slightly different in the decimal columns--illustrate that for each design, the performance is identical on all three bands. The excess decimals are given because NEC reports them and because they show how little different the performance varies as we correctly scale a beam from one band to another.</p>
|
||||
<p>However, the front-to-back figures can mislead us into thinking that the short-boom model has an almost miraculously better front-to-back performance than the other beams, as good as they are. The figure given is for the 180-degree front-to-back ratio, which is usually a very convenient figure to obtain. That figure may not be indicative of the performance of the antenna in all its rearward directions.</p>
|
||||
<center>
|
||||
<img src="3lyg-2.gif" width="639" height="303" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 2</b> provides an overlay of the 28.5 MHz free space azimuth patterns of all three antenna designs. We may note the forward gain peaks, which just about correlate to the differentials in boom length. The rear lobes give us much more valuable information. Here we see that the long-boom design has three small lobes of roughly equal size so that the front-to-back ratio in the table gives a fair appraisal of the overall front-to-rear performance. In contrast, the other two models have deeper 180-degree "dimples," combined with much stronger angling lobes. When averaged out, none of the antennas has much of an advantage over the other. Indeed, anything above 20-dB front-to-back ratio overall might be considered outstanding.</p>
|
||||
<p>To this point, it may seem that the decision we need to make is one of gain advantage vs. mechanical disadvantage to a longer boom. However, let us not be so swift to judgment. For I have once more misled you by giving you the peak or nearly peak performance of the antenna at one frequency. The next question is how each of these designs performs across an entire ham band.</p>
|
||||
<p>Since the first MHz of 10 meters is the greatest stretch for these designs (because 20 and 15 are proportionately narrower), let's sweep the antennas across the 28 to 29 MHz range and see how the performance of each holds up.</p>
|
||||
<center>
|
||||
<img src="3lyg-3.gif" width="646" height="529" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>In <b>Fig. 3</b>, we have the gain of the three designs across the pass band. Note that the gain figures in the tables are intermediate values. The gain across the band can vary by as much as 0.4 dB in at least one of the designs. In selecting a design, we must also ask whether the lowest gain--which occurs at the low end of the band for 3-element Yagi designs--is an acceptable figure (when weighed against all of the other factors involved in our decision to build).</p>
|
||||
<center>
|
||||
<img src="3lyg-4.gif" width="646" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 4</b> records the 180-degree front-to-back ratios across the pass band. Here we can either be impressed by the peak values, or we can take a closer look at the values at the band edges. The short-boom design actually excels in this department, with no value falling below 20 dB. The other two designs are remarkably close in their band edge values, which are about 17 dB at the low end and 15-16 dB at the high end. Before settling on a design for the two lower bands, we should remember that these bands are only 68% to 70% as wide as the 10-meter passband. Hence, we can achieve better band-edge results by centering the peaks in our final design.</p>
|
||||
<center>
|
||||
<img src="3lyg-5.gif" width="644" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>A rough measure of whether we shall be able to effect a wide-band match to a 50-Ohm cable can be derived from the SWR curve of the antenna related to the resonant impedance before matching. Since the feedpoint impedance of all three models is so close to 25 Ohms, we can use that number as representative of the resonant feedpoint impedance. Then we can evaluate the SWR curves in <b>Fig. 5</b>.</p>
|
||||
<p>For all three designs, the SWR climb more quickly above the resonant design frequency than below it. On 10 meters, this means exceeding a 2:1 SWR value at the upper end of the pass band for two of the three designs. However, all three designs will likely show under 2:1 SWR across the narrower reaches of 20 and 15 meters. The short boom design shows the flattest curve.</p>
|
||||
<p>Although the designs were set up for a resonant feedpoint impedance (little or no reactance), one can extend or shrink the driver length as necessary to use any of the common matching system: beta, gamma, Tee. Of course, a 1/4 wl section of 35-37 Ohm coax will provide a matching section to a 50-Ohm main line if we leave the feedpoints resonant. We shall look in more detail at typical matching systems before we are done.</p>
|
||||
<p>So it looks like we are almost ready to make a decision. Seemingly, all we need to do is to look at how the elements might change if we use several diameters of tubing for each one.</p>
|
||||
<p>Not so fast. We have not yet looked at truly wide-band designs to see if they hold any advantages and what those advantages might be. So while we are still in this preliminary stage of design evaluation, let's look next month at a pair of wide-band Yagi designs.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 07-31-99. © L. B. Cebik, W4RNL. This item appeared in</i> antenneX<i>, April, 1999. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
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|
||||
<img src="../images/colorbar.gif" width="591" height="3">
|
||||
</center>
|
||||
<p><a href="3lyg2.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Part 2: How Wide-Band Shall I Make It?</a></p>
|
||||
<p><a href="3lyg.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Index</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
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|
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|
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<head>
|
||||
<meta charset="UTF-8">
|
||||
<meta name="keywords" content="Yagi, HF Upper">
|
||||
<title>Build a 3-Element Yagi Part 2: How Wide-Band Shall I Make It?</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
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||||
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|
||||
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||||
<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"> I Want to Build a 3-Element Yagi<br>
|
||||
Part 2: How Wide-Band Shall I Make It?</h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
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|
||||
<br>
|
||||
<p>Last time, we looked at the basic design of high-performance 3-element Yagis. All had similar front-to-back and feedpoint impedance characteristics. They differed in gain in accord with the length of their booms. For 10-meter models, here are the design center figures as a reference for this month's investigation:</p>
|
||||
<pre>10-Meters: 28.5 MHz
|
||||
Antenna Design Free Space Front-to- Feed Impedance
|
||||
Gain dBi Back dB R +/- jX Ohms
|
||||
Long-Boom 12' 8.11 27.15 25.70 - j 0.80
|
||||
Medium-Boom 10' 7.80 34.77 26.51 + j 1.69
|
||||
Short-Boom 8' 7.12 41.66 27.45 - j 0.01</pre>
|
||||
<p>Each of these designs is capable of performance close to that at the design frequency across the first MHz of 10 meters. When scaled to 20 and 15 meters, the designs will cover the entire bands--and, of course, cover the WARC bands when scaled to those frequencies.</p>
|
||||
<p>However, there is an alternative design philosophy. This philosophy sacrifices some gain in favor of wide-band characteristics. For example, the high-performance models might vary in gain by almost 0.5 dB across the span from 28 to 29 MHz. A wide-band design might vary in gain across the same span by only 0.15 dB. Likewise, a high-performance design with a feedpoint impedance in the mid-20s might just barely allow a 2:1 SWR at the band edges. A wide-band design might show those figures over the entire 10-meter band, with a corresponding shallow SWR curve within the first MHz of 10 meters.</p>
|
||||
<p>Consequently, before we freeze our design decisions, let's look at the basic wide-band 3-element Yagi design a little more closely.</p>
|
||||
<p><b>Wide-Band 3-Element Yagi Design</b></p>
|
||||
<p>To create a wide-band 3-element Yagi requires more boom length for a given gain than used in the high-performance designs we surveyed last time. A 12' boom on 10 meters will yield about 1 dB less gain. What, then, is the rationale for such a design?</p>
|
||||
<p>The fundamental reasons for going to a wide-band design are two:</p>
|
||||
<p>1. The wide-band design provides reasonably smooth and quite adequate performance figures for the entire 10-meter band, which is 1.7 MHz wide. Hence, a single beam will cover the CW/SSB and the FM/satellite portions of the band. When scaled to 20 or 15 meters, the beam will show less change in gain, front-to-back ratio, and feedpoint impedance across those bands.</p>
|
||||
<p>2. The wide-band design can be set for a feedpoint impedance very close to 50 Ohms, thus eliminating the need for any sort of matching network at the feedpoint. (However, a choke or 1:1 choke balun remains recommended to attenuate common mode currents.)</p>
|
||||
<p>When these two reasons are combined, the result is a very close match to a standard 50-Ohm coaxial cable feedline with very small changes in SWR across the band of choice. This eases not only matching problems, but as well the sensitivity of some transmitting equipment to even low SWR levels that often result in automated power output reductions. Accompanying this feature is consistent gain and front-to-back ratio performance across the band.</p>
|
||||
<p>There is an additional feature occasionally anecdotally noted by some operators. They claim superior performance results from Yagis with higher input impedances. Since these claims do not show themselves in NEC models, the vehicle we are using to evaluate performance potential, no comment can be made upon those experiences in this context.</p>
|
||||
<p>Since the performance of wide-band models shows up most clearly on 10-meter models, we shall focus solely upon them. You can extrapolate the most relevant portions of performance curves for the other HF bands.</p>
|
||||
<p>Two notable designs for wide-band 3-element Yagis have appeared in US antenna literature in the last decade. In the May, 1990, issue of <i>Ham Radio</i>, Bill Orr, W6SAI presented a wide-band design in one of his columns. In the Winter, 1998, issue of <i>Communications Quarterly</i>, Joe Reisert, W1JR, present a similar design, along with scalable design equations.</p>
|
||||
<center>
|
||||
<img src="3lyg-6.gif" width="638" height="253" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 6</b> shows schematically both the Orr and the Reisert designs, set for 10 meters. As we noted in connection with the high-performance designs, do NOT commit to using these dimensions to construct either design. Both are predicated on uniform diameter elements: 0.5" for the Reisert design, 1.0" for the Orr design. In common practice, Yagi elements consist of two or more sections of tubing each side of center, each section having a smaller diameter than the preceding one. This tapered-diameter schedule results in a need to lengthen elements relative to uniform-diameter models. We shall explore this topic next time.</p>
|
||||
<p>For the moment, we want to look at the wide-band designs as concepts rather than as finished products. Both the designs require 12' booms (with allowance at each end for element-mounting hardware). The dimensions of the two antennas are sufficiently close to each other that we suspect similar performance will emerge from the antennas. And we get it.</p>
|
||||
<center>
|
||||
<img src="3lyg-7.gif" width="639" height="321" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The free-space azimuth patterns in <b>Fig. 7</b> overlay the Orr and Reisert designs at their resonant frequencies. The differences are too small to require comment. Gain is within 0.02 dB; front-to-back difference is less than 1 dB; and the feed point impedances are within 1 Ohm of each other at resonance. For reference, here is a handy table of values:</p>
|
||||
<pre>Antenna Resonant Free Space Front-to- Feed Impedance
|
||||
Freq. MHz Gain dBi Back dB R +/- jX Ohms
|
||||
Orr, 1990 28.80 7.11 21.60 47.08 + j 0.98
|
||||
Reisert, 1998 28.85 7.13 20.92 46.01 + j 0.20</pre>
|
||||
<p>The slight differential in resonant frequency owes to the slight design differences, which in turn yield slightly different performance curves on 10 meters. Moving the resonant point allows one to obtain a satisfactory SWR curve across the band. We can easily sample the performance by running frequency sweeps. In the following graphs, I have used 29.5 MHz as an arbitrary upper limit in order to limit the number of data points.</p>
|
||||
<center>
|
||||
<img src="3lyg-8.gif" width="644" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The free-space gain figures for the two designs appear in <b>Fig. 8</b>. The Reisert model shows the smoother gain curve, although the differences are marginal. Note that in wide-band designs, there is often a dip in the gain at the low end of the band. The dip reaches minimum for the Orr design below the lower end of 10 meters.</p>
|
||||
<p>If we translate the first MHz of the band into roughly equivalent performance across 20 or 15 meters, the Reisert design, especially, shows a very small change in gain. The consistency of performance across the band is one of the advantages of a wide-band design.</p>
|
||||
<center>
|
||||
<img src="3lyg-9.gif" width="645" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>What the Orr design gives up in gain, it reacquires in front-to-back performance, as shown in <b>Fig. 9</b>. If we use a 20-dB front-to-back ratio as a plateau figure of merit, then the Orr design would show this across all of 20 and 15 meters. However, the Reisert design averages only about 1 dB less performance in this category, an amount unlikely to be either noticed or measurable.</p>
|
||||
<p>With either design, the 180-degree front-to-back ratio used in the graph is a reasonable approximation of overall front-to-rear performance. The smooth rear quadrant lobe shown earlier in the azimuth patterns ensures a reasonable correlation between the 180-degree front-to-back ratio and the other two common measure of rearward performance: an averaged front-to- rear ratio and a worst-case front-to-back performance figure.</p>
|
||||
<center>
|
||||
<img src="3lyg-10.gif" width="645" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Where the wide-band designs shine is in the flatness of the SWR curves. As shown in <b>Fig. 10</b>, the 50-Ohm SWR curves for both designs permit a ready match directly to 50-Ohm feedline across 10 meters. If the curve centers are translated into equivalent 20 and 15 meter versions, achieving a maximum band-edge SWR of 1.35:1 is no real challenge.</p>
|
||||
<p>For those willing to accept a slightly lower gain on the HF band of choice, the wide-band design offers consistent performance across the band with an ease of matching to 50-Ohm feedlines systems that the high-performance models cannot duplicate. The wide-band models, however, offer only the gain that the high-performance models achieve with a boom 2/3 as long.</p>
|
||||
<p>Before we close out this preliminary survey of designs, let's take a moment to review what we are seeking to do when we decide that we want to build a 3-element Yagi.</p>
|
||||
<p><b>A Pair of 2-Element Yagi Designs</b></p>
|
||||
<p>Although we can design a 2-element driver-director Yagi of excellent performance and very compact size for the WARC bands, covering the wider HF bands requires that we look to 2-element driver-reflector designs. In order to see just why we might want a 3-element version, let's look at two versions of the 2-element Yagi. One uses narrow spacing, the other wide spacing.</p>
|
||||
<center>
|
||||
<img src="3lyg-11.gif" width="407" height="365" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 11</b> shows the dimensions of both models, which use 1" uniform diameter elements. In these very standard designs, the element lengths are quite similar, with only small changes needed to accommodate the difference in boom length. At the design frequency of 28.5 MHz, the short boom is just under 1/8 wavelength long, while the long boom is somewhat over 1/6 wavelength long. At the design frequency, the modeled performance characteristics are these:</p>
|
||||
<pre>Antenna Design Free Space Front-to- Feed Impedance
|
||||
Freq. MHz Gain dBi Back dB R +/- jX Ohms
|
||||
Narrow Space 28.50 6.27 11.24 32.65 - j 0.32
|
||||
Wide Space 28.50 6.13 10.65 51.71 + j 9.23</pre>
|
||||
<p>As the table makes clear, the design frequency gain is about 1 dB less than that of the wide-band 3-element Yagi and 2 dB less than the 12' boom high- performance version.</p>
|
||||
<center>
|
||||
<img src="3lyg-12.gif" width="645" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The gain curve in <b>Fig. 12</b> shows that, contrary to the curves for Yagis with a director, the gain for a driver-reflector Yagi descends as the frequency increases. Were we inclined to mislead you, we might do one of two things. First, we might suggest--based on the gain value for ONLY the low end of 10 meters--that there is little difference in performance between 2- and 3-element Yagis, using either the short boom high-performance or the wide-band 3-element Yagi as a comparator. Second, using ONLY the gain value for the high end of the pass band graphed in the figure, we might try to convince you that a 3-element Yagi has an inordinately high gain advantage over the 2-element version. Both claims would be equally inaccurate. Only a graph of values over a relevant frequency range tells a full story.</p>
|
||||
<p>With respect to the 2 2-element designs, the narrow-spaced version has the higher gain across the pass band. For this exercise, only the first MHz of 10 meters is shown, although the wide-spaced version will be usable across the entire 10-meter band.</p>
|
||||
<center>
|
||||
<img src="3lyg-13.gif" width="646" height="529" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The overall front-to-back ratio of a 2-element driver-reflector Yagi is fairly weak, failing to reach 12 dB as shown in <b>Fig. 13</b>. (Note: this weaker front-to-back ratio can be an advantage in certain kinds of net and contest operations where total exclusion of signals from the rear quadrants can be a hindrance to efficient operation.) The narrow-spaced version has the stronger values, but the curve is clearly sharper than the gentler slope of the values for the wide-spaced version. The wide-spaced model front-to-back ratio is about 10 dB at the upper limit of the 10-meter band.</p>
|
||||
<center>
|
||||
<img src="3lyg-14.gif" width="647" height="528" border="2" vspace="5" hspace="9">
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||||
</center>
|
||||
<p>The SWR curves in <b>Fig. 14</b> for the two models tell different stories. The sharp curve for the narrow-spaced model has a resonant point of about 32.5 Ohms. Hence, the antenna will require a matching network for a 50-Ohm main feedline. In contrast, the curve for the wide-spaced model is based on 50 Ohms. The curve has been intentionally shifted downward in frequency to show the shallow rise in value above the lowest value. By moving the center point of the curve upward in frequency simply by shortening the driver element slightly, the SWR can be held below 2:1 across the entire 10-meter band. This permits direct matching to a coax line with no matching network needed (but with the recommended choke or choke balun in place, as always).</p>
|
||||
<p>With the 2-element Yagi curves in mind, we can make more precise our reasons for wanting to move to a 3-element Yagi.</p>
|
||||
<p>1. Most prominently, for operations that need it, the front-to-back values for all versions of the 3-element Yagi are considerably superior to those of the 2-element driver-reflector Yagi.</p>
|
||||
<p>2. The gain of a high-performance, long-boom 3-element Yagi show about a 2 dB advantage over the 2-element Yagis. This amount of gain is significant. The short-boom and the wide-band 3-element Yagis show about half that value of extra gain, and the significance of the difference is reduced accordingly.</p>
|
||||
<p>3. Both 2- and 3-element Yagis can be designed for 50-Ohm feedpoint impedances that hold up even across the wide reaches of 10-meters. However, in both 2- and 3-element designs, narrower spacing allows more gain per unit-length of boom.</p>
|
||||
<center>
|
||||
<img src="3lyg-15.gif" width="640" height="380" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 15</b> shows comparative free-space azimuth patterns for the narrow-spaced 2-element Yagi, the wide-band 3-element Yagi, and the 12' boom high-performance Yagi. The steps of performance improvement are clear in the graphic. What the patterns cannot show is that, where narrower band operation and a 25-Ohm feedpoint impedance are acceptable, the short-boom 3-element Yagi will provide roughly the same gain and front-to-back ratio as the wide-band model with 2/3 the boom length.</p>
|
||||
<p>In the end, the final decision as to which design meets our needs will use some mixture of the performance figures, plus a measure of the physical properties of the structure we propose to build. Hence, every such decision will be very individualized. The various models we have presented are intended only to provide some comparative measures as a background against which to make the final decision.</p>
|
||||
<p>Still, we have not addressed some of the structural considerations that go into the decision. Boom length so far is a matter of a number, not a real physical property. As well, we need to look at what real tapered-diameter elements may imply for the antenna design. Both of these questions mean that we shall have to add a "Part 3" to this series--next time.</p>
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<p><i>Updated 07-31-99. © L. B. Cebik, W4RNL. This item appeared in</i> antenneX<i>, May, 1999. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="3lyg3.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Part 3: What Real Dimensions Shall I Use?</a></p>
|
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<p><a href="3lyg.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Index</a></p>
|
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<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>Build a 3-Element Yagi Part 3: What Real Dimensions Shall I Use?</title>
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"> I Want to Build a 3-Element Yagi<br>
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Part 3: What Real Dimensions Shall I Use?</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<img src="../images/colorbar.gif" align="middle" width="540" height="4">
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<br>
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<p>Had the 3-element Yagis we explored in the first two parts of this investigation been designed for VHF, we might have been able to use the dimensions as given. At VHF and above, we often use elements with uniform diameters. However, at HF, the most common practice is to use elements composed of tubing whose diameter decreases as we move outward from the element center. That makes a big difference--perhaps the difference between a beam that works and one that only seems to work.</p>
|
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<p><b>Tapered-Diameter Elements</b></p>
|
||||
<p>The relationship between uniform-diameter and tapered-diameter elements has long been guessed at, but only in the last decade or two has it been fully appreciated. Perhaps the most complete treatment of that relationship appears in David Leeson, W6QHS, <i>Physical Design of Yagi Antennas</i>. Leeson's conversion equations are built into some antenna modeling software (EZNEC and NEC-Win Plus).</p>
|
||||
<p>When an element in the vicinity of 1/2 wavelength long is composed of materials which taper downward in diameter away from the center point, the required length for resonance increases relative to an element of equivalent uniform diameter. Moreover, the equivalent uniform diameter is not simply the average diameter of the materials used.</p>
|
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<center>
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<img src="3lyg-16.gif" width="632" height="416" border="2" vspace="5" hspace="9">
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</center>
|
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<p><b>Fig. 16</b> gives us some examples to explore. Each shows 1/2 of a resonant aluminum element for 28.5 MHz. At the top, the element is roughly equally divided. The second element uses the same sequence of tubing, but in unequal lengths. The bottom element uses the same inner section lengths as the first example, but with a steeper taper.</p>
|
||||
<p>In this example, and in all element-diameter taper schedules, only the exposed part of each tubing size is shown. Where one tube fits inside another, an additional overlap length of about 3" is generally recommended. Anything more tends to add weight without adding strength, and much less than the 3" overlap may jeopardize junction strength.</p>
|
||||
<p>The examples show that the required length for resonance changes if we change the relative lengths for a given taper schedule or if we change the taper schedule itself. The following table can make the changes even clearer.</p>
|
||||
<pre>Example Required Average Equivalent Length of Feed Z
|
||||
(Fig. 16) Length in Dia. in. Dia. inches Equiv. El. R +/- jX
|
||||
Top 100.8 0.623 0.631 98.58 71.8-j0.1
|
||||
Middle 100.6 0.634 0.598 98.67 71.8-j0.1
|
||||
Bottom 102.4 0.742 0.739 98.39 71.8-j0.2</pre>
|
||||
<p>The more radical the taper, the longer the required length for resonance. Variable length sections alter the length and the equivalent uniform diameter. Sometimes the equivalent diameter is close to the average--and sometimes not.</p>
|
||||
<p>Many home Yagi builders tend to freely substitute materials in a design, unconcerned about the changes in element length that may be required to get the same performance as the original. These same builders often lack any means of checking performance other than an SWR meter. Hence, they assumed that if the antenna has the same feedpoint characteristics as the original, it must yield the same performance. The assumption is very often a bad one.</p>
|
||||
<p>Without sophisticated antenna test range equipment, the best method available to check a Yagi without any element loading for performance is antenna modeling software. Both MININEC and NEC (the latter with tapered- diameter corrections built in) have been calibrated and tested extensively, and Yagi designs that pass the modeling test generally perform as modeled. There are almost always slight variations between the model and the physical version of the antenna, but in most cases, they are far smaller and less detrimental to performance than the guess work that marked the first 30 years of home Yagi design after World War II.</p>
|
||||
<p>Let's look at some dimensions for 3-element monoband Yagis using various diameter taper schedules for 10, 15, and 20 meters. The differences from our uniform-diameter study models may be instructive. The following notes will use common US aluminum tubing sizes, which tend to come in 1/8" increments. A common wall size of 0.58" permits succeeding tube sizes to mate closely and still have enough play to slide one within another with ease. Since builders in other parts of the world must work with available sizes, usually specified in millimeters, here is a small table of equivalencies.</p>
|
||||
<pre>US Size Size in mm US Size Size in mm
|
||||
in inches in inches
|
||||
0.375 9.5 1.0 25.4
|
||||
0.5 12.7 1.125 28.6
|
||||
0.625 15.9 1.25 31.8
|
||||
0.75 19.1 1.5 38.1
|
||||
0.875 22.2 2.0 50.8</pre>
|
||||
<p>Because non-US antenna elements will likely have a different set of element diameter transitions, it is especially important to remodel a Yagi design when changing the element dimensions from English to metric.</p>
|
||||
<p><b>Some 10-Meter Designs</b></p>
|
||||
<p>We shall look at only two of the many possible element taper schedules one might use for 10-meter beams. See <b>Fig. 17</b>.</p>
|
||||
<center>
|
||||
<img src="3lyg-17.gif" width="508" height="356" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Following the work of K6STI, we shall call the upper schedule the high- loading schedule, because it was thought to be better able to withstand higher winds. The lower version is called the medium wind load schedule. Other schedules are not only possible, but are in use by both home builders and commercial manufacturers. Some beam-makers prefer heavier, more rigid element designs, while others prefer more "aggressive" tapers that flex without breaking. End diameters of 7/16" and 3/8" are used in some commercial designs. A builder who wishes to depart from one of the "ordinary" element designs may wish to consult a computer program called YagiStress to check the survivability potential of a proposed element diameter schedule.</p>
|
||||
<p>As a consequence of the many possible element designs, we can only sample a couple, with the goal of ultimately comparing tapered element lengths to the uniform lengths used in our initial look at different Yagi design philosophies. Our point will be, in part, to verify that the performance promised by the uniform-diameter designs can be obtained with tapered elements.</p>
|
||||
<p>10 meters is a good starting point, because the diameter changes in a given element are the fewest. Let's begin by comparing the dimensions of our short boom study design with version built from the taper schedules shown in <b>Fig. 17</b>. Dimensions will be in inches. The "end" section length will simply be the difference of half the total length and the distance from element center up to the start of the last tubing section.</p>
|
||||
<pre>Antenna Uniform-Dia (0.5") High Load Medium Load
|
||||
Dimension
|
||||
Refl. Length 211.8 216.0 215.0
|
||||
Refl.-DE Space 36.0 36.0 36.0
|
||||
DE Length 193.8 203.6 202.0
|
||||
DE-Dir. Space 54.0 54.0 54.0
|
||||
Dir. Length 184.9 189.0 187.2</pre>
|
||||
<p>All of these antennas use the same element spacing. The parasitical element dimension extension of the tapered-element designs runs from 3 to 5 inches, but the driven element required for resonance is 8 to 10 inches longer than the uniform element.</p>
|
||||
<p>One evidence that the antennas are comparable is the feedpoint impedance (R +/- jX Ohms) at 28, 28.5, and 29 MHz.</p>
|
||||
<pre>Antenna Uniform-Dia (0.5") High Load Medium Load
|
||||
Frequency
|
||||
28.0 27.0 - j 11.5 28.8 - j 11.0 30.0 - j 10.6
|
||||
28.5 27.5 - j 0.0 28.9 - j 0.3 30.1 + j 0.2
|
||||
29.0 22.6 + j 14.0 23.5 + j 13.6 24.7 + j 13.9</pre>
|
||||
<p>We can make a similar comparison between the uniform-diameter wide-band Yagi and a tapered version. Let's use the Reisert design, along with a "high-load" tapered version of the same antenna.</p>
|
||||
<pre>Antenna Uniform-Dia (0.5") High Load
|
||||
Dimension
|
||||
Refl. Length 211.8 214.8
|
||||
Refl.-DE Space 69.2 70.0
|
||||
DE Length 193.8 199.2
|
||||
DE-Dir. Space 63.8 63.0
|
||||
Dir. Length 177.8 184.8</pre>
|
||||
<p>In this case, we have made some slight but important changes in the element spacing to achieve performance across the entire 10-meter band, mainly in centering the front-to-back curve so that performance at the band edges is comparable. Both the original and the tapered version of the antenna provide under 2:1 50-Ohm SWR across the band. However, the dimensions of the tapered version differ significantly from the uniform-diameter version. Thus, even if a designer provides neat equations for calculating uniform- diameter elements, those equations disappear from utility once one moves to tapered-diameter elements.</p>
|
||||
<p>To establish that the modified beams perform like the originals, let's look at a pair of graphs.</p>
|
||||
<center>
|
||||
<img src="3lyg-18.gif" width="661" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 18</b> provides gain curves from 28 to 29 MHz for all 5 Yagis in the tables above. The gain values for the high-performance, short boom models are within a few hundredths of a dB of each other. The difference between the tapered and untapered wide-band Yagis averages under 0.1 dB.</p>
|
||||
<center>
|
||||
<img src="3lyg-19.gif" width="656" height="525" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The front-to-back curves in <b>Fig. 19</b> tell a very similar story. All three of the high-performance, short boom Yagis show the same high mid-band peak in 180-degree front-to-back ratio, with similar pass band edge performance. The wide-band models show insignificant differences and also end up at very similar values at the wider band edges of the design. In short, with some care in adjusting element lengths--and occasionally the element spacing-- one can achieve the same basic performance from a tapered-diameter Yagi as from a uniform-diameter version of the same design.</p>
|
||||
<p><b>Some 15-Meter Designs</b></p>
|
||||
<p>In <b>Fig. 20</b>, we can examine the differences between high-load and medium- load element tapers for 15 meters.</p>
|
||||
<center>
|
||||
<img src="3lyg-20.gif" width="635" height="322" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Obviously, each level requires an additional, larger tubing segment to accommodate the longer element. Corresponding design dimensions for the original uniform-tape design and tapered-diameter designs appear in the following table.</p>
|
||||
<pre>Antenna Uniform-Dia (0.75") High Load Medium Load
|
||||
Dimension
|
||||
Refl. Length 284.4 289.0 286.0
|
||||
Refl.-DE Space 48.4 48.0 48.0
|
||||
DE Length 260.5 274.0 271.0
|
||||
DE-Dir. Space 72.6 92.0 92.0
|
||||
Dir. Length 248.5 254.0 251.0</pre>
|
||||
<p>The difference in overall boom length is immediately noticeable--a matter of 19" or over 10% of the total length. In some respects, the tapered- element beams are not the same design as the study model, although the longer elements required by element-diameter tapering are apparent. The 15-meter study model was scaled directly from the 10-meter model, while the taper-diameter models came from work by K6STI.</p>
|
||||
<p>One consequence of the longer boom with about the same reflector-driven element spacing is a lower feedpoint impedance. The following table shows the difference:</p>
|
||||
<pre>Antenna Uniform-Dia (0.75") High Load Medium Load
|
||||
Frequency
|
||||
21.0 27.4 - j 6.2 20.5 - j 9.2 21.4 - j 9.9
|
||||
21.225 26.6 - j 0.0 20.4 + j 1.0 22.1 + j 0.3
|
||||
21.45 23.5 + j 9.0 20.7 + j 10.0 22.5 + j 9.4</pre>
|
||||
<p>The shorter, uniform-diameter model would permit more versatility in matching and would likely be amenable even to a 35-Ohm 1/4 wl matching section. The lower impedances of the longer tapered-diameter models would likely require a beta match or similar, with suitable adjustments to the length of the driven element.</p>
|
||||
<p>However, there may be compensation for the lower source impedance.</p>
|
||||
<center>
|
||||
<img src="3lyg-21.gif" width="646" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The gain curves in <b>Fig. 21</b> show the nearly half dB gain advantage accrued from using a longer boom. Not only is the gain higher, but it changes less across the 15-meter band.</p>
|
||||
<center>
|
||||
<img src="3lyg-22.gif" width="646" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>In <b>Fig. 22</b>, we discover that all three models have excellent front-to-back ratios, never worse than 20 dB. Since the peak value reflects a deep dimple in the rearward lobes, differences have less importance: roughly equally high marks go to all three designs.</p>
|
||||
<p>In the end, the decision comes down to whether the lower feedpoint impedance and the 19" of additional boom are worth the added gain. The shorter length boom with tapered elements can be developed into a beam having characteristics very close to the study model--as we saw in the case of 10-meter beams.</p>
|
||||
<p><b>Some 20-Meter Designs</b></p>
|
||||
<p>Tapered element design can become quite complex as we move to the 20-meter band. See <b>Fig. 23</b>.</p>
|
||||
<center>
|
||||
<img src="3lyg-23.gif" width="587" height="379" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The medium load element adds one more section to the corresponding 15-meter element. However, the high-load model adds two sections. In addition, the 1/4" jump in diameter as we enter the center section of the element generally suggests that two tubing sections of the same length are used: a 1.125" diameter section inside the 1.25" section.</p>
|
||||
<p>Once more, if we turn to our short-boom model, we obtain dimensions of the following values.</p>
|
||||
<pre>Antenna Uniform-Dia (1.0") High Load Medium Load
|
||||
Dimension
|
||||
Refl. Length 425.9 447.3 438.0
|
||||
Refl.-DE Space 72.4 80.0 90.0
|
||||
DE Length 389.6 421.6 414.0
|
||||
DE-Dir. Space 108.6 106.0 106.0
|
||||
Dir. Length 371.7 393.3 386.0</pre>
|
||||
<p>Despite the use of larger diameter sections in each element, the high-load element has the more radical taper and results in a greater required element length at each position than the thinner but less radically tapered medium-load element. The element position adjustments, necessary to sustain performance across the 20-meter band, elevate the source impedance from about 25 Ohms to about 30 Ohms.</p>
|
||||
<p>A similar significant lengthening occurs with the long-boom (24') high performance Yagi design.</p>
|
||||
<pre>Antenna Uniform-Dia (1.0") High Load
|
||||
Dimension
|
||||
Refl. Length 414.7 433.8
|
||||
Refl.-DE Space 125.5 125.5
|
||||
DE Length 396.0 415.0
|
||||
DE-Dir. Space 145.1 145.1
|
||||
Dir. Length 372.6 391.0</pre>
|
||||
<p>Note that the boom length is here preserved. The average element length increase is about 19" for the radically tapered high-load element or nearly 5% of the total element length.</p>
|
||||
<center>
|
||||
<img src="3lyg-24.gif" width="645" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The gain curves in <b>Fig. 24</b> show that for each type of beam, we obtain similar gain curves relative to untapered and tapered models.</p>
|
||||
<center>
|
||||
<img src="3lyg-25.gif" width="646" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>In <b>Fig. 25</b>, we can see the front-to-back performance. The two high- performance models have values that closely overlap. The short-boom models place their front-to-back peaks at different frequencies. If the high-load model peak were moved upward in frequency by just a bit, its band-edge performance would closely parallel the other two models.</p>
|
||||
<p><b>Some Miscellany</b></p>
|
||||
<p>The point of our exercises has been to familiarize you with the effects of element-diameter tapering on overall Yagi dimensions. Moving from a uniform-diameter design to one that includes multiple tubing sizes requires careful redesign to take into account the actual element-diameter taper used. This is no casual design process, but rather a task of optimizing dimensions throughout the design. Programs such as YO and YagiMax can mechanize the process to a great degree.</p>
|
||||
<p>The particular models used in this exercise do not mean that any particular design is recommended. However, the range of models does suggest a set of standards, relative to performance expectations and boom length.</p>
|
||||
<p>Several items have not be covered. For example, the models surveyed so far all assume insulated mounting relative to the boom. We have not looked at the so-called "plumber's delight" element mounting system. One technique for dealing with the effects of direct electrical mounting to the boom is to create a short, large-diameter wire at the very center of the element.</p>
|
||||
<p>Also missing is a consideration of the requirements and the hazards of long booms. When viewed from the mast-to-boom mounting assembly, the boom is a pair of lever arms. The longer the arm, the greater force exerted by an element of some preset size. Seemingly small increases in boom length can require close attention to boom strength. The increase in boom length can also create larger wind forces as viewed from mast. Nonetheless, very durable assemblies have been created for Yagis far larger then our 3-element models.</p>
|
||||
<p>Although the high number of variables involved in the physical structure of 3-element Yagis would make a short discussion less than useful, there is a subject to which we might devote one more part in this series: feeding and matching the antenna to a standard 50-Ohm coaxial cable feedline.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 07-31-99. © L. B. Cebik, W4RNL. This item appeared in</i> AntenneX<i>, June, 1999. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" width="591" height="3">
|
||||
</center>
|
||||
<p><a href="3lyg4.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Part 4: How Shall I Feed the Antenna?</a></p>
|
||||
<p><a href="3lyg.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Index</a></p>
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<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>Build a 3-Element Yagi Part 4: How Shall I Feed the Antenna?</title>
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<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10"> I Want to Build a 3-Element Yagi<br>
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Part 4: How Shall I Feed the Antenna?</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Because our purpose has been to look at the basic electronic design aspects of the 3-element Yagi, we have omitted structural details. We shall also bypass matters of supporting and rotating the antenna, even though they involve some interesting facets of horizontal antenna performance at various heights above ground. Most of these items are relevant to almost any horizontal antenna, and we wish to remain focused on the 3-element monoband Yagi.</p>
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<p>One detail that we dare not overlook is the method of feeding the Yagi. Actually, the term "feeding" is a bit of a misnomer, since it involves the establishment of an impedance match between the antenna and the main feedline. We shall assume that the main feedline is a standard 50-Ohm coaxial cable. (You may, of course, feed the Yagi with any parallel feedline, using an antenna tuner at the shack end of the line. This would permit use of the antenna on many bands, although directional characteristics would be optimal on only one band. However, some remnant directivity may exist on bands adjacent to the one for which the Yagi is designed.)</p>
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<p>Feeding the wide-band designs is simplicity itself. To attenuate common mode currents, a choke or 1:1 choke balun is recommended at the antenna feedpoint. Designs ranging from 1:1 toroidal baluns to ferrite-bead baluns to a coil of coax sized for the band in use appear in the standard references, such as the <i>ARRL Antenna Book</i>.</p>
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<p>Feeding the high-performance beams we have surveyed is another matter entirely. For these designs, the antenna feedpoint impedance ranged from 20 to 30 Ohms. All of the driven elements were set to resonance, but we can make any adjustments necessitated by the type of matching system we use. Changing the length of a driver by 2 to 3% makes no operational difference in the antenna performance, although it may change the resistive component of feedpoint impedance by a few Ohms. However, it will be the change in the reactive component of the feedpoint impedance that will be most critical to adjusting the matching system.</p>
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<p>One common criticism of using matching networks with Yagis is that they introduce losses. However, calculations show that with reasonable care in design, these losses no where exceed 2% of the applied power, a loss of only about 0.08 dB. By contrast, most 3-element Yagi designs that do not need a matching network have gains that are near to or exceed 1 dB lower than those of the high-performance designs. Hence, the use of a matching system should not be considered a negative aspect of a design, so long as the feedpoint impedance does not go too low. Good construction and maintenance practice is, of course, assumed in this note.</p>
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<p><b>Tees and Gammas</b></p>
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<p>Two related forms of feeding a Yagi and at the same time elevating the feedpoint impedance to 50 Ohms are the Tee and the Gamma. (The "Tee" is actually named the "T" for its shape, but adding the "ee" ensures that the single letter is not missed by the reader.) Both methods of feed permit the direct physical and electrical connection of the driven element to the boom.</p>
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<img src="3lyg-26.gif" width="531" height="382" border="2" vspace="5" hspace="9">
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<p>As <b>Fig. 26</b> shows, we make our feedline connection to a rod or double rod that parallels the element for a distance, at which point we connect a shorting strap to the rod and the element. The gamma match permits a direct conversion of the antenna feedpoint impedance to 50 Ohms. In the gamma match, the coax braid is connected directly to the electrical center of the element.</p>
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<p>The Tee double rod system is most effective in raising the impedance to a much higher value than 50 Ohms. 200 Ohms is a convenient value, since a 4:1 balun then provides a fine 50-Ohm match, while performing the task of attenuating common mode currents at the same time.</p>
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<p>In VHF beams, many builders have noted and measured some pattern distortion with the gamma match. Hence, the Tee has almost become a standard for beams having other than a 50-Ohm feedpoint impedance. HF beam makers have more confidence in the gamma match, whose parts are a relatively smaller proportion of the driver structure than they are at VHF and UHF. However, the Tee is inherently balanced.</p>
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<p>The dimensions of both matching systems depend upon the relative sizes of and the spacing between the element and the rod material. Calculations are best done with software specifically designed for the task. YO (Yagi Optimizer), by K6STI, for example, has such auxiliary software, and independent utility calculation programs are available as freeware from many sources.</p>
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<p>Note that the sketches in <b>Fig. 26</b> did not include a series capacitor at the feedpoint and rod junction, a standard feature in many basic system sketches. In most instances, variable capacitors do not withstand well the rigors of weather. Moreover, the reactance of the system can be set (or nulled, depending upon one's perspective) by adjusting the length of the driven element.</p>
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<p><b>The Beta Match</b></p>
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<p>If you are willing to use a driven element that is insulated from the boom, matching networks become very much simplified. Let's begin with the beta match. Many hams distrust the beta match because the small "hairpin" they see across the feedpoint looks like a short circuit. They just do not understand how the beta match works.</p>
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<img src="3lyg-27.gif" width="538" height="422" border="2" vspace="5" hspace="9">
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<p>In <b>Fig. 27</b>, we see a common L-network to match a source impedance that is high to a load impedance that is lower. One way to accomplish this is the place a shunt (or parallel) inductor across the source and add a series capacitor to the load. The network arrangement assumes that both the source and the load impedance are purely resistive.</p>
|
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<p>The equations for calculating the proper values for an L-network are very simple. First, let's calculate delta, a figure of "merit" (sometimes called network Q) for the proposed network. For this purpose, we can use the equation</p>
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<p>where Rs is the source resistive impedance and Ra is the load resistive impedance. As noted., Rs is higher than Ra. From delta and the values for Rs and Ra, we can calculate the reactances of the shunt and series components of the network:</p>
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<p>Xa, the series reactance, can be converted into a value of capacitance for the design frequency, although we shall not have to do this in our application. The value designated as Xl gives the reactance the shunt component. In principle, we can use a series inductor and a parallel capacitor as easily as we might use as series capacitor and a shunt inductor. In most, but not all, antenna applications, the series capacitive reactance and shunt inductive reactance is more usual--and is most apt to our 3-element high-performance Yagi designs.</p>
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<p>As the lower part of <b>Fig. 27</b> indicates, we can obtain the series capacitive reactance without using a separate capacitor. We simply shorten the driven element which changes the resonant resistive feedpoint impedance to a complex value having a capacitive reactance component. How much do we shorten the element?</p>
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<p>The answer to this question has two parts. First, we need to know what amount of series capacitive reactance we need. If we assume that Rs is 50 Ohms, the characteristic impedance of our main feedline and the transmitter connected to it, then we can calculate the required value of both the series and the shunt reactances. Since all of our beams had feedpoint impedances between 20 and 30 Ohms, we can reduce the problem to a little chart.</p>
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<pre>Load Res. Source Resistance, Rs = 50 Ohms
|
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Ra (Ohms) 30 25 20
|
||||
delta 0.817 1.0 1.225
|
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Series Xa (Ohms) 24.5 25 24.5
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Shunt Xl (Ohms) 61.2 50.0 40.8</pre>
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<p>The chart tells us several things, including the answer to the second part of our question about the series capacitive reactance. We need to shorten the driven element by an amount that will yield a series capacitive reactance that is near to 25 Ohms. For a 20-meter driven element using aluminum tubing, the overall shortening of the element will be about 10" or so. On 10 meters, half the shortening will yield about the same capacitive reactance at the feedpoint. Shortening the element by this much may lower the resistive component of the feedpoint impedance by an Ohm or perhaps two, but, as you can see from the chart, the required series capacitive reactance will not change by a significant amount.</p>
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<p>The value that changes most with changes in the resistive component of the feedpoint impedance is the shunt inductive reactance necessary to effect the match to the 50-Ohm source.</p>
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<p>We can provide a shunt inductive reactance in two normal ways. We can convert the inductive reactance for the design frequency into a value of inductance using standard handbook equations. Then we can design a simple coil (a single layer solenoid, as it is called) of #14 to #10 wire having the right inductance. For the small values involved, it is fairly easy to design an inductor with a Q between 200 and 300. Since the network losses are roughly (but not exactly) equal to the value of delta divided by the coil Q, we can see that in ordinary circumstances, losses will be under 1%. Moreover, we can adjust the precise inductance of the small coil by spreading or squeezing the turns until the 50-Ohm SWR at the network terminals is 1:1.</p>
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<p>An alternative to the inductor is the shorted transmission line stub. For the most compact assembly and widest bandwidth, a wide-spaced "hairpin" made of wire or rod creates a shorted parallel transmission line section with a high characteristic impedance. The required length is a function of the wire diameter, the spacing, the frequency, and the required reactance. Handbook equations can be used for this calculation, although independent utility software exists to combine all of the calculations into one exercise. One such program was created by WA1SPI and is included in the <a href="../radio.html#HAMCALC">HAMCALC</a> collection available from VE3ERP.</p>
|
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<p>Although some builders prefer to have an adjustable shorting bar as the rear part of the hairpin, my own preference is for a single hairpin with longer legs. We adjust the hairpin length by clamping down at the antenna feedpoint on the legs at the right points. This system reduces the number of mechanical connections by 2, which in turn reduces the number of potential resistive loss points.</p>
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<p>The hairpin tends not to answer directly to standard Q formulations, since it changes the value of reactance in a manner different from the way in which coils change reactance values as we move off the design frequency. However, a hairpin with a characteristic impedance above 600 Ohms tends to have a similar bandwidth characteristic to a beta coil with a Q of 300 or more.</p>
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<img src="3lyg-29.gif" width="640" height="238" border="2" vspace="5" hspace="9">
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<p><b>Fig. 29</b> shows the 50-Ohm SWR values across 20 meters for one of our 3-element models set up with a beta match consisting of a 600-Ohm shorted transmission line a little over 11" long. Modeling software does not show transmission line losses, but parallel lines that are about 1/70 of a wavelength do not have much loss when composed of wires or rods sufficiently thick to handle the currents involved.</p>
|
||||
<p>One caution is necessary with beta matches. It is possible to find a setting for a coil or a hairpin that effects a match when the series capacitive reactance is not optimal. Often, these settings result in additional and unnecessary losses from the network. Hence, it is important that the builder set the driven element as close as possible to the correct length for the required value of capacitive reactance at the feedpoint.</p>
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<p><b>The Quarter-Wavelength Matching Section</b></p>
|
||||
<p>For designs that have a resonant feedpoint impedance close to 25 Ohms, we can use the driven element as given and effect a match to a 50-Ohm line in another way: the 1/4 wavelength matching section.</p>
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<img src="3lyg-30.gif" width="545" height="382" border="2" vspace="5" hspace="9">
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<p>In <b>Fig. 30</b>, we can see the simplicity of the system. Although 35-Ohm transmission line runs about $3 a foot, it provides a set-and-forget matching system (except, of course, at our semi-annual inspection and maintenance sessions).</p>
|
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<p>The use of 35-Ohm cable for the matching section is based on the standard impedance transformation effects of 1/4 wavelength transmission line sections, which answer to the equations</p>
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<img src="3lyge3.gif" width="257" height="81">
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<p>where Zo is the characteristic impedance of the matching system, Zin is one of the two resistive impedance values being matched, and Zout is the other value being matched. The 1/4 wave line length, of course, is the electrical length, and the physical length must be adjusted for the velocity factor of the line--which yields for all coaxial lines a physical length that is shorter than the electrical length. In many cases, the 1/4 wavelength section can be coiled to act as a common mode current choke while still performing its matching duties.</p>
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<p>The losses of a 1/4 wavelength matching section will be as low as those of any other matching system. Programs such as N6BV's TLA or TLW can be used to check line losses both at the resonant frequency and at the band edges, using projected feedpoint impedance values supplied by an antenna modeling program.</p>
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<img src="3lyg-31.gif" width="640" height="220" border="2" vspace="5" hspace="9">
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<p>As shown in <b>Fig. 31</b>, a quarter wavelength matching section is capable of performance very similar to a beta match when it comes to 50-Ohm SWR values across 20 meters. The values in Fig. 31 are insignificantly different from those recorded in <b>Fig. 29</b>.</p>
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<p>However, the two methods of impedance transformation are sufficiently different to yield actual impedance values that are also different, despite the fact that the values result in quite similar SWR values. The following table shows the modeled feedpoint impedance values for the same Yagi model with a beta match in one instance and with a quarter wavelength matching section in the other.</p>
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||||
<pre>Matching Source Impedance (R +/- jX Ohms)
|
||||
System 14.0 14.175 14.35
|
||||
Beta 79.6 + j 11.3 50.9 - j 2.7 27.9 - j 0.5
|
||||
50-Ohm SWR 1.65 1.06 1.79
|
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1/4 WL Section 35.8 + j 17.8 47.6 + j 1.7 39.5 - j 22.9
|
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50-Ohm SWR 1.70 1.06 1.75</pre>
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<p><b>Conclusions (Of a Sort)</b></p>
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<p>In the end, the selection of a Matching system for a high-performance 3- element Yagi with a feedpoint impedance in the 20-30-Ohm range will depend on a number of factors outside the limits of electrical design. First, the selection of construction design will limit the matching network options. Direct connection of the driven element to the boom will eliminate both the beta and the quarter wavelength matching systems. The Tee can be used with insulated or uninsulated drivers (relative to the boom). In general, most but not all commercial designs have gravitated toward the used of drivers that are insulated from the boom. In fact, a number of available designs insulate all elements from the boom.</p>
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<p>If cost is a factor, the beta match using either a home-made hairpin or coil is likely the cheapest matching system available--but it is no less effective for its low cost. (Although the hairpin is about as cheap as you can get, it is no license to skimp on the quality of hardware used anywhere in the beam. Low quality hardware tends to result in both mechanical and electrical problems with any beam antenna.)</p>
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<p>The beta is also fairly easy to adjust, since it is located very close to the hub of the antenna boom-to-mast region. Hence, readjustment at the tower top is easier than with Tees or Gammas. Often, settings chosen in near-ground preliminary assembly phases hold good at the final antenna height.</p>
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<p>This entire series has been devoted to some of the electrical design options available to the builder of his or her first 3-element Yagi. A Yagi of this size and performance makes an excellent project for the home builder, since most of the materials can be obtained with reasonable ease. Multi-band Yagis, on the other hand, are best left for later exercises, once one has become thoroughly familiar with all of the parameters of Yagi design--or they are best left to the professional designers who can spend 8 hours or more each day on this difficult task.</p>
|
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<p>If you have never used an antenna as good as a 3-element monoband Yagi, you are in for some real surprises and adventures in operating--once you complete the project. Where your dream designs take you after that will be your own responsibility.</p>
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<p><i>Updated 07-31-99. © L. B. Cebik, W4RNL. This item appeared in</i> antenneX<i>, July, 1999. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
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<p><a href="3lyg.html"><img src="../images/rtnhome.gif" align="bottom" width="40" height="40"> Return to Index</a></p>
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<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>Wide-Band 40-Meter Yagis Part 1: Standard and Non-Standard Designs</title>
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<h1>Wide-Band 40-Meter Yagis<br>
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Part 1: Standard and Non-Standard Designs</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>In my <i>Notes on Wide-Band Yagis</i>, I discussed several 40-meter designs for full-band coverage (Volume 2, Chapter 7). Since the appearance of those notes, <i>QEX</i> has published an interesting new design by J. V. Evans, N3HBX. It promises (and delivers) the performance of a full-size wide-band 3-element Yagi in about 2/3 the boom length (31' vs. 45'). The key feature is the absence of a reflector element. Some years back, I noted in converting a 2-element pair of phased elements into a beam that a reflector was unnecessary, since the directors control both the gain and the front-to-back ratio. N3HBX provides further confirmation of this fact with his beam.</p>
|
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<p>In addition, the beam also calls to our attention a facet of designing antennas by computer modeling techniques, one usually overlooked in the modeling process. One facet of converting a computer design into a physical reality is the sensitivity of the design to the inevitable small construction variations from the computer design. The more able a design is to absorb these variables without changing essential characteristics, the more buildable is the design. As we review some standard wide-band designs and the N3HBX design, we shall have an opportunity to examine this matter more closely and to show how we can use the design software to arrive at a probable answer to the question: "How buildable is the antenna?"</p>
|
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<p><b>Some Essential Background on 40-Meter Yagis</b></p>
|
||||
<p>The U.S. 40-meter allocation runs from 7.0 to 7.3 MHz. If we measure the band in kHz, it is narrower than 20, 15, and 10 meters. However, in terms of bandwidth, as the term applies to antennas (and other components and circuits), we cannot use a simple kHz-count. Instead, we should measure the bandwidth as a percentage of the center frequency. If we divide the 300 kHz bandwidth by 7.15 MHz (and multiply by 100, of course), we arrive at a bandwidth of 4.2%. Compare this value to the 2.5% bandwidth of 20, the 2.1% bandwidth of 15, and the 3.5% bandwidth of the first MHz of 10 meters. Obviously, covering the full width of the U.S. 40-meter band with a single effective Yagi is a challenge.</p>
|
||||
<p>However, the challenge only begins with bandwidth. To an important degree, the element diameter influences the available bandwidth of a Yagi design. Suppose that we begin with a wide-band 10-meter design capable of covering the first MHz of that band. One way to obtain the coverage with relatively smooth performance figures is to use larger tubing. On 10, we might use an element taper schedule the begins with 3/4" tubing and goes down to 3/8" tubing. Such elements often have equivalent uniform diameters close to 0.5". If we scale the design to 40 meters, we must have an equivalent uniform diameter of about 2.0".</p>
|
||||
<p>Unfortunately, we cannot simply scale the 10-meter elements. Rather, we must build tubular elements from many tubing sizes so that the element can support itself in a reasonable wind. The beefiest 40-meter element structure that I have so far encountered in U.S.-made beams begins with 2.5" tubing at the center and gradually reduces to 0.375" tubing. <b>Fig. 1</b> shows the steps in this element. You may assume that where the steps are greater than 1/8", tubing has been doubled or tripled for greater strength in the sections that bear the highest loads. I shall not perform a full stress analysis on this element structure. However, I shall use it in all of the models that we shall review here. In that way, each model is subject to the same set of potentials and limitations related to potential physical structure.</p>
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<img src="40wb-1.gif" width="629" height="216" border="2" vspace="5" hspace="9">
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<p>The taper schedule of our standardized element yields an equivalent uniform diameter between 1.3" and 1.6", depending on the exact length of the element. This value falls far short of the requisite 2" to obtain equivalent operating bandwidth to a wide-band design that might appear at 10 meters. Therefore, we shall have to substitute design ingenuity for the convenience of electrically fat elements.</p>
|
||||
<p>The extreme taper of the element presents a small challenge for modeling it. MININEC has no significant difficulties with stepped-diameter element construction, but NEC-2 is notorious for the errors that such element construction produces. NEC-4 revised the current calculation algorithms to overcome the problem, but it does so only if the steps are small and if the region of highest current has the fewest and smallest steps. If we look at the half-element diagram, we find that the inner half of the half-element decreases by a full inch in diameter, in steps that reach as high as 3/8". NEC-4 alone cannot handle this structure without error. Hence, whether using NEC-2 or NEC-4, one must invoke Leeson corrections and calculate on the basis of equivalent uniform-diameter elements.</p>
|
||||
<p>We complete our list of design challenges with one more peculiar fact about wide-band Yagis on 40 meters. The operators who are most likely to invest high dollars in a large 40-meter Yagi, along with the massive support structure to hold it at an advantageous height and to rotate it around the horizon, tend to be avid contesters. Their equipment usually includes power amplifiers capable of the full legal amateur limit using any mode of operation. Many of these amplifiers contain protection circuits to ensure long life from the expensive components within them. One of those protection mechanisms is a circuit that cuts off the amplifier when the return voltage sample indicates an SWR value above a certain level. In amateur circles, we are most used to using a 2:1 SWR level as a convenient marker for operating bandwidth that will satisfy the modern generation of amateur transceivers. However, in many amplifiers, the cut-off limit is 1.5:1. Therefore, we must design with that limit in mind.</p>
|
||||
<p><b>The Designs That We Shall Review</b></p>
|
||||
<p>In the course of these notes, we shall examine the characteristics of 4 Yagi designs ranging from 2 to 4 elements. <b>Fig. 2</b> shows the 4 designs in outline. The outlines are close to being in scale with each other. Hence, you can see the growth in the overall boomlength of each design as we move from 2 to 4 elements. The shortness of the N3HBX design becomes clearly evident.</p>
|
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<center>
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<img src="40wb-2.gif" width="770" height="490" border="2" vspace="5" hspace="9">
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<p>Since every element uses essentially the same structure, we can supply the necessary additions to let you replicate the models. The following table lists the progressive element spacing, using the reflector as the baseline. It then lists the element half-length. Multiply by 2 for the total element length. The final column lists the tip length. A zero or a negative entry means that the element has no 3/8" tip section, and the value in the column represents an adjustment to the 0.5" element section shown in <b>Fig. 1</b>. You may also use that graphic to fill in the requisite wires for any models that you create from the following table.</p>
|
||||
<pre>
|
||||
Dimensions of the Yagis Discussed in These Notes
|
||||
|
||||
Note: All dimensions are in inches. Multiply by 2.54 for centimeters,
|
||||
0.0254 for meters.
|
||||
|
||||
Antenna Element Spacing Half-Length Tip Length
|
||||
2-Element Reflector 0 451 65
|
||||
Driver 282 412 26
|
||||
3-Element Reflector 0 463 77
|
||||
Driver 305 428 42
|
||||
Director 540 386 0
|
||||
4-Element Reflector 0 449 63
|
||||
Driver 245 436 50
|
||||
Director 1 329 407 21
|
||||
Director 2 622 379 -7
|
||||
4-El N3HBX Slaved Driver 1 0 456.5 70.5
|
||||
Fed Driver 62 440 54
|
||||
Slaved Driver 2 107 420 34
|
||||
Director 1 367 383.5 -2.5
|
||||
</pre>
|
||||
<p>For any given design, the required element lengths will change if you a. change the equivalent uniform diameter or b. change the taper schedule for the tubing. If the effective diameter does not change by much, you likely will not need to adjust the element spacing, since spacing is relatively less sensitive than is element length. Just how sensitive the element lengths are in a given design will be one of our later subjects.</p>
|
||||
<p><b>2-, 3-, and 4-Element "Standard" Yagi Designs</b></p>
|
||||
<p>In the 1980s, Bill Orr, W6SAI, presented a pair of designs for 10 meters. I have generally ascribed the designs to him, since he did not specify if he obtained them or generated them himself. The basic principle underlying both his 2-element and his 3-element design is straightforward. The reflector-driver spacing in a standard Yagi largely determines the feedpoint impedance level of the array. In the 1980s, most Yagis used fairly narrow spacing and ended up with feedpoint impedances ranging from about 18 to 25 Ohms.</p>
|
||||
<p>In a 2-element Yagi that uses a reflector and a driver, widening the spacing actually has 3 effects. First, it lowers the gain and the front-to-back ratio by a small amount, but not enough to be operationally significant if 2-element performance is useful. Second, it raises the feedpoint impedance. A spacing of about 1/8 wavelength yields about the best performance compromise in a normal 2-element Yagi, but the feedpoint impedance is between 30 and 35 Ohms. Raising that spacing to a range of 0.14 to 0.17 wavelength increases the impedance to a good match for 50-Ohm coaxial cable. Third, the wider spacing also increases the SWR bandwidth. So the sample 2-element 40-meter Yagi uses a spacing of 0.17 wavelength. As a result, we obtain the following table of results, sampled at the band edges and at mid-band.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 6.50 9.89 40.7 - j 19.4 1.60
|
||||
7.15 5.96 10.70 53.1 + j 3.5 1.09
|
||||
7.3 5.52 9.71 63.5 + j 24.2 1.63
|
||||
</pre>
|
||||
<p>To convert the raw numbers into a more graphical form, <b>Fig. 3</b> presents curves that track the free-space gain, front-to-back ratio, resistance, reactance, and 50-Ohm SWR across the 40-meter band.</p>
|
||||
<center>
|
||||
<img src="40wb-3.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>2-element driver-reflector Yagi gain naturally decreases with increasing frequency. The design creates a front-to-back peak ratio about mid-band. The feedpoint resistance rises only 23 Ohms across the band, while the reactance shows a 44-Ohm spread. As a result, we obtain acceptable 2-element Yagi performance with an SWR well below 2:1. However, we have missed the more desirable 1.5:1 ratio at the band edges. One standard commercial spacing used for 40-meter 2-element Yagis is 20'. Our wide-band Yagi uses a 23.5' boom.</p>
|
||||
<p>The 3-element Yagi also uses an Orr-derived 10-meter design as its foundation. Once more, wider reflector-to-driver spacing increases the feedpoint impedance. By properly sizing and positioning the director, we can obtain good patterns and reasonable forward gain. However, the gain level is more appropriate to a shorter boomlength when designed for a lower impedance. For example, a boom length of about 0.22 wavelength in a conventional Yagi will yield about 7 dBi free-space gain, while a boomlength of about 0.33 wavelength will yield about 8 dBi gain, both with feedpoint impedances in the 25-Ohm range. Our 50-Ohm direct feed Yagi design yields about 7 dBi gain on the longer boom. However, we need that 45' boom to give us the wide-band performance values shown in the following table.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 6.92 19.16 51.6 - j 17.6 1.41
|
||||
7.15 7.03 21.54 48.9 - j 1.3 1.04
|
||||
7.3 7.37 19.00 41.6 + j 18.8 1.57
|
||||
</pre>
|
||||
<p>We just miss having a 20-dB front-to-back ratio from one end of the band to the other. The 3-element Yagi almost reaches the goal of a 1.5:1 50-Ohm SWR all across the band. <b>Fig. 4</b> provides the graphs and patterns to establish the utility of the design, even if the Yagi is twice as long as the 2-element version.</p>
|
||||
<center>
|
||||
<img src="40wb-4.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The gain curve is apropos to any Yagi having a single driver and at least one director: it rises as we increase frequency. The front-to-back ratio peaks about mid-band. The resistance and reactance excursions are between half and two-thirds of the amounts shown by the 2-element Yagi. Nevertheless, we have only come close to the most desired SWR limit.</p>
|
||||
<p>The third design uses 4 elements on a 52' (0.38 wavelength) boom. The added element and increased boom length yields about a full dB of additional gain while preserving the front-to-back ratio levels. The design is an adaptation of a 10-meter design that I developed several years ago. The element called Director 1 in <b>Fig. 2</b> also functions as a slaved driver to broaden the operating passband of the antenna. With the dual driver or master-slave system, the reflector tends to exert some control on the pattern properties at the low end of the band, while the remaining forward director has greater control of the high end of the band. The following table samples the modeled values.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 7.90 18.08 46.8 - j 9.0 1.22
|
||||
7.15 8.00 23.99 50.2 - j 0.2 1.01
|
||||
7.3 8.21 18.64 44.9 + j 7.0 1.20
|
||||
</pre>
|
||||
<p>The most notable feature of the design is the SWR curve in association with the values of feedpoint resistance and reactance. <b>Fig. 5</b> shows the data in graphical form. The 50-Ohm SWR never reaches 1.25:1 anywhere within the band. To obtain this level of performance, we must use a very long boom with very heavy elements on its ends. Hence, the support system for this beam would likely require heavy-duty trusses for both the individual elements and for the boom itself.</p>
|
||||
<center>
|
||||
<img src="40wb-5.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The 4-element wide-band Yagi has appeared in many guises, ranging from the lower HF range up to UHF. With suitably fat elements on 70 cm, the antenna is capable of covering the entire band with usable--if less than optimum--performance. The drawback of all of the designs is that they pay little heed to boomlength. On 40-meters, the shorter the boom, the more easily that we may convert a design into a physical antenna.</p>
|
||||
<p><b>The 4-element N3HBX Yagi</b></p>
|
||||
<p>N4HBX experimented with modeling 40-meter Yagi designs using a pair of slaved drivers that surround the fed driver. The triple combination, he reasoned, would offer even greater operating bandwidth. In the process of his design work, he discovered that he could eliminate the reflector. He lost some gain in the process, but the amount by which the boom became shorter more than offset the loss. His final boomlength is only about 0.22 wavelength or 31'.</p>
|
||||
<p>Surrounding the fed driver with slaved drivers that use relatively tight spacing tends to give the slaved drivers control over the array's properties. Hence, the gain shows a mid-band dip (insignificantly small) as well as dips in the SWR curve. The front-to-back curve shows a small mid-band peak, but remains at 20 dB or better across 40 meters.</p>
|
||||
<p>The cost is an impedance curve that varies around the 25-Ohm level. In the data table and impedance graphs, I have attached a quarter wavelength section of 35-Ohm transmission line to match the impedance to a main 50-Ohm cable. You can construct the matching section from 35-Ohm coax or from paralleled lengths of 70-Ohm cable. The following table lists both the pre-match and post-match impedance and SWR values.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Pre-Match Z 25-Ohm Post-match Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR R +/- jX Ohms SWR
|
||||
7.0 7.21 20.91 16.3 - j 2.8 1.57 67.7 + j 20.5 1.59
|
||||
7.15 7.18 25.20 31.7 + j 0.4 1.27 38.7 - j 0.1 1.29
|
||||
7.3 7.46 20.69 15.7 + j 4.8 1.69 72.6 - j 20.4 1.65
|
||||
</pre>
|
||||
<p>The SWR values for the sampling points do not show the SWR minimum values that occur about 50 kHz from each band edge. These appear clearly in the graph in <b>Fig. 6</b>. The figure also includes the gain and front-to-back curves, as well as sampled patterns that correspond to those shown for the other beams in the collection. The gain reaches its minimum value at about the same frequency on which we find the maximum value of front-to-back ratio.</p>
|
||||
<center>
|
||||
<img src="40wb-6.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The N3HBX design achieves well under 2:1 matched SWR at the band edges. However, it does not achieve the ideal 1.5:1 level. The reason is that slaved drivers generally have a narrower operating bandwidth than a fed driver. Hence, the two slaved drivers lose control of the feedpoint impedance before we arrive at the band edges. Optimizing the driver lengths for a lower SWR value at the band edges results in a much higher mid-band SWR value, since the pair of slaved drivers effectively prevents the fed driver from controlling that portion of the band with respect to antenna properties. Therefore, we see only 2 SWR minimums and 2 gain maximums rather than 3. Nevertheless, the beam has generally very even performance from one end of 40 meters to the other--all on a relatively short boom.</p>
|
||||
<p>Perhaps the key design element in the N3HBX design is the removal of the reflector. Many radio amateurs are wedded to the notions of "reflector" and "director" that suggest to the imagination that some real reflection and direction is occurring. In fact, all elements are directors in the sense of being positioned and sized so that the signal is highly directional. The reflector does not reflect in the sense that a flashlight reflector reflects. Rather, the length and position of the reflector sets up a current magnitude and phase angle that contributes to the overall directionality of the antenna's radiation field. Where we have only a driver and a reflector, the parasitic rear element plays a key role in the pattern shape. But as we saw in the 2-element Yagi design, the reflector only helps the gain and the front-to-back ratio a little bit. Directors are the key elements in controlling both the gain and the front-to-back ratio of a standard design Yagi. Once we add directors, the reflector's role is reduced largely to establishing the general level of the feedpoint impedance on the driver. Without a reflector, the N3HBX design could not achieve a direct 50-Ohm feedpoint connection, although the 25-Ohm level proves to be perfectly matchable to coaxial cable.</p>
|
||||
<p>While exploring the properties of phased elements about 5 years ago (August, 1998), I experimented with adding a director to a pair of phased elements. I set the phased pair of elements to be nearly the same length and used a phasing line that set them up for very close to the maximum gain possible from 2 elements--a little over 7 dBi in free space. As <b>Fig. 7</b> shows, that configuration yields a very poor front-to-back ratio compared to the very high ratio we can obtain from the same pair of elements at the same spacing just by altering the relative current magnitude and phase angles. (However, when set for maximum front-to-back ratio, the gain drops to just about 6 dBi.) The next step was to add a single director without altering the phased element pair. The 17.2' director (at 12 meters) is 9' from the phased element pair. The figure shows the resulting free-space E-plane pattern with 8.6 dBi gain and over 30-dB front-to-back ratio.</p>
|
||||
<center>
|
||||
<img src="40wb-7.gif" width="600" height="400" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The experimental beam has quite narrow-band properties and requires a reverse beta match for a 50-Ohm cable (that is, a capacitor across the feedpoint to complete the L-network). The narrow-band properties of the phagi (or phased-Yagi combination) follow from the fact that the setting of a phased element pair for maximum gain is itself a narrow-band property. (For further information on the experimental beam, see <a href="../phase/phagi.html">Horizontal Phased Arrays with Parasitic Directors</a>.) If this analysis is correct, then the driver set of the N3HBX array--without the director--should show some pattern properties that are as wide-band as the total Yagi itself. See <b>Fig. 8</b> for confirmation of this condition. The patterns display values that would be quite usable in a 2-element Yagi. Unfortunately, in the absence of the director, the feedpoint impedances are not practical.</p>
|
||||
<center>
|
||||
<img src="40wb-8.gif" width="600" height="250" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>It does not matter whether we call the two un-fed elements slaved drivers or parasitic elements functioning as a very close-spaced set of reflector and director elements. In both cases, the elements have positions and lengths to effect current magnitudes and phase angles that result in a directional pattern. The director plays the key role in altering the patterns to their final desired shapes within the limits of what any parasitic element might do.</p>
|
||||
<p>I am not advocating the general removal of reflectors. An operational Yagi is a blend and balance of many properties. Where one or more directors are present, the reflector remains a primary method of setting the general level of the feedpoint impedance to a convenient level for use with common feedlines. In most designs, that function is significant. In the case of the N3HBX array, shortening the overall boomlength proved to have a higher priority, and the resulting un-matched feedpoint proved to be quite usable without the reflector. For many other designs, doing away with the reflector would be highly impractical.</p>
|
||||
<p><b>Moving from Model to Reality</b></p>
|
||||
<p>One question normally overlooked by individuals who are new to designing antennas via computer software is the "buildability" of the resulting design. Even if we follow the computer-generated plan as closely as possible, the physical elements will rarely match the design dimensions exactly. Hardware will create "lumps and bumps" on elements. Even mounting hardware that we insulate from the elements may detune them by a very slight amount. Unless we have a very extensive or commercial-grade shop, our element-section lengths may vary a fractional amount from the model. Finally, even if we insulate the boom from the elements, we may encounter a small amount of coupling that the model cannot show due to its inability to handle transverse as well as axial current along elements.</p>
|
||||
<p>Despite these potentials for a divergence between the model and reality, innumerable successful antennas have emerged from computer design processes. In most cases, the differences turn out to be negligible when measured in terms of predicted and actual antenna performance. However, that fact is not true of all designs. Therefore, the designer needs to develop a system for evaluating the likelihood that a given antenna will be sensitive to construction variables.</p>
|
||||
<p>The need for such a system emerges from the modeling process itself. As we approach the dimensions of the "final" design, we tend to sneak up on the element lengths and positions. In other words, we vary each dimension in very small amounts until we optimize performance or at least achieve acceptable performance. Rarely do we keep track of the increments we use in varying element lengths. More important, we rarely take note of the significance of those design-variation increments. If we do not normally take this step during design, then we should systematically evaluate the design once we have frozen it.</p>
|
||||
<p>No single system of pre-testing a design will satisfy every antenna on every band built in every situation. Commercial makers can usually cut tubing within a few thousandths of an inch, while the home-shop builder may be restricted to the accuracy of his eyes and the markings on the tape measure he uses. A half-inch variation limit in element length on 10 meters is equivalent to about 2" as a limit on 40 meters. Both figures represent about a quarter of 1% for the bands involved. Therefore, whatever tests we perform on the design models are likely to vary from one antenna design to another and from one part of the frequency spectrum to the next.</p>
|
||||
<p>To obtain a first-order measure of how sensitive the 40-meter wide-band antennas are to construction variables, I set up a very simple test. Since the Yagis in this collection are set on 40 meters, I varied the individual element lengths by +2" and then by -2" from their design values. For each variation, I recorded the most telling data at the design frequency limits (7.0 and 7.3 MHz). We already have the values for the design dimensions in the tables associated with each Yagi. Of course, after looking at each element, I returned it to its design length before varying the length of the next element. After looking at the variations due to element length, I also varied the spacing between element by the same +2" and -2". Since element spacing is less sensitive to change than element length, I did not expect to find significant performance variations in that department. However, since some of the designs use parasitic or slaved drivers, the checks were necessary.</p>
|
||||
<p>The "standard" designs all turn out to be quite buildable within the limits of the test. That is, changes of the order used in the test did not materially affect the radiation or the impedance performance by more than very small amounts. In no case did they jeopardize the utility of the antenna for its intended purpose. However, the N3HBX design raises a few question marks.</p>
|
||||
<p>To abbreviate the test report, we may compare the N3HBX design with the longer 3-element Yagi that has similar performance. <b>Fig. 9</b> shows the general parameters of the test runs.</p>
|
||||
<center>
|
||||
<img src="40wb-9.gif" width="650" height="250" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The charts of values for the two antennas cover several sheets of paper. However, we can summarize the results by taking the maximum variation in each performance category. Very often a single element of spacing will account for the variation in a given category, and we may annotate the table accordingly. The following table provides the results for the standard 3-element design.</p>
|
||||
<pre>
|
||||
Results of Varying Dimension by +/-2" for the 3-Element Wide-Band 40-Meter Yagi
|
||||
|
||||
Category Variation Variation Notes
|
||||
Range 7.0 MHz Range 7.3 MHz
|
||||
Gain (dBi) 0.05 0.11 Equal range from reflector and director length variations
|
||||
Front-Back Ratio (dB) 0.62 0.60 Sources variable
|
||||
FP Resistance (Ohms) 1.11 2.68 Director length
|
||||
FP Reactance (Ohms) 4.16 4.29 Driver length
|
||||
50-Ohm SWR 0.115 0.117 Driver length
|
||||
</pre>
|
||||
<p>As expected, changes in the element spacing did not yield value changes that approached the high and low values that define the range of variation in each category. More significantly, the range of change in each category is very small. For that reason, I have characterized the 3-element wide-band design as quite buildable in the average home shop--assuming that the shop can handle the basic materials required. A commercial-grade shop or factory is not necessary to build this antenna successfully, if we again assume the same quality of components and basic construction techniques.</p>
|
||||
<p>When we turn to the N3HBX design, we obtain a different set of values in each sampled category. The following table provides the data from the modeled variations. The model used contained the quarter wavelength matching section. Hence, typical band-edge feedpoint resistance values are all above 50 Ohms.</p>
|
||||
<pre>
|
||||
Results of Varying Dimension by +/-2" for the 4-Element N3HBX Wide-Band 40-Meter Yagi
|
||||
|
||||
Category Variation Variation Notes
|
||||
Range 7.0 MHz Range 7.3 MHz
|
||||
Gain (dBi) 0.08 0.06 Rear-driver length at 7.0; director length at 7.3
|
||||
Front-Back Ratio (dB) 3.78 0.98 Rear-driver length at 7.0; director length at 7.3
|
||||
FP Resistance (Ohms) 9.11 18.99 Rear-driver and fed driver length at 7.0;
|
||||
front-driver length at 7.3
|
||||
FP Reactance (Ohms) 13.84 20.32 Fed-driver length at 7.0; front-driver length at 7.3
|
||||
50-Ohm SWR 0.290 0.643 Rear-driver length at 7.0; front-driver length at 7.3
|
||||
</pre>
|
||||
<p>If we compare the 4-element Yagi table to the table for the 3-element design, we find greater variations among band-edge values in every category except forward gain. Gain, however, is rarely critical in a wide-band design. The N3HBX design shows modest changes in the front-to-back ratio at the upper band edge, since the director largely controls that value. At the low end of the band, the reflector normally performs the same function. However, the design lacks a true reflector, and the front-to-back value changes by almost 4 dB due to changes in the length of the rear slaved driver.</p>
|
||||
<p>The feedpoint resistance changes by a factor of 9 at both ends of the band, relative to the changes we saw in the 3-element design. The rear slaved driver and the fed driver contribute equally to the resistance changes at 7.0 MHz, while the forward slaved driver accounts for the larger range of change we see at 7.3 MHz. The fed driver is the source of the widest reactance range at 7.0 MHz. At 7.3 MHz, the forward slaved driver again yields an even wider range of reactance values as we alter its length by the +/-2" amount defined for the test. Note that the forward slaved driver' length contributes most to the wider range of both resistance and reactance values. These values move in the same direction, resulting the very large change in SWR value. In fact, if we decrease the forward slaved driver length by 2", the band-edge 50-Ohm SWR drops to 1.39:1, while a 2" length increase yields an SWR increase to 2.03:1.</p>
|
||||
<p>The overall picture for the N3HBX design is especially interesting, since a 2" length change to any one of the elements becomes the source of at least one tabulated value reaching a surveyed limit. The element with the most influence on the performance curves for the antenna is the forward slaved driver. The tests suggest that construction variables may well move the performance at 7.3 MHz outside the useful range of values.</p>
|
||||
<p>The test establishes that the N3HBX 4-element design is more sensitive to slight variations in construction than we find for the 3-element design. We might have intuited this situation from the compressed spacing within the driver cell. Very closely spaced elements change their mutual coupling with only small changes in length and in spacing. Experience with master-slaved driver arrangements would also have taught us that the forward slaved driver tends to be the most sensitive element in any array using virtually any variation on that system. (Many wide-band VHF beams use first directors that in fact function as slaved drivers over half or more of the Yagi's passband, even though the spacing may not suggest this function. However, the function usually becomes clear if we track the relative current magnitudes on the fed driver and the first director across a given passband.)</p>
|
||||
<p>These notes on sensitivity to change certainly do not condemn the N3HBX design. Indeed, it is potentially a highly competent beam that is competitive with 3-element designs that require much long booms. Instead, the notes simply offer a few cautions. A commercial shop could easily fabricate successful replicas of a field-adjusted prototype. In the home shop, where 1 and only 1 beam will emerge, the lesson is that we cannot simply build and mount the antenna with expectations that it will necessarily perform to specification. We have to be ready for field adjustments to bring the performance to specification. Since this procedure is wise for any Yagi, no matter how forgiving the computer design, we might alter the forewarning to this: for the N3HBX design, we should be prepared for extensive field adjustments.</p>
|
||||
<p>The prospective size of the field adjustment task is one of the factors that does or should go into the building plans of anyone who constructs his own beams. How much weight we give this task varies from one builder to the next. The more sensitive a beam design is to small changes in its dimensions is a measure of the likely size of the adjustment task. These notes do not intend to establish the weight we give the task. Instead, they only provide one method of evaluating the size of the prospective task while the antenna design is still within the computer.</p>
|
||||
<p><b>Conclusion</b></p>
|
||||
<p>Adding the N3HBX 4-element reflector-less wide-band 40-meter Yagi to the collection of extant wide-band designs promises 3-element performance from a boom that is not much longer than we find on 2-element full-size 40-meter Yagis. The design is a good example of using a self-contained driver system--in this case, a master-slaved driver system--to achieve wide-band performance. Unexplored alternatives to the N3HBX drive system are direct driver coupling via very short lengths of transmission line and possibly a phased pair or triplet of drivers. Both systems--if successful--would add a few feet to the boomlength but still result in a beam that is quite a bit shorter than the comparably performing 3-element design.</p>
|
||||
<p>The 4-element reflector-less design with a closely spaced driver cell has also provided me with a pretext for reminding home-designers of Yagi antennas to check the sensitivity of the antenna design to small changes in its dimensions. The results--especially when compared to results for a beam known to be forgiving of small dimension shifts--allows us to estimate the ease or difficulty of replicating the design using real aluminum tubes and stainless steel hardware. That information leads to better planning and decision-making within the overall task of constructing our own Yagis.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 02-01-2006. © L. B. Cebik, W4RNL. This item originally appeared in</i> antenneX<i>, (Jan., 2006). Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="40wb2.html"><img src="../images/fwdrd.gif" align="bottom" width="36" height="33"> Go to Part 2</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Main Index</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-lower'>HF Lower</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
||||
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|
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<title>Wide-Band 40-Meter Yagis Part 2: Alternative 4-Element Designs</title>
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<h1>Wide-Band 40-Meter Yagis<br>
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Part 2: Alternative 4-Element Designs</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>In Part 1 of our expanded notes on wide-band 40-meter Yagis, we looked at some standard and non-standard Yagi design ideas. The 2- and 3-element Orr-derived Yagis employed wide element spacing to accomplish 2 goals: to raise the general feedpoint impedance to the 50-Ohm level and to increase the operating passband to cover all of 40 meters with under 2:1 SWR. However, only the relatively long-boom 4-element Yagi that used a master-slaved dual driver system achieved the most desirable end of having well under 1.5:1 SWR across the band while raising the gain level to about 8 dBi in free space. We also examined the recent N4HBX reflector-less design that employed a fed driver and bracketing slaved drivers to obtain about 7 dBi gain with under 2:1 SWR across the band.</p>
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<p>In the interest of making fair comparisons among the designs, we employed the same element-diameter schedule in all elements of all beam designs. <b>Fig. 1</b> replicates the taper schedule. As we proceed through the new notes, the structure tables will list only the element spacing (using the reflector as the baseline), along with the total element half-length and the tip length. You only need to add the 3/8"-diameter tip length to the schedule in <b>Fig. 1</b> in order to replicate any model within the series.</p>
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<p>As we closed the first part of these notes, we noted that we had not exhausted all of the design options that we might apply to Yagis in order to achieve full band coverage. In the following notes, we shall take up the design challenge in 2 directions. One path uses direct-coupled dual drivers as a substitute for the open-sleeve coupling system used in two of the Part-1 designs. The other roadway leads to phased pairs of elements used as drivers. In both directions, we shall restrict ourselves to 4-element designs that produce between 7 and 7.4 dBi gain with a good front-to-back ratio across the band. We shall aim at both shorter and longer versions of each design both to display the performance differences and to once again stay below 1.5:1 50-Ohm SWR from 7.0 to 7.3 MHz--if possible. A shorter Yagi version will be one that is between 31 and 36 feet long, while a longer version will be between 36 and 42 feet. All arrays will be shorter than the 45' boom required by the standard 3-element wide-band Yagi that also yields forward gain in the same low-7-dBi free-space range.</p>
|
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<p>Before we move into precise design details, however, we should look more closely at all of the varied Yagi driving schemes for widening the operating bandwidth of a beam. Each type of driving system achieves its goal by different means.</p>
|
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<p><b>How Drivers Drive a Yagi</b></p>
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<p>A standard Yagi has a single driving or fed element, along with a one or more parasitic elements. By convention, any element behind the driver--relative to the direction of the antenna's main lobe--is a reflector, and any parasitic element ahead of the driven element is a director. In an open-sleeve coupled driving system, we directly feed only one element. Hence, various sources may call the slaved driving element either part of the driver or a director. Either title is technically correct, since a slaved driver obtains its energy parasitically. It becomes a driver by dominating the current magnitude from which other elements derive energy parasitically. In the notes that immediately follow, I shall identify the driving elements, whether directly connected to the source or not.</p>
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<p>In most standard designs, the reflector and director elements are sufficiently distant from the driven element to qualify strictly as parasitic but not driving elements. Again, the distinction is more conventional than strictly technical, since every parasitic element contributes to forming the antenna's radiation pattern. Let's consider the 3-element standard wide-band Yagi from Part 1. If we provide the driver with a relative current magnitude of 1.0, and then, if we move the antenna across the band, we find that the current magnitude levels on the parasitic elements changes along the way. The following table shows the relative current magnitudes on all of the element centers at 7.0, 7.15, and 7.3 MHz.</p>
|
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<pre>
|
||||
Relative Current Magnitude on the Elements of a 3-Element Wide-Band 40-Meter Yagi
|
||||
Element 7.0 MHz 7.15 MHz 7.3 MHz
|
||||
Reflector 0.385 0.289 0.219
|
||||
Driver 1.0 1.0 1.0
|
||||
Director 0.525 0.625 0.734
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</pre>
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<p>The table does not show the phase angle of the currents. The driver, by model set-up, is always at 0 degrees. The reflector normally shows a positive phase angle that falls anywhere between 100 and 150 degrees relative to the driver. The director phase angle is normally negative relative to the driver and may be anywhere between 80 and 150 degrees. The exact phase angle for each parasitic element derives largely from the position and size of each element relative to the driver. If you see only a current table for a set of elements without designations such as "reflector" and "director," you can still tell the direction of the main beam by looking at the phase-angle entries.</p>
|
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<p><b>Fig. 2</b> replicates the data in the table using a more graphical presentation. The pink vertical lines represent the relative current magnitude on each element at each sampled frequency. The figures may make the progression of current values more readily apparent.</p>
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<p>Very noticeable in the table for the standard 3-element Yagi is the fact that as the frequency increases, the relative reflector current magnitude decreases. In Part 1, I noted in connection with certain designs that the reflector size has a greater effect on lower frequency performance than on higher frequency performance. Equally, the director size has a larger effect on the higher-frequency performance. As the table shows, the director current magnitude increases as we raise the beam's operating frequency.</p>
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<p>The pattern of current magnitudes for the 4-element open-sleeve coupled design in Part 1 shows a similar progression of current values for the reflector and the most forward element, which I shall simply call the director. The exact values differ from those of the 3-element Yagi because the element lengths and position differ between the two designs. The center 2 elements form the master and slaved driving elements, and we should pay close attention to them. Since we feed only one element, its current magnitude is always 1.0. In the following table, note the relative current magnitude on the slaved driving element as we increase frequency. Also follow the current magnitudes in the graphical representation in <b>Fig 3</b>, which appears just below the table.</p>
|
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<pre>
|
||||
Relative Current Magnitude on the Elements of a 4-Element Wide-Band 40-Meter Yagi
|
||||
Element 7.0 MHz 7.15 MHz 7.3 MHz
|
||||
Reflector 0.569 0.413 0.276
|
||||
Master (Fed) Driver 1.0 1.0 1.0
|
||||
Slaved Driver 0.683 0.909 1.120
|
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Director 0.435 0.544 0.646
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</pre>
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<p>As we increase the frequency, the current magnitude on the slaved driver reaches and then exceeds the current on the fed driver element, the one to which we have connected the feedline. (Remember that the current levels are relative to an assumed value of 1.0 at the feedpoint.) If we were to remove the slaved driver, then, at 7.3 MHz, the current level on the director would fall considerably--or its phase angle would change radically--with the net result that the beam's performance would decline in one, and usually several, categories. In designs like this one, we tend to cut the directly fed driver long to obtain good low-end performance and then to cut the slaved driver high to extend the performance at the high end of the band. If we carefully select the reflector spacing, we obtain a very wide SWR curves with a low value across the band. Essentially, the slaved driver's control of the beam's pattern at the higher end of the band--where the relative current equals or exceeds the driver current magnitude--extends back to the feedpoint so that the impedance at the junction with the transmission line remains stable. You may recall that the 4-element design in Part 1 showed a 50-Ohm SWR that did not grow to 1.25:1 anywhere within the 40-meter band.</p>
|
||||
<p>Many classic Yagi designs have used the open-sleeve dual driver system without realizing that fact. Early literature on the famous DL6WU VHF/UHF Yagi series used a reflector-to-driver space of 0.2 wavelength and a driver-to-director-1 spacing of about 0.05 wavelength. The result was a wide-band Yagi that covered all of the 70-cm band with an acceptably low SWR. Current analyses of the driver and first director elements show that the two form a master-slaved driver pair. Other well-known designs have followed this lead, using other spacing combinations. In the HF range, the OWA (optimized wideband antenna) Yagis used the same type of driving system, with different relative element spacings. However, the OWA series of beams also used special treatments of the second and third directors that stabilized a number of other performance properties. Hence, a Yagi may be a wide-band Yagi by using open-sleeve coupling between a fed driver and the next forward element without being an OWA Yagi. Since we shall not have enough directors to provide the special OWA treatment, all of our examples will simply be wide-band Yagis.</p>
|
||||
<p>The N3HBX reflector-less Yagi uses a 3-element driving cell, with slaved driving elements behind and ahead of the fed driver. Since the driving cell is an example of open-sleeve coupling among the elements, we should expect a pattern of changing current on each element, with the rear driver showing reduced current and the forward driver showing elevated current as we increased frequency. The following table and <b>Fig. 4</b> confirm our suspicions.</p>
|
||||
<pre>
|
||||
Relative Current Magnitude on the Elements of a 4-Element N3HBX Wide-Band 40-Meter Yagi
|
||||
Element 7.0 MHz 7.05 MHz 7.15 MHz 7.25 MHz 7.3 MHz
|
||||
Rear Slaved Driver 0.985 1.014 0.686 0.422 0.221
|
||||
Master (Fed) Driver 1.0 1.0 1.0 1.0 1.0
|
||||
ForwardSlaved Driver 0.520 0.930 1.092 1.541 1.282
|
||||
Director 0.286 0.458 0.481 0.625 0.433
|
||||
</pre>
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<img src="40wb2-4.gif" width="330" height="340" border="2" vspace="5" hspace="9">
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<p>The relative driver current magnitude levels do not disappoint our expectations. I have added 2 extra columns to the table because we noted that the impedance and SWR curves for the Yagi design showed peak performance values just inside each band-edge. The current values reflect the performance data. At 7.05 MHz, the rear slaved driver has close to its highest value. Likewise, the forward driver shows nearly peak magnitude at about 7.25 MHz.</p>
|
||||
<p>Many antenna designers use open-sleeve coupling for multi-band antennas, tuning each slaved driver to a band that is usually higher in frequency than the band to which they tune the fed driver. The technique has seen successfully use in both vertical monopole systems and in multi-band Yagis. One of the earliest conscious in-band uses of open-sleeve coupling involved the design of a dipole to cover all of the 80-75-meter band, or at least selected portions of the band.</p>
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<p>An alternative to open-sleeve coupling for multi-band antennas is direct coupling, sometimes called closed-sleeve coupling. Optimbeam Yagis use such a system, and the Bencher Skyhawk changed its feed system from open- to closed-sleeve coupling a few years back. One advantage of direct coupling over open-sleeve coupling is a broader bandwidth on frequencies higher than the lowest covered by the antenna system. The principle of direct coupling is simple-sounding, but electrically somewhat complex. Let's take a pair of driver elements. Let one be connected to the main feedline. Between that element and the other, we shall make a direct connection with a fairly low-impedance transmission line. The line will be short and not reversed. The close proximity of the two drivers produces parasitic or high mutual coupling similar to that found in open sleeve coupling system. However, we have also fed power directly to both elements. One result is that directly coupled drivers usually use element spacing that is greater than for driver pairs that use only parasitic coupling. A second result is the need to carefully balance the connecting transmission line impedance with the element size and spacing.</p>
|
||||
<p>Most direct-coupled driver system emerge from computer experimentation, which is far more rapid than physically varying the components with a prototype antenna. When successful, the multi-band driver set--amid the other elements forming a Yagi--shows a good match to the main feedline on each band. However, where we connect the feedline may vary, with some feedlines connecting to the higher-frequency element and some to the lower-frequency element.</p>
|
||||
<p>When we translate a directly coupled driver pair from multi-band to in-band service, we may encounter some surprises in the sense of needing a configuration unlike those that we saw in open-sleeve coupled driver systems. The following table and <b>Fig. 5</b> track element currents for a shorter 4-element 40-meter wide-band Yagi that uses a pair of directly coupled drivers, with a short, direct 50-Ohm transmission line between the driving elements. Note from the graphic that the feedpoint is on the rear driver of the pair. This driver is also the shorter of the two interconnected elements.</p>
|
||||
<pre>
|
||||
Relative Current Magnitude on the Elements of a 4-Element Wide-Band 40-Meter Yagi
|
||||
with a Directly Coupled Driver Pair
|
||||
Element 7.0 MHz 7.15 MHz 7.3 MHz
|
||||
Reflector 0.505 0.395 0.300
|
||||
Rear Driver 0.194 0.144 0.300
|
||||
Forward Driver 0.780 0.935 1.357
|
||||
Director 0.398 0.548 0.783
|
||||
</pre>
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<center>
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<img src="40wb2-5.gif" width="300" height="340" border="2" vspace="5" hspace="9">
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<p>The rear driver appears to carry a very low current magnitude, but the directly coupled pair yields a large operating bandwidth--as large at least as the bandwidth of the long-boom 3-element Yagi, but on a boom that is 10' shorter. How the directly coupled pair operate in this particular design is quite interesting. The phase angle of the rear driver is positive and almost as large as the positive phase angle of the reflector. The parasitic reflector element shows a decrease in relative current magnitude as we increase the frequency, but the rear driver shows an increase in current over that same span. The net effect is fairly smooth performance over the entire 40-meter band. Since we have a division of current at the main feedpoint, neither element shows a relative current magnitude of 1.0. As well, we also have changes in the phase angle from the feedpoint or source value of 0 degrees. Since even a short low-impedance transmission line will show impedance-transforming properties, the driver magnitudes do not add arithmetically. The directly coupled driver pair used in this design exercise is not the only one that we might employ, but it is satisfactory to our goal of producing a wide-band 40-meter beam.</p>
|
||||
<p>Our final example involves the use of a driver pair arranged to form a system traditionally called phased feeding. The two driver elements are more widely spaced and use a reversed (or half-twist) transmission line section to achieve proper phasing. Phasing does not directly concern the feedpoint impedance at each element in the pair. Rather, we arrange the element lengths and their spacing, as well as the line impedance between them, to achieve a desired relative current magnitude and phase on each element. Since the phased pair of elements forms a 2-element LPDA arrangement, the resultant beam that we produce when we add a reflector and a director often bears the name "log-cell Yagi." However, for small log cells (fewer than 4 elements), the driver pair usually emerges from experimentation rather than the application of LPDA equations. The systems used in our wide-band 40-meter designs employ a 250-Ohm phase line, and the feedpoint is on the forward element of the pair. The following table and <b>Fig. 6</b> provide data on the shorter of the two versions of the wide-band log-cell Yagi that we shall later examine in more detail.</p>
|
||||
<pre>
|
||||
Relative Current Magnitude on the Elements of a 4-Element Wide-Band 40-Meter Yagi
|
||||
with a Pair of Phased Driver Elements
|
||||
Element 7.0 MHz 7.15 MHz 7.3 MHz
|
||||
Reflector 0.223 0.155 0.102
|
||||
Rear Driver 0.654 0.655 0.453
|
||||
Forward Driver 0.674 0.824 1.030
|
||||
Director 0.610 0.905 1.163
|
||||
</pre>
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<img src="40wb2-6.gif" width="315" height="340" border="2" vspace="5" hspace="9">
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<p>The reflector shows decreasing relative current, while the director shows increasing relative current as we increase the operating frequency. The rear driver trend is downward, while the forward driver trend is upward. However, perhaps the most notable aspect of the system is the relative unimportance of the reflector and the very high importance of the director in terms of their relative current levels all across the band. We can remove the reflector and sustain the feedpoint impedance at nearly the 50-Ohm level and the overall operating bandwidth. The director plays the most important role in setting the radiation pattern behavior. You may relate this discussion of relative current levels to the Part-1 discussion of reflector-less Yagis using a phased pair of driver elements.</p>
|
||||
<p>Although our excursion into Yagi element currents has delayed our introduction of additional wide-band Yagi designs, it has shown us a few things, and has potential for showing us others. The diversity of parasitic arrangements by which we can achieve a set of operating goals within an overall beam structure continues to grow as designers bring their ingenuity to bear on their challenges. An understanding of the current patterns that emerge as we track a design through its operating range contribute to our better understanding of the design, even in the absence of explicit design equations. In fact, we might even use the information that we have just surveyed to understand some of the behavioral quirks in the designs to come.</p>
|
||||
<p><b>The Design Challenge</b></p>
|
||||
<p>In Part 1, we examined a 3-element standard Yagi on a 45' boom. It provided the performance listed in the following table from that discussion.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 6.92 19.16 51.6 - j 17.6 1.41
|
||||
7.15 7.03 21.54 48.9 - j 1.3 1.04
|
||||
7.3 7.37 19.00 41.6 + j 18.8 1.57
|
||||
</pre>
|
||||
<p>The Yagi averages about 7.1-dBi forward gain and close to 20-dB front-to-back ratio. It shows a rising gain curve as we increase frequency. The 50-Ohm SWR curve is very shallow and just misses the 40-meter grail of being under 1.5:1 at the upper end of the band. (We shall not here debate the merits of calculating the SWR at the end of some arbitrary length of lossy coaxial cable.) Boomlength remains the key drawback to the design and is not eliminable. A shorter boom would narrow the operating bandwidth and lower the resistive component of the feedpoint impedance, thus increasing the SWR over a large part of the band.</p>
|
||||
<p>In contrast, the 4-element N3HBX design requires only 31' of boom. The triple-driver section with a single director produces the performance shown in the following table, repeated from Part 1.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Pre-Match Z 25-Ohm Post-match Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR R +/- jX Ohms SWR
|
||||
7.0 7.21 20.91 16.3 - j 2.8 1.57 67.7 + j 20.5 1.59
|
||||
7.15 7.18 25.20 31.7 + j 0.4 1.27 38.7 - j 0.1 1.29
|
||||
7.3 7.46 20.69 15.7 + j 4.8 1.69 72.6 - j 20.4 1.65
|
||||
</pre>
|
||||
<p>Forward gain averages about 7.25 dBi, with a 22-dB front-to-back ratio. The average feedpoint impedance is in the vicinity of 25 Ohms. So the array requires a matching network. The quarter wavelength section of 35-Ohm transmission line yields the final columns of impedance and SWR values. Although the antenna does not produce as low a set of SWR values as the 3-element array, it remains well under 2:1, with a savings of 14' of boom but a cost of the weight of one more element.</p>
|
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<p>Obviously, the challenge facing any competing design is to improve upon one or more sets of figures while maintaining the shortest possible boomlength. We shall look at 2 different ways of approaching the challenge.</p>
|
||||
<p><b>4-Elements and Directly Coupled Drivers</b></p>
|
||||
<p>The first design direction involves the use of a driver pair with directly coupled elements. A length of 50-Ohm transmission line connects the 2 drivers. The rear driver is the shorter one and also serves as the feedpoint for a direct connection to 50-Ohm cable. To this pair of drivers we shall add a reflector and a director. The following table provides dimensions for the shorter version of the 4-element wide-band Yagi with directly coupled drivers.</p>
|
||||
<pre>
|
||||
Dimensions of the 4-Element Wide-Band Yagis with Directly Coupled Drivers
|
||||
|
||||
Note: All dimensions are in inches. Multiply by 2.54 for centimeters,
|
||||
0.0254 for meters.
|
||||
|
||||
Antenna Element Spacing Half-Length Tip Length
|
||||
Short Reflector 0 461 75
|
||||
Rear Driver 112 397 11
|
||||
Forward Driver 177 425 39
|
||||
Director 2 418 386 0
|
||||
Long Reflector 0 461 75
|
||||
Rear Driver 192 398 12
|
||||
Forward Driver 257 425 39
|
||||
Director 2 498 386 0
|
||||
</pre>
|
||||
<p>The short version of the array has a boomlength just under 35'. Within those limits, it produces the performance figures that appear in the following table.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 7.05 22.35 28.4 - j 7.1 1.81
|
||||
7.15 7.07 24.54 36.1 + j 2.6 1.39
|
||||
7.3 7.31 22.06 45.2 + j 22.0 1.60
|
||||
</pre>
|
||||
<p>The curves in <b>Fig. 7</b> translate the sample values into a full-band picture of performance. Below the graphs are free-space E-plane patterns at the sampled frequencies.</p>
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||||
<center>
|
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<img src="40wb2-7.gif" width="740" height="860" border="2" vspace="5" hspace="9">
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</center>
|
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<p>The gain and front-to-back figures are very comparable to those for the N3HBX beam without some of the dimensional finickiness that goes along with the triple-driver that uses a master and 2 slaved driving elements. The directly coupled system provide well under 2:1 50-Ohm SWR on a boom only 4' long than the N3HBX boom, but it does not result in the desired 1.5:1 50-Ohm SWR maximum.</p>
|
||||
<p>Since the resistive component of the impedance values is uniformly low, we can increase it by increasing the distance between the reflector and the rear driver. As we increase this distance, the feedpoint resistance does not rise as fast as it might with a single driver. As noted earlier, the rear driver exerts reflector-like influence on the radiation pattern due to the positive phase angle of the element-center current, an angle that almost equals the phase angle of the reflector current. Therefore, we must increase the distance by nearly 7' to achieve the desired SWR curve. So the final boomlength of the long version of the 4-eleent directly coupled driver Yagi is about 41.5'--still short of the boomlength for the simple 3-element wide-band Yagi. The following table samples the performance.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 7.13 21.01 36.9 - j 10.9 1.48
|
||||
7.15 7.19 24.23 42.1 - j 1.5 1.19
|
||||
7.3 7.42 21.59 49.6 + j 18.8 1.45
|
||||
</pre><b>Fig. 8</b> converts the sampled data into full-band curves and provides the e-plane patterns.
|
||||
<center>
|
||||
<img src="40wb2-8.gif" width="740" height="860" border="2" vspace="5" hspace="9">
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</center>
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<p>The only changes in the long version, relative to the shorter one, involve moving the reflector and changing the length of the rear driver by 2" (1" per half-element length). Since the director position and size do not change, the radiation pattern performance does not change, although the gain does increase by a paltry 0.1 dB. However, the array does not exceed a 50-Ohm SWR value of 1.5:1 across the whole band. Although shorter than the 3-element Yagi, the array is considerably longer than the N3HBX beam (by 10.5'). However, the long directly coupled Yagi is about 10' shorter than the original 4-element Yagi with its added dB of gain and its impressive SWR curve.</p>
|
||||
<center>
|
||||
<img src="40wb2-9.gif" width="500" height="215" border="2" vspace="5" hspace="9">
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||||
</center>
|
||||
<p>The use of a directly coupled driver pair puts the potential builder in a quandary. <b>Fig. 9</b> compares the outlines of the 2 beams and reveals the key difference: the driver-to-reflector spacing needed to achieve the better SWR performance. At the short length, and if a 2:1 SWR curve is satisfactory, the design would be competitive with the N3HBX and show less sensitivity to construction variables.</p>
|
||||
<p><b>4-Elements and Phased Drivers</b></p>
|
||||
<p>We may also use a log-cell driver system consisting of 2 fed element connected by a reversed transmission line. Although related to LPDA structures, small log cells of 2 to 3 elements usually emerge from trial-and-error experimentation when used in conjunction with parasitic elements to form log-cell Yagis. Such cells are normally used for monoband Yagis, but may also appear in some multi-band Yagi designs. A log cell--even if only 2 elements long--generally lengthens the overall length of a Yagi without adding to its overall gain or front-to-back potential. However, the cell is capable of widening the effective bandwidth of virtually all of the Yagi's properties.</p>
|
||||
<p>For our experimental 40-meter designs using the prescribed element-diameter schedule, I created a 2-element cell with a reflector and a director to form a 4-element beam. The log cell has a normal form, with a longer rear element and a shorter forward element connected by a 250-Ohm phase line using a single half twist. The forward element center serves as the connection point for the main (50-Ohm) feedline. The following table provides the critical dimensions.</p>
|
||||
<pre>
|
||||
Dimensions of the 4-Element Wide-Band Yagis with Phased Drivers
|
||||
|
||||
Note: All dimensions are in inches. Multiply by 2.54 for centimeters,
|
||||
0.0254 for meters.
|
||||
|
||||
Antenna Element Spacing Half-Length Tip Length
|
||||
Short Reflector 0 462 76
|
||||
Rear Driver 194 439 53
|
||||
Forward Driver 290 412 26
|
||||
Director 2 427 399 13
|
||||
Long Reflector 0 463 77
|
||||
Rear Driver 192 432 46
|
||||
Forward Driver 290 401 15
|
||||
Director 2 467 388 2
|
||||
</pre>
|
||||
<p>Between the two versions, numerous dimensions change by greater or lesser amounts. However, the reflector-to-driver spacing remains almost unchanged, with the major boomlength change between the forward driver and the director. The shorter version is just over 35.5', while the longer version is just under 39'.</p>
|
||||
<p>As the following table reveals, the shorter version of the antenna achieves virtually all of the array goals except one: it falls just under the desired 20-dB front-to-back ratio at the band edges. <b>Fig. 10</b> shows the curves and patterns that permit a fuller evaluation of the antenna's potential performance/</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 7.01 18.41 36.5 + j 7.0 1.42
|
||||
7.15 7.07 21.78 47.1 + j 5.8 1.14
|
||||
7.3 7.26 19.83 40.9 - j 14.5 1.46
|
||||
</pre>
|
||||
<center>
|
||||
<img src="40wb2-10.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Like the other models that we have surveyed, the performance across the band is very smooth, with only operationally undetectable changes in the radiation pattern from 7.0 to 7.3 MHz. The chief merit in the phased driver system is the achievement of the more rigorous 50-Ohm SWR standard in a beam under 36' long.</p>
|
||||
<p>The longer version of the phased-driver Yagi extends the boom by 40", making a few changes in the element lengths in order to re-center the performance curves. Since the SWR curve of the shorter design was satisfactory, the reflector-to-rear-driver spacing remained almost unchanged. The additional boomlength or driver-to-director spacing gave the array an unnoticeable increment of additional gain. More importantly, it raised the overall level of the front-to-back ratio by an average of over 5 dB. In the process, the SWR curve improved, mostly due to a narrowing of the total range of reactance change across the band. The following table samples the values, followed by <b>Fig. 11</b>, which presents the data in graphical form.</p>
|
||||
<pre>
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 7.10 22.34 40.4 - j 1.7 1.24
|
||||
7.15 7.15 31.49 44.4 - j 1.6 1.13
|
||||
7.3 7.35 25.35 43.8 - j 9.9 1.28
|
||||
</pre>
|
||||
<center>
|
||||
<img src="40wb2-11.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The longer phased-driver Yagi's SWR curve does not exceed 1.3:1 across the band and hence rivals the SWR performance of the long-boom (51') 4-element Yagi described in Part 1 (but without the 8+ dBi forward gain). It also provides a detectable improvement in the front-to-back ratio. Even though the cited figure is for the 180-degree ratio, the rearward performance generally holds good for the whole of the rear quadrants.</p>
|
||||
<center>
|
||||
<img src="40wb2-12.gif" width="500" height="200" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 12</b> compares the outlines of the short and long Yagis with phased drivers. The increased spacing from the driver to the director is clear in the long version sketch.</p>
|
||||
<p>Optibeam of Germany makes available a 4-element Yagi with a phased driver pair. The boomlength is similar to the one used in this design. I do not know the element taper schedule or precise dimensions used by the commercial offering. It does use some inductive loading to shorten the elements. This fact should not alter the SWR curve or the front-to-back ratio in any noticeable way. However, it may reduce forward gain slightly, depending upon the actual degree of element shortening and the inductor Q. These notes are neither for or against the Optibeam design. Rather I note them in passing to confirm that the phased driver system is a viable technique for obtain relatively high performance across the entirety of the (U.S) 40-meter band.</p>
|
||||
<p><b>3-Elements and Phased Drivers</b></p>
|
||||
<p>Let's review a couple of facts about 40-meter beams as presently used on the band. First, the most common varieties use boomlengths of about 20-22 feet, most of the time with only 2 elements. The wide-band design in Part 1 calls for 2 elements on a 23.5' boom. For reference, let's repeat the dimensions and the performance table.</p>
|
||||
<pre>
|
||||
Dimensions of the 2-Element Wide-Band Yagi
|
||||
|
||||
Note: All dimensions are in inches. Multiply by 2.54 for centimeters,
|
||||
0.0254 for meters.
|
||||
|
||||
Antenna Element Spacing Half-Length Tip Length
|
||||
2-Element Reflector 0 451 65
|
||||
Driver 282 412 26
|
||||
|
||||
Performance
|
||||
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 6.50 9.89 40.7 - j 19.4 1.60
|
||||
7.15 5.96 10.70 53.1 + j 3.5 1.09
|
||||
7.3 5.52 9.71 63.5 + j 24.2 1.63
|
||||
</pre>
|
||||
<p>The gain and front-to-back values are typical for a 2-element driver-reflector Yagi. We obtained full-band coverage by widening the element spacing from the usual 40-meter 20' value.</p>
|
||||
<p>The second fact from Part 1 is the ability of Yagis to operate satisfactorily without a reflector, if the driver cell can handle the chore of setting the basic feedpoint impedance level. In the phased driver cell used in the preceding pair of designs, we have such a cell. If we remove the reflector, the impedance does not change beyond our ability to adjust by lengthening the director by a few inches on each end. We may leave all other elements just as they occur in the 4-element design and also retain the spacing values. The result is a 3-element Yagi with a phased pair of drivers and a single director on a 22.9' boom. <b>Fig. 13</b> shows the outline.</p>
|
||||
<center>
|
||||
<img src="40wb2-13.gif" width="320" height="170" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The following table and <b>Fig. 14</b> surveys the potential performance of the shorter array across 40 meters.</p>
|
||||
<pre>
|
||||
Dimensions of the 3-Element Wide-Band Yagi with Phased Drivers
|
||||
|
||||
Note: All dimensions are in inches. Multiply by 2.54 for centimeters,
|
||||
0.0254 for meters.
|
||||
|
||||
Antenna Element Spacing Half-Length Tip Length
|
||||
3-Element Rear Driver 0 432 46
|
||||
Forward Driver 98 401 15
|
||||
Director 275 393 7
|
||||
Performance
|
||||
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 6.13 13.56 38.2 + j 5.5 1.35
|
||||
7.15 6.57 15.30 48.4 + j 1.8 1.05
|
||||
7.3 7.05 13.49 45.2 - j 15.2 1.40
|
||||
</pre>
|
||||
<center>
|
||||
<img src="40wb2-14.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The data suggest a Yagi with performance intermediate between a standard 2-element driver-reflector design and the full 4-element phased-driver design with a reflector. Indeed, the performance is similar to that of an optimized driver-director 2-element Yagi, but without the usual restriction that comes with such designs--an exceptionally narrow operating bandwidth. Both the gain and front-to-back ratio exceed what is possible in the 2-element Yagi, but do not quite reach the level achieved by the 4-element design. However, the patterns are very well-behaved, and the 50-Ohm SWR curve never rises to 1.5:1.</p>
|
||||
<p>Adding the weight of a third element to a 2-element Yagi is not a small adjustment. The overall beam will weigh about 30% more and call for support strengthening. However, the shorter boomlength reduces overall structural stressing in the wind relative to one of the longer boom 4-element designs. With a 23' boom, the 3-element phased-driver arrangement may be the most promising way to achieve the desired contester SWR curve with quite reasonable performance everywhere in the band. Indeed, the short boom allows room for further tweaking of the phased drivers to allow even further improvements in the SWR curve, as well as further adjustments to the length and position of the director for small performance improvements. The design improves on the sample 12-meter antenna (with a loss in gain and front-to-back ratio) by beginning with a wide-band phased pair of elements rather than the maximum gain pair used in the earlier experiment. In addition, the design also shows what the reflector in the 4-element long design adds to performance--assuming that its 39' boomlength is not a hindrance to implementation.</p>
|
||||
<p><b>Conclusion</b></p>
|
||||
<p>These designs complete our small survey of wide-band 40-meter Yagis. They do suggest that there are a number of alternative designs capable of covering the band with adequately high performance with boomlengths under 40', with not much lost as we shorten the array down to 35' or so. The new additions appear to have the additional merit of not being as sensitive to changes in some critical elements as the N3HBX design. Indeed, the final 3-element Yagi with phased drivers seems to be the most practical among all of the designs</p>
|
||||
<p>Along the way, we have had occasion to probe various Yagi properties often overlooked by both Yagi users and home Yagi builders. Since few radio amateurs have the wherewithal to build or sustain a full size 3- or 4-element 40-meter Yagi, these additional discussions may prove in the long run to have more utility than the design suggestions. Indeed, the designs would require considerable revision and refinement to meet whatever one chose as a usable element taper schedule for implementation. Hence, they are only first-order design examples useful for comparisons among the collection of Yagi ideas.</p>
|
||||
<p>The U.S. 40-meter amateur band is a special case within the HF spectrum. It is just narrow enough to lead us to believe that we can cover the entire band with a single antenna. However, the mechanical requirements for tubular elements restrict the achievable bandwidth with standard designs. Full band coverage calls for design ingenuity and some specialized techniques for extending coverage while maintaining a manageable boomlength (assuming that element length presents no insurmountable problems). As a consequence, it is a good band for learning one or two new things about wide-band Yagi performance and design.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 03-01-2006. © L. B. Cebik, W4RNL. This item originally appeared in</i> antenneX<i>, (Feb., 2006). Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="40wb3.html"><img src="../images/fwdrd.gif" align="bottom" width="36" height="33"> Go to Part 3</a></p>
|
||||
<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Main Index</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-lower'>HF Lower</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
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<meta name="keywords" content="Yagi, HF Lower">
|
||||
<title>Wide-Band 40-Meter Yagis Part 3: A 3-Element Wire Design</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
<body>
|
||||
<center>
|
||||
<h1>Wide-Band 40-Meter Yagis<br>
|
||||
Part 3: A 3-Element Wire Design</h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center><br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p>At the end of Part 2 in this series, we introduced a 3-element Yagi beam that used phased drivers and a single director to provide reasonably good performance on 40 meters. <b>Fig. 1</b> shows the general outline.</p>
|
||||
<center>
|
||||
<img src="40wb3-1.gif" width="320" height="170" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The phased-driver and director system offers a number of advantages over other designs.</p>
|
||||
<ul>
|
||||
<li>1. The boom length is shorter than a driver-reflector Yagi with a similar operating bandwidth.</li>
|
||||
<li>2. The 3-element arrangement offers higher gain (overall) and a better front-to-back ratio than a two-element design.</li>
|
||||
<li>3. The performance is only a bit shy of the performance of a wide-band 3-element Yagi expressly designed for broadband operation, but uses a much shorter boom.</li>
|
||||
<li>4. The phased-driver system allows full-band coverage with a 50-Ohm SWR that is below 1.5:1 across the entire band.</li>
|
||||
</ul>
|
||||
<p><b>Fig. 2</b> graphically summarizes the beam's performance, with selected numerical data in <b>Table 1</b>.</p>
|
||||
<pre>
|
||||
Table 1. Dimensions of the 3-Element Wide-Band Yagi with Phased Drivers
|
||||
|
||||
Note: All dimensions are in inches. Multiply by 2.54 for centimeters,
|
||||
0.0254 for meters.
|
||||
|
||||
Antenna Element Spacing Half-Length Tip Length
|
||||
3-Element Rear Driver 0 432 46
|
||||
Forward Driver 98 401 15
|
||||
Director 275 393 7
|
||||
Performance
|
||||
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 6.13 13.56 38.2 + j 5.5 1.35
|
||||
7.15 6.57 15.30 48.4 + j 1.8 1.05
|
||||
7.3 7.05 13.49 45.2 - j 15.2 1.40
|
||||
</pre>
|
||||
<center>
|
||||
<img src="40wb3-2.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Like all Yagis with directors, the beam shows a rising gain curve with increased operating frequency. The front-to-back ratio peaks near the band center with roughly equal values at the band edges. The phased drivers require no matching system for compatibility with a standard 50-Ohm coaxial cable feedline (although a common-mode attenuation device is always in order).</p>
|
||||
<p>The dimensions for the beam presume a very husky aluminum tubing taper schedule. The two driver elements require insulation and isolation from the boom for the feedline and phase line connections. Hence, the gap insulation structure and the mechanical boom connections demand special attention to ensure strength. Even within the genre of tubing-based 40-meter full size beams, the addition of a third element represents major increases in beam weight and wind loading. Still, the boom stress and the wind loading are not as high as would be the case for a wide-band standard 3-element design with its much longer boom.</p>
|
||||
<p>Full-size heavy Yagis with all of the required supporting mechanisms and structures lie beyond the capabilities of a very large number of amateurs. Many members of the excluded group would be happy with a fixed position beam composed of wire. However, obtaining very wide-band performance requires considerable real estate, especially since single-wire element designs cannot match the performance of fat-element tubular designs. Hence, wire-beam designers often add elements or use very wide element spacing to obtain good results across the entire 40-meter band (as defined for the U.S., that is, from 7.0 to 7.3 MHz).</p>
|
||||
<p>So we have an unanswered question: can the 3-element phased-driver design undergo adaptation to the needs of the fixed-position wire-beam user? The answer is affirmative, if we are willing to make only a small set of concessions.</p>
|
||||
<p><b>The 3-Element Phased-Driver-Director Yagi in Wire Form</b></p>
|
||||
<p>Adapting a Yagi design intended for use with aluminum tubing in a complex taper schedule to a uniform wire size requires multiple steps. There is no absolute order to them, but a good first effort is to find the uniform-diameter length and diameter for the elements in the original design. One may use either the facility within a modeling program or an external calculator (program or spreadsheet) to arrive at the values. For the 3-element beam described above, we find the following equivalent uniform-diameter elements:</p>
|
||||
<ul>
|
||||
<li>Rear Driver: Length: 801.3", Diameter: 1.38"</li>
|
||||
<li>Forward Driver: Length: 750.3", Diameter: 1.50"</li>
|
||||
<li>Director: Length: 737.4", Diameter: 1.54"</li>
|
||||
</ul>
|
||||
<p>When we use an invariant set of diameter sets for all elements and vary only the length of the outer tip, the diameter value progression is not unusual. The longer rear element tip results in a smaller equivalent uniform diameter than we find for the director with its shorter tip section. Nevertheless, the variation in diameters is less significant in this case than the fact that all three diameters are nearly 1.5".</p>
|
||||
<p>The most commonly used antenna wire size is AWG #12. With a diameter of 0.0808", this wire is only about 0.05 of the tubular element diameter. Such a small-diameter wire has two consequences of special import in this context. One is the fact that the mutual coupling between adjacent elements is much less than for the tubular elements. The phased drivers are just as dependent upon mutual coupling (in conjunctions with the phase line between them) as are the director and the drivers in a conventional parasitic arrangement. Therefore, a single-wire element set would require very significant compression of the element spacing, as well as adjustments to the element lengths to arrive at a working beam of the current design.</p>
|
||||
<p>The second consequence is that the small diameter wire will result in a shrinking operating bandwidth that includes gain and front-to-back ratio as well as SWR. One of the goals of the adaptation is to see how much of the large-element performance we can preserve using wire elements. The single-wire route is not promising.</p>
|
||||
<p>We can simulate larger diameter wires by using multiple wires in each element. We need not resort to cages, since we can approximate the full tubular element diameter with a 2-wire pair using a spacing of about 3" or so. While 3" spacing is feasible, we might also try closer spacing and see how a commonly available material might work. For example, we can purchase open-wire transmission line (ladder line) composed of AWG #12 conductors and about a 1" spacing. The rough equivalent single-wire diameter would be about 1/2". <b>Fig. 3</b> provides an outline of how we would use the line for the elements in the beam.</p>
|
||||
<center>
|
||||
<img src="40wb3-3.gif" width="483" height="281" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The two drivers require a center insulator to provide a gap between element halves. The 50-Ohm cable connects to the forward driver. The 250-Ohm phase line with a single half twist runs between the forward and the rear drivers. For a wire beam, one should use home-built line. <b>Table 2</b> provides dimensions for 250-Ohm lines using various wire diameters.</p>
|
||||
<pre>
|
||||
Table 2. 250-Ohm open-wire transmission line dimensions
|
||||
AWG Wire Size Wire Diameter Center-to-Center Spacing
|
||||
#14 0.0641" 0.262"
|
||||
#12 0.0808" 0.330"
|
||||
#10 0.1019" 0.416"
|
||||
#8 0.1285" 0.525"
|
||||
</pre>
|
||||
<p>You will need spacers about every 3" to maintain the wire spacing accurately. The best way to make spacers is to drill the wire holes in a long strip of polycarbonate, Plexiglas, or similar plastic. Cut the spacers to size after you complete the drilling. Do not make the holes too large; you want a tight fit. A drop of one of the superglues at each hole will lock the spacers in place. The velocity factor of this phase-line will be very close to 1.0. You will need just under 8' of the phase line.</p>
|
||||
<p>You can construct the ladder-line elements in a similar manner, or you can purchase commercial ladder line if the conductors are hard-drawn copper. Using this type of material, the required dimensions with both length and spacing adjustments appear in <b>Table 3</b>. Both drivers are slightly longer than the uniform-diameter equivalent lengths, as fits the smaller equivalent diameter of the ladder-line elements. To compensate for the reduced mutual coupling, the spacing between the drivers shrinks by 5". The director is considerably longer than the uniform-diameter equivalent and about 10" closer to the driver set. The resulting "boom" length is a total of 260", about 15" shorter than the boom length of the tubular version of the beam.</p>
|
||||
<p>The selected data in <b>Table 3</b> and the graphs in <b>Fig. 4</b> show that some compromises are at work in preserving as much performance as possible. The resistance, reactance, and SWR data show that the minimum SWR is not quite as ideal as in the tubular version, although the maximum SWR remains below 1.5:1 across the band. It is possible to tweak the dimensions further. For example, reducing the director spacing from the drivers will increase the gain and front-to-back slightly, but at the cost of the broad SWR curve.</p>
|
||||
<pre>
|
||||
Table 3. Dimensions of the 3-Element Wide-Band Wire Yagi with Phased Drivers
|
||||
|
||||
Note: All dimensions are in inches. Multiply by 2.54 for centimeters,
|
||||
0.0254 for meters.
|
||||
|
||||
Antenna Element Spacing Half-Length Full Length
|
||||
3-Element Rear Driver 0 403 806
|
||||
Forward Driver 93 379 758
|
||||
Director 260 377 754
|
||||
Performance
|
||||
|
||||
Frequency Free-Space Front-Back Feedpoint Z 50-Ohm
|
||||
MHz Gain dBi Ratio dB R +/- jX Ohms SWR
|
||||
7.0 5.92 12.89 35.8 + j 4.0 1.42
|
||||
7.15 6.42 15.48 50.9 + j 5.2 1.11
|
||||
7.3 6.97 13.22 54.5 - j 20.0 1.48
|
||||
</pre>
|
||||
<center>
|
||||
<img src="40wb3-4.gif" width="740" height="860" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The average gain across 40 meters for the wire version of the antenna is about 0.15 dB less than for the very heavy tubular array. The band-edge front-to-back ratio declines by about a half-dB. One could not detect these amounts in operations for beams at the same height. A comparison of the E-plane pattern shapes at the bottom of <b>Fig. 2</b> and of <b>Fig. 4</b> shows that the ladder-line version of the antenna preserves the beam width of the forward lobe and the rear lobe shape across the band.</p>
|
||||
<p>On this basis, one may judge the ladder-line elements to be a successful adaptation of the original design. Of course, there are no rules against constructing one's own wider 2-wire elements. Every increase in wire spacing will improve the operating bandwidth of the array. However, the task is somewhat daunting, since the elements require a total of about 2400" (200') of 2-wire element line. As well, widening the spacing between wires in the elements will require adjustments to the element lengths and spacing values. In most cases, the adjusted values will fall between the values for the tubular beam and those for the ladder-line version.</p>
|
||||
<p><b>Installation and Expectations</b></p>
|
||||
<p>Installing a wire Yagi or other flattop array of any sort involves establishing 4 corner vertical supports and then developing a perimeter rope assembly to hold the elements both at the desired height and also in relationship to each other. <b>Fig. 5</b> sketches the barest outline of such a system. Actual systems will be as varied as the installation sites.</p>
|
||||
<center>
|
||||
<img src="40wb3-5.gif" width="400" height="200" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The vertical posts can be anything that is tall and sturdy. In some areas, trees are plentiful. The distance from the tree to the array corner may vary at each corner so long as the installer adjusts overall tension to hold the wires in the correct position. One may also use existing towers for upper-HF beams. Other arrangements are possible. Stresses in the direction of the beam are generally less critical within the limits of strength of the non-conductive ropes used. However, the stress along each element should be sufficient to prevent undue catenary sag without pressing the limits of the wire strength.</p>
|
||||
<p>The feedpoint at the forward driver deserves extra thought for support. The coaxial cable feedline and any common-mode attenuator tend to be heavy compared to other portions of the beam. Adding a vertical post beneath the feedpoint to minimize the unsupported cable run is one way to minimize the stress.</p>
|
||||
<p>Most 40-meter beams will serve at lower heights--as measured in fractions of a wavelength--than upper HF beams. This fact will be true regardless of the beam construction. The rule of height ("the higher, the better") remains in effect at 40 meters. <b>Fig. 6</b> overlays the elevation patterns for the 40-meter 3-element Yagi at 35', 70', 105', and 140'. The last height is about 1/2 wavelength. The patterns show the lowering of the lowest lobe as the height increases, thus improving low-angle long-distance skip transmission and reception.</p>
|
||||
<center>
|
||||
<img src="40wb3-6.gif" width="457" height="180" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The use of phased drivers does not wholly eliminate the effects of ground on the feedpoint impedance. However, it does minimize them to a generally acceptable degree. <b>Fig. 7</b> provides the 50-Ohm SWR sweeps for all four heights. The SWR remains well below 2:1 at all heights. With some element adjustment, one may be able to hold the value at less than 1.5:1 at the band edges at any of the heights with the 2-wire construction.</p>
|
||||
<center>
|
||||
<img src="40wb3-7.gif" width="432" height="226" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Generally, when adjusting the phased driver lengths, the following rules of thumb will apply. To lower the frequency of minimum SWR either lengthen the rear driver or shorten the forward driver--or both. To raise the frequency of minimum SWR, do the opposite. Be prepared also to adjust the length of the director to center the SWR curve within the passband for roughly equal band-edge SWR values. In most cases, these adjustments will be so minor as not to affect the general beam performance across the band. These rules are not absolute, since they do not take into account possible interactions with other objects at the installation site.</p>
|
||||
<p><b>Conclusion</b></p>
|
||||
<p>These notes do not add anything new to the general ideas on wide-band 40-meter beams covered in Parts 1 and 2. They do show that it is possible to adapt such designs to wire construction. In general, single wire elements tend not to provide the desired wire operating bandwidth when we include gain and front-to-back ratio to the usual SWR conception of operating bandwidth. However, even the relatively narrow spacing of ladder line is sufficient to preserve the performance of fat tubular element designs to a degree that makes a wire beam in a fixed position worthwhile.</p>
|
||||
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|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
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|
||||
<p><i>Updated 12-31-2007. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
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||||
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<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Main Index</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-lower'>HF Lower</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>Modeling 6 Long-Boom Yagis</title>
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<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10"> Modeling 6 Long-Boom Yagis <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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Over the last year or so, I accumulated designs for 5 different 20- meter Yagis with boom between 45 and 55 feet long and having either 5 or 6 elements. After the appearance of a preliminary version of these notes, a 6th was donated. All display an average free space gain from 14.0 to 14.35 MHz of over 10 dBi, with good front-to-back ratios averaging well over 20 dB across the band. It seemed to be an interesting project to model all 6 on the same versions of NEC (in this case, NEC-2 with Leeson corrections and NEC-4, using EZNEC Pro) and do some comparative assessment.
|
||||
<p>What actually emerged is a lesson in some of the difficulties in making such assessments. Some of those difficulties arise from limitations of the programs (and every version of NEC/MININEC has limitations). Others emerge from differences among models and specification sheets that cannot be resolved without additional data not at hand. Hasty modeling and simplistic conclusions can yield fog where clarity is desired.</p>
|
||||
<h3>The Six Beams</h3>The six beams are the following:
|
||||
<ul>
|
||||
<li>1. A 5-element, 45' boom design by W6NGZ, published in <i>CQ</i> for October, 1996, p. 22.</li>
|
||||
<li>2. A 5-element, 48' boom design by K6STI, published among the beams in YA, which accompanies the <i>ARRL Antenna Book</i>--this is a medium strength model.</li>
|
||||
<li>3. A 5-element, 48' boom design by W3LPL, published on the World Wide Web.</li>
|
||||
<li>4. A 5-element, 55' boom design by K4SB, originally developed in monotaper for 10 meters, modified for this study by me for a somewhat arbitrary element taper (used by permission).</li>
|
||||
<li>5. A 6-element, 48' boom design by NW3Z, published on the World Wide Web.</li>
|
||||
<li>4. A 5-element, 48' sent to me by WB0DGF and once published on the Hy-Gain web site as HG205XLB.</li>
|
||||
</ul>
|
||||
<p>The beams are interesting for their differing design features, not the least of which is the choice in element spacing, as shown in the figure below.</p>
|
||||
<center>
|
||||
<img src="5l20-6.gif" width="617" height="503" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The K6STI has the lowest design feedpoint impedance, which is indicated in the reflector-driver spacing. The W6NGZ design aims for a higher feedpoint impedance and uses a correspondingly wider reflector-driver spacing. The W3LPL design has a comparable feedpoint impedance and hence a similar reflector-driver spacing to the W6NGZ design, despite the differences in boom length. These three designs are similar beyond the driver in that the directors are proportionally spaced, relative to each other. However, do not ignore the ability of D2 to affect feedpoint impedance. The WB0DGF design demonstrates how a different taper schedule may influence the required spacing for a design, since its reflector-drive spacing is greater than the K6STI antenna, but the DGF version has only a moderately higher feedpoint impedance.</p>
|
||||
<p>The longest model (K4SB) tries for a direct 50-Ohm feed and uses the widest reflector-driver spacing. D1 is not especially different in spacing from the driver relative to the first three models. The truly different design is the NW3Z 6-element Yagi, which uses a moderately wide spacing between the reflector and driver combined with the closely spaced D1 to set the basic parameters of the antenna. However, D2 also plays a crucial role in setting the feedpoint impedance: at correct spacing, the feedpoint impedance is a good coax match; with D2 40" closer to D1, the feedpoint impedance drops to the 25-Ohm neighborhood without seriously affecting the gain and front-to-back characteristics of the antenna.</p>
|
||||
<p>These notes are not designed to recommend one design over another, either overtly or tacitly. That would require a detailed construction analysis plus the resolution to some modeling questions which I intend to leave open.</p>
|
||||
<h3>Some Modeling Preliminaries</h3>Unfortunately, modeling Yagis within the amateur community has become somewhat of a casual process, without the care and attention to detail used by serious designers, such as the author's of the designs shown here. Whatever the source of the design--YO, Yagi-Max, or simply manual design on AO or EZNEC--evaluation of a design requires good modeling practice.
|
||||
<p>NEC-2 without a correction factor cannot accurately model a linear element using a tapered-diameter schedule. For that reason, some implementations of NEC-2 include Leeson or similar corrections, which substitute a carefully calculated mono-tapered element for each tapered-diameter element in an array. However, it is possible to disturb the substitute element by paying too little attention to segment length equality. Especially at current nodes (the center of near-1/2 wl elements), segment length needs to be equalized along a linear element whether it is a single dipole or a part of a larger array). Since the Leeson corrections modify the length and diameter of a wire but not its segmentation, segment lengths should be equalized in advance. This factor plays an especially important role where the designer may place a very-large diameter short segment at the element center to simulate boom-to-element mounting plates and hardware. Often--as with the K6STI and WB0DGF models--the length of this segment sets the approximate length of segments in the adjoining sections of the element.</p>
|
||||
<p>Therefore, for the models at hand, segmentation was increased to yield segment lengths between 8" and 12", depending upon the model. The K6STI and WB0DGF models use the shorter lengths due to their center "lumps." (Note that the WB0DGF model omits the center "lump" on the driven element, while the K6STI model preserves it.) The W6NGZ model uses 12" segments, because they came out most even with the element sections in the design.</p>
|
||||
<p>Other beams used 8" (W3LPL) or 10" (K4SB, NW3Z) per segment for similar reasons: the chosen segment length turned out to be the best way to achieve roughly equal length segments on the most elements. Despite best efforts, some variations inevitably result. Designers will always have, for any segment length chosen, a section that comes out to be n.5 segments long. Some will choose center sections that for any reasonable segment length seem to require an even number of segments. Adding extra segment is necessary for the driver--and hence, for the other elements in order to keep segment junctions as well aligned as possible.</p>
|
||||
<p>NEC-4, by its initial reports, overcomes the NEC-2 weakness with tapered-diameter elements. Unfortunately, the initial reports have proven overly optimistic. Although NEC-4 is a vast improvement over uncorrected NEC-2, it falls short of the standard set by NEC-2-corrected, which also correlates with MININEC results. (MININEC does not have a tapered-diameter element problem, although it has other limitations.)</p>
|
||||
<p>The NEC-4 limitation is somewhat minor for general purpose modeling, unless the element shows too large a diameter change between wires in the element, especially close to the current node, with the driven element the most crucial. This condition exists with the K6STI model.</p>
|
||||
<p>In order to show the modeling differences between NEC-2-corrected and NEC-4, I have modeled all 6 beams in both programs via EZNEC Pro. (I could have applied the Leeson equations to the NEC-4 runs, but that would have produced the same results as the NEC-2 runs.) I graphed data comparatively for NEC-2-corrected and for NEC-4, to include free space gain in dBi, 180-degree front-to-back ratio, feedpoint resistance, and feedpoint reactance. SWR data is also shown wherever the source impedance is close enough to 50-Ohms to make such data relevant. All graphs run from 13.8 to 14.4 MHz in order to show downward slides of NEC-4 curves.</p>
|
||||
<p>Let's look at the designs individually.</p>
|
||||
<h3>The W6NGZ 5-element, 45' boom model</h3>
|
||||
<center>
|
||||
<img src="ngz.gif" width="596" height="319" border="2" vspace="5" hspace="9">
|
||||
</center>The W6NGZ model is the shortest of the five, but it sustains a free space gain of over 10 dBi across the band, with better than 20 dB front-to-back ratio everywhere on 20. The element taper is from the <i>CQ</i> article, and it contains no compensation for element mounting plates at the element center. Within these constraints, NEC-4 and NEC-2-corrected tell very similar graphical stories about the beam. Differentials are marginal at best, although a wee bit of excess gain estimation and a slide of the curves downward in frequency are both evident.
|
||||
<center>
|
||||
<img src="ngz1.gif" width="651" height="509" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="ngz2.gif" width="646" height="509" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="ngz3.gif" width="635" height="518" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="ngz4.gif" width="644" height="523" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="ngz5.gif" width="660" height="517" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>As the graphics make plain, the antenna close being a very good match for 50-ohm cable.</p>
|
||||
<h3>The K6STI 5-element, 48' boom model</h3>
|
||||
<center>
|
||||
<img src="sti.gif" width="594" height="311" border="2" vspace="5" hspace="9">
|
||||
</center>The K6STI design, with its center "bulge" in place, models quite well in NEC-2-corrected. It also displays very well the weakness of NEC-4 in dealing with the large change of diameter so close to the source or highest current point on the elements, despite extensive efforts to equalize segment lengths. NEC-4 gain data is almost a full dB higher than the more reasonable NEC-2 data, with a curve displacement lower in frequency, which also shows up on the front-to-back graph. The source resistance and reactance graphs show a similar slide, with the resistance values low in the NEC-4 run. Since current, impedance, and far field strength are related, the coincidence of these value differences from those in the NEC-2 run are not unexpected.
|
||||
<center>
|
||||
<img src="sti1.gif" width="645" height="518" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="sti2.gif" width="645" height="514" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="sti3.gif" width="638" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="sti4.gif" width="638" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>No SWR curve is shown, since the antenna is designed for a matching system such as a beta/hairpin match.</p>
|
||||
<h3>The W3LPL 5-element, 48' boom model</h3>
|
||||
<center>
|
||||
<img src="lpl.gif" width="589" height="311" border="2" vspace="5" hspace="9">
|
||||
</center>The W3LPL beam was designed for use with a Tee match, which neither NEC-2 nor NEC-4 can physically model accurately. Interestingly, W3LPL designed the antenna with non-conductive boom-to-element mounting plates in mind. Nonetheless, the curves--plus a look at the antenna characteristics just below 14 MHz--suggests that there is an approximate 50 kHz offset in frequency, with the elements in the model again playing short in the NEC-4 run, relative to the NEC-2-corrected run. Gain is still a little high in NEC-4. Given the W6NGZ model runs, this much difference is somewhat unexpected. The chief difference in the modeled data is that the W3LPL antenna shows considerable and constant inductive reactance, due to its intended use with a Tee-matching system.
|
||||
<center>
|
||||
<img src="lpl1.gif" width="644" height="522" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="lpl2.gif" width="648" height="517" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="lpl3.gif" width="636" height="524" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="lpl4.gif" width="642" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="lpl5.gif" width="643" height="517" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>I have included the 50-Ohm SWR curve for the W3LPL antenna because it makes a good contrast with that of the W6GNZ antenna, even though the antenna designer specifically introduced the inductive reactance in the driver to set up conditions for a no-capacitor Tee match system for a 200-Ohm feed impedance. Shortening the driver to resonance in an attempt to reduce the SWR will result in a decrease in the resistive component so that no great improvement actually occurs. However, those wishing a direct 50-Ohm feed without a Tee match may wish to consider a beta match using a shunt capacitor, instead of the usual shunt inductor, which is applied when the driver element is capacitively reactive. A capacitor value of about 90 Ohms reactance (about 125 pF) with sufficient voltage and current handling capabilities would come close to serving the need.</p>
|
||||
<h3>The K4SB 5-element, 55' boom model</h3>
|
||||
<center>
|
||||
<img src="sb.gif" width="594" height="330" border="2" vspace="5" hspace="9">
|
||||
</center>I received this model from K4SB (with whose permission it is used here) in simplified form, using monotapered elements on 10-meters. I frequency-scaled it to 20-meters and then assigned it an element taper schedule, readjusting the element lengths until it closely approximated the monotaper performance. The antenna shows the greatest gain and front-to-back differential across the band of all the models, and this exhibits the effects of using wide reflector spacing alone to raise the feedpoint impedance of the beam.
|
||||
<p>This beam is for me simply a modeling exercise and does not take into account any variations in building technique. It does not compensate for the effects of boom-to-element hardware. Considerable model tweaking would likely be required before construction.</p>
|
||||
<center>
|
||||
<img src="sb1.gif" width="646" height="516" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="sb2.gif" width="649" height="521" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="sb3.gif" width="632" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="sb4.gif" width="646" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="sb5.gif" width="650" height="518" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The SWR curve is quite good. There is a significant variation in reactance across the band, to go along with the variations in gain and front-to-back ratio, although the beam demonstrates close to the optimum of what may be achieved using reflector spacing alone to determine a 50-Ohm feedpoint match.</p>
|
||||
<h3>The NW3Z 6-element, 48' boom model</h3>
|
||||
<center>
|
||||
<img src="z.gif" width="589" height="312" border="2" vspace="5" hspace="9">
|
||||
</center>The NW3Z design adds a director close to the driven element. Along with the spacing of director D2 for impedance control and the spacing of the remaining directors for performance characteristics, this design provides a very tight near-50-Ohm feedpoint impedance with very little variation in other antenna performance characteristics across 20 meters. Indeed, everything remains stable for a little ways below the lower end of 20 meters.
|
||||
<p>As with all other designs, NEC-4 tends to overestimate gain and move the performance characteristics lower in frequency relative to NEC-2 curves. Since this model uses a 96" constant diameter center section in each element, the effect is not too extreme, but still greater than for the W6NGZ design.</p>
|
||||
<center>
|
||||
<img src="z1.gif" width="649" height="519" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="z2.gif" width="647" height="516" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="z3.gif" width="641" height="524" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="z4.gif" width="638" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="z5.gif" width="643" height="525" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Beware of misreading the steep SWR curves: first read the axis values. Only then can you compare the numbers with those appearing in the other beam graphs.</p>
|
||||
<h3>The WB0DGF 5-element, 48' boom model</h3>
|
||||
<center>
|
||||
<img src="dgf.gif" width="586" height="351" border="2" vspace="5" hspace="9">
|
||||
</center>The WB0DGF design, provides an interesting contrast to both the K6STI and the NW3Z designs with respect to modeled properties. It contains center "bulge" sections that limit segment length to 8". However, these occur on only the parasitical elements. Moreover, the design uses a wider reflector spacing than some of the other designs. The result is a far less extreme overestimation of gain, but a significant shift in the NEC-4 curves lower in frequency than the NEC-2 curves. These phenomena are clear on all four of the graphs.
|
||||
<p>Since this antenna design was set up for potential use with a beta match system, no SWR curve is shown.</p>
|
||||
<center>
|
||||
<img src="dgf1.gif" width="645" height="521" border="1" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="dgf2.gif" width="650" height="518" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="dgf3.gif" width="637" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="dgf4.gif" width="649" height="525" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h3>Conclusions</h3>Because of the open questions left along the way, no evaluative assessment is warranted by this exercise alone. However, two reminders are in order.
|
||||
<p>1. The beams do reveal something of the broader aspects of large Yagi behavior in the means we use to design specific feedpoint impedances while retaining the highest levels of gain and front-to-back performance. Study of the spacing sketch alone (made more precise by the EZNEC Pro file descriptions appended to this note) may give some insight into future Yagi design directions. Yagi design innovations are far from exhausted.</p>
|
||||
<p>2. Modeling comparisons are not to be undertaken lightly or hastily. Analyzing the foundations of received models as well as those inherent in one's own creations is a necessary step if models are to be compared against some set of design or analysis goals. It would have been very easy to see two of the designs above as simply inadequate to cover the entirety of 20 meters by looking solely at NEC-4 curves, when what is more likely at work is a frequency offset. Add to this the limitations of each core used to model the structures unambiguously, and the simple conjunction of models would become a disservice to understanding the antennas involved.</p>
|
||||
<p>Within these handicaps, the comparison of some of the best of extant long- boom 20-meter Yagi design provides an interesting and potentially productive exercise. It can assist us in understanding both antenna design and antenna modeling just a little bit better.</p>
|
||||
<h3>EZNEC Wire Tables for the 5 Yagis</h3>
|
||||
<pre> EZNEC/4 ver. 2.0
|
||||
|
||||
5L45' W6NGZ CQ 10-96 p 22 07-19-1998 07:44:08
|
||||
|
||||
|
||||
Frequency = 14.175 MHz.
|
||||
|
||||
Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
|
||||
|
||||
|
||||
--------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : in) Conn. --- End 2 (x,y,z : in) Dia(in) Segs
|
||||
|
||||
1 -215.60, 0.000, 0.000 W2E1 -156.00, 0.000, 0.000 6.25E-01 5
|
||||
2 W1E2 -156.00, 0.000, 0.000 W3E1 -120.00, 0.000, 0.000 7.50E-01 3
|
||||
3 W2E2 -120.00, 0.000, 0.000 W4E1 -72.000, 0.000, 0.000 8.75E-01 4
|
||||
4 W3E2 -72.000, 0.000, 0.000 W5E1 72.000, 0.000, 0.000 1.00E+00 13
|
||||
5 W4E2 72.000, 0.000, 0.000 W6E1 120.000, 0.000, 0.000 8.75E-01 4
|
||||
6 W5E2 120.000, 0.000, 0.000 W7E1 156.000, 0.000, 0.000 7.50E-01 3
|
||||
7 W6E2 156.000, 0.000, 0.000 215.605, 0.000, 0.000 6.25E-01 5
|
||||
8 -205.95, 79.800, 0.000 W9E1 -156.00, 79.800, 0.000 6.25E-01 4
|
||||
9 W8E2 -156.00, 79.800, 0.000 W10E1 -120.00, 79.800, 0.000 7.50E-01 3
|
||||
10 W9E2 -120.00, 79.800, 0.000 W11E1 -72.000, 79.800, 0.000 8.75E-01 4
|
||||
11 W10E2 -72.000, 79.800, 0.000 W12E1 72.000, 79.800, 0.000 1.00E+00 13
|
||||
12 W11E2 72.000, 79.800, 0.000 W13E1 120.000, 79.800, 0.000 8.75E-01 4
|
||||
13 W12E2 120.000, 79.800, 0.000 W14E1 156.000, 79.800, 0.000 7.50E-01 3
|
||||
14 W13E2 156.000, 79.800, 0.000 205.950, 79.800, 0.000 6.25E-01 4
|
||||
15 -198.21,155.160, 0.000 W16E1 -156.00,155.160, 0.000 6.25E-01 4
|
||||
16 W15E2 -156.00,155.160, 0.000 W17E1 -120.00,155.160, 0.000 7.50E-01 3
|
||||
17 W16E2 -120.00,155.160, 0.000 W18E1 -72.000,155.160, 0.000 8.75E-01 4
|
||||
18 W17E2 -72.000,155.160, 0.000 W19E1 72.000,155.160, 0.000 1.00E+00 13
|
||||
19 W18E2 72.000,155.160, 0.000 W20E1 120.000,155.160, 0.000 8.75E-01 4
|
||||
20 W19E2 120.000,155.160, 0.000 W21E1 156.000,155.160, 0.000 7.50E-01 3
|
||||
21 W20E2 156.000,155.160, 0.000 198.209,155.160, 0.000 6.25E-01 4
|
||||
22 -196.55,337.920, 0.000 W23E1 -156.00,337.920, 0.000 6.25E-01 3
|
||||
23 W22E2 -156.00,337.920, 0.000 W24E1 -120.00,337.920, 0.000 7.50E-01 3
|
||||
24 W23E2 -120.00,337.920, 0.000 W25E1 -72.000,337.920, 0.000 8.75E-01 4
|
||||
25 W24E2 -72.000,337.920, 0.000 W26E1 72.000,337.920, 0.000 1.00E+00 13
|
||||
26 W25E2 72.000,337.920, 0.000 W27E1 120.000,337.920, 0.000 8.75E-01 4
|
||||
27 W26E2 120.000,337.920, 0.000 W28E1 156.000,337.920, 0.000 7.50E-01 3
|
||||
28 W27E2 156.000,337.920, 0.000 196.548,337.920, 0.000 6.25E-01 3
|
||||
29 -189.90,530.400, 0.000 W30E1 -156.00,530.400, 0.000 6.25E-01 3
|
||||
30 W29E2 -156.00,530.400, 0.000 W31E1 -120.00,530.400, 0.000 7.50E-01 3
|
||||
31 W30E2 -120.00,530.400, 0.000 W32E1 -72.000,530.400, 0.000 8.75E-01 4
|
||||
32 W31E2 -72.000,530.400, 0.000 W33E1 72.000,530.400, 0.000 1.00E+00 13
|
||||
33 W32E2 72.000,530.400, 0.000 W34E1 120.000,530.400, 0.000 8.75E-01 3
|
||||
34 W33E2 120.000,530.400, 0.000 W35E1 156.000,530.400, 0.000 7.50E-01 3
|
||||
35 W34E2 156.000,530.400, 0.000 189.900,530.400, 0.000 6.25E-01 4
|
||||
|
||||
-------------- SOURCES --------------
|
||||
|
||||
Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
|
||||
Seg. Actual (Specified)
|
||||
|
||||
1 7 11 / 50.00 ( 11 / 50.00) 1.000 0.000 I
|
||||
|
||||
No loads specified
|
||||
|
||||
No transmission lines specified
|
||||
|
||||
Ground type is Free Space
|
||||
</pre>
|
||||
<br>
|
||||
<pre> EZNEC/4 ver. 2.0
|
||||
|
||||
5L48' K6STI YA 07-19-1998 07:44:45
|
||||
|
||||
|
||||
Frequency = 14.175 MHz.
|
||||
|
||||
Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
|
||||
|
||||
|
||||
--------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : in) Conn. --- End 2 (x,y,z : in) Dia(in) Segs
|
||||
|
||||
1 -216.50, 0.000, 0.000 W2E1 -138.00, 0.000, 0.000 5.00E-01 10
|
||||
2 W1E2 -138.00, 0.000, 0.000 W3E1 -96.000, 0.000, 0.000 6.25E-01 6
|
||||
3 W2E2 -96.000, 0.000, 0.000 W4E1 -48.000, 0.000, 0.000 7.50E-01 6
|
||||
4 W3E2 -48.000, 0.000, 0.000 W5E1 -4.000, 0.000, 0.000 8.75E-01 5
|
||||
5 W4E2 -4.000, 0.000, 0.000 W6E1 4.000, 0.000, 0.000 3.42E+00 1
|
||||
6 W5E2 4.000, 0.000, 0.000 W7E1 48.000, 0.000, 0.000 8.75E-01 5
|
||||
7 W6E2 48.000, 0.000, 0.000 W8E1 96.000, 0.000, 0.000 7.50E-01 6
|
||||
8 W7E2 96.000, 0.000, 0.000 W9E1 138.000, 0.000, 0.000 6.25E-01 6
|
||||
9 W8E2 138.000, 0.000, 0.000 216.500, 0.000, 0.000 5.00E-01 10
|
||||
10 -203.50, 72.000, 0.000 W11E1 -138.00, 72.000, 0.000 5.00E-01 8
|
||||
11 W10E2 -138.00, 72.000, 0.000 W12E1 -96.000, 72.000, 0.000 6.25E-01 6
|
||||
12 W11E2 -96.000, 72.000, 0.000 W13E1 -48.000, 72.000, 0.000 7.50E-01 6
|
||||
13 W12E2 -48.000, 72.000, 0.000 W14E1 -4.000, 72.000, 0.000 8.75E-01 5
|
||||
14 W13E2 -4.000, 72.000, 0.000 W15E1 4.000, 72.000, 0.000 3.42E+00 1
|
||||
15 W14E2 4.000, 72.000, 0.000 W16E1 48.000, 72.000, 0.000 8.75E-01 5
|
||||
16 W15E2 48.000, 72.000, 0.000 W17E1 96.000, 72.000, 0.000 7.50E-01 6
|
||||
17 W16E2 96.000, 72.000, 0.000 W18E1 138.000, 72.000, 0.000 6.25E-01 6
|
||||
18 W17E2 138.000, 72.000, 0.000 203.500, 72.000, 0.000 5.00E-01 8
|
||||
19 -201.75,160.000, 0.000 W20E1 -138.00,160.000, 0.000 5.00E-01 8
|
||||
20 W19E2 -138.00,160.000, 0.000 W21E1 -96.000,160.000, 0.000 6.25E-01 6
|
||||
21 W20E2 -96.000,160.000, 0.000 W22E1 -48.000,160.000, 0.000 7.50E-01 6
|
||||
22 W21E2 -48.000,160.000, 0.000 W23E1 -4.000,160.000, 0.000 8.75E-01 5
|
||||
23 W22E2 -4.000,160.000, 0.000 W24E1 4.000,160.000, 0.000 3.42E+00 1
|
||||
24 W23E2 4.000,160.000, 0.000 W25E1 48.000,160.000, 0.000 8.75E-01 5
|
||||
25 W24E2 48.000,160.000, 0.000 W26E1 96.000,160.000, 0.000 7.50E-01 6
|
||||
26 W25E2 96.000,160.000, 0.000 W27E1 138.000,160.000, 0.000 6.25E-01 6
|
||||
27 W26E2 138.000,160.000, 0.000 201.750,160.000, 0.000 5.00E-01 8
|
||||
28 -198.88,359.000, 0.000 W29E1 -138.00,359.000, 0.000 5.00E-01 8
|
||||
29 W28E2 -138.00,359.000, 0.000 W30E1 -96.000,359.000, 0.000 6.25E-01 5
|
||||
30 W29E2 -96.000,359.000, 0.000 W31E1 -48.000,359.000, 0.000 7.50E-01 6
|
||||
31 W30E2 -48.000,359.000, 0.000 W32E1 -4.000,359.000, 0.000 8.75E-01 6
|
||||
32 W31E2 -4.000,359.000, 0.000 W33E1 4.000,359.000, 0.000 3.42E+00 1
|
||||
33 W32E2 4.000,359.000, 0.000 W34E1 48.000,359.000, 0.000 8.75E-01 6
|
||||
34 W33E2 48.000,359.000, 0.000 W35E1 96.000,359.000, 0.000 7.50E-01 6
|
||||
35 W34E2 96.000,359.000, 0.000 W36E1 138.000,359.000, 0.000 6.25E-01 5
|
||||
36 W35E2 138.000,359.000, 0.000 198.875,359.000, 0.000 5.00E-01 8
|
||||
37 -191.62,570.000, 0.000 W38E1 -138.00,570.000, 0.000 5.00E-01 7
|
||||
38 W37E2 -138.00,570.000, 0.000 W39E1 -96.000,570.000, 0.000 6.25E-01 5
|
||||
39 W38E2 -96.000,570.000, 0.000 W40E1 -48.000,570.000, 0.000 7.50E-01 6
|
||||
40 W39E2 -48.000,570.000, 0.000 W41E1 -4.000,570.000, 0.000 8.75E-01 6
|
||||
41 W40E2 -4.000,570.000, 0.000 W42E1 4.000,570.000, 0.000 3.42E+00 1
|
||||
42 W41E2 4.000,570.000, 0.000 W43E1 48.000,570.000, 0.000 8.75E-01 6
|
||||
43 W42E2 48.000,570.000, 0.000 W44E1 96.000,570.000, 0.000 7.50E-01 6
|
||||
44 W43E2 96.000,570.000, 0.000 W45E1 138.000,570.000, 0.000 6.25E-01 5
|
||||
45 W44E2 138.000,570.000, 0.000 191.625,570.000, 0.000 5.00E-01 7
|
||||
|
||||
-------------- SOURCES --------------
|
||||
|
||||
Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
|
||||
Seg. Actual (Specified)
|
||||
|
||||
1 1 14 / 50.00 ( 14 / 50.00) 1.000 0.000 V
|
||||
|
||||
No loads specified
|
||||
|
||||
No transmission lines specified
|
||||
|
||||
Ground type is Free Space</pre>
|
||||
<br>
|
||||
<pre> EZNEC/4 ver. 2.0
|
||||
|
||||
5L48' W3LPL WWW 07-19-1998 07:45:37
|
||||
|
||||
|
||||
Frequency = 14.175 MHz.
|
||||
|
||||
Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
|
||||
|
||||
|
||||
--------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : in) Conn. --- End 2 (x,y,z : in) Dia(in) Segs
|
||||
|
||||
1 -217.00, 0.000, 0.000 W2E1 -150.00, 0.000, 0.000 7.50E-01 8
|
||||
2 W1E2 -150.00, 0.000, 0.000 W3E1 -117.00, 0.000, 0.000 8.75E-01 4
|
||||
3 W2E2 -117.00, 0.000, 0.000 W4E1 -84.000, 0.000, 0.000 1.00E+00 4
|
||||
4 W3E2 -84.000, 0.000, 0.000 W5E1 -33.750, 0.000, 0.000 1.12E+00 6
|
||||
5 W4E2 -33.750, 0.000, 0.000 W6E1 33.750, 0.000, 0.000 1.25E+00 9
|
||||
6 W5E2 33.750, 0.000, 0.000 W7E1 84.000, 0.000, 0.000 1.12E+00 6
|
||||
7 W6E2 84.000, 0.000, 0.000 W8E1 117.000, 0.000, 0.000 1.00E+00 4
|
||||
8 W7E2 117.000, 0.000, 0.000 W9E1 150.000, 0.000, 0.000 8.75E-01 4
|
||||
9 W8E2 150.000, 0.000, 0.000 217.000, 0.000, 0.000 7.50E-01 8
|
||||
10 -212.00, 86.000, 0.000 W11E1 -150.00, 86.000, 0.000 7.50E-01 8
|
||||
11 W10E2 -150.00, 86.000, 0.000 W12E1 -117.00, 86.000, 0.000 8.75E-01 4
|
||||
12 W11E2 -117.00, 86.000, 0.000 W13E1 -84.000, 86.000, 0.000 1.00E+00 4
|
||||
13 W12E2 -84.000, 86.000, 0.000 W14E1 -33.750, 86.000, 0.000 1.12E+00 6
|
||||
14 W13E2 -33.750, 86.000, 0.000 W15E1 33.750, 86.000, 0.000 1.25E+00 9
|
||||
15 W14E2 33.750, 86.000, 0.000 W16E1 84.000, 86.000, 0.000 1.12E+00 6
|
||||
16 W15E2 84.000, 86.000, 0.000 W17E1 117.000, 86.000, 0.000 1.00E+00 4
|
||||
17 W16E2 117.000, 86.000, 0.000 W18E1 150.000, 86.000, 0.000 8.75E-01 4
|
||||
18 W17E2 150.000, 86.000, 0.000 212.000, 86.000, 0.000 7.50E-01 8
|
||||
19 -199.38,171.000, 0.000 W20E1 -135.88,171.000, 0.000 7.50E-01 8
|
||||
20 W19E2 -135.88,171.000, 0.000 W21E1 -102.88,171.000, 0.000 8.75E-01 4
|
||||
21 W20E2 -102.88,171.000, 0.000 W22E1 -69.875,171.000, 0.000 1.00E+00 4
|
||||
22 W21E2 -69.875,171.000, 0.000 W23E1 -22.000,171.000, 0.000 1.12E+00 6
|
||||
23 W22E2 -22.000,171.000, 0.000 W24E1 22.000,171.000, 0.000 1.25E+00 5
|
||||
24 W23E2 22.000,171.000, 0.000 W25E1 69.875,171.000, 0.000 1.12E+00 6
|
||||
25 W24E2 69.875,171.000, 0.000 W26E1 102.875,171.000, 0.000 1.00E+00 4
|
||||
26 W25E2 102.875,171.000, 0.000 W27E1 135.875,171.000, 0.000 8.75E-01 4
|
||||
27 W26E2 135.875,171.000, 0.000 199.380,171.000, 0.000 7.50E-01 8
|
||||
28 -196.25,349.000, 0.000 W29E1 -119.25,349.000, 0.000 7.50E-01 10
|
||||
29 W28E2 -119.25,349.000, 0.000 W30E1 -86.250,349.000, 0.000 8.75E-01 4
|
||||
30 W29E2 -86.250,349.000, 0.000 W31E1 -53.250,349.000, 0.000 1.00E+00 4
|
||||
31 W30E2 -53.250,349.000, 0.000 W32E1 -22.000,349.000, 0.000 1.12E+00 4
|
||||
32 W31E2 -22.000,349.000, 0.000 W33E1 22.000,349.000, 0.000 1.25E+00 5
|
||||
33 W32E2 22.000,349.000, 0.000 W34E1 53.250,349.000, 0.000 1.12E+00 4
|
||||
34 W33E2 53.250,349.000, 0.000 W35E1 86.250,349.000, 0.000 1.00E+00 4
|
||||
35 W34E2 86.250,349.000, 0.000 W36E1 119.250,349.000, 0.000 8.75E-01 4
|
||||
36 W35E2 119.250,349.000, 0.000 196.250,349.000, 0.000 7.50E-01 10
|
||||
37 -183.75,570.000, 0.000 W38E1 -131.75,570.000, 0.000 7.50E-01 7
|
||||
38 W37E2 -131.75,570.000, 0.000 W39E1 -98.750,570.000, 0.000 8.75E-01 4
|
||||
39 W38E2 -98.750,570.000, 0.000 W40E1 -65.750,570.000, 0.000 1.00E+00 4
|
||||
40 W39E2 -65.750,570.000, 0.000 W41E1 -22.000,570.000, 0.000 1.12E+00 5
|
||||
41 W40E2 -22.000,570.000, 0.000 W42E1 22.000,570.000, 0.000 1.25E+00 5
|
||||
42 W41E2 22.000,570.000, 0.000 W43E1 65.750,570.000, 0.000 1.12E+00 5
|
||||
43 W42E2 65.750,570.000, 0.000 W44E1 98.750,570.000, 0.000 1.00E+00 4
|
||||
44 W43E2 98.750,570.000, 0.000 W45E1 131.750,570.000, 0.000 8.75E-01 4
|
||||
45 W44E2 131.750,570.000, 0.000 183.750,570.000, 0.000 7.50E-01 7
|
||||
|
||||
-------------- SOURCES --------------
|
||||
|
||||
Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
|
||||
Seg. Actual (Specified)
|
||||
|
||||
1 5 14 / 50.00 ( 14 / 50.00) 1.000 0.000 V
|
||||
|
||||
No loads specified
|
||||
|
||||
No transmission lines specified
|
||||
|
||||
Ground type is Free Space</pre>
|
||||
<br>
|
||||
<pre> EZNEC/4 ver. 2.0
|
||||
|
||||
K4SB 5-element, 55' boom 07-19-1998 07:46:17
|
||||
|
||||
|
||||
Frequency = 14.175 MHz.
|
||||
|
||||
Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
|
||||
|
||||
|
||||
--------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : in) Conn. --- End 2 (x,y,z : in) Dia(in) Segs
|
||||
|
||||
1 -217.00, 0.000, 0.000 W2E1 -152.00, 0.000, 0.000 5.00E-01 7
|
||||
2 W1E2 -152.00, 0.000, 0.000 W3E1 -116.00, 0.000, 0.000 6.25E-01 4
|
||||
3 W2E2 -116.00, 0.000, 0.000 W4E1 -72.000, 0.000, 0.000 7.50E-01 4
|
||||
4 W3E2 -72.000, 0.000, 0.000 W5E1 -48.000, 0.000, 0.000 8.75E-01 2
|
||||
5 W4E2 -48.000, 0.000, 0.000 W6E1 48.000, 0.000, 0.000 1.00E+00 9
|
||||
6 W5E2 48.000, 0.000, 0.000 W7E1 72.000, 0.000, 0.000 8.75E-01 2
|
||||
7 W6E2 72.000, 0.000, 0.000 W8E1 116.000, 0.000, 0.000 7.50E-01 4
|
||||
8 W7E2 116.000, 0.000, 0.000 W9E1 152.000, 0.000, 0.000 6.25E-01 4
|
||||
9 W8E2 152.000, 0.000, 0.000 217.000, 0.000, 0.000 5.00E-01 7
|
||||
10 -207.50,182.504, 0.000 W11E1 -152.00,182.504, 0.000 5.00E-01 6
|
||||
11 W10E2 -152.00,182.504, 0.000 W12E1 -116.00,182.504, 0.000 6.25E-01 4
|
||||
12 W11E2 -116.00,182.504, 0.000 W13E1 -72.000,182.504, 0.000 7.50E-01 4
|
||||
13 W12E2 -72.000,182.504, 0.000 W14E1 -48.000,182.504, 0.000 8.75E-01 2
|
||||
14 W13E2 -48.000,182.504, 0.000 W15E1 48.000,182.504, 0.000 1.00E+00 9
|
||||
15 W14E2 48.000,182.504, 0.000 W16E1 72.000,182.504, 0.000 8.75E-01 2
|
||||
16 W15E2 72.000,182.504, 0.000 W17E1 116.000,182.504, 0.000 7.50E-01 4
|
||||
17 W16E2 116.000,182.504, 0.000 W18E1 152.000,182.504, 0.000 6.25E-01 4
|
||||
18 W17E2 152.000,182.504, 0.000 207.500,182.504, 0.000 5.00E-01 6
|
||||
19 -195.53,269.544, 0.000 W20E1 -152.00,269.544, 0.000 5.00E-01 4
|
||||
20 W19E2 -152.00,269.544, 0.000 W21E1 -116.00,269.544, 0.000 6.25E-01 4
|
||||
21 W20E2 -116.00,269.544, 0.000 W22E1 -72.000,269.544, 0.000 7.50E-01 4
|
||||
22 W21E2 -72.000,269.544, 0.000 W23E1 -48.000,269.544, 0.000 8.75E-01 2
|
||||
23 W22E2 -48.000,269.544, 0.000 W24E1 48.000,269.544, 0.000 1.00E+00 9
|
||||
24 W23E2 48.000,269.544, 0.000 W25E1 72.000,269.544, 0.000 8.75E-01 2
|
||||
25 W24E2 72.000,269.544, 0.000 W26E1 116.000,269.544, 0.000 7.50E-01 4
|
||||
26 W25E2 116.000,269.544, 0.000 W27E1 152.000,269.544, 0.000 6.25E-01 4
|
||||
27 W26E2 152.000,269.544, 0.000 195.530,269.544, 0.000 5.00E-01 4
|
||||
28 -192.48,448.494, 0.000 W29E1 -152.00,448.494, 0.000 5.00E-01 4
|
||||
29 W28E2 -152.00,448.494, 0.000 W30E1 -116.00,448.494, 0.000 6.25E-01 4
|
||||
30 W29E2 -116.00,448.494, 0.000 W31E1 -72.000,448.494, 0.000 7.50E-01 4
|
||||
31 W30E2 -72.000,448.494, 0.000 W32E1 -48.000,448.494, 0.000 8.75E-01 2
|
||||
32 W31E2 -48.000,448.494, 0.000 W33E1 48.000,448.494, 0.000 1.00E+00 9
|
||||
33 W32E2 48.000,448.494, 0.000 W34E1 72.000,448.494, 0.000 8.75E-01 2
|
||||
34 W33E2 72.000,448.494, 0.000 W35E1 116.000,448.494, 0.000 7.50E-01 4
|
||||
35 W34E2 116.000,448.494, 0.000 W36E1 152.000,448.494, 0.000 6.25E-01 4
|
||||
36 W35E2 152.000,448.494, 0.000 192.480,448.494, 0.000 5.00E-01 4
|
||||
37 -189.42,656.474, 0.000 W38E1 -152.00,656.474, 0.000 5.00E-01 4
|
||||
38 W37E2 -152.00,656.474, 0.000 W39E1 -116.00,656.474, 0.000 6.25E-01 4
|
||||
39 W38E2 -116.00,656.474, 0.000 W40E1 -72.000,656.474, 0.000 7.50E-01 4
|
||||
40 W39E2 -72.000,656.474, 0.000 W41E1 -48.000,656.474, 0.000 8.75E-01 2
|
||||
41 W40E2 -48.000,656.474, 0.000 W42E1 48.000,656.474, 0.000 1.00E+00 9
|
||||
42 W41E2 48.000,656.474, 0.000 W43E1 72.000,656.474, 0.000 8.75E-01 2
|
||||
43 W42E2 72.000,656.474, 0.000 W44E1 116.000,656.474, 0.000 7.50E-01 4
|
||||
44 W43E2 116.000,656.474, 0.000 W45E1 152.000,656.474, 0.000 6.25E-01 4
|
||||
45 W44E2 152.000,656.474, 0.000 189.420,656.474, 0.000 5.00E-01 4
|
||||
|
||||
-------------- SOURCES --------------
|
||||
|
||||
Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
|
||||
Seg. Actual (Specified)
|
||||
|
||||
1 5 14 / 50.00 ( 14 / 50.00) 1.000 0.000 V
|
||||
|
||||
No loads specified
|
||||
|
||||
No transmission lines specified
|
||||
|
||||
Ground type is Free Space</pre>
|
||||
<br>
|
||||
<pre> EZNEC/4 ver. 2.0
|
||||
|
||||
NW3Z 6 el, 48' boom WWW 07-19-1998 07:47:18
|
||||
|
||||
|
||||
Frequency = 14.175 MHz.
|
||||
|
||||
Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
|
||||
|
||||
|
||||
--------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : in) Conn. --- End 2 (x,y,z : in) Dia(in) Segs
|
||||
|
||||
1 -217.73, 0.000, 0.000 W2E1 -152.00, 0.000, 0.000 5.00E-01 7
|
||||
2 W1E2 -152.00, 0.000, 0.000 W3E1 -116.00, 0.000, 0.000 6.25E-01 4
|
||||
3 W2E2 -116.00, 0.000, 0.000 W4E1 -72.000, 0.000, 0.000 7.50E-01 4
|
||||
4 W3E2 -72.000, 0.000, 0.000 W5E1 -48.000, 0.000, 0.000 8.75E-01 2
|
||||
5 W4E2 -48.000, 0.000, 0.000 W6E1 48.000, 0.000, 0.000 1.00E+00 9
|
||||
6 W5E2 48.000, 0.000, 0.000 W7E1 72.000, 0.000, 0.000 8.75E-01 2
|
||||
7 W6E2 72.000, 0.000, 0.000 W8E1 116.000, 0.000, 0.000 7.50E-01 4
|
||||
8 W7E2 116.000, 0.000, 0.000 W9E1 152.000, 0.000, 0.000 6.25E-01 4
|
||||
9 W8E2 152.000, 0.000, 0.000 217.730, 0.000, 0.000 5.00E-01 7
|
||||
10 -210.70, 90.000, 0.000 W11E1 -152.00, 90.000, 0.000 5.00E-01 6
|
||||
11 W10E2 -152.00, 90.000, 0.000 W12E1 -116.00, 90.000, 0.000 6.25E-01 4
|
||||
12 W11E2 -116.00, 90.000, 0.000 W13E1 -72.000, 90.000, 0.000 7.50E-01 4
|
||||
13 W12E2 -72.000, 90.000, 0.000 W14E1 -48.000, 90.000, 0.000 8.75E-01 2
|
||||
14 W13E2 -48.000, 90.000, 0.000 W15E1 48.000, 90.000, 0.000 1.00E+00 9
|
||||
15 W14E2 48.000, 90.000, 0.000 W16E1 72.000, 90.000, 0.000 8.75E-01 2
|
||||
16 W15E2 72.000, 90.000, 0.000 W17E1 116.000, 90.000, 0.000 7.50E-01 4
|
||||
17 W16E2 116.000, 90.000, 0.000 W18E1 152.000, 90.000, 0.000 6.25E-01 4
|
||||
18 W17E2 152.000, 90.000, 0.000 210.700, 90.000, 0.000 5.00E-01 6
|
||||
19 -200.80,139.520, 0.000 W20E1 -152.00,139.520, 0.000 5.00E-01 5
|
||||
20 W19E2 -152.00,139.520, 0.000 W21E1 -116.00,139.520, 0.000 6.25E-01 4
|
||||
21 W20E2 -116.00,139.520, 0.000 W22E1 -72.000,139.520, 0.000 7.50E-01 4
|
||||
22 W21E2 -72.000,139.520, 0.000 W23E1 -48.000,139.520, 0.000 8.75E-01 2
|
||||
23 W22E2 -48.000,139.520, 0.000 W24E1 48.000,139.520, 0.000 1.00E+00 9
|
||||
24 W23E2 48.000,139.520, 0.000 W25E1 72.000,139.520, 0.000 8.75E-01 2
|
||||
25 W24E2 72.000,139.520, 0.000 W26E1 116.000,139.520, 0.000 7.50E-01 4
|
||||
26 W25E2 116.000,139.520, 0.000 W27E1 152.000,139.520, 0.000 6.25E-01 4
|
||||
27 W26E2 152.000,139.520, 0.000 200.800,139.520, 0.000 5.00E-01 5
|
||||
28 -194.62,266.700, 0.000 W29E1 -152.00,266.700, 0.000 5.00E-01 4
|
||||
29 W28E2 -152.00,266.700, 0.000 W30E1 -116.00,266.700, 0.000 6.25E-01 4
|
||||
30 W29E2 -116.00,266.700, 0.000 W31E1 -72.000,266.700, 0.000 7.50E-01 4
|
||||
31 W30E2 -72.000,266.700, 0.000 W32E1 -48.000,266.700, 0.000 8.75E-01 2
|
||||
32 W31E2 -48.000,266.700, 0.000 W33E1 48.000,266.700, 0.000 1.00E+00 9
|
||||
33 W32E2 48.000,266.700, 0.000 W34E1 72.000,266.700, 0.000 8.75E-01 2
|
||||
34 W33E2 72.000,266.700, 0.000 W35E1 116.000,266.700, 0.000 7.50E-01 4
|
||||
35 W34E2 116.000,266.700, 0.000 W36E1 152.000,266.700, 0.000 6.25E-01 4
|
||||
36 W35E2 152.000,266.700, 0.000 194.620,266.700, 0.000 5.00E-01 4
|
||||
37 -194.63,388.440, 0.000 W38E1 -152.00,388.440, 0.000 5.00E-01 4
|
||||
38 W37E2 -152.00,388.440, 0.000 W39E1 -116.00,388.440, 0.000 6.25E-01 4
|
||||
39 W38E2 -116.00,388.440, 0.000 W40E1 -72.000,388.440, 0.000 7.50E-01 4
|
||||
40 W39E2 -72.000,388.440, 0.000 W41E1 -48.000,388.440, 0.000 8.75E-01 2
|
||||
41 W40E2 -48.000,388.440, 0.000 W42E1 48.000,388.440, 0.000 1.00E+00 9
|
||||
42 W41E2 48.000,388.440, 0.000 W43E1 72.000,388.440, 0.000 8.75E-01 2
|
||||
43 W42E2 72.000,388.440, 0.000 W44E1 116.000,388.440, 0.000 7.50E-01 4
|
||||
44 W43E2 116.000,388.440, 0.000 W45E1 152.000,388.440, 0.000 6.25E-01 4
|
||||
45 W44E2 152.000,388.440, 0.000 194.630,388.440, 0.000 5.00E-01 4
|
||||
46 -187.39,570.000, 0.000 W47E1 -152.00,570.000, 0.000 5.00E-01 4
|
||||
47 W46E2 -152.00,570.000, 0.000 W48E1 -116.00,570.000, 0.000 6.25E-01 4
|
||||
48 W47E2 -116.00,570.000, 0.000 W49E1 -72.000,570.000, 0.000 7.50E-01 4
|
||||
49 W48E2 -72.000,570.000, 0.000 W50E1 -48.000,570.000, 0.000 8.75E-01 2
|
||||
50 W49E2 -48.000,570.000, 0.000 W51E1 48.000,570.000, 0.000 1.00E+00 9
|
||||
51 W50E2 48.000,570.000, 0.000 W52E1 72.000,570.000, 0.000 8.75E-01 2
|
||||
52 W51E2 72.000,570.000, 0.000 W53E1 116.000,570.000, 0.000 7.50E-01 4
|
||||
53 W52E2 116.000,570.000, 0.000 W54E1 152.000,570.000, 0.000 6.25E-01 4
|
||||
54 W53E2 152.000,570.000, 0.000 187.390,570.000, 0.000 5.00E-01 4
|
||||
|
||||
-------------- SOURCES --------------
|
||||
|
||||
Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
|
||||
Seg. Actual (Specified)
|
||||
|
||||
1 5 14 / 50.00 ( 14 / 50.00) 1.000 0.000 V
|
||||
|
||||
No loads specified
|
||||
|
||||
No transmission lines specified
|
||||
|
||||
Ground type is Free Space</pre>
|
||||
<br>
|
||||
<pre> EZNEC/4 ver. 2.0
|
||||
|
||||
Hy-Gain 205-XLB, 48' Boom (WB0DGF) 07-19-1998 07:55:56
|
||||
|
||||
|
||||
Frequency = 14.175 MHz.
|
||||
|
||||
Wire Loss: Aluminum -- Resistivity = 4E-08 ohm-m, Rel. Perm. = 1
|
||||
|
||||
|
||||
--------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : in) Conn. --- End 2 (x,y,z : in) Dia(in) Segs
|
||||
|
||||
1 -286.00,-223.75, 0.000 W2E1 -286.00,-166.00, 0.000 4.38E-01 7
|
||||
2 W1E2 -286.00,-166.00, 0.000 W3E1 -286.00,-142.00, 0.000 6.25E-01 3
|
||||
3 W2E2 -286.00,-142.00, 0.000 W4E1 -286.00,-91.500, 0.000 8.75E-01 6
|
||||
4 W3E2 -286.00,-91.500, 0.000 W5E1 -286.00,-45.500, 0.000 1.12E+00 6
|
||||
5 W4E2 -286.00,-45.500, 0.000 W6E1 -286.00, -3.625, 0.000 1.25E+00 5
|
||||
6 W5E2 -286.00, -3.625, 0.000 W7E1 -286.00, 3.625, 0.000 4.00E+00 1
|
||||
7 W6E2 -286.00, 3.625, 0.000 W8E1 -286.00, 45.500, 0.000 1.25E+00 5
|
||||
8 W7E2 -286.00, 45.500, 0.000 W9E1 -286.00, 91.500, 0.000 1.12E+00 6
|
||||
9 W8E2 -286.00, 91.500, 0.000 W10E1 -286.00,142.000, 0.000 8.75E-01 6
|
||||
10 W9E2 -286.00,142.000, 0.000 W11E1 -286.00,166.000, 0.000 6.25E-01 3
|
||||
11 W10E2 -286.00,166.000, 0.000 -286.00,223.750, 0.000 4.38E-01 7
|
||||
12 -170.00,-209.50, 0.000 W13E1 -170.00,-156.50, 0.000 4.38E-01 7
|
||||
13 W12E2 -170.00,-156.50, 0.000 W14E1 -170.00,-132.50, 0.000 6.25E-01 3
|
||||
14 W13E2 -170.00,-132.50, 0.000 W15E1 -170.00,-82.000, 0.000 8.75E-01 6
|
||||
15 W14E2 -170.00,-82.000, 0.000 W16E1 -170.00,-36.000, 0.000 1.12E+00 6
|
||||
16 W15E2 -170.00,-36.000, 0.000 W17E1 -170.00,-10.000, 0.000 1.25E+00 3
|
||||
17 W16E2 -170.00,-10.000, 0.000 W18E1 -170.00, 10.000, 0.000 1.25E+00 3
|
||||
18 W17E2 -170.00, 10.000, 0.000 W19E1 -170.00, 36.000, 0.000 1.25E+00 3
|
||||
19 W18E2 -170.00, 36.000, 0.000 W20E1 -170.00, 82.000, 0.000 1.12E+00 6
|
||||
20 W19E2 -170.00, 82.000, 0.000 W21E1 -170.00,132.500, 0.000 8.75E-01 6
|
||||
21 W20E2 -170.00,132.500, 0.000 W22E1 -170.00,156.500, 0.000 6.25E-01 3
|
||||
22 W21E2 -170.00,156.500, 0.000 -170.00,209.500, 0.000 4.38E-01 7
|
||||
23 -80.000,-206.25, 0.000 W24E1 -80.000,-156.50, 0.000 4.38E-01 6
|
||||
24 W23E2 -80.000,-156.50, 0.000 W25E1 -80.000,-132.50, 0.000 6.25E-01 3
|
||||
25 W24E2 -80.000,-132.50, 0.000 W26E1 -80.000,-82.000, 0.000 8.75E-01 6
|
||||
26 W25E2 -80.000,-82.000, 0.000 W27E1 -80.000,-36.000, 0.000 1.12E+00 6
|
||||
27 W26E2 -80.000,-36.000, 0.000 W28E1 -80.000, -3.625, 0.000 1.25E+00 4
|
||||
28 W27E2 -80.000, -3.625, 0.000 W29E1 -80.000, 3.625, 0.000 4.00E+00 1
|
||||
29 W28E2 -80.000, 3.625, 0.000 W30E1 -80.000, 36.000, 0.000 1.25E+00 4
|
||||
30 W29E2 -80.000, 36.000, 0.000 W31E1 -80.000, 82.000, 0.000 1.12E+00 6
|
||||
31 W30E2 -80.000, 82.000, 0.000 W32E1 -80.000,132.500, 0.000 8.75E-01 6
|
||||
32 W31E2 -80.000,132.500, 0.000 W33E1 -80.000,156.500, 0.000 6.25E-01 3
|
||||
33 W32E2 -80.000,156.500, 0.000 -80.000,206.250, 0.000 4.38E-01 6
|
||||
34 86.000,-203.25, 0.000 W35E1 86.000,-156.50, 0.000 4.38E-01 6
|
||||
35 W34E2 86.000,-156.50, 0.000 W36E1 86.000,-132.50, 0.000 6.25E-01 3
|
||||
36 W35E2 86.000,-132.50, 0.000 W37E1 86.000,-82.000, 0.000 8.75E-01 6
|
||||
37 W36E2 86.000,-82.000, 0.000 W38E1 86.000,-36.000, 0.000 1.12E+00 6
|
||||
38 W37E2 86.000,-36.000, 0.000 W39E1 86.000, -3.625, 0.000 1.25E+00 4
|
||||
39 W38E2 86.000, -3.625, 0.000 W40E1 86.000, 3.625, 0.000 4.00E+00 1
|
||||
40 W39E2 86.000, 3.625, 0.000 W41E1 86.000, 36.000, 0.000 1.25E+00 4
|
||||
41 W40E2 86.000, 36.000, 0.000 W42E1 86.000, 82.000, 0.000 1.12E+00 6
|
||||
42 W41E2 86.000, 82.000, 0.000 W43E1 86.000,132.500, 0.000 8.75E-01 6
|
||||
43 W42E2 86.000,132.500, 0.000 W44E1 86.000,156.500, 0.000 6.25E-01 3
|
||||
44 W43E2 86.000,156.500, 0.000 86.000,203.250, 0.000 4.38E-01 6
|
||||
45 286.000,-193.12, 0.000 W46E1 286.000,-138.25, 0.000 4.38E-01 7
|
||||
46 W45E2 286.000,-138.25, 0.000 W47E1 286.000,-114.25, 0.000 6.25E-01 3
|
||||
47 W46E2 286.000,-114.25, 0.000 W48E1 286.000,-63.750, 0.000 8.75E-01 6
|
||||
48 W47E2 286.000,-63.750, 0.000 W49E1 286.000,-17.750, 0.000 1.12E+00 6
|
||||
49 W48E2 286.000,-17.750, 0.000 W50E1 286.000, -3.625, 0.000 1.25E+00 2
|
||||
50 W49E2 286.000, -3.625, 0.000 W51E1 286.000, 3.625, 0.000 4.00E+00 1
|
||||
51 W50E2 286.000, 3.625, 0.000 W52E1 286.000, 17.750, 0.000 1.25E+00 2
|
||||
52 W51E2 286.000, 17.750, 0.000 W53E1 286.000, 63.750, 0.000 1.12E+00 6
|
||||
53 W52E2 286.000, 63.750, 0.000 W54E1 286.000,114.250, 0.000 8.75E-01 6
|
||||
54 W53E2 286.000,114.250, 0.000 W55E1 286.000,138.250, 0.000 6.25E-01 3
|
||||
55 W54E2 286.000,138.250, 0.000 286.000,193.125, 0.000 4.38E-01 7
|
||||
|
||||
-------------- SOURCES --------------
|
||||
|
||||
Source Wire Wire #/Pct From End 1 Ampl.(V, A) Phase(Deg.) Type
|
||||
Seg. Actual (Specified)
|
||||
|
||||
1 2 17 / 50.00 ( 17 / 50.00) 0.707 0.000 V
|
||||
|
||||
No loads specified
|
||||
|
||||
No transmission lines specified
|
||||
|
||||
Ground type is Free Space</pre>
|
||||
<br>
|
||||
<p>Any transcription errors in the above models are strictly my own fault, and I shall gladly correct any found.<br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p><i>Updated 7-20-98. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Amateur Radio Page</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
||||
</body>
|
||||
</html>
|
||||
@@ -0,0 +1,96 @@
|
||||
<!DOCTYPE html>
|
||||
<html lang="en">
|
||||
<head>
|
||||
<meta charset="UTF-8">
|
||||
<meta name="keywords" content="Yagi, HF Lower">
|
||||
<title>Wire Beam for 80 and 75 Meters</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
<body>
|
||||
<img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10">
|
||||
<center>
|
||||
<h1>Notes on a Wire Beam for 80 and 75 Meters</h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
Wire beams for the lower HF bands still have a vital place for those who cannot field the hardware necessary for rotatable arrays at those frequencies. However, the CW and SSB ends of the 3.5-4.0 MHz amateur band are so far apart that a single beam will not have the necessary bandwidth to perform respectably at both ends. Add the bandwidth narrowing thinness of wire, in contrast to large aluminum tubing, and the situation seems to call for complex measures.
|
||||
<p>Hanging a wire beam for 80 or 75 meters is difficult enough without having also to support complex matching networks. Separate beams appear to call for separate antennas and feedlines--again, a double weight. Enter open sleeve coupling. It is possible to construct beams for 80 and 75 and to position them so that feeding only the 80 meter driven element will provide adequate performance at both ends of the band.</p>
|
||||
<p>The general outlines of such a 2-element wire beam appear in Figure 1. The higher frequency driven element--the one not connected to the feedline--is ahead of the driven 80-meter element, with the corresponding reflectors appropriately aligned.</p>
|
||||
<center>
|
||||
<img src="80751.gif" width="681" height="293" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>To see what such a beam might look like in practice, I modeled a couple of versions using slightly different driven element spacings and slightly different basic driven-element-to-reflector spacings. In common, both version use #12 copper wire and both beams are 136' above ground, approximately 1/2 wavelength. At that height, the angle of maximum radiation is approximately 27 degrees, assuming as NEC does, level terrain.</p>
|
||||
<p>As a suggestive beginning for refinement by builders, here are the dimensions of each version and the NEC-derived anticipated performance from one end of the band to the other.</p>
|
||||
<pre>Version 1: Driven element spacing 3'; overall front-to-back distance 42'9"
|
||||
|
||||
Dimensions:
|
||||
|
||||
75 DE (no connex) 122'5" long space = 0
|
||||
80 DE (coax conx) 134'0" - 3'
|
||||
75 REF 128'4" -36'8"
|
||||
80 REF 139'0" -42'9"
|
||||
|
||||
Predicted performance:
|
||||
FQ Gain dBi F-B dB Feed Z ohms SWR @ 75 ohm line
|
||||
3.5 11.4 11.5 47 - j 18 1.7
|
||||
3.55 11.0 13.0 76 - j 5 1.1
|
||||
3.6 10.3 8.0 88 - j 9 1.2
|
||||
3.65 9.2 3.7 79 - j 11 1.2
|
||||
3.7 8.7 2.5 52 + j 7 1.5 Reverse direction
|
||||
3.75 9.5 1.1 45 + j 81 4.0 Reverse direction
|
||||
3.8 11.6 11.7 158 + j 73 2.7
|
||||
3.85 11.2 11.6 107 + j 3 1.4
|
||||
3.9 10.8 9.8 51 + j 24 1.7
|
||||
3.95 10.1 9.1 21 - j 64 6.3
|
||||
4.0 8.2 9.7 8 + j106 28.0
|
||||
</pre>
|
||||
<br>
|
||||
<pre>Version 2: Driven element spacing 3'6"; overall front-to-back distance 39'6"
|
||||
|
||||
Dimensions:
|
||||
|
||||
75 DE (no connex) 121'8" long space = 0
|
||||
80 DE (coax conx) 133'0" - 3'6"
|
||||
75 REF 127'5" -33'2"
|
||||
80 REF 138'0" -39'6"
|
||||
|
||||
Predicted performance:
|
||||
FQ Gain dBi F-B dB Feed Z ohms SWR @ 75 ohm line
|
||||
3.5 11.2 6.8 28 - j 37 3.4
|
||||
3.55 11.3 14.0 55 - j 7 1.4
|
||||
3.6 11.7 10.3 82 - j 2 1.1
|
||||
3.65 9.9 5.9 86 - j 10 1.2
|
||||
3.7 8.4 1.6 68 - j 8 1.2
|
||||
3.75 9.9 7.3 37 + j 25 2.3 Reverse direction
|
||||
3.8 10.9 4.9 70 + j114 4.2
|
||||
3.85 11.4 12.4 147 + j 34 2.1
|
||||
3.9 10.9 10.6 72 + j 11 1.2
|
||||
3.95 10.3 9.5 29 + j 49 3.9
|
||||
4.0 9.0 9.7 11 + j 93 17.6
|
||||
</pre>
|
||||
<p>The two versions are peaked at slightly different points on the band, but there is little to choose between them. Indeed, with appropriate tweaking of reflector lengths and spacings, performance improvements can be garnered. However, improvements are more likely to be those suited to an individual's desired "main" frequency rather than to overall performance across the band.</p>
|
||||
<p>The phrase "Reverse direction" refers to the forward lobe of the beam being in the reverse direction or toward the reflectors for the frequencies indicated. The 80-75 meter ham band is sufficiently wide, relative to the frequencies involved, that with a wire antenna, 3 beams would be required for full band coverage without reversal or excessive SWR somewhere along the way.</p>
|
||||
<p>One limitation of these antennas relative to reality is that they were modeled flat, without the usual droop of wire. However, the absence of major differences between the two versions, despite the changes in driven element spacing and the D.E-to-reflector spacing suggest that droop alone will not seriously change performance.</p>
|
||||
<p>75-ohm coax, with a choke or bead balun, seems the most versatile feed system for these antennas. At 80-75 meters, moderately high SWRs do not incur nearly the level of line losses as they do in the upper HF region; consequently, an ATU should allow matching to rigs with SWRs of 5:1 and under.</p>
|
||||
<p>These notes are not intended to be definitive, but only suggestive of one more way to obtain gain and directivity in the 80-75 meter band. construction details will be dictated by available support structures, and final design will fit the refinements occasioned by specific operating goals. The models do suggest very strongly that open-sleeve coupling is not confined to upper HF beams and lower HF dipoles. A single-feed wire beam for both the CW and SSB portions of the 80-75 meter amateur band is both possible and practical.<br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p><i>Updated 4-9-97. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Amateur Radio Page</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-lower'>HF Lower</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
||||
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|
||||
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|
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<head>
|
||||
<meta charset="UTF-8">
|
||||
<meta name="keywords" content="Yagi, HF Upper">
|
||||
<title>Director/Driven Element 2-Element Yagis for 12 and 17 Meters</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
<body>
|
||||
<center>
|
||||
<h1><img src="../images/yagicad.gif" align="left" width="125" height="62" border="2" vspace="5" hspace="25"><img src="../images/yagicad.gif" align="right" width="125" height="62" border="2" vspace="5" hspace="25">Director/Driven Element 2-Element Yagis<br>
|
||||
Some Ideas for 12 and 17 Meters</h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" width="591" height="3">
|
||||
</center><br>
|
||||
<br>
|
||||
I have in other notes made mention of the Director/Driven Element configuration of the 2-element Yagi. When pressed to maximum gain, these antennas are capable of over 7 dBi free space gain, about a dB higher than the conventional Reflector/Driven Element configuration tuned for maximum front-to-back ratio. However, these antennas have a low feedpoint impedance and fairly narrow band widths.
|
||||
<p>Jerry Haigwood, W5JH, reminded me that with a beta match, the low impedance can be overcome, and on the WARC bands, narrow bandwidth is not a real concern. Jerry is exactly correct, and his comments and other good ideas raised some interesting design possibilities.</p>
|
||||
<p>Jerry prefers spacing his director about 0.08 wl from the driven element. The range from 0.07 to 0.09 wl is a good choice. Even though one might get a bit more gain from the antenna, the 1/12th wl spacing holds the feedpoint impedance of a D/DE Yagi set for a good front-to-back ratio at about 20 ohms, which is quite workable.</p>
|
||||
<center>
|
||||
<img src="bb1.gif" width="592" height="288" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Figure 1</b> shows the construction of a reasonable D/DE Yagi. The 1" elements of this 10-meter model are feasible hardware store material, but the model is designed less to build than to set the properties of the genre of beam. With wider spacing, the antenna gain drops off, although the feedpoint impedance goes up. Closer spacing raises gain for a while, but continuously drops the feedpoint impedance. The model antenna, set for 28.5 MHz, has a feedpoint impedance of 20.5 - j23.5 ohms, just about right for a hairpin or inductor beta match of very conventional design.</p>
|
||||
<p>Incidentally, <a href="../radio.html#HAMCALC">HAMCALC</a> has a very nice program for calculating beta matches, including the hairpin. The equivalent inductance is also given, which permits you to use another program on HAMCALC to create a coil instead. See the basic "Radio" page for the address of VE3ERP, the master of this suite of GW BASIC utility programs.</p>
|
||||
<p>With only slight readjustments, the basic 2-element D/DE Yagi can be swept through a variety of radiation patterns. Three of them are illustrated in <b>Figure 2</b>. The Maximum Front-to-Back settings will yield a modest gain (about 6.5 dBi free space) and a very good (greater than 20 dB) front-to-back ratio with a feedpoint impedance of about 20 ohms. At the other extreme, maximum gain provides over 7 dBi gain but under 10 dB front-to-back and a feedpoint impedance of about 10 ohms. There is a midpoint setting shown in the patterns, where the gain is intermediate, the front-to-back is respectable, and the feedpoint Z is about 15 ohms.</p>
|
||||
<center>
|
||||
<img src="bb2.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>These setting are not so very far apart, and a frequency difference of about 1.5% will sweep you through them. A little further, and you will experience pattern reversal. On 10 meters, this is under 400 kHz. So unless your needs are very frequency specific, the D/DE Yagi may not be a good design choice.</p>
|
||||
<p>However, on the WARC bands, no such problems occur. On 12 and 17 meters, the available bandwidth is about 1/2 of 1% of the frequency, and the D/DE design can easily cover this spread with stable characteristics.</p>
|
||||
<h3>Back-to-Back 12-17-Meter Yagis</h3>
|
||||
<p>One of the chief attractions of the D/DE design is the short boom length required for the antenna. For example, 0.07 wl is only 3.8' at 17 meters (with elements between 26 and 28') and 2.75' at 12 meters (with elements between 18 and 20'). Modeling suggests that individual antennas for these two bands show very little interaction when placed back-to-back and separated by about a foot or more. Hence, an 8' boom would hold back-to- back individual antennas.</p>
|
||||
<p>Individual antennas, however, require separate feedlines or a switching system. We can make life even simpler and cut the feedline needs down to 1 feedline. By placing the driven elements close together, we can use open-sleeve coupling. This involves connecting the feedline permanently to the 17 meter driven element and letting it excite the 12-meter element when fed with 12 meter energy. The required spacing is just about 4" which requires that we retune the 12 meter elements for this configuration. Once done, however, the two beams do their jobs with few signs of other interactions. Moreover, the two antennas require only a 7' boom.</p>
|
||||
<p>To test this idea, I created models of a double D/DE Yagi using open sleeve coupling and placed back-to-back. I used an aggressive stepped diameter tubing schedule to keep the antenna array as light as possible. Figure 3 shows a sketch of the overall dimensions of the antenna.</p>
|
||||
<center>
|
||||
<img src="bb3.gif" width="602" height="339" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Since the tubing schedule would not show well in the sketch, here is the antenna description file with a detailed list of lengths and diameters of aluminum tubing.</p>
|
||||
<pre> --------------- WIRES ---------------
|
||||
|
||||
Wire Conn.--- End 1 (x,y,z : ft) Conn. -- End 2 (x,y,z : ft) Dia(in)
|
||||
|
||||
12-meter director
|
||||
1 -9.570, 3.080, 0.000 W2E1 -6.500, 3.080, 0.000 3.75E-01
|
||||
2 W1E2 -6.500, 3.080, 0.000 W3E1 -2.500, 3.080, 0.000 5.00E-01
|
||||
3 W2E2 -2.500, 3.080, 0.000 W4E1 -1.000, 3.080, 0.000 6.25E-01
|
||||
4 W3E2 -1.000, 3.080, 0.000 W5E1 1.000, 3.080, 0.000 7.50E-01
|
||||
5 W4E2 1.000, 3.080, 0.000 W6E1 2.500, 3.080, 0.000 6.25E-01
|
||||
6 W5E2 2.500, 3.080, 0.000 W7E1 6.500, 3.080, 0.000 5.00E-01
|
||||
7 W6E2 6.500, 3.080, 0.000 9.570, 3.080, 0.000 3.75E-01
|
||||
12-meter driven element
|
||||
8 -10.120, 0.330, 0.000 W9E1 -6.500, 0.330, 0.000 3.75E-01
|
||||
9 W8E2 -6.500, 0.330, 0.000 W10E1 -2.500, 0.330, 0.000 5.00E-01
|
||||
10 W9E2 -2.500, 0.330, 0.000 W11E1 -1.000, 0.330, 0.000 6.25E-01
|
||||
11 W10E2 -1.000, 0.330, 0.000 W12E1 1.000, 0.330, 0.000 7.50E-01
|
||||
12 W11E2 1.000, 0.330, 0.000 W13E1 2.500, 0.330, 0.000 6.25E-01
|
||||
13 W12E2 2.500, 0.330, 0.000 W14E1 6.500, 0.330, 0.000 5.00E-01
|
||||
14 W13E2 6.500, 0.330, 0.000 10.120, 0.330, 0.000 3.75E-01
|
||||
17-meter director
|
||||
15 -13.000, -3.785, 0.000 W16E1 -9.000, -3.785, 0.000 5.00E-01
|
||||
16 W15E2 -9.000, -3.785, 0.000 W17E1 -3.500, -3.785, 0.000 6.25E-01
|
||||
17 W16E2 -3.500, -3.785, 0.000 W18E1 -1.500, -3.785, 0.000 7.50E-01
|
||||
18 W17E2 -1.500, -3.785, 0.000 W19E1 1.500, -3.785, 0.000 8.75E-01
|
||||
19 W18E2 1.500, -3.785, 0.000 W20E1 3.500, -3.785, 0.000 7.50E-01
|
||||
20 W19E2 3.500, -3.785, 0.000 W21E1 9.000, -3.785, 0.000 6.25E-01
|
||||
21 W20E2 9.000, -3.785, 0.000 13.000, -3.785, 0.000 5.00E-01
|
||||
17-meter driven element
|
||||
22 -13.850, 0.000, 0.000 W23E1 -9.000, 0.000, 0.000 5.00E-01
|
||||
23 W22E2 -9.000, 0.000, 0.000 W24E1 -3.500, 0.000, 0.000 6.25E-01
|
||||
24 W23E2 -3.500, 0.000, 0.000 W25E1 -1.500, 0.000, 0.000 7.50E-01
|
||||
25 W24E2 -1.500, 0.000, 0.000 W26E1 1.500, 0.000, 0.000 8.75E-01
|
||||
26 W25E2 1.500, 0.000, 0.000 W27E1 3.500, 0.000, 0.000 7.50E-01
|
||||
27 W26E2 3.500, 0.000, 0.000 W28E1 9.000, 0.000, 0.000 6.25E-01
|
||||
28 W27E2 9.000, 0.000, 0.000 13.850, 0.000, 0.000 5.00E-01</pre>
|
||||
<p>The feedpoint is on wire #25, the center of the 17-meter driven element. Do not expect the model to be exact on the spacing for the open sleeve coupling distance. Rather, experiment and measure the impedance on both bands as you work.</p>
|
||||
<p>The center of gravity should be just on the 17-meter side of the 17-meter driven element, minimizing the need for coax to run along the boom. Almost any boom in the 1.25" range should do, even a length of the lighter TV mast (which is too light for mast use), as long as it is weather protected so that it does not rust out in a year. Even hardware store aluminum with a wall thickness of 0.055" should handle the job, although inserts at the element clamping points should be used to prevent tube crush. Alternatively, you can use short sections of the next tubing size up as strengtheners at the element clamping points. The model is designed for elements insulated from the boom, so if you use direct clamping to the boom, expect to adjust element lengths a bit.</p>
|
||||
<p>The feedpoint impedance is just about 21 ohms resonant for both frequency ranges. This permits the use of a broad-band balun to feed the antenna and effect an impedance transformation to the 50-ohm coax line. Alternatively, the antenna lengths can be reset to show capacitive reactance. A Beta match usually is effective not only at the frequency for which it is designed, but as well at higher frequencies, and with a little juggling of dimensions, a 2-band match should be obtained.</p>
|
||||
<center>
|
||||
<img src="bb4.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Figure 4</b> shows the patterns of the two antennas in one back-to-back pattern at free space. At 70' up, the gain is in the neighborhood of 12 dBi, a little under 5 dB better than a dipole, and with a strong front-to-back ratio.</p>
|
||||
<p>The back-to-back open-sleeve D/DE Yagis are not world beaters, but they are a. inexpensive to build, and b. lots better than some of the antennas being pressed into service for these bands. The entire antenna, boom and all, should weight in at less than 20 pounds, making it a good candidate for stacking on top of an existing antenna.</p>
|
||||
<p>If you separate the two antennas, you will have to adjust the dimensions--especially of the 12-meter model--for independent use. Likewise, if you use different materials or a different schedule of diameter steps, you will also have to adjust the dimensions to restore the pattern and the feedpoint impedance. But that comes with the territory of antenna experimenting and home-brewing.</p>
|
||||
<h3>12-17-Meter 2-Element Yagis Facing the Same Way</h3>
|
||||
<p>Is it feasible to place the two antennas in the same plane using the open-sleeve coupling system for a single feed line? The answer is yes and no. Yes, it is possible to develop a set of dimensions that will produce good performance at a desirable feed impedance for one frequency within the upper band. (The lower band is not affected.) However, both elements for the upper band are inside the elements for the lower band, producing a "cage" effect. The chief problem created is an extreme narrow bandwidth for desirable characteristics. In terms of this design, when the beams face in opposite directions, they show an operating bandwidth for under 2:1 SWR in excess of the 100 kHz width of the bands. When caged (or facing in the same direction), the operating bandwidth of the upper band beam is far less than the width of the band. For building and tuning, this also means that hitting the precisely needed dimensions is very tricky--and the settings are very susceptible to the need for change with changes of antenna height below about 1.5 wl. Hence, my recommendation is for opposing directions or for combinations of beam types, where the upper band antenna is "uncaged." (Yes, I know, I watch too many wildlife documentaries and it is affecting my choice of terminology.)</p>
|
||||
<p>There is an alternative that achieves the uncaging. It has the disadvantages, relative to the Janus-faced design above, of requiring a longer boom (10') and sacrificing a good bit of 17-meter front-to-back ratio. On the other hand, the beams face the same direction and require a single directly-matched 50-Ohm feed to cover both bands.</p>
|
||||
<p>The design is a DE-Reflector for 17 and a DE-Director for 12. By keeping the elements for each band together, they remain uncaged. In addition, there is a slight forward stagger effect so that the beams have marginally higher values than when they are independent. With the spacing used, the feedpoint impedance on both bands is close to 50 Ohms. The outline dimensions are shown in <b>Figure 5</b>.</p>
|
||||
<center>
|
||||
<img src="bb5.gif" width="558" height="310" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The same caution about spacing of the open-sleeve coupled driven elements applies to both designs: the builder will have to adjust both length and spacing of the 12-meter slaved driver, since the modeling program is close to its limits for handling closely spaced elements of different lengths. However, the model should be quite close.</p>
|
||||
<p>In addition, the antenna was designed with a tapering schedule in place. Here is the detailed wire table.</p>
|
||||
<pre> --------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in) Segs
|
||||
12-meter Director
|
||||
1 -9.500, 3.130, 0.000 W2E1 -6.500, 3.130, 0.000 3.75E-01 5
|
||||
2 W1E2 -6.500, 3.130, 0.000 W3E1 -2.500, 3.130, 0.000 5.00E-01 5
|
||||
3 W2E2 -2.500, 3.130, 0.000 W4E1 -1.000, 3.130, 0.000 6.25E-01 2
|
||||
4 W3E2 -1.000, 3.130, 0.000 W5E1 1.000, 3.130, 0.000 7.50E-01 3
|
||||
5 W4E2 1.000, 3.130, 0.000 W6E1 2.500, 3.130, 0.000 6.25E-01 2
|
||||
6 W5E2 2.500, 3.130, 0.000 W7E1 6.500, 3.130, 0.000 5.00E-01 5
|
||||
7 W6E2 6.500, 3.130, 0.000 9.500, 3.130, 0.000 3.75E-01 5
|
||||
12-meter Driver
|
||||
8 -10.070, 0.380, 0.000 W9E1 -6.500, 0.380, 0.000 3.75E-01 5
|
||||
9 W8E2 -6.500, 0.380, 0.000 W10E1 -2.500, 0.380, 0.000 5.00E-01 5
|
||||
10 W9E2 -2.500, 0.380, 0.000 W11E1 -1.000, 0.380, 0.000 6.25E-01 2
|
||||
11 W10E2 -1.000, 0.380, 0.000 W12E1 1.000, 0.380, 0.000 7.50E-01 3
|
||||
12 W11E2 1.000, 0.380, 0.000 W13E1 2.500, 0.380, 0.000 6.25E-01 2
|
||||
13 W12E2 2.500, 0.380, 0.000 W14E1 6.500, 0.380, 0.000 5.00E-01 5
|
||||
14 W13E2 6.500, 0.380, 0.000 10.070, 0.380, 0.000 3.75E-01 5
|
||||
17-meter Driver
|
||||
15 -13.100, 0.000, 0.000 W16E1 -9.500, 0.000, 0.000 3.75E-01 4
|
||||
16 W15E2 -9.500, 0.000, 0.000 W17E1 -6.750, 0.000, 0.000 5.00E-01 4
|
||||
17 W16E2 -6.750, 0.000, 0.000 W18E1 -4.000, 0.000, 0.000 6.25E-01 4
|
||||
18 W17E2 -4.000, 0.000, 0.000 W19E1 4.000, 0.000, 0.000 7.50E-01 11
|
||||
19 W18E2 4.000, 0.000, 0.000 W20E1 6.750, 0.000, 0.000 6.25E-01 4
|
||||
20 W19E2 6.750, 0.000, 0.000 W21E1 9.500, 0.000, 0.000 5.00E-01 4
|
||||
21 W20E2 9.500, 0.000, 0.000 13.100, 0.000, 0.000 3.75E-01 4
|
||||
17-meter Reflector
|
||||
22 -14.100, -6.800, 0.000 W23E1 -9.500, -6.800, 0.000 3.75E-01 4
|
||||
23 W22E2 -9.500, -6.800, 0.000 W24E1 -6.750, -6.800, 0.000 5.00E-01 4
|
||||
24 W23E2 -6.750, -6.800, 0.000 W25E1 -4.000, -6.800, 0.000 6.25E-01 4
|
||||
25 W24E2 -4.000, -6.800, 0.000 W26E1 4.000, -6.800, 0.000 7.50E-01 11
|
||||
26 W25E2 4.000, -6.800, 0.000 W27E1 6.750, -6.800, 0.000 6.25E-01 4
|
||||
27 W26E2 6.750, -6.800, 0.000 W28E1 9.500, -6.800, 0.000 5.00E-01 4
|
||||
28 W27E2 9.500, -6.800, 0.000 14.100, -6.800, 0.000 3.75E-01 4</pre>
|
||||
<p>As always, since the taper schedule affects performance, you will have to adjust dimensions to suit the materials you have on hand. Four light-weight elements on a 10' boom should not stress an extended mast used to stack this simple beam above a heavy-weight tri-bander.</p>
|
||||
<p>As <b>Figure 6</b> shows, the gain of the two beams is about the same--between 6.3 and 6.4 dBi in free space. However, the 17-meter front-to-back ratio is reduced considerably. However, until the WARC bands become seriously over-crowded, this may not be a major problem.</p>
|
||||
<center>
|
||||
<img src="bb6.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<h3>Back-to-Back with a 30-Meter Dipole Between</h3>A couple of requests came in for an odd combo: a 30-m dipole wit the back to back 17-12 Yagi combo. This combo would fit on an 8' boom, with the big dipole (48.6') centered at the mast--but perhaps due to the greater weight of the 17-meter Yagi and the longer boom to that side, offset so that the dipole did not sit right on the mast. Such an antenna is possible, and the sketch shows its outline (<b>Figure 7</b>)
|
||||
<center>
|
||||
<img src="bb7.gif" width="617" height="201" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The dipole is now the fed element, with both Yagi drivers open-sleeve coupled. The frequency ratio for the 17-meter driver (10.125:18.118) is low enough to make it easy to preserve the full performance of the 2-element DE-Dir beam (6.5 dBi/28 dB F-B) with a 53-Ohm feedpoint impedance. The frequency ratio for the 12-meter driver (10.125:24.95) is large enough to adversely affect performance to a small degree (6.3 dBi/20 dB F-B) with a 47-Ohm feedpoint impedance. The dipole pattern is slightly flattened as the Yagis stretch it in the planar directions--the amount is about 0.2 dB. The dipole impedance in free space showed about 78 Ohms. In all cases, the impedance figures are good enough for standard coax feed systems.</p>
|
||||
<p>Here is the modeled wire table for the 3-band antenna. Unfortunately, I lack ftp capabilities and cannot provide a downloadable .NEC or .EZ file.</p>
|
||||
<pre> --------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in) Segs
|
||||
12-m director
|
||||
1 -9.600, 3.340, 0.000 W2E1 -6.500, 3.340, 0.000 3.75E-01 5
|
||||
2 W1E2 -6.500, 3.340, 0.000 W3E1 -2.500, 3.340, 0.000 5.00E-01 5
|
||||
3 W2E2 -2.500, 3.340, 0.000 W4E1 -1.000, 3.340, 0.000 6.25E-01 2
|
||||
4 W3E2 -1.000, 3.340, 0.000 W5E1 1.000, 3.340, 0.000 7.50E-01 3
|
||||
5 W4E2 1.000, 3.340, 0.000 W6E1 2.500, 3.340, 0.000 6.25E-01 2
|
||||
6 W5E2 2.500, 3.340, 0.000 W7E1 6.500, 3.340, 0.000 5.00E-01 5
|
||||
7 W6E2 6.500, 3.340, 0.000 9.600, 3.340, 0.000 3.75E-01 5
|
||||
12-m driver
|
||||
8 -10.050, 0.600, 0.000 W9E1 -6.500, 0.600, 0.000 3.75E-01 5
|
||||
9 W8E2 -6.500, 0.600, 0.000 W10E1 -2.500, 0.600, 0.000 5.00E-01 5
|
||||
10 W9E2 -2.500, 0.600, 0.000 W11E1 -1.000, 0.600, 0.000 6.25E-01 2
|
||||
11 W10E2 -1.000, 0.600, 0.000 W12E1 1.000, 0.600, 0.000 7.50E-01 3
|
||||
12 W11E2 1.000, 0.600, 0.000 W13E1 2.500, 0.600, 0.000 6.25E-01 2
|
||||
13 W12E2 2.500, 0.600, 0.000 W14E1 6.500, 0.600, 0.000 5.00E-01 5
|
||||
14 W13E2 6.500, 0.600, 0.000 10.050, 0.600, 0.000 3.75E-01 5
|
||||
17-m driver
|
||||
15 -13.780, -0.620, 0.000 W16E1 -9.000, -0.620, 0.000 5.00E-01 6
|
||||
16 W15E2 -9.000, -0.620, 0.000 W17E1 -3.500, -0.620, 0.000 6.25E-01 6
|
||||
17 W16E2 -3.500, -0.620, 0.000 W18E1 -1.500, -0.620, 0.000 7.50E-01 3
|
||||
18 W17E2 -1.500, -0.620, 0.000 W19E1 1.500, -0.620, 0.000 8.75E-01 5
|
||||
19 W18E2 1.500, -0.620, 0.000 W20E1 3.500, -0.620, 0.000 7.50E-01 3
|
||||
20 W19E2 3.500, -0.620, 0.000 W21E1 9.000, -0.620, 0.000 6.25E-01 6
|
||||
21 W20E2 9.000, -0.620, 0.000 13.780, -0.620, 0.000 5.00E-01 6
|
||||
17-m director
|
||||
22 -13.050, -4.480, 0.000 W23E1 -9.000, -4.480, 0.000 5.00E-01 6
|
||||
23 W22E2 -9.000, -4.480, 0.000 W24E1 -3.500, -4.480, 0.000 6.25E-01 6
|
||||
24 W23E2 -3.500, -4.480, 0.000 W25E1 -1.500, -4.480, 0.000 7.50E-01 3
|
||||
25 W24E2 -1.500, -4.480, 0.000 W26E1 1.500, -4.480, 0.000 8.75E-01 5
|
||||
26 W25E2 1.500, -4.480, 0.000 W27E1 3.500, -4.480, 0.000 7.50E-01 3
|
||||
27 W26E2 3.500, -4.480, 0.000 W28E1 9.000, -4.480, 0.000 6.25E-01 6
|
||||
28 W27E2 9.000, -4.480, 0.000 13.050, -4.480, 0.000 5.00E-01 6
|
||||
30-m dipole
|
||||
29 -24.300, 0.000, 0.000 W30E1 -17.500, 0.000, 0.000 5.00E-01 5
|
||||
30 W29E2 -17.500, 0.000, 0.000 W31E1 -12.650, 0.000, 0.000 6.25E-01 5
|
||||
31 W30E2 -12.650, 0.000, 0.000 W32E1 -7.150, 0.000, 0.000 7.50E-01 5
|
||||
32 W31E2 -7.150, 0.000, 0.000 W33E1 -5.150, 0.000, 0.000 8.75E-01 2
|
||||
33 W32E2 -5.150, 0.000, 0.000 W34E1 -2.000, 0.000, 0.000 1.00E+00 3
|
||||
34 W33E2 -2.000, 0.000, 0.000 W35E1 2.000, 0.000, 0.000 1.12E+00 3
|
||||
35 W34E2 2.000, 0.000, 0.000 W36E1 5.150, 0.000, 0.000 1.00E+00 3
|
||||
36 W35E2 5.150, 0.000, 0.000 W37E1 7.150, 0.000, 0.000 8.75E-01 2
|
||||
37 W36E2 7.150, 0.000, 0.000 W38E1 12.650, 0.000, 0.000 7.50E-01 5
|
||||
38 W37E2 12.650, 0.000, 0.000 W39E1 17.500, 0.000, 0.000 6.25E-01 5
|
||||
39 W38E2 17.500, 0.000, 0.000 24.300, 0.000, 0.000 5.00E-01 5</pre>
|
||||
<p>As with the other designs, you will have to experiment for other taper schedules to the elements. Likewise, the exact lengths and spacing of the drivers from the dipole will also be am tter of experimentation.</p>
|
||||
<p>If you shorten the dipole by any other means than a capacity hat on each end, the performance of the Yagis will be thrown off due to the altered current levels along parts of the dipole providing the drive for the coupled Yagi drivers. Even mid-element loads throw everything off. Capacity hats do preserve the current levels along the shortened dipole at the same magnitude as with a full size dipole; however, the hats will necessarily by large enough to possible couple to the Yagi driver ends.</p>
|
||||
<p>No doubt further modeling time could yield improved performance from the 12-m Yagi, with many iterations of element length, spacing, and spacing from the dipole changes. But the figures shown here should suffice as a starting point for individual experimentation.</p>
|
||||
<h3>A Forward-Facing 17-12 Combo with Higher Gain on 12</h3>It is possible to obtain greater gain on 12 meters with a forward-facing combination of a Driver-Reflector Yagi for 17 and a driver-director Yagi for 12, using the same forward-stagger and open-sleeve principles of the earlier 4-element (2 per band) we looked at earlier. The way to more gain is to add another director for 12 meters. The general outline looks like <b>Figure 8</b>.
|
||||
<center>
|
||||
<img src="bb8.gif" width="484" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>If we compare this 5-element Yagi to the forward-facing 4-element version earlier, we shall see several differences. First, the reflector spacing has been increased. The chief result of this move is to increase the resistive component of the feedpoint impedance. The essential performance on 17 is not changed. You can feel free to use any spacing from the close spacing of the 4-element version to the wide spacing of this version.</p>
|
||||
<p>Second, adding a second director changes the director dimensions and lengthens the boom considerable--a total of 17' for the entire array. However, besides obtaining better gain on 12, the array is less sensitive to minor changes of length of the 2 directors. What you might change slightly by a small alteration of one element's length can be restored by slightly altering the length of the other director.</p>
|
||||
<p>The NEC-4 modeled performance of this array is given in the following table:</p>
|
||||
<pre>Freq Gain FS F-B R +/- jX SWR
|
||||
18.068 6.30 11.89 62.1 - j 4.7 1.26
|
||||
18.118 6.25 11.92 64.2 - j 3.1 1.29
|
||||
18.168 6.20 11.91 66.3 - j 1.6 1.32
|
||||
|
||||
24.89 7.16 13.75 57.1 + j 6.9 1.20
|
||||
24.94 7.21 13.82 54.9 + j 8.9 1.22
|
||||
24.95 7.26 13.87 52.8 + j11.1 1.25</pre>
|
||||
<p>It is possible to optimize this design further. The 17-meter section can be set closer to 50 Ohms, although significant increases in gain and front-to-back ratio are not possible with only 2 elements. The gain and front-to-back ratio on 12 can also be altered, but improving one results in decreases to the other. This version has opted for gain and only a modest front-to-back ratio. Comparative patterns for the 2 bands appear in <b>Figure 9</b>.</p>
|
||||
<center>
|
||||
<img src="bb9.gif" width="640" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>As always, models of open sleeve coupling are approximate only. Therefore, expect to play a bit with the length of the 12-meter slaved driver and its spacing from the 17-meter fed driver to achieve the desired 12-meter feedpoint impedance. Such adjustments have little or no effect on the 17-meter feedpoint impedance or other operating parameters on that band. Also, if you select a different tapering schedule for your elements, do two things. 1. Check out the potential durability of the element, using a program like YagiStress or use a model of element taper that has a proven record. 2. Adjust the element lengths to suit the Leeson corrected substitute uniform diameter elements that are equivalent to the ones shown here. This latter task can be handled with one of the antenna modeling programs. The details of the model on which this note is based appear in the following EZNEC antenna description.</p>
|
||||
<pre> --------------- WIRES ---------------
|
||||
|
||||
Wire Conn. --- End 1 (x,y,z : ft) Conn. --- End 2 (x,y,z : ft) Dia(in) Segs
|
||||
17-Meter Reflector
|
||||
1 -14.100, 0.000, 0.000 W2E1 -12.250, 0.000, 0.000 5.00E-01 2
|
||||
2 W1E2 -12.250, 0.000, 0.000 W3E1 -9.500, 0.000, 0.000 6.25E-01 3
|
||||
3 W2E2 -9.500, 0.000, 0.000 W4E1 -6.750, 0.000, 0.000 7.50E-01 3
|
||||
4 W3E2 -6.750, 0.000, 0.000 W5E1 -4.000, 0.000, 0.000 8.75E-01 3
|
||||
5 W4E2 -4.000, 0.000, 0.000 W6E1 4.000, 0.000, 0.000 1.00E+00 9
|
||||
6 W5E2 4.000, 0.000, 0.000 W7E1 6.750, 0.000, 0.000 8.75E-01 3
|
||||
7 W6E2 6.750, 0.000, 0.000 W8E1 9.500, 0.000, 0.000 7.50E-01 3
|
||||
8 W7E2 9.500, 0.000, 0.000 W9E1 12.250, 0.000, 0.000 6.25E-01 3
|
||||
9 W8E2 12.250, 0.000, 0.000 14.100, 0.000, 0.000 5.00E-01 2
|
||||
17-Meter Fed Driver
|
||||
10 -13.167, 9.158, 0.000 W11E1 -11.250, 9.158, 0.000 5.00E-01 2
|
||||
11 W10E2 -11.250, 9.158, 0.000 W12E1 -8.500, 9.158, 0.000 6.25E-01 3
|
||||
12 W11E2 -8.500, 9.158, 0.000 W13E1 -5.750, 9.158, 0.000 7.50E-01 3
|
||||
13 W12E2 -5.750, 9.158, 0.000 W14E1 -4.000, 9.158, 0.000 8.75E-01 3
|
||||
14 W13E2 -4.000, 9.158, 0.000 W15E1 4.000, 9.158, 0.000 1.00E+00 9
|
||||
15 W14E2 4.000, 9.158, 0.000 W16E1 5.750, 9.158, 0.000 8.75E-01 3
|
||||
16 W15E2 5.750, 9.158, 0.000 W17E1 8.500, 9.158, 0.000 7.50E-01 3
|
||||
17 W16E2 8.500, 9.158, 0.000 W18E1 11.250, 9.158, 0.000 6.25E-01 3
|
||||
18 W17E2 11.250, 9.158, 0.000 13.167, 9.158, 0.000 5.00E-01 2
|
||||
12-meter "Slave" Driver
|
||||
19 -9.833, 9.575, 0.000 W20E1 -6.250, 9.575, 0.000 3.75E-01 4
|
||||
20 W19E2 -6.250, 9.575, 0.000 W21E1 -2.500, 9.575, 0.000 5.00E-01 4
|
||||
21 W20E2 -2.500, 9.575, 0.000 W22E1 -0.500, 9.575, 0.000 6.25E-01 3
|
||||
22 W21E2 -0.500, 9.575, 0.000 W23E1 0.500, 9.575, 0.000 7.50E-01 1
|
||||
23 W22E2 0.500, 9.575, 0.000 W24E1 2.500, 9.575, 0.000 6.25E-01 3
|
||||
24 W23E2 2.500, 9.575, 0.000 W25E1 6.250, 9.575, 0.000 5.00E-01 4
|
||||
25 W24E2 6.250, 9.575, 0.000 9.833, 9.575, 0.000 3.75E-01 4
|
||||
12-Meter Director 1
|
||||
26 -9.500, 10.683, 0.000 W27E1 -6.250, 10.683, 0.000 3.75E-01 4
|
||||
27 W26E2 -6.250, 10.683, 0.000 W28E1 -2.500, 10.683, 0.000 5.00E-01 4
|
||||
28 W27E2 -2.500, 10.683, 0.000 W29E1 -0.500, 10.683, 0.000 6.25E-01 3
|
||||
29 W28E2 -0.500, 10.683, 0.000 W30E1 0.500, 10.683, 0.000 7.50E-01 1
|
||||
30 W29E2 0.500, 10.683, 0.000 W31E1 2.500, 10.683, 0.000 6.25E-01 3
|
||||
31 W30E2 2.500, 10.683, 0.000 W32E1 6.250, 10.683, 0.000 5.00E-01 4
|
||||
32 W31E2 6.250, 10.683, 0.000 9.500, 10.683, 0.000 3.75E-01 4
|
||||
12-Meter Director 2
|
||||
33 -8.833, 16.967, 0.000 W34E1 -6.250, 16.967, 0.000 3.75E-01 4
|
||||
34 W33E2 -6.250, 16.967, 0.000 W35E1 -2.500, 16.967, 0.000 5.00E-01 4
|
||||
35 W34E2 -2.500, 16.967, 0.000 W36E1 -0.500, 16.967, 0.000 6.25E-01 3
|
||||
36 W35E2 -0.500, 16.967, 0.000 W37E1 0.500, 16.967, 0.000 7.50E-01 1
|
||||
37 W36E2 0.500, 16.967, 0.000 W38E1 2.500, 16.967, 0.000 6.25E-01 3
|
||||
38 W37E2 2.500, 16.967, 0.000 W39E1 6.250, 16.967, 0.000 5.00E-01 4
|
||||
39 W38E2 6.250, 16.967, 0.000 8.833, 16.967, 0.000 3.75E-01 4</pre>
|
||||
<p>The element taper used here is an adaptation of a commercial design for other bands. 6061-T6 tubing or its equivalent is available from numerous sources. This beam is far from optimized to its maximum possible performance. However, it may provide a further alternative for experimentation. Since numerous commercial beams use booms in the vicinity of 18', a design such as this makes a possible conversion project once you spring for the very long boom tri-bander to use on 20-15-10 meters</p>
|
||||
<h3>The End Result is This . . .</h3>
|
||||
<p>As W5JH reminded me, the D/DE 2-element Yagi has a natural home on the WARC bands, especially 12 and 17. Matching is straight forward, and performance will be stable across these narrow bands. 0.07 to 0.09 wl makes a good element spacing for a 20-ohm feedpoint impedance, an easy beta match. Combining DE-Dir and DE-Ref designs is certainly feasible and can effect simplicities of matching and use. Combining back-to-back Yagis with a 30-meter dipole is also possible.</p>
|
||||
<p>Whether you go 2-faced or long-boomed, these designs and innumerable possible variations on them should serve many needs on 12 and 17. Moreover, as this little exercise has shown, the possibilities for experimentation are endless.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" width="591" height="3">
|
||||
</center><br>
|
||||
<br>
|
||||
<p><i>Updated 7-25-99. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" width="591" height="3">
|
||||
</center>
|
||||
<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Return to Amateur Radio Page</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
||||
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<head>
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<meta charset="UTF-8">
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||||
<meta name="keywords" content="Yagi, HF Upper">
|
||||
<title>What Can We Expect from a 2-Element Beam? Index</title>
|
||||
<link rel="stylesheet" href="../styles.css">
|
||||
</head>
|
||||
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|
||||
<center>
|
||||
<h1><img src="../images/yagicad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10">What Can We Expect from a 2-Element Beam?<img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="10"></h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p>I have heard lots of 2-element Yagi performance numbers bandied about over the years. Some are reasonable and well explained. Some are reasonable, but not well-explained. Some are unreasonable and demand explanation. The numbers are disparate enough to keep 2-element Yagi performance a mystery to the relative newcomer.</p>
|
||||
<p>Having modeled and built a very large number of 2-element Yagis over the years, I thought a refresher might be in order, if only to clarify what count as reasonable expectations. The information is available in almost any solid reference, but perhaps a different type of presentation might be useful. If this is your first entry to these notes, I recommend that you read the parts in order.</p>
|
||||
<p>The refresher is long and broken into a number of sections. The original version of this small series had no illustrations, but only tabular data. Since 9 years have passed since the original 1997 version of the notes, I have decided to rewrite and expand some of the coverage. In the process, I have re-modeled everything, since software has advanced considerably over the near-decade since I last examined this work in detail. Mostly, I have added some graphics to make the notes easier to digest. For example, <b>Fig. 1</b>.</p>
|
||||
<center>
|
||||
<img src="beam-1.gif" width="350" height="210" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The sketch shows the elements (but not the boom or the mast) of 3 types of 2-element beams, only 2 of which are Yagis. 2-element Yagi beams come in two varieties: reflector-driver arrays and driver-director arrays. Do not think of a beam like a flashlight. Consider the beam's forward direction, that is the direction in which signals are strongest (both transmitting and receiving). The reflector is simply any element to the rear of the driver, while a director is simply any element forward of the driver. For most purposes, but certainly not for all, the reflector-driver version is the more popular--and for some very good reasons that we shall note along the way. The driver-director version has some interesting properties that suit it to special purposes.</p>
|
||||
<p>The third 2-element beam is not a Yagi. Rather, it uses a current phasing line between the elements. Only one of several possible versions appears here--the ZL-Special variety. However, one might redesign the array to have the feedpoint at the center, at the rear, or somewhere else along the phasing line. Since I have dealth extensively with phased arrays in another series of notes, we shall stick to 2-element Yagis in this series. (For information on horizontal phased arrays, see "<a href="../phase/ph.html">Some Notes on Two-Element Horizontal Phased Arrays</a>" In fact, most of the notes will apply to the reflector-driver type of 2-element Yagi because it is so versatile.</p>
|
||||
<center>
|
||||
<h3>Contents</h3>
|
||||
</center>
|
||||
<blockquote>
|
||||
<b><a href="beam1.html">Part 1: Method, Units of Measure, and the Dipole Standard of Reference</a></b>
|
||||
<p><b><a href="beam2.html">Part 2: The Full-Size 2-Element Yagi</a></b></p>
|
||||
<p><b><a href="beam3.html">Part 3: Shortened Dipoles and Capacity Hat Yagis</a></b></p>
|
||||
<p><b><a href="beam4.html">Part 4: Loaded Yagis</a></b></p>
|
||||
<p><b><a href="beam5.html">Part 5: Strategies for Improving Forward and Rearward Performance</a></b></p>
|
||||
</blockquote>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 05-09-1997, 04-25-2006. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><a href="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> Go to Amateur Radio Page</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
||||
</body>
|
||||
</html>
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