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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<meta name="keywords" content="Doublet, Lazy H, HF Multi Band">
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<title>Back-Up Antenna for 80-20 Meters</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/wirecad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10"> The "Ideal" Back-Up Antenna for 80-20 Meters <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>
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<p>Everyone needs a back-up antenna in case one or more of the main arrays at the antenna farm becomes inoperative. The requirements for the back-up antenna are very straightforward:</p>
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<ul>
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<li>1. It should be a simple, multi-band design--in order to replace any one or more of the main systems. We shall accept the need for an antenna tuner (ATU).</li>
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<li>2. It should also be mechanically simple--to make maintenance a relatively easy matter.</li>
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<li>3. It should cover all bands of main interest--here defined arbitrarily as 80 through 20 meters.</li>
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</ul>
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<p>For this exercise, I shall confine myself to horizontal antennas, with the proviso that they be as high as possible. 70' is not very high on 80 meters for a horizontal antenna, and I shall use that as my minimum height. However, if the back-up is to replace wounded high-altitude horizontal beams, 100' is not unrealistic. If you live in the right kind of forest, these heights can be attained using trees instead of towers as the end support points.</p>
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<p>These notes do not imply that a vertical does not make a good back-up for the main antenna systems. In fact, I use a multi-band vertical myself for just such purposes. However, it is too difficult to cover both vertical and horizontal possibilities in one small article, so I shall confine the discussion to horizontal antennas.</p>
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<h3>The "Best" Single Wire</h3>
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<p>If we begin with a single horizontal wire, placed as high as we can achieve, only one question remains: how long? <b>Figure 1</b> suggests the answer I would give.</p>
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<center>
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<img src="88-1.gif" width="430" height="228" border="2" vspace="5" hspace="9">
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</center>
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<p>Notice that I have by-passed the standard multi-band doublet lengths of 135' and 102' (or thereabouts). The reason is simple. As we increase frequency, the azimuth patterns for these antennas break into many lobes, with much reduced radiation broadside to the wire. I shall take the following condition to be desirable for a back-up antenna: we know where the main lobes of the pattern go. The best way to guarantee consistency for all the bands we wish to cover with the back-up antenna is to ensure that the lobes on every band are broadside to the wire.</p>
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<p>88'--plus or minus a non-critical bit--is the longest wire we can use to ensure broadside lobes on 20 meters. The antenna length is about 1.25 wavelengths on 20, which makes it an extended double Zepp. At the same time, the chosen length is between one- third and three-eighths wavelength on the low end of 80 meters. With care, that length is usable at a lower level of performance than for the other bands.</p>
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<center>
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<img src="88-2.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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</center>
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<p><b>Figure 2</b> shows the free-space azimuth patterns for the back-up antenna for 80 through 20 meters. You may correlate the patterns with the data in <b>Table 1</b>.</p>
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<pre>Table 1. Free-Space Performance of an 88' #12 Copper Wire Doublet
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Freq. F-S Gain Horizontal Feedpoint Z
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MHz dBi B/W (deg.) R +/- jX Ohms
|
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3.6 1.77 85 25 - j 615
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||||
3.9 1.82 84 30 - j 500
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5.37 2.05 79 71 - j 20
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7.0 2.38 71 185 + j 510
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10.1 3.36 53 3360 + j2245
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14.0 5.03 32 155 - j 805</pre>
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<p>Of course, these NEC-4 modeling numbers are more precise than would be operationally significant. However, they do clearly show the trends in performance. An often overlooked figure of merit is the beamwidth, which gives us a measure of relative coverage for an antenna. Note that the 60-meter values place the antenna at close to resonant-dipole length on this band.</p>
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<p>No one has ever decreed that we cannot make an 88' doublet out of aluminum tubing. Before we dream of rotating such an antenna, let's examine the free-space performance figures for a version with an average effective diameter of 1". <b>Table 2</b> tells the story.</p>
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<pre>Table 2. Free-Space Performance of an 88' 1" Aluminum Tubing Doublet
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Freq. F-S Gain Horizontal Feedpoint Z
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MHz dBi B/W (deg.) R +/- jX Ohms
|
||||
3.6 1.90 85 24 - j 425
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||||
3.9 1.93 84 29 - j 340
|
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5.37 2.13 78 72 - j 2
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7.0 2.44 71 197 + j 385
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10.1 3.43 53 2560 + j 220
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14.0 5.02 31 125 - j 495</pre>
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<p>There are some interesting differences between the numbers in the two tables. First, the trend with the fatter element is higher (but not significantly higher) gain--except for 20 meters. Here, the gain actually decreases (although insignificantly), because the fatter wire more closely approaches an electrical length where the EDZ ears come to dominate the azimuth pattern. At an electrical length of 1.5 wavelengths, the antenna would show 6 nearly equal lobes.</p>
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<p>Second, the fatter wire tends to reduce the feedpoint impedance, especially the reactive components. This effect can be very useful in easing the burden on the ATU. On 80 and 75, there is a disproportionately high ratio of reactance to resistance in the feedpoint impedance. Hence, even with very high efficiency parallel lines, expect line losses to add to the reduced performance from the already short antenna length (about 1/3 wavelength). That is an important reason why I call this antenna a back-up rather than a prime station antenna.</p>
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<p>One of the limiting factors for every ATU is the range of reactance it is able to compensate for at any given frequency. Of course, if we choose the "wrong" line length, we may encounter such cases due to the impedance transformation properties of every transmission line. One easy solution is to change the line length until we reach the best compromise setting. This technique--plus tuning up using very low power--can be critical on the lowest band (80/75 meters).</p>
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<center>
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<img src="88-3.gif" width="429" height="311" border="2" vspace="5" hspace="9">
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</center>
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<p>For those who would like the benefits of tubing but the low cost and lighter weight of wire, <b>Figure 3</b> offers a couple of the many alternatives. We can simulate the diameter of any size tubing with a pair of wires spaced by a certain distance. The spacing would have the value that allows the antenna to be naturally resonant on the same frequency at which the tubular antenna is resonant. This is an easy modeling task that takes the work out of field adjustment.</p>
|
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<p>An alternative to the paired wire arrangement (shorted at both the outer end and at the feedpoint) is the old-fashioned cage. Since everything old becomes new again, cage antennas for low-band dipole use have gained a certain popularity, especially as the newer polycarbonate plastics have become generally available. As these figures suggest, the cage may also have some utility for multi-band doublets.</p>
|
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<p>Even more significant are the figures for the doublet's performance over ground. <b>Table 3</b> and <b>Table 4</b> list the figures for heights of 70' and 100'. Added to the table is the TO angle (the take-off angle or elevation angle of maximum radiation) and the vertical beamwidth. Together, these figures give us a view of the range of incoming and outgoing skip angles over which we may effect communications. Like the horizontal beamwidth, the vertical beamwidth is a much overlooked valuable piece of data.</p>
|
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<pre>
|
||||
Table 3. 70' Performance of an 88' #12 Copper Wire Doublet
|
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Freq. F-S Gain TO Angle Vertical Horizontal Feedpoint Z
|
||||
MHz dBi (deg.) B/W (deg.) B/W (deg.) R +/- jX Ohms
|
||||
3.6 5.90 59 130 180 30 - j 610
|
||||
3.9 5.86 53 134 180 35 - j 495
|
||||
5.37 6.41 37 51 97 80 - j 30
|
||||
7.0 7.84 28 33 79 165 + j 485
|
||||
10.1 8.66 19 21 55 3810 + j2160
|
||||
14.0 10.81 14 15 33 155 - j 820
|
||||
|
||||
Table 4. 100' Performance of an 88' #12 Copper Wire Doublet
|
||||
Freq. F-S Gain TO Angle Vertical Horizontal Feedpoint Z
|
||||
MHz dBi (deg.) B/W (deg.) B/W (deg.) R +/- jX Ohms
|
||||
3.6 6.12 38 57 107 30 - j 620
|
||||
3.9 6.40 36 47 102 35 - j 505
|
||||
5.37 7.93 25 29 85 60 - j 25
|
||||
7.0 7.83 20 21 74 185 + j 530
|
||||
10.1 9.16 14 14 54 3115 + j2450
|
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14.0 10.50 10 10 32 165 - j 810</pre>
|
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<p>The feedpoint impedances at the two levels fall well within the margins of rough equality. The most important differences show up in the gain and TO angle columns. Although usable, the 70' model shows very high TO angles and lower gain on 80 meters. The 180-degree beamwidths on 80 meters indicate nearly circular patterns. In contrast, the TO angles and vertical beamwidths for 80 meters in the 100' model promise significantly better DX performance, with much more oval patterns. On 40 through 20 meters, the gain differentials disappear, but the higher model shows the expected lower TO angles.</p>
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<center>
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<img src="88-4.gif" width="640" height="270" border="2" vspace="5" hspace="9">
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||||
</center>
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<center>
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||||
<img src="88-5.gif" width="640" height="270" border="2" vspace="5" hspace="9">
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||||
</center>
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<p>In order to get a better perspective on the significance of these figures, examine <b>Figure 4</b> and <b>Figure 5</b>. They show the elevation patterns of the antenna at the two heights: 100' and 70', respectively. Besides illustrating the notes just given, they also reveal the growth of secondary lobes at the upper frequencies as we raise the antenna by 30' or so. 20 meters has grown a third elevation lobe. The high dome pattern gravitates from 10.1 MHz down to 7 MHz in the move from 70' to 100'. The shape difference at the two heights in the 80/75 meter patterns is self-evident.</p>
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<p>I have not added the azimuth patterns, since they resemble too closely the patterns in <b>Figure 2</b>. The key difference is that as we reduce the frequency of use, the side rejection decreases. The decrease is more radical at the lower height, where it disappears almost completely at 80 meters. In addition, the high X:R ratio at 80 meters tends to yield higher line losses that do not appear in the basic antenna patterns. The gain of the antenna along remains unchanged, although a high-loss line situation means that less power will reach the antenna.</p>
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<h3>A Pair of Semi-Eternal Triangles</h3>
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<p>Although it is not likely to be true, let me assume that I have convinced you that an 88' doublet is the best single-wire back-up antenna for 80 through 20 meters. Once we have gone this far for the sake of the argument, we can pose the question of how to derive the best world-wide coverage with such a wire antenna. The answer is as simple as the triangle.</p>
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<p>Actually, I want to explore two versions of the triangle: a Y-array and a true triangle. For no particular reason, I shall begin with the Y-array, shown in <b>Figure 6</b>.</p>
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<center>
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<img src="88-6.gif" width="381" height="327" border="2" vspace="5" hspace="9">
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||||
</center>
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<p>As the figure shows, the ends of the wires are set 12' from a center-point for the array. The array would require a rectangle about 100' by 175' for implementation. A 4- post construction method seems most obvious (1 at the center and three on the perimeter). However, those with special skills in high-strength wire trussing might manage with only the perimeter posts. A slight dip in antenna height toward the center point would create no significant performance problems</p>
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<p>There is no special magic to my choice of element separation from the center point. The goal was to minimize interaction between the active element and the inactive ones. 18' between adjacent ends is sufficient to achieve this goal, as evidenced by the modeled data in <b>Table 5</b>. The data are for a 100' array height.</p>
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<pre>Table 5. 100' Performance of a Y-Array of 88' #12 Copper Wire Doublets
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Freq. F-S Gain TO Angle Vertical Horizontal Feedpoint Z
|
||||
MHz dBi (deg.) B/W (deg.) B/W (deg.) R +/- jX Ohms
|
||||
3.6 6.11 38 58 108 31 - j 620
|
||||
3.9 6.38 36 47 104 35 - j 505
|
||||
5.37 9.18 26 29 135 130 - j 30
|
||||
7.0 7.87 19 21 73 185 + j 535
|
||||
10.1 9.14 13 14 55 3110 + j2470
|
||||
14.0 10.39 10 10 33 165 - j 810</pre>
|
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<p>The data are insignificantly different from those of <b>Table 4</b>, which gives modeled values for an independent 88' doublet. 60 meters is the exception. On that band, the near-resonance of the elements yields a beam pattern with an 11-dB front-to-back ratio. The main lobe is away from the inactive elements, which form a composite parasitic reflector. The significantly different feedpoint impedance relative to the value for a single 88' wire at the same height is another indicator of the odd behavior on this band. Whether that odd behavior is an advantage or a disadvantage depends upon operating needs.</p>
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<center>
|
||||
<img src="88-7.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
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</center>
|
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<p>To effect world wide-communications, we should understand how the patterns for the triangle overlap. As well we should examine the nulls in the pattern. As <b>Figure 7</b> attests, on 80 meters, there is no significant null (<1 dB). On 40 meters (<b>Figure 8</b>), the nulls are only about 2 dB, which is likely small enough not to occasion any repositioning of wires.</p>
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<center>
|
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<img src="88-8.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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</center>
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<center>
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<img src="88-9.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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</center>
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<p>The first band on which we discover nulls deep enough to cause concern is 30 meters (<b>Figure 9</b>). The nulls become very much deeper on 20 meters (<b>Figure 10</b>), where the EDZ narrow beamwidth becomes quite significant in antenna planning.</p>
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<center>
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<img src="88-10.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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</center>
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<p>In modeling exercises, no negative effects resulted from warping the Y array from its perfect 120-degree separation. Changes up to 20 degrees appear not to create any noticeable consequences for the radiation patterns from the individual antenna wires. Consequently, the array designer can position the three wires in an approximate Y, with each wire broadside to the most favored contact directions, whether those are domestic or DX.</p>
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<p>It would be incorrect to say that the wires do not interact at all. <b>Figure 10</b> shows the very slight interaction by the manner in which the secondary lobes of the three patterns overlap in slightly irregular ways. In fact, the main lobes depart from the true broadside by about 1 degree in the direction of the other wires. These facts, however, do no more than complete the record. Their operational significance is negligible, and the three wires may be considered as aiming in a true broadside direction for all practical purposes. As well, the wires more severely interact on 60 meters so that the result is a directional pattern with an 11-dB front-to-back ratio. The interaction level is a function of the near-resonant length of the wires in the triangle.</p>
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<p>The Y-array presumes that all of the parallel feedlines will be brought to a central switching point, from which a single parallel feedline will proceed to the shack. Switching would be by a remote system (unless the shack is located approximately under the center-point of the array. <b>Figure 11</b> provides the basic elements of such a remote switching system.</p>
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<center>
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<img src="88-11.gif" width="518" height="446" border="2" vspace="5" hspace="9">
|
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</center>
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<p>The system shown can be considered a "bare-bones" version that can be embellished in innumerable ways. Since the array shows no significant differences between open and closed centers for the inactive antennas, shorting the transmission lines of the unused antennas can serve useful functions. For example, you might want to add RF chokes to the shorted contacts with a subsequent lead to ground. The result will be to bleed off any static charges on these wires.</p>
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<p>In fact, you may wish to add a fourth "Off" position to the switching system, for use when the array is wholly inactive. This option would simply remove all power from the relays and bleed all three wires of static charges that may build up from winds and other weather forces. (I shall assume that there will be provisions nearer to the shack for a total disconnect and grounding of both the relay power lines and the parallel transmission line coming from the switching unit. Add other safety features as the spirit moves you.)</p>
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<p>In addition to safety features, it also pays to decouple the relay power lines from RF right at the switching unit. You may add rf chokes and by-pass capacitors to each power lines inside the weatherproof relay box. Alternatively, you can place ferrite cores over each power lines. Be sure to include the common in this treatment. Although shown with a ground connection at the shack, this line at the remote switching unit is ripe for RF pick-up and distribution. Additional decoupling at the shack end of the line is also a wise precaution.</p>
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<center>
|
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<img src="88-12.gif" width="487" height="474" border="2" vspace="5" hspace="9">
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</center>
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<p>The same switching unit can be used with the second configuration of the array: the true triangle shown in <b>Figure 12</b>. This arrangement requires only 3 posts, about 112' apart. The outer dimensions allows for a 12' spacing of the wire from the post, which results in 12' of separation between adjacent wire ends. The true triangle requires less space than the Y-array. A rectangle about 112' by 97' will contain all of the wires. Like the Y- array, I shall assume that the feedlines are brought to a central point for switching among the antennas.</p>
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<p>The true triangular array in theory shows more tendency toward interaction between the active antenna and the inactive wires. Hence, there is likely to be a more significant difference in performance based on whether the center points of the inactive wires are open or shorted.</p>
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<p>The interaction does not greatly affect the overall performance of the antenna, as shown in <b>Table 6</b> and <b>Table 7</b>. With the unused centers closed or shorted, there is a very slight front-to-back ratio. It is never greater than 0.3 dB and the "stronger" lobe is in the direction of the unused wires. With the unused wire centers open, the front-to-back effect drops to less than 0.1 dB on all but 30 meters.</p>
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<pre>
|
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Table 6. 100' Performance of a Triangle-Array of 88' #12 Copper Wire Doublets
|
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(Unused Centers Closed)
|
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Freq. F-S Gain TO Angle Vertical Horizontal Feedpoint Z
|
||||
MHz dBi (deg.) B/W (deg.) B/W (deg.) R +/- jX Ohms
|
||||
3.6 6.25 38 55 109 28 - j 615
|
||||
3.9 6.57 35 46 103 32 - j 500
|
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7.0 7.61 20 21 71 185 + j 525
|
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10.1 9.31 13 14 56 3320 + j2655
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14.0 10.22 10 10 32 165 - j 820
|
||||
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Table 7. 100' Performance of a Triangle-Array of 88' #12 Copper Wire Doublets
|
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(Unused Centers Open)
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Freq. F-S Gain TO Angle Vertical Horizontal Feedpoint Z
|
||||
MHz dBi (deg.) B/W (deg.) B/W (deg.) R +/- jX Ohms
|
||||
3.6 6.14 38 57 108 29 - j 620
|
||||
3.9 6.42 35 47 101 34 - j 505
|
||||
7.0 7.88 19 21 76 185 + j 535
|
||||
10.1 9.38 13 14 62 3050 + j2490
|
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14.0 10.16 10 10 32 170 - j 820</pre>
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<center>
|
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<img src="88-13.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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</center>
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<p>In reality, a noticeable difference in pattern shape occurs only on 30 meters. <b>Figure 13</b> compares the 30-meter patterns for a. the Y-array (which is virtually identical to the pattern of the independent doublet), b. the triangle with the unused centers closed, and c. the triangle with the unused centers open. I chose a different orientation for each antenna, since the key pattern alteration occurs near pattern center. The side "bulges" in the 30-meter patterns for the true triangle do not materially affect the gain of the main lobes. However, the open condition does yield a front-to-back ratio of 0.6 dB, which can be noticed on patterns, but not in operation.</p>
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<p>In the end, then, you may take your choice of open or closed unused elements. Since the choice is so close in performance, my own preference would be to opt for the safest choice: lines closed at the remote switching center. Whether that choice results in electrically closed or open feedpoints for the unused elements depends on the exact length of the line used between the elements and the switching box. Quarter wavelength lines will result in open centers, while half wavelength lines will yield shorted centers. Shorted lines that are longer than 1/4 wavelength will add capacitive reactance to the wires and possible increase the directive effect in the direction of the unused wires. Shorted lines shorter than 1/4 wavelength or longer than 1/2 wavelength will likely add inductive reactance to the unused wires, converting them into reflectors of sorts. The effect, of course, will vary from band-to-band, since the line length to the switching box will change its electrical length with changes of frequency.</p>
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||||
<p>For many applications, minor directive and reflective effects may be no hindrance, and the smaller footprint of the true triangle will be the overriding consideration. For some applications, maximum isolation of each antenna will be the dominant concern: in such cases, the Y-array should be the configuration of choice. Before making this decision, it would likely be wise to model the system--with the proposed feedline as part of the model--to gain a better view of actual effects.</p>
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||||
<h3>The "Expanded" Lazy-H</h3>
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<p>Before leaving the field of back-up wire antennas that are about 88' long and that cover 80 through 20 meters, we should take note of the expanded lazy-H, which likely first appeared in print in CQ in an article by W2EEY. <b>Figure 14</b> shows the essential elements of the array: wires vertically spaced 44' apart. Although there are monoband schemes for bottom feeding the array, multi-band use tends to require balanced in phase feeding of the system, as shown in the sketch.</p>
|
||||
<center>
|
||||
<img src="88-14.gif" width="357" height="249" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>As we did with the independent doublet and the triangular arrays, let's place this antenna at a top height of 100', with the lower wires at 56'. For the data in <b>Table 8</b>, I used a 450-Ohm phasing line and took that feedpoint impedance readings from the junction of the two 22' lines.</p>
|
||||
<pre>
|
||||
Table 8. 100' Performance of an Expanded Lazy-H of 88' #12 Copper Wire Doublets
|
||||
Freq. F-S Gain TO Angle Vertical Horizontal Feedpoint Z
|
||||
MHz dBi (deg.) B/W (deg.) B/W (deg.) R +/- jX Ohms
|
||||
3.6 5.95 46 138 131 10 - j 95
|
||||
3.9 6.05 43 141 118 15 - j 60
|
||||
5.37 8.81 30 37 137 150 - j 115
|
||||
7.0 9.02 23 27 76 455 - j 445
|
||||
10.1 11.69 16 18 56 20 - j 45
|
||||
14.0 14.87 11 12 32 50 + j 385</pre>
|
||||
<p>We should divide our discussion of the performance between the lower bands and the upper bands for this antenna. The lower bands show lesser gain and higher TO angles than the independent doublet. This phenomenon results from the fact that the elevation angle at any frequency is a composite angle of the upper and lower wires. The lower wire at 56' significantly increases the 80/75-meter TO angle. The vertical beamwidth is very wide, which may compensate to a degree by offering low angle performance not far down from maximum. However, on the lowest band, the high angle reception is likely to increase the overall noise in relationship to signal strength. Hence, for 80 and 75 meters, finding a way to disable the lower wire and using only the upper wire would make good operating sense. On 40 meters, the added gain compared to a single doublet may more fully compensate for the slightly higher TO angle.</p>
|
||||
<p>The performance on 60 meters is similar to the performance of a single wire triangle. The front-to-back ratio is 13 dB and the main forward lobe is in a direction away from the "inactive" reflector wires.</p>
|
||||
<p>Upper band performance of the Lazy-H is marked by very significant increases in gain over a single doublet without decreases in the horizontal beamwidth. Where the gain comes from appears in <b>Figure 15</b>, an elevation plot for 30 meters.</p>
|
||||
<center>
|
||||
<img src="88-15.gif" width="640" height="270" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Compare this plot to the 30-meter trace in <b>Figure 4</b>. That plot shows a very strong second elevation lobe at about a 45-degree elevation angle, along with the "dome" that indicates the development of third lobes. The expanded Lazy-H (which is a little under 1 wavelength long on 30 meters) shows strong first elevation lobes with second lobes about 10 dB lower in strength. The 20-meter elevation pattern would show a similar plot, with more ripples but no added strength above the first lobe. The added gain in the lowest lobes comes from the reduction in high angle gain--a very nice trade indeed.</p>
|
||||
<p>The impedances of the triangle of lazy-H arrays suggest the use of a lower-impedance parallel line. Due to the variation in values from band to band, you can expect some tuning difficulties with common tuner designs. You may use inserts of added line length to arrive at the best set of impedance values for each band. On any band where the tuning is very sharp at the recommended low-power initial tune-up point, raise the power level slowly, checking for any necessary returning as you go. 20 and 80 meters may present the most difficulties due to the relatively high ratio of reactance to resistance.</p>
|
||||
<p>I have not modeled a Y-array of three expanded lazy-Hs, but I would suspect that the end-to-end isolation of the antennas would be similar to that of the doublets. If we have three (or 4) tall poles, towers, or trees for the support of a doublet array, we might wish to think about the expanded lazy-H as an alternative--at least for all of the bands except 80/75 meters.</p>
|
||||
<h3>Updating the Expanded Lazy-H for 80-20 Meters</h3>
|
||||
<p>Some of the impedance values for the feedpoint junction of the phasing lines to the elements show similarities to those of the basic 88' doublet, although with a somewhat smaller X:R ratio. The most troublesome feedpoint impedance values occur when the resistive component is very low and the line impedance is quite high. The result is en exceptionally high SWR that even the low-loss reputation of parallel lines cannot overcome. That is, the line losses will be very considerable and in some cases prohibitive.</p>
|
||||
<p>The are several strategies to lower the line losses. One route is to lower the impedance of the feedline to the lowest practical level while maintaining an open-wire construction to preserve the very low level of matched-line loss. 300-Ohm lines will go a long way toward reducing line losses to a more (but not completely) acceptable level when the feedpoint resistance is very low and the reactance is significant. Vinyl-coated lines aqre most common in this category, although we must use two precautions with them. First, the loss level of a vinyl coated line is higher than for open wires. Second, we must use line that is rated for transmitting duty and not the inexpensive low-power TV reception line.</p>
|
||||
<p>We may also use other techniques to modify the arrangement of the antenna (including phase lines). One relatively easy technique is to raise the impedance of the phase line to the highest practical level, perhaps 600 Ohms. <b>Table 9</b> compares the 88' lazy-H at a top height of 100' for 450-Ohm and 600-Ohm phasing lines with respect to the feedpoint impedance.</p>
|
||||
<pre>
|
||||
Table 9. Feedpoint Impedanceof an Expanded Lazy-H of 88'
|
||||
#12 Copper Wire Doublets at 100'
|
||||
450-Ohm Line 600-Ohm Line
|
||||
Freq. Feedpoint Z Feedpoint Z
|
||||
MHz R +/- jX Ohms R +/- jX Ohms
|
||||
3.6 10 - j 95 15 - j 95
|
||||
3.9 15 - j 60 20 - j 50
|
||||
5.37 150 - j 115 90 + j 215
|
||||
7.0 455 - j 445 990 - j 260
|
||||
10.1 20 - j 45 40 - j 100
|
||||
14.0 50 + j 385 60 + j 450
|
||||
</pre>
|
||||
<p>In most cases, the ratio between the resistance and reactance changes very litttle, but the 80- and 75-meter resistance values do rise slightly.</p>
|
||||
<p>An alternative is to use a slightly longer element length. Chuck Gerarden, W0DLE, has constructed a lazy-H of the present type on a tall tower using 92' elements. Chuck's elements use an interesting techniques of employing aluminum tubing for the inner sections and thin-wall fiberglass tubing with aluminum wire inside for the outer sections. <b>Fig. 16</b> shows the antenna. At present, only the upper element rotates, although he will add a rotator for the lower element. He also has the ability to switch between upper-only, lower-only, and both elements in phase. For the present, he has the array aligned for bi-dirctional coverage of both coasts, but can rotate the upper element on the lower bands where the single element gain exceeds that of the combination of upper and lower elements. When he activates only the upper element, he can use the tower as a top-hat loaded vertical for 160 meters.</p>
|
||||
<center>
|
||||
<img src="88-16.jpg" border="2" vspace="5" hspace="9" width="480" height="640">
|
||||
</center>
|
||||
<p>If we combine the higher-Z phase-line with the longer element, we obtain the probable feedpoint impedances shown in <b>Table 10</b>. Note that the table does not account for the fatter elements used in Chuck's antenna.</p>
|
||||
<pre>
|
||||
Table 10. Feedpoint Impedanceof an Expanded Lazy-H of 92'
|
||||
#12 Copper Wire Doublets at 100'
|
||||
600-Ohm Line
|
||||
Freq. Feedpoint Z
|
||||
MHz R +/- jX Ohms
|
||||
3.6 15 - j 75
|
||||
3.9 25 - j 30
|
||||
5.37 125 + j 265
|
||||
7.0 865 - j 595
|
||||
10.1 40 - j 75
|
||||
14.0 130 + j 670
|
||||
</pre>
|
||||
<p>We can extend both techniques--raising the phase-line impedance and elxtending the element length--and effect some further small improvements. We might try for 100' elements and check phase-lines of 600 Ohms and 800 Ohms for this type of lazy-H. <b>Table 11</b> provides the results of this experiment.</p>
|
||||
<pre>
|
||||
Table 11. Feedpoint Impedanceof an Expanded Lazy-H of 100'
|
||||
#12 Copper Wire Doublets at 100'
|
||||
600-Ohm Line 800-Ohm Line
|
||||
Freq. Feedpoint Z Feedpoint Z
|
||||
MHz R +/- jX Ohms R +/- jX Ohms
|
||||
3.6 25 - j 20 30 + j 26
|
||||
3.9 35 + j 35 40 + j 92
|
||||
5.37 320 + j 435 295 + j 585
|
||||
7.0 240 - j 450 555 - j 745
|
||||
10.1 30 + j 30 55 + j 45
|
||||
14.0 320 - j 810 225 - j 895
|
||||
</pre>
|
||||
<p>The effects are small, but may make the difference between whether a tuner can handle the resulting impedance at the shack end of the line. Of course, for tower-mounted elements, the user can mount a remote weatherproofed tuner at the feedpoint and eliminate all line losses except a. the small losses in the phase lines and b. the matched-line losses of coax running from the feedpoint to the equipment in the shack.</p>
|
||||
<p>Extending the length of the element has a drawback on 20 meters. Remember that the premise of the 88' back-up antenna was to have the main lobes of the bi-directional pattern broadside to the element. <b>Fig. 17</b> shows that about 100' is the absolute limit of an element length that will cover the 20-meter band in this fashion. Even so, the sidelobes that we see at the 88' length grow until they are about equal in strength to the broaside lobes. As well, the broadside lobes suffer further reduction in their beamwidth.</p>
|
||||
<center>
|
||||
<img src="88-17.gif" width="600" height="250" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>These alternatives to the 88' lazy-H for 80-10 meters with a standard 450-Ohm line prove the old saying that there may be no such thing as a perfect antena--or at least a perfect simple antenna. As we squeeze out a slightly more convenient feedpoint impedance at 80 meters, we begin to see a decay of the desired 20-meter performance.</p>
|
||||
<h3>Conclusion</h3>
|
||||
<p>The horizontal wire back-up antennas and arrays I have described depend on height for good performance on all bands. If you have the structures in place--or if you are thinking about how to place towers for a major antenna farm for the lower bands, you might seriously consider setting them up so as to support one or more of the suggested antennas as the system back-up. They offer to take up the slack in operations when the main systems are down.</p>
|
||||
<p>However, if you only have the trees or poles with no present antennas, you may wish to give one of these arrays another kind of serious consideration. Although I have called the 88' doublet (and its variations) a good back-up antenna relative to larger systems, there is no reason that it cannot form the basis of a very good main system on its own, at least from 60 meters on up through 20. It is at least worthy of thoughtful investigation during the planning stages of a low-band antenna farm.</p>
|
||||
<p>The key to the system is the property of the 88' doublet to have true bi-directional patterns on all of the bands from 80 through 20 meters. (We can do--and I have elsewhere done--a similar exercise with a 44' doublet to cover 40 through 10 meters.) Pattern control is a key element in serious operation, and the 88' doublet offers flexibility and reliability if we are willing to dust off that old link-coupled tuner and invest in some high quality parallel transmission line. Whether you keep it simple with an independent doublet, get bold with a Y-array or triangle, or go totally wild with the expanded lazy-H--alone or in an array--the performance is likely to be surprisingly good. However, do not develop high expectations from 80 and 75 from the short wire element and possible line losses. However, as a back-up antenna or array, the 88' length may prove serviceable as a single compromise length with determinate pattern directions.</p>
|
||||
<p><i>Updated 04-14-2000, 11-01-2004, 09-08-2005. © L. B. Cebik, W4RNL. This item first appeared in volume 2 of the</i> Top Band Anthology<i>, pp. 19-33. 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 Amateur Radio Page</a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#doublet'>Doublet</a>, <a href='../topics.html#hf-multi-band'>HF Multi Band</a>, <a href='../topics.html#lazy-h'>Lazy H</a></p><br>
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<title>Suppose I Could Have Only One Wire Antenna. . .</title>
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<h1>Suppose I Could Have Only One Wire Antenna. . .</h1>
|
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</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
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<br>
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<p>. . .What would I choose? Let me be a bit more specific. Suppose I wanted to work 40 through 10 meters. And further suppose that I want to know where my signal is going. Now what would I choose.</p>
|
||||
<p><b>The 40-Meter Dipole Starting Point</b></p>
|
||||
<p>The most common answer to the problem I just posed is a 40-meter #12/#14 copper wire dipole, fed with parallel transmission line for use as a multi-band doublet. <b>Fig. 1</b> tells the simple construction tale.</p>
|
||||
<center>
|
||||
<img src="aledz-1.gif" width="389" height="171" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Placed high enough in the air, the 67' doublet performs very well. Its bi-directional pattern on 40 has good side QRM rejection, and with enough altitude, the elevation angle makes DX a real potential with every band opening. Let's place the antenna at a height of 66' (about 20 m) up and see what the models tell us that we can expect by way of performance.</p>
|
||||
<pre>Freq. Max. Gain TO VBW HBW Feedpoint Z
|
||||
MHz dBi Deg Deg Deg R +/- jX Ohms
|
||||
7.15 7.3 28 35 86 70 - j 10
|
||||
10.1 8.1 20 23 70 275 + j 800
|
||||
14.15 9.0 15 16 51 4670 - j 345
|
||||
18.1 10.5 11 12 33 175 - j 860
|
||||
21.2 8.4 10 10 33 100 - j 115
|
||||
24.95 9.3 8 9 33 375 + j 730
|
||||
28.5 9.5 7 8 28 3265 + j 375</pre>
|
||||
<p>The TO angle is the elevation angle of maximum radiation. I have also provided information on the vertical and horizontal beamwidths (measured to the -3 dB points from the maximum strength bearing). We often neglect this information, but the data tell us some important facts. The vertical beam width is a rough measure of the range of elevation angles that we can count on for good communications. The horizontal beamwidth tells us how broad or narrow our signal is and hence how careful we must be in aiming the antenna--either when we build it or when we rotate it. (One of the amusing facets of reading lots of e-mail is discovering how many beam users demand 1-degree aiming accuracy when their beamwidths are well over 50 degrees.)</p>
|
||||
<p>The 67' doublet shows the anticipated lowering of the TO angle as we increase frequency. As we increase the frequency, the antenna is increasing in electrical height, that is, its height as a fraction of a wavelength. So we expect the beam angle to be lower on the upper bands. The beam widths--both vertical and horizontal--narrow with rising frequency. Still, the vertical beamwidth is wide enough on all bands to catch the main stream of long-range skip. And the horizontal beamwidth is sufficiently broad to make aiming non-critical (but not unimportant).</p>
|
||||
<p>The range of impedances at the antenna suggests that with a parallel transmission line and an ATU, we should be able to effect a match on all bands. 20 and 10 meters might present slight problems, but changing the line length will likely solve them by presenting the ATU with impedance values it can handle.</p>
|
||||
<p>However, the gain column presents us with a small problem. The maximum gain of the 67' doublet occurs on 17-meters, takes a large dip above that band, and then slowly rises once more. For a simple wire, the actual gain numbers are not the problem. The question we want to ask is this: why does the dip occur?</p>
|
||||
<center>
|
||||
<img src="aledz-2.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 2</b> can help us figure out what has occurred. The azimuth patterns overlaid on the plot are for 40, 15, and 10 meters. The 40-meter pattern is the typical oval. As we increase frequency, the oval grows narrower (decreasing horizontal beamwidth) while the gain increases--up to 17 meters. On this band, the 67' doublet is about 1.25 wl long: an extended double Zepp (EDZ). We expect about 3 dB gain over a dipole from an EDZ, and if we compare the 40-meter and 17-meter gain figures, we can see that we get it.</p>
|
||||
<p>Above 17 meters, the antenna is longer than 1.25 wl. At 15 meters, the antenna is about 1.5 wl long. The main lobe is no longer broadside to the antenna, but, as shown in <b>Fig. 2</b>, it is broken into 6 distinct lobes. The lobes broadside to the wire are no longer the strongest. As we move into the 10-meter region, where the antenna is 2 wl long, we have a pattern composed of 4 lobes at roughly 40-degree angles to the wire.</p>
|
||||
<p>For those unfamiliar with pattern development as an antenna becomes multiple wavelengths long, the following rules of thumb apply. For an antenna that is N wavelengths long, where N is an integer (like, 1, 2, 3), the number of lobes is twice the value of N. So a 2 wl antenna has 4 lobes, and a 1 wl antenna has only 2. For antenna lengths that are N.5 (like 1.5, 2.5, etc.), the number of lobes will be the sum of the number of lobes we get at N and at N+1. At 15 meters, where the antenna is 1.5 wl long, 1 wl gives us 2 lobes and 2 wl gives us 4 lobes, for a total of 6. The higher number at N.5 wl values arises because the new lobes are growing and the old ones shrinking--and they are nearly equal strength at the N.5 wl points.</p>
|
||||
<p><b>The 44' Wire Solution</b></p>
|
||||
<p>The problem with wire lengths over 1.25 wl is that we are no longer sure that we have a good signal broadside to our antenna. Suppose I put up a wire in Tennessee, running it NW to SE. That makes it broadside to Europe in one direction and to VK/ZL-land in the other. Not a bad set up. However, the main lobes for frequencies from 21 MHz up are no longer going where I want them to go. (Where they go may result in interesting contacts, but we set up our problem at the beginning so that it includes the need to keep our signals where we want them.)</p>
|
||||
<p>There is a simple solution to this problem, but it may not be the one some folks would expect. Conventional wisdom tells us always to make antennas bigger and longer. However, the solution to our problem is to make our doublet shorter. Let's try a doublet that is 44' long, as in <b>Fig. 3</b>. Again, #14 or #12 AWG copper wire will do just fine for the antenna.</p>
|
||||
<center>
|
||||
<img src="aledz-3.gif" width="389" height="171" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The first question we may now ask is whether we lose anything with the shorter wire. Let's find part of the answer in another table, modeled with the copper wire (#14 AWG) antenna 66' above average ground.</p>
|
||||
<pre>Freq. Max. Gain TO VBW HBW Feedpoint Z
|
||||
MHz dBi Deg Deg Deg R +/- jX Ohms
|
||||
7.15 7.0 29 35 94 25 - j 580
|
||||
10.1 7.6 20 23 83 55 - j 100
|
||||
14.15 7.7 15 16 72 195 + j 485
|
||||
18.1 8.6 12 12 60 920 + j1565
|
||||
21.2 9.0 10 10 51 4160 + j 155
|
||||
24.95 10.4 8 9 40 520 - j1545
|
||||
28.5 10.4 7 8 31 140 - j 650</pre>
|
||||
<p>The gain figures begin about a quarter dB below the figures for the 67' doublet on 40 meters and climb steadily. The elevation angles of maximum radiation and the vertical beamwidths are virtually identical to those for the longer doublet. The shorter antenna provides a broader horizontal beamwidth on every band, which makes aiming less critical. The pattern of impedances offered at the feedpoint differs in detail from that of the longer doublet, but the values are manageable. However, see the cautionary note below.</p>
|
||||
<p>To see one of the major advantages of our short doublet, we should look at <b>Fig. 4</b>.</p>
|
||||
<center>
|
||||
<img src="aledz-4.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The composite azimuth patterns for the doublet on all of the bands for which it is intended have their major lobes exactly broadside to the wire. The 44' length was no accident. On 10 meters, this length is about 1.25 wl long, the standard EDZ length. The 10-meter pattern shows the anticipated strong main lobes plus the emerging "ears," secondary lobes that will become the major lobes at higher frequencies. On 15 meters, the antenna is about 1 wl long, and on 30 meters it is just under the right length for a half wl dipole (which is indicated by the low resistive impedance and the capacitive reactance for 10.1 MHz). On 40 meters, the antenna is between 1/3 and 3/8 wl long, about the minimum length we should use. Anything shorter would show very low resistance values and very high reactance values--a difficult situation for any ATU to handle.</p>
|
||||
<p>If I wanted to aim at both Europe and at Australia and New Zealand on all bands from 40 through 10 meters, then the 44' doublet is the superior antenna to the longer 67' doublet. Of course, larger (102' or 135') all-band doublets break into fragmented lobe patterns at lower frequencies than the 40-meter dipole with which we started. So, if we make aiming one of the criteria for our antenna, the 44' doublet may be the way to go.</p>
|
||||
<p>Two side notes. First, if we want to cover the bands only up through 20 meters, but want to include 80 and 75 meters within the frequencies for which we are well-aimed, then an 88' doublet might meet our needs. Of course, there is nothing magic in the precise length numbers chosen, since a length change of a foot or two will change almost nothing in terms of performance. The bands with the highest reactances at the feedpoint might show the greatest change in value as we alter the antenna length, but the feedpoint values on the other bands would hardly change enough for an ATU to notice.</p>
|
||||
<p>For either the 88' or the 44' doublet, the lowest band of use (80/75 and 40, respectively) present challenges in line losses and matching at the shack end of the line. Hence, the short doublet--only 1/3 to 3/8 wavelength on the lowest band--should be considered as a back-up antenna on those bands.</p>
|
||||
<p>Second, height is a major consideration with this sort of antenna. Perhaps 66' is not feasible for everyone. However, every foot of (safe) height that you can add to the antenna, the better it will work. This principle goes back to the days of George Grammer of ARRL, who preferred to add height rather than elements to his antennas. The idea is no less true today, although there are some limits. Once we get above 1 wl, there may be some holes in our DX elevation-angle coverage at certain antenna heights. However, in Grammer's day, only on 10 meters and VHF did most hams think about heights above 1 wl or so.</p>
|
||||
<p><b>The Aluminum Alternative</b></p>
|
||||
<p>There is no good reason why a single element antenna must be constructed from thin wire--excepting cost and ease of construction. If there is only one support that is high enough, then we might well consider constructing an aluminum tubing version of the 44' doublet. <b>Fig. 5</b> shows one of many possible schemes for constructing the element. The wind survival rating for this scheme is about 70 mph.</p>
|
||||
<center>
|
||||
<img src="aledz-5.gif" width="458" height="286" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>In tabular form, the element structure looks like the following partial antenna model description.</p>
|
||||
<pre> --------------- WIRES ---------------
|
||||
|
||||
Wire Conn.--- End 1 (x,y,z : ft) Conn.--- End 2 (x,y,z : ft) Dia(in) Segs
|
||||
|
||||
1 -22.000, 0.000, 66.000 W2E1 -19.000, 0.000, 66.000 5.00E-01 3
|
||||
2 W1E2 -19.000, 0.000, 66.000 W3E1 -15.250, 0.000, 66.000 6.25E-01 4
|
||||
3 W2E2 -15.250, 0.000, 66.000 W4E1 -11.500, 0.000, 66.000 7.50E-01 4
|
||||
4 W3E2 -11.500, 0.000, 66.000 W5E1 -7.750, 0.000, 66.000 8.75E-01 4
|
||||
5 W4E2 -7.750, 0.000, 66.000 W6E1 -4.000, 0.000, 66.000 1.00E+00 4
|
||||
6 W5E2 -4.000, 0.000, 66.000 W7E1 4.000, 0.000, 66.000 1.25E+00 9
|
||||
7 W6E2 4.000, 0.000, 66.000 W8E1 7.750, 0.000, 66.000 1.00E+00 4
|
||||
8 W7E2 7.750, 0.000, 66.000 W9E1 11.500, 0.000, 66.000 8.75E-01 4
|
||||
9 W8E2 11.500, 0.000, 66.000 W10E1 15.250, 0.000, 66.000 7.50E-01 4
|
||||
10 W9E2 15.250, 0.000, 66.000 W11E1 19.000, 0.000, 66.000 6.25E-01 4
|
||||
11 W10E2 19.000, 0.000, 66.000 22.000, 0.000, 66.000 5.00E-01 3</pre>
|
||||
<p>The construction shown in the figure should be designated light to medium for the element length involved. It is based upon purchasing 8' lengths of aluminum and cutting them in half. Prices for 6061-T6 aluminum tubing from sources such as Texas Towers and others are quite reasonable. The tapering scheme allows for about 3" of tubing overlap at every junction. Any less overlap would jeopardize physical strength, while too much more overlap will unnecessarily increase the element weight. The double section of 1.25" and 1.125" diameter tubing at the center provides reinforcement for most mounting schemes.</p>
|
||||
<p>Some builders prefer to alternate longer and shorter sections of tubing for greater overall strength. Whatever scheme one uses, analyzing it through a program such as YagiStress by Kurt Andress can go a long way toward ensuring a mechanically sound antenna. Nonetheless, the 44' doublet is only about 9' longer than most 20-meter Yagi reflectors and a good bit shorter than the elements used for 30-meter and 40-meter Yagis. Hence, the use of an aluminum tubing element is certainly feasible.</p>
|
||||
<p><b>Special Note</b>: Some time after I placed this note at the site, Carroll Allen, AA2NN, pointed out the the taper schedule suggested would have a wind survival rating of only about 70 mph. He developed a spread sheet for EXCEL to calculate the stress on the tubing. For commonly used antenna tubing, such as 6061-T6, with a wall thickness of 0.058", the maximum stress for each section should be 40,000 psi or less. He kindly redesigned the sections for a 100 mph wind survival rating. The following table presents the revised taper schedule. Like the original schedule, the 1.125" diameter section is presumed to run all the way through the 1.25" section, but also to have its own exposure length.</p>
|
||||
<pre> 44' Aluminum Doublet Half-Element Structure
|
||||
for 100 MPH Wind Survival
|
||||
Diameter (") Section L (") Cumulative L (")
|
||||
1.25 72 72
|
||||
1.125 19 91
|
||||
1.0 20.5 111.5
|
||||
0.875 21.5 133
|
||||
0.75 23 156
|
||||
0.625 24 180
|
||||
0.5 84 264</pre>
|
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<p>The use of aluminum tubing provides a small but determinate increase in electrical performance for the doublet. As we increase the diameter of an element, the RF resistance decreases considerably. Although copper wire is certainly efficient enough for most purposes, the tubing version of the antenna shows an increase in efficiency, despite the fact that the tubing version user aluminum, which has a higher resistivity than copper.</p>
|
||||
<pre>Frequency #14 Copper Wire Stepped-Dia. Aluminum
|
||||
MHz Efficiency (%) Efficiency (%)
|
||||
7.15 96.98 99.70
|
||||
28.50 98.35 99.77</pre>
|
||||
<p>Notice that the differential in efficiency grows less as the frequency increases, that is, as the wire diameter becomes a greater fraction of a wavelength. Nonetheless, the tubing version shows systematically higher gain values for each band than the wire version of the 44' doublet. Compare the following table with the copper wire table given earlier.</p>
|
||||
<pre>Freq. Max. Gain TO VBW HBW Feedpoint Z
|
||||
MHz dBi Deg Deg Deg R +/- jX Ohms
|
||||
7.15 7.2 29 35 94 20 - j 410
|
||||
10.1 7.7 20 23 83 50 - j 85
|
||||
14.15 7.8 15 16 72 195 + j 295
|
||||
18.1 8.7 12 12 60 1005 + j 845
|
||||
21.2 9.1 10 10 51 1700 - j 705
|
||||
24.95 10.5 8 9 40 285 - j 795
|
||||
28.5 10.5 7 8 31 100 - j 375</pre>
|
||||
<p>The gain differences are certainly not large enough to make any kind of operational difference in using the 44' doublet. At most, they help us better understand some of the variables involved in antenna structures.</p>
|
||||
<p>Notice also that the feedpoint impedance figures vary from the wire values more radically as the frequency increases--and also where the resistance or reactance values are high to begin with. The values shown--which will vary considerably as one changes the precise length of the finished antenna--are nonetheless quite manageable by most ATUs.</p>
|
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<p>Supporting an aluminum doublet of the size we are suggesting is a considerable project. <b>Fig. 6</b> shows the main aspects of the things we should consider.</p>
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<img src="aledz-6.gif" width="645" height="453" border="2" vspace="5" hspace="9">
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</center>
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<p>1. We shall need a support tower or pole as tall as we can safely make it. Although I know some inveterate climbers much older than I am who regularly scale high towers, my own experience tends to decrease my tower height by one section for every decade older I get. Safety comes first; antenna height second.</p>
|
||||
<p>2. As I doodled the sketch in <b>Fig. 6</b>, I added a rotator for the fun of the prospect. We only need about 180 degrees rotation for this doublet, so a side mounting on an existing tower would likely be good enough. For sample photos of one version of this antenna constructed from tubing and able to rotate with a CD44 rotator, see the web site of <a href="http://www.n4ekv.com/antennas.asp" target="_blank">Adam, N4EKV</a>. (Today's cheap TV rotators are no longer able to handle this antenna, but 25 years ago. . .)</p>
|
||||
<p>3. For elements 40' and longer, consider adding a top-mounted truss to help support the element. A truss system will require a longer mast than truss-less elements. The exact position on the element to place the ends of the truss depends a great deal on the precise element diameter schedule chosen. Use a weather and UV resistant material for the truss rope.</p>
|
||||
<p>4. Since the antenna will use parallel transmission line, stand-off insulators will be necessary. If the antenna is to rotate, the position of the line from the feedpoint to the stationary supports on the pole will require considerable planning. The line should avoid close proximity to any metal in the mounting region.</p>
|
||||
<p>5. The parallel transmission line is low loss inherently when the runs follow standard handbook recommendations. However, for lowest losses, consider true open-wire line rather than vinyl-coated lines. Even lines with "windowed" openings in the vinyl between the wires tend to show higher losses when wet than when dry. Line losses will generally be low except for the lowest band in the range shown.</p>
|
||||
<p>6. When the antenna feedpoint impedance is disproportionately reactive, you can expect difficulties effecting a match with an antenna tuner. On the lowest band, the 3/8 wavelength wire has an impedance where the reactance is 20 times the resistance. Under these conditions, the ability of a tuner to effect a match between the impedance at its terminals and the standard 50-Ohm coax to the rig will depend both on the line and the tuner configuration. Line concerns involve an interaction between the line length and the characteristic impedance. As well, even high-efficiency parallel lines will add losses to the already lower performance on the lowest band in the usable antenna range. To avoid tuner damage, tune up using low power and then raise power to the operating level.</p>
|
||||
<p>Like all antennas, preventive maintenance at least once per year (every 6 months is better) will go a long way toward preventing unexpected catastrophic failures.</p>
|
||||
<p>However, you decide to construct a single-element antenna, the 44' doublet has some interesting properties that provide advantages over other types of multi-band doublets. It is an antenna worth considering--if you can have only one wire.</p>
|
||||
<p>The use of the 44' length--or more properly, the approximate length of a 1.25 wavelength wire at 10 meters--is not new. It dates back to at least the middle of the 20th century. For example, Gene Fuller, W2LU, in the July, 1966, issue of <i>CQ</i> wrote an article on using 42' wires in a set of phased arrays ("Beam Antennas for the H.F. Range," page 12).</p>
|
||||
<p>The 44'-wire exercise is designed mainly to encourage antenna builders to think "outside the box" of antennas that are naturally resonant at some operating frequency or at the lowest operating frequency. Resonance is not a requirement of good performance. Hence, we are free to set some one or more other goals for our antenna. In this case, we have set the goal of having radiation patterns that are broadside to the wire. For 40-10 meters, the 44' wire fulfills this goal, however, close to or far from resonance its length may be on any one of the bands covered.</p>
|
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<p><i>Updated 03-01-2000, 07-01-2002, 03-26-2003, 07-30-2003. © L. B. Cebik, W4RNL. This item appeared in</i> AntenneX<i>, February, 2000. 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="../radio.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> <b>Go to Main Index</b></a></p>
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<p class="topics-footer">Topics: <a href='../topics.html#doublet'>Doublet</a>, <a href='../topics.html#hf-multi-band'>HF Multi Band</a></p><br>
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<title>Some Notes on EDZ Beams</title>
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<h1><img src="../images/wirecad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="25"> Some Notes on EDZ Beams <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="25"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Some time back, I wrote a piece for <i>Communications Quarterly</i> on the Extended Double Zepp ("Modeling and Understanding Small Beams: <a href="../mu/mu3a.html">Part 3: The EDZ Family of Antennas</a>," Fall, 1995, 53-71). My hope was to improve our understanding of the EDZ and look at some of its possibilities.</p>
|
||||
<p>The EDZ in its simplest form is a non-resonant wire antenna about 5/4 wavelengths long. Being non-resonant, exact length is non-critical. Shorter versions have smaller side lobes but higher capacitive reactance; longer versions the reverse. Feedpoint impedance ranges from 100 to 150 ohms resistive with well over 600 ohms capacitive reactance. The chief reason for using the EDZ is its 1.5+ dB gain over a dipole comparably situated.</p>
|
||||
<p>Brian Egan, ZL1LE and I had been discussing EDZ potentials since about 1991. He initially suggested a 2-element beam consisting of an EDZ driven element plus two Yagi-type reflector elements spaced a few feet behind the driven element and each pushed sideways to the wire end limits of the driven element. Modeling this configuration seemed to make a different arrangement preferable. From this arose the 2-element (driven element- reflector) beam noted in the article. The center of each element is inductively loaded, one for matching the feedline, the other for optimizing the rear element as a reflector. With 1/2 wavelength parallel lines down to near ground level, the two matching/loading units could be reversed, reversing the direction of the beam. This installation was tested for a couple of years at W4RNL and worked quite credibly.</p>
|
||||
<p>Well, folks, as the old song says, "Everything old is new again." Bill McDowell, K4CIA, sent me a copy of an article from the June, 1938, <i>QST</i>, "The Extended Double-Zepp Antenna." In the back pages is a description of how to add a parasitical element to the EDZ. Author Hugo Romander, W2NB, describes a 0.2 wavelength spaced array. The driven element is stub matched to the source feedline. The other element is stub loaded inductively, but at two points: one for use as a reflector, the other for use as a director. Hence, a different system for a reversible beam--and a perfectly competent one. W2NB's system has the advantage of simplicity, while ours has the advantage of convenience. It can be fun to discover that one has reinvented the wheel. Fortunately, the information I added to the end of my article reviewing the principles of stub matching and loading would aid one to replicate the W2NB EDZ beam, so I do not feel totally disconnected from the 1938 work.</p>
|
||||
<p>From time-to-time, folks discuss the possibilities for a 3-element EDZ beam. Henry Pollock, WB4HFL, is actually planning to build one. The idea led me to try to verify his modeling results and to compare his configuration to an alternative.</p>
|
||||
<p>As with the 2-element EDZ, one has two main choices of configuration: 3 long elements or a long driven element with pairs of Yagi elements (directors and reflectors) at the extreme limits of the EDZ driving element.</p>
|
||||
<center>
|
||||
<img src="edz3.gif" width="444" height="339" border="2" vspace="5" hspace="10">
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</center>
|
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<p>Henry has chosen the double-Yagi version, and very likely wisely so. Since he plans to put it at 60' for 10 meters, let's look at modeling results for both arrangements centered at 28.5 MHz.</p>
|
||||
<p>The 3-wire EDZ beam can be built from 3 41'8" lengths of #12 wire, each with a center load. The director requires about 800 ohms, the reflector 1150 ohms, and the driven element 980 ohms. The resulting antenna has a resonant feed resistance of about 90-95 ohms, just about right for a 1/4 wl matching section of 70 ohm coax to the regular 50-ohm feedline. You can make the director and reflector inductive loads from coils or from 450-ohm parallel vinyl covered feedline stubs 5.5' and 6.25' long each. If you do not use a split coil for the driven element feed point, you may wish to design a stub matching system--or perhaps use an ATU and parallel line all the way.</p>
|
||||
<p>The potential beam performance at 60' is quite good: 15.5 dBi gain with about 30 dB front-to-back ratio. Adding the director to our old 2-element EDZ beam really improves the front-to-back ratio more than it helps raw gain.</p>
|
||||
<p>But here is the rub:</p>
|
||||
<center>
|
||||
<img src="edz1.gif" width="640" height="480" border="2" vspace="5" hspace="10">
|
||||
</center>
|
||||
<p>The 3-wire EDZ beam is quite narrow-band in pattern--and even more so in feedpoint impedance and loading. At 28 and 29 MHz, the front-to-back ratio drops to the 7-9 dB range. The feedpoint shows a 250-ohm reactance change across the band. This is a beam that needs adjustment of all 3 elements if one hopes to cover more than 100 kHz of 10 meters. (For lower band versions, narrow the bandwidth in proportion of the ratio of the desired lower frequency to 28.5 MHz.)</p>
|
||||
<p>The double Yagi EDZ tells a somewhat different story. WB4HFL did not give me detailed dimensions, so I modeled my own version, with directors and reflectors spaced 5' from the driven element. Parasitical elements had outer limits in line with the end of the EDZ element (41'8"). Reflectors were 17'1" and directors were 16'0.5" long. #12 wire, of course, for consistency throughout.</p>
|
||||
<p>The results at the design center frequency were interesting: 16.0 dBi gain with a front-to-back ratio of 38 dB at 60' height. The front-to-rear quadrants had a minimum ratio of about 21 dB. What the 3-wire gives us in a slightly better front-to-rear ratio at design center is offset by the added gain of the double-Yagi version. Feedpoint impedance of the double Yagi version is about 90 - j1000 ohms, calling for a stub match or an ATU feed system.</p>
|
||||
<p>However, the WB4HFL-style design has two hidden advantages, partially revealed by a frequency sweep:</p>
|
||||
<center>
|
||||
<img src="edz2.gif" width="640" height="480" border="2" vspace="5" hspace="10">
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||||
</center>
|
||||
<p>First, the double-Yagi version retains a better figure from 28 to 29 MHz, never sinking below 11 dB front-to-back ratio at those extremes. While not superlative, the ratio climbs to nearly 20 dB at the 28.25 and 28.75 marks. These numbers are far superior to the 3-wire beam.</p>
|
||||
<p>Second, the only adjustment needed across the band is the driven element tuning. Like its counterpart, the double-Yagi version shows a 250-ohm reactance excursion across the band, along with a 75-ohm change in the resistive component. However, parallel feedline and a good ATU would take care of the problem. Because of the high reactance-to-resistance ratio, one might have to carefully select the line length in order to present the ATU with a load it can handle most efficiently. Nonetheless, no other adjustments are necessary, a plus for the double-Yagi design.</p>
|
||||
<p>Now why does the double-Yagi version work?</p>
|
||||
<center>
|
||||
<img src="edz4.gif" width="438" height="187" border="2" vspace="5" hspace="10">
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</center>
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<p>If you look at the current distribution along the EDZ, it consists of a small central peak with outward dips. The outer 1/2 wavelength of the EDZ wire on either side is a perfect dipole current distribution pattern. Parasitical elements aligned with these peaks perform just as they would with independent in-phase separate driven elements. The 3-element double Yagi EDZ beam is actually a form of two in-phase-fed side-by-side 3-element Yagis with ¼ wavelength tip-to-tip spacing. And the performance is just about the same.</p>
|
||||
<p>One question that often arises with EDZ beams is how do we get rid of the ears in the pattern, those quartering side lobes. W1GQL, David Billheimer, sent me a design that accomplishes just that--an ear-less EDZ beam. Figure W1GQL-1 shows the azimuth pattern of this 20-meter 2-element wire beam.</p>
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<center>
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||||
<img src="edz5.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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</center>
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<p>Dave's technique is to create what looks to me like a "gull-wing" design: the driven element is drawn in to midway between the 2 elements, while the parallel sections are drooped like a Vee. See Figure W1GQL-2.</p>
|
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<center>
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<img src="edz6.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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</center>
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<p>Dave's 20-meter CW beam has a 50' maximum height. The elements are about 5' apart. The two reflectors begin about 14.5' apart and droop to the 33.6' level at a maximum width of 39.4' each side of center. The driven element legs (beginning at center) move outward and forward to a little over 8' each side of center and then parallel the reflectors. (I have rounded the dimension numbers.)</p>
|
||||
<p>The natural feedpoint of this ear-less EDZ beam is about 60 ohms resistive and -j1000 ohms reactive. Dave uses stub-tuning to match 50-ohm coax for his narrow-band CW needs. However, the pattern of the beam holds up across 20-meters and can be fed with parallel feedline and an ATU quite effectively.</p>
|
||||
<p>I have added Dave's beam to demonstrate that we have not yet exhausted all the possibilities with either EDZs or wire beams. Among these notes and two articles, we have look at EDZs, stacked EDZs, parasitical EDZs, bent EDZs, and phased EDZs. I must be overlooking something. . .</p>
|
||||
<p>Of course: co-linear EDZs.George Goldstone, W8AP, sent me some correspondence he had with Hugo Romander, W2NB (W6CH in the 1960s), and Henry Pollock, WB4HFL, sent me a copy of a <i>Ham Radio</i> article by Alvan Mitchell, W6QVI. The article was an update on Hugo's co-linear EDZ array, which the <i>ARRL Antenna Book</i> of 1943 still carried. Since few folks have access to either Hugo's 1938 article or the 1943 <i>Antenna Book</i>, let's take a look at this version of the EDZ, outlined in the figure below.</p>
|
||||
<center>
|
||||
<img src="edzarray.gif" width="606" height="312" border="2" vspace="5" hspace="9">
|
||||
</center>
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||||
<p>The antenna is given in terms of electrical degrees. The phasing lines are shorted parallel-line stubs. Here are Hugo's 20 meter dimensions and Alvin's 15 meter dimensions as samples: dimensions are in feet.</p>
|
||||
<pre>
|
||||
Band A B C
|
||||
14 43 53.5 11.5
|
||||
21 27.5 35.5 7.7
|
||||
</pre>
|
||||
<p>The 15 meter version, which I modeled extensively, is 126' long, about the length of a 75-meter dipole. So what do you get for all that linear space on 15 meters.</p>
|
||||
<p>You get almost precisely what Hugo predicted in 1938: about 7 dB gain over a dipole similarly place in a bi-directional pattern that is very narrow: 16°-17° between -3 dB points to be as exact as my model will permit. Modeled gain is about 14.3 dBi in a pattern like this one:</p>
|
||||
<center>
|
||||
<img src="edzarr2.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
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||||
<p>The best points for installing the parallel line stubs in the 15-meter model were actually 1.5' farther inboard than W6QVI suggests: about 29' from the ends and 34' from the antenna center. 67° proved the length required for maximum gain. The feedpoint impedance is about 170 - j740 ohms, requiring an ATU, stub matching, or something similar.</p>
|
||||
<p>As W2NB pointed out, careful aiming is required. (The EDZ beams earlier noted had beamwidths between 32 and 38 degrees, while a standard Yagi or quad has a beamwidth between 50 and 60 degrees, depending on the number of elements.) This is no antenna for casual worldwide DXing. Rather, it is a serious point-to-point antenna. Within that context, it is an antenna that proves that narrow beamwidths are not impossible at HF. I have expensive flashlights with wider beamwidths.</p>
|
||||
<p>If you want a bi-directional antenna with a gain of about 18 dBi each way, while retaining the narrow (16-17°) beamwidth, try stacking two of these antennas at 5/8 wl separation. For the 15-meter sample, that places the two at 50' and 79' respectively. The feedpoint impedance, when fed in phase with 450-ohm line at the midpoint between the two, is about 65 + j350 ohms. A pair of series capacitors would cancel the reactance, providing a direct connection to 70-75-ohm cable (through a choke balun).</p>
|
||||
<p>Let's carry the experiment one step further. To each of the vertically stacked EDZ arrays, one might arrange a series of 1/2 wl reflectors to achieve some further forward gain and reduce the rear lobe. Alternatively, one might place a second vertically stacked array 1/8 wl behind the original vertical stack. Then, feed the rear array with a current magnitude and phase to maximize forward gain and front-to-back ratio. At 21.2 MHz, the spacing would be just about 5.8'. With proper feeding of the rear elements, we might achieve an azimuth pattern like this:</p>
|
||||
<center>
|
||||
<img src="4edz.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>21 dBi forward gain, over 26 dB front-to-back ratio, and a 16-17° beamwidth are figures that amateur radio operators rarely see from antenna arrays whose bases are about 1 wl up and whose tops are 1.7 wl up. Of course, for most operations, such figures are contrary to what is need for effective ham operation, but they might be useful for some specialized operations.</p>
|
||||
<p>Antennas spaced 1/8 wl apart call for 135° phase differences in the elements on the currents: this is the received wisdom. Unfortunately, it is wrong. Dipoles with this spacing might call for something close to this figure. However, for any two elements spaced more than a small fraction of a wavelength but less than 1/4 wl, there will be for each spacing a relative current magnitude and phase for the rear element that will yield maximum gain and maximum front-to-back ratio. With real materials, these two maxima may occur on very slightly different frequencies. For the array shown here, the maxima occurred with the rear elements fed at 0.75 the forward element current at a phase angle of 142 degrees. Slightly better performance might have been obtained if the upper and lower phased pairs had been individually optimized.</p>
|
||||
<p>It is unlikely that standard ZL Special techniques would achieve perfect phasing. Instead, one should most likely use phasing networks to establish the operating conditions. Since the arrays are identical, one could use this method to flip the direction of the beam electronically. "But in the end, this is all hypothetical, isn't it? No one could or would build such an array." Given that all the techniques needed are standard in the field, I am not so sure of this. Some folks might engineer such an array just to say that they have an array with 21 dBi forward gain and to listen to the long-path echoes of their own signals. Hams have done far stranger things in the history of the service. I wonder what might be heard if one of these reversible arrays were aimed directly toward one of the poles.</p>
|
||||
<p>We are still far from done with the EDZ. I am looking forward to the next step. These notes are simply the update so far. I'll add more as soon as I learn more about this interesting antenna. If I read enough old articles, I could learn more very shortly.</p>
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<p><i>Updated 5-10-97 © 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></p>
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<h1><img src="../images/wirecad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="25"> Feeding the EDZ <img src="../images/w4rnlwwr.gif" align="middle" width="99" height="83" border="2" vspace="5" hspace="25"></h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Typically, Extended Double Zepp (EDZ) users employ one of two methods in feeding this higly capacitively reactive 1.25 wl long antenna. Some users, especially those who employ the antenna as a simple center-fed long wire on bands other than the design band, simply use parallel feedline and an ATU. Others, with single-band use in mind, use a matching stub arrangement to find a 50-ohm point for a coax feedline. Of course, one can also place a split coil at the feedpoint to provide the inductive reactance necessary to cancel out the antenna's inherent capacitive reactance, although the resulting resistive impedance will still be 100 ohms or greater.</p>
|
||||
<p>In the Summer, 1997, issue of <i>Communications Quarterly</i>, Rick Littlefield, K1BQT, presents a 2-meter EDZ that bears close examination. Besides an interesting construction method, designed to make a very durable EDZ for vertical use in hearing 2-meter repeaters, the key unique feature of Rick's design is the match and feed system that eliminates the usual center inductor to cancel out the heavy capacitive reactance at the feedpoint.</p>
|
||||
<p>An EDZ at almost any frequency has a variable feedpoint impedance and capacitive reactance, depending on the exact length to which it is cut. However, without altering performance by more than a tenth or so of dB gain, one can cut the antenna for a feedpoint impedance in the 100-140 ohm resistive range, which gives a capacitive reactance in the 500-600 ohm range.</p>
|
||||
<p>Rick applied a technique used with 5/8 wl vertical gp antennas: instead of an inductor, he uses a length of coax about 10.875" long, with a rod element beyond that point. Let's think of a 5/8 wl vertical and then simply place 2 of them feedpoint to feedpoint to get the final EDZ. The feed goes to the center conductor of the coax length. At the feed end, the braid is not connected to anything. At the far end, the center conductor and the braid are connected together and this junction goes to the 38" rod element. Rick calls this a delay line.</p>
|
||||
<p>When two of these assemblies are put end to end, the coax center conductors are the two terminals making up the feedpoint. The braid ends are about an inch or two apart (and must NOT be connected together). The result is a nearly purely resistive feedpoint impedance of 100 ohms in the 2-meter model. Rick uses a 75-ohm 1/4 wl (+ 1/2 wl added) to make a combination matching section and balun for a 50-ohm coax feedline.</p>
|
||||
<p>The delay line is interesting, because the name does not describe its function. Actually, it is a simple shorted feedline stub providing the inductive reactance necessary to cancel the antenna's capacitive reactance. Let's look at the figure to see the evolution of the arrangement.</p>
|
||||
<center>
|
||||
<img src="edzfeed1.gif" width="514" height="350" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Part A shows the conventional split coil arrangement so familiar to EDZ builders (who do not use either open feedline to an ATU or a stub tuning method for a 50-ohm match). Part B replaces the inductors with a pair of shorted feedline stubs calculated to provide the same inductive reactance as the coil sections. Note that I have designated the outer part of the antenna line as "a" and the inner part as "b" in the sketch. When I substituted the parallel feedline, I designated on side of it as "c." "C" is as long as "b".</p>
|
||||
<p>There is no rule that says you cannot sometimes make a wire do double duty. There is no incompatibility between wire "c" and wire length "b" so that we can combine them together as in Part C of the sketch. And Part C is essentially the arrangement used in Rick's coax "delay" line. Because the coax is now doing double duty, the exact length may change from simple calculations for stubs, but it is very close.</p>
|
||||
<p>Although the final arrangement looks like a Tee match, it is not. The center must have a gap, forming at best a split-T top. The open ends of the non-feedpoint center are actually the ends of the inductive stubs away from the feedpoint and thus must be kept independent of each other by a gap.</p>
|
||||
<p>The use of coax was possible in the original model, because the resistive portion of the feedpoint impedance was near 50 ohms on each side of center (for the 100-ohm total). Hence, the use of 50-ohm coax did not alter the feedpoint impedance.</p>
|
||||
<p>If we use feedlines of higher impedance, two things will happen. First, the feedpoint impedance will be reduced. Second, the length of the stub will increase, placing the junction of "c-b" and "a" farther outward on each side of center. The next figure is a sketch of a model I developed while exploring this subject.</p>
|
||||
<center>
|
||||
<img src="edzfeed2.gif" width="516" height="213" border="2" vspace="5" hspace="9">
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||||
</center>
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||||
<p>To create as reliable a NEC model as possible, I used 0.125" diameter aluminum for all parts. This kept the diameter constant, thus allowing a greater reliability of the result with NEC-4. I modeled the parallel lines 1" apart, give about a 330-ohm characteristic impedance for the resultant line. With a connection point about 14.5" outward from center and a 2" gap between the open ends, the stub/line or split-T match provides about 54 ohms resistive impedance and no reactance at the feedpoint. Hence, direct 50-ohm coax feed is now possible.</p>
|
||||
<p>The revised feed provides both capacitive reactance cancellation and resistive impedance transformation together. The length of the section is about the same as the combination of a 330-ohm shorted stub plus a length of the same line necessary to transform the overall impedance to nearly 50 ohms on its own, although treating this way of looking at the sections as a correct analysis of actions and interactions involved is far from certain at this point.</p>
|
||||
<p>However, the model's parallel feedline section is close enough to 300 ohms to suggest that experiments may be useful with twinlead, taking into account the line's velocity factor of 0.8, of course. The technique may also be applicable using 450-ohm window line or 600 ohm ladder line, experimentally finding the correct length to use. And the technique is likely applicable at all HF and VHF frequencies at which EDZs are in use.</p>
|
||||
<p>One advantage of the technique is that it is fairly broad-banded, giving full 2-meter coverage either in Rick's coax version or in the modified parallel line version shown here. I have not explored the consequences of this feed for use of the EDZ as a simple long wire on other HF bands--yet.</p>
|
||||
<p>Henry Pollock, WB4HFL, took up the challenge of creating a #14 copper wire HF version of the impedance transformation "delay" line EDZ. His initial version was 42' long with 450-ohm match lines either side of center, each 66.5" long. The antenna resonated at 28.9 MHz on this first try. Modeling the exercise suggested that lengthening the antenna a bit (to 44.7') and shortening the match sections to about 65.5" (adjusted after physical modeling for velocity factor) would likely bring the antenna closer to a 28.5 MHz target. The 2:1 SWR bandwidth of the model appears to be about 600 kHz, although the use of a coax feeder will likely widen the bandwidth operationally at the shack end of the line. These figures are not unlike the bandwidth numbers for stub-tuned versions of the EDZ.</p>
|
||||
<p>As a general rule, lengthening the antenna wire tends to reduce the resistive component of the feedpoint impedance, while lengthening and shortening the match section changes the reactance without affecting the resistance much. In general, it appears that the higher the characteristic impedance of the feedline used to effect the impedance transformation, the narrower the 2:1 SWR bandwidth of the antenna.</p>
|
||||
<p>So here is one more experimental way to feed an EDZ. The K1BQT and WB4HFL experimental antennas prove that the principle works, yielding an EDZ with no need for stubs or ATUs: matching is built into the antenna structure itself. Have fun creating some interesting prototype EDZs for 50-ohm feeds.</p>
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<p><i>Updated 8-27-97. © 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></p>
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<p><a href="../radio.html"><img src="../images/return.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#doublet'>Doublet</a>, <a href='../topics.html#edz-zepp'>EDZ / Zepp</a>, <a href='../topics.html#hf-multi-band'>HF Multi Band</a></p><br>
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<title>An Almost Universal HF Back-Up Antenna</title>
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<img src="../images/wirecad.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">
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<h1>An Almost Universal HF Back-Up Antenna<br>
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For the Antenna Farm That Has Everything Else:<br>
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Some Preliminary Notes</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>I have had occasion in the past to write on the W2EEY Expanded/Extended Lazy-H array that first appeared in the 1960s as a mono-band wire array. W6SAI later discovered that the antenna had available gain on frequencies in a range of at least 2:1. In other words, a 10-meter version of the antenna might be still useful on 20 meters.</p>
|
||||
<p>My own looks into this antenna suggest an even wider range of utility, even though performance tapers off steadily as one lowers frequency. The chief drawback of the antenna has so far been the fixation on wire construction. I wondered what tubular elements might do, and the result is this preliminary note.</p>
|
||||
<p>Large antenna farms for serious DX and contest use often have far more than one antenna per band. Hence, back-up on many bands is almost a matter of course. However, there is often only on antenna for each of the following bands: 40, 30, 17, and 12.</p>
|
||||
<p>Now suppose there were a single rotatable antenna of relatively easy maintenance (compared to a 5-element Yagi or similar) that might provide emergency performance on 40 through 10 meters, performance that was not stellar, but usable in a crunch. Further suppose that the antenna had a top height of not more than say 70' (about 1/2 wl on 40 meters) and has elements no longer than some of the half-size 40-meter beams that are commercially available. Finally, suppose that the antenna required no boom, but looked more like one of those 40-meter beams flopped over to point straight up.</p>
|
||||
<p>Such an antenna is not a mechanically simple installation, since 44' foot aluminum elements are not for the beginner. However, once installed, the antenna poses fewer stress problems than horizontally positioned arrays that require a boom. If it happened also be to bi-directional, then it might be laid close in to a pole or tower, since the total required rotation is 180 degrees.</p>
|
||||
<p>There might be a niche for such an antenna, so the performance potentials and the installation challenges seem worth exploring, at least on a preliminary basis.</p>
|
||||
<h3>The W2EEY Expanded/Extended Lazy-H</h3>The basic array is an extension of the Lazy-H: two 1 wl elements vertically separated by 1/2 wl and fed in phase. The W2EEY innovation was to extend the elements to 1.25 wl, extended double Zepp length. He also expanded the separation to 5/8 wl to maximize in-phase gain.
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<img src="lal-1.gif" width="504" height="356" border="2" vspace="5" hspace="9">
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<p><b>Fig. 1</b> shows the outlines of a typical extended Lazy-H in typical wire form. For many years, the Editors and Engineers <i>Radio Handbook</i> (edited by W6SAI) has carried that standard Lazy-H, fed at the bottom with a stub for coax matching for mono-band use. However, for multi-band use, a center junction of equal lengths of feedline is the simplest route to in-phase feeding on many bands.</p>
|
||||
<p>The dimensions of the array for 10 meters are a modest 44' element length and 22' vertical separation. The EDZ element lengths represent the practical limit for use on HF bands, including 10 meters. Longer elements will yield a multi-lobe pattern on 10 meters.</p>
|
||||
<p>The antenna height selected for study was a 66' top height. It can be mounted higher or lower with standard changes in the elevation angle of maximum radiation. However, a height of at least 66' seems advisable for reasonable 40-meter use.</p>
|
||||
<h3>From Wire to Tubing</h3>I have elsewhere looked at the performance of the W2EEY version of the antenna across many bands. The question that came to mind regarding rotating the array required a change in element material. So I redesigned the antenna for aluminum tubing having an aggressive tapering schedule.
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<center>
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<img src="lal-2.gif" width="528" height="362" border="2" vspace="5" hspace="9">
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</center>
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<p><b>Fig. 2</b> shows the arrangement used to develop models. There is no assurance that this particular schedule meets appropriate standards. Any actual elements that one might contemplate constructing should be taken through an exercise or two on YagiStress to determine their mechanical feasibility. Although the schedule used here has proven very useful in comparing wire versions of the antenna to the most severe gradations I could think of, the element design is hypothetical only in mechanical terms.</p>
|
||||
<p><b>Special Note</b>: Some time after I placed this note at the site, Carroll Allen, AA2NN, pointed out the the taper schedule suggested would have a wind survival rating of only about 70 mph. He developed a spread sheet for EXCEL to calculate the stress on the tubing. For commonly used antenna tubing, such as 6061-T6, with a wall thickness of 0.058", the maximum stress for each section should be 40,000 psi or less. He kindly redesigned the sections for a 100 mph wind survival rating. The following table presents the revised taper schedule. Like the original schedule, the 1.125" diameter section is presumed to run all the way through the 1.25" section, but also to have its own exposure length.</p>
|
||||
<pre> 44' Aluminum Doublet Half-Element Structure
|
||||
for 100 MPH Wind Survival
|
||||
Diameter (") Section L (") Cumulative L (")
|
||||
1.25 72 72
|
||||
1.125 19 91
|
||||
1.0 20.5 111.5
|
||||
0.875 21.5 133
|
||||
0.75 23 156
|
||||
0.625 24 180
|
||||
0.5 84 264</pre>
|
||||
<p>The chief differences between the wire and tubing versions of the antenna were two. First, the models had many more wires. Second, the source impedances (as taken at the junction of the two feedlines from the elements to a center small segment) varied somewhat from those associated with the wire version.</p>
|
||||
<p>For those unfamiliar with the extended Lazy-H, let's run a series of tabular entries and some patterns to check the potential performance. As with all models, these assume level, uncluttered terrain (average Sommerfeld-Norton ground) in NEC-4.1. Any serious antenna farmer has already run terrain analysis using N6BV or K6STI software and can therefore adjust the numbers for gain and elevation angles accordingly.</p>
|
||||
<p>The figures in the first table focus on performance. The frequencies are band centers. Gain is maximum at the elevation angle of maximum radiation (TO angle). note that gain is bi-directional. The horizontal beamwidth is to the -3 dB points on the maximum gain curve.</p>
|
||||
<pre>
|
||||
Frequency Gain TO Angle Horizontal Notes
|
||||
MHz dBi degrees Beamwidth (deg)
|
||||
28.5 15.2 8 37 EDZ-type side lobes
|
||||
24.94 14.7 10 41
|
||||
21.225 12.6 11 52 Standard Lazy-H
|
||||
18.118 11.0 13 61
|
||||
14.175 9.1 17 73
|
||||
10.125 8.2 24 85
|
||||
7.15 6.5 33 99
|
||||
7.15 7.2 29 89 Using top wire only</pre>
|
||||
<p>As the table shows, gain decreases steadily with frequency, since the elements grow shorter and the spacing narrower. The 10-meter gain is similar to a long-boom 5-element Yagi (without the front-to-back ratio). On 15, gain performance is similar to some 3-element Yagis, dropping to 2- element performance on 17. Below that frequency, the antenna becomes the equivalent of a rotatable dipole.</p>
|
||||
<p>The patterns to follow, band by band, show the elevation patterns along the axis of maximum gain. Only spot azimuth patterns (for 10, 15 and 30 meters) are shown, since the evolution of the azimuth pattern with frequency changes is perfectly normal. One key advantage for the array is the relative absence of very strong high angle radiation, especially on the upper bands, thus reducing a potential noise source.</p>
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||||
<center>
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||||
<img src="lal10.gif" width="640" height="765" border="2" vspace="5" hspace="9"><br>
|
||||
10-Meters
|
||||
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||||
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|
||||
<img src="lal12.gif" width="639" height="286" border="2" vspace="5" hspace="9"><br>
|
||||
12-Meters
|
||||
</center>
|
||||
<center>
|
||||
<img src="lal15.gif" width="640" height="765" border="2" vspace="5" hspace="9"><br>
|
||||
15-Meters
|
||||
</center>
|
||||
<center>
|
||||
<img src="lal17.gif" width="640" height="285" border="2" vspace="5" hspace="9"><br>
|
||||
17-Meters
|
||||
</center>
|
||||
<center>
|
||||
<img src="lal20.gif" width="640" height="285" border="2" vspace="5" hspace="9"><br>
|
||||
20-Meters
|
||||
</center>
|
||||
<center>
|
||||
<img src="lal30.gif" width="640" height="765" border="2" vspace="5" hspace="9"><br>
|
||||
30-Meters
|
||||
</center>
|
||||
<center>
|
||||
<img src="lal40.gif" width="640" height="285" border="2" vspace="5" hspace="9"><br>
|
||||
40-Meters
|
||||
</center>
|
||||
<p>The 40 meter graphic shows two patterns, one for the use of both wires in phase, the other for the use of only the top wire. The latter is harder to implement in practice, but increases gain while lowering the take-off angle. On 30 meters, the use of only the top wire gives the appearance of lowering the take-off angle, but in fact, the bottom lines of the two patterns overlap, with the 2-wire system showing more gain. The seemingly unexpected result comes from a difference between vertical beamwidths for the two arrangements. Hence, on 30, the phased 2-wire system appears to offer superior overall performance.</p>
|
||||
<p>The decrease in gain for each reduction in frequency is clearly apparent. given the array of antennas that inhabit some of the most extensive antenna farms, one might judge 20-meter performance to be the weakest in comparative terms. WARC band antennas tend to be simpler, and relative to them, the array is down by only a couple of dB at most. (If one has designed against these tendencies, then the differentials will also be different.) From 15 meters upward, the array gain is quite good--indeed, competitive might be a reasonable term. However, on 20, 5-element Yagis are fairly common (as are stacks of beams that give similar performance). The array on 20 yields performance similar to that of a 1 wl wire, perhaps 2 S-units weaker than the high-performance main antenna(s).</p>
|
||||
<p>Nonetheless, the overall performance of the array relative to all of the antennas in use for every band must be accounted quite good in view of what the antenna is in this application: it is an emergency back-up capable of being switch in to replace any antenna system that goes dead when Murphy dictates. With a size similar to a half-size 40-meter 2-element Yagi pointing straight up, it is also much simpler than its most cogent competitor: a log-periodic for 40-10 meters.</p>
|
||||
<h3>Phase Lines</h3>The models developed here used 600-Ohm, VF=1.0 lines for phasing. Each line was cut to 14' even though the distance to the center point was 11' only. A 3' buffer was allowed for each line to account for routing that would clear any metallic structures that might disrupt standard performance of the lines.
|
||||
<p>With these constraints, the models yield the following source impedance calculations.</p>
|
||||
<pre>
|
||||
Frequency Source Impedance Notes
|
||||
MHz R +/- jX Ohms
|
||||
28.5 140 - j 655 Highly variable with change of Zo
|
||||
24.94 335 + j 655 Somewhat variable with changes of Zo
|
||||
21.225 110 + j 85 Decreases with Zo decreases
|
||||
18.118 165 - j 135 Decreases with Zo decreases
|
||||
14.175 880 - j 136 Highly variable with change of Zo
|
||||
10.125 65 + j 250 Reasonably stable
|
||||
7.15 14 + j 10 Low and stable
|
||||
7.15 14 + j 30 Top-wire only in use, with line.</pre>
|
||||
<p>The number of cases in which the source impedance decreases with lower characteristic impedance phasing lines suggest that the highest feasible value of Zo be used for the antenna. Indeed, with lower values of Zo, the 40-meter source impedance can decrease below 10 Ohms, rapidly escalating the negative effects of loss sources in any installation.</p>
|
||||
<p>High variability with changes in phasing line type tend to indicate that the builder of any antenna of this type may encounter quite different source impedances owing to minor local variations. Although the phased pair of 44' doublets yields lower ratios of reactance to resistance, relative to a single 44' doublet, the low impedance at 40 meters may prove difficult to match at the tuner end of the line. The reactance will vary more widely than the resistance over a greater section of each half wavelength of line. Hence, careful line-length selection may prove necessary, although this factor will vary with the particular tuner configuration and internal components.</p>
|
||||
<p>On the basis of these results (typical of several different models), there appear to be three initially plausible feed systems.</p>
|
||||
<p><b>1. Parallel feedline and an ATU</b>: The simplest feed system is to use parallel feedline from the juncture of the phasing lines to the operating position, with an ATU providing the requisite matching. for minimum loss in the matching system and for maximum isolation of equipment from common mode currents, a link-coupled tuner is recommended (with due construction precautions against unintended common mode paths). Without careful measurement of the actual source impedances encountered and equally careful measurement of the feedline length, there may be cases in which the impedance presented to the tuner terminals falls outside the range for which the tuner can effect a match while compensating for reactances at that point. Ordinarily, changing the line length in small increments will overcome this problem without incurring significant losses.</p>
|
||||
<p><b>2. Remote switching of matching circuits--Version 1</b>: A box of fair proportions installed at or very near the junction of the two phase lines might contain an array of circuits matching the source impedance on each band to standard 50-Ohm coaxial cable (or 75-Ohm hardline). The system would require a power source to control separate input and output sides of each network. The design of such a system would have to be a custom installation based on the actual source impedances of the system.</p>
|
||||
<p><b>3. Remote switching of matching circuits--Version 2</b>: A remotely switched matching network box mounted precisely at the junction of phase lines may present mechanical problems if the installation has a rotating mast joining the two elements. Moreover, the impedances to be matched may not be the most desirable. A length of open-wire parallel feeder from the phase line junction to some point further down the support structure may provide more desirable values and a more convenient mechanical installation. Since the variables of this modified system are so many, I have not explored any particular line lengths to check feasibility. Hence, electrically, this system can only be classed as equivalent to the first version.</p>
|
||||
<p>Either remote matching system serves a certain preference among antenna farmers: the desire to present sensitive rigs and amplifiers with loads that require no in-shack tuning during intensive operations. All adjustments are made during installation and remade during routine maintenance. Those willing to make real-time adjustments and who can correctly install open-wire feeders may wish to use the simplest of the systems outlined here.</p>
|
||||
<p>Of the three systems, only the remote box at the phase-line junction is amenable to easy switch-over to using only the top wire on 40 meters. Adapting the other systems to such use will be an exercise in ingenuity.</p>
|
||||
<h3>Mechanical Considerations</h3>
|
||||
<p><img src="lal-3.gif" align="left" width="273" height="556" border="2" vspace="5" hspace="9"></p>
|
||||
<p>The basic elements of the antenna structure appear in <b>Fig. 3</b>.</p>
|
||||
<p>No support structure is shown, since this aspect of the installation is subject to so many variations. The antenna itself consists of two 44' long elements of considerable weight. The top one will be 22' above the lower one, which might be considered a moderate stacking challenge to those with large antenna farms. Since the elements are mounted close to the line of the rotating support mast, the stresses they impose upon the assembly may be less than those of elements at the ends of a boom. A mast extension to handle a truss system to add support farther out along the elements is certainly feasible and may well be advisable.</p>
|
||||
<p>If the lower element is (or both elements are) are side-mounted relative to a supporting pole or tower, then there will be a dead zone in the rotation. The size of this zone in degrees will vary inversely with the distance of the mast from the support structure. If we assume that there is a direction from the station that can be ignored, side-mounting may offer a means of installing the emergency antenna on an existing tower or pole. However, the elements may well interfere with existing guy wires.</p>
|
||||
<p>These mechanical notes are offered on the premise that anyone thinking about this antenna has considerable experience with tall installations and can integrate the structure into a considerable backlog of diverse variables involved in high pole and high tower work. Those who may be reading these very preliminary notes without requisite experience should perhaps review some of the very considerable stack of reprints offer by Champion (K7LXC) before becoming too much attracted to the ideas noted here.</p>
|
||||
<p>The aluminum extended Lazy-H is a bi-directional array with rotational capabilities that may make use of its reasonably narrow horizontal beamwidth on 40 through 10 meters. As such, it may fill a special niche, which I have termed the nearly-universal back-up for giant antenna farms. The feed system challenges are more electronic than they are mechanical--except for mounting and water-proofing a box for two of the suggested systems. Otherwise, the array presents fewer mechanical problems than most very large Yagis.</p>
|
||||
<p>Still, the array not a cure-all for whatever ails. Nor is it a magic elixir for all antenna installations. The notes presented here simply suggest that for some installations, the antenna may provide that nearly universal back-up which is ever handy: ready to go on 40 to 10 meters when Murphy strikes down the primary antenna(s). Just do not tell Murphy you have installed one of these or he will take down your antennas two at a time.</p>
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<p><i>Updated 2-12-99. © 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>
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<p class="topics-footer">Topics: <a href='../topics.html#hf-multi-band'>HF Multi Band</a>, <a href='../topics.html#lazy-h'>Lazy H</a></p><br>
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<title>The Expanded Lazy-H</title>
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<img src="../images/wirecad.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">
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<h1>Not Everything Good is New:<br>
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The Expanded Lazy-H</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Very effective antennas do not have to be exceptionally large or expensive. The latest designs and construction methods have their advantages--and also their costs. They tend to obscure some older designs of high merit as we forget to remember them.</p>
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<p>Rotatable antennas are very effective, but for those unwilling or unable to put a tower, rotator, and sizable aluminum structure in the air, fixed position wire arrays can provide excellent gain. Most designs are bi- directional, but the side rejection is often sufficient to eliminate most QRM. If we have the trees or the poles to support the ends, and if we take the trouble to align the antenna in the most favorable directions for our intended operation, a wire array can work wonders. For example, a great circle drawn through my QTH in Tennessee with one end in VK-ZL land will have its other end in Europe. A bi-directional array might be just the ticket for much of my operating.</p>
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||||
<p>Many broadside arrays are flat-tops--that is, they require at least two wires with considerable horizontal space between them. For most purposes, I would need 4 supports. However, a design that has been around since wire became popular is the Lazy-H, a vertical stack of two wires fed in phase. The standard Lazy-H consisted of two 1 wl wires spaced 1/2 wl apart and elevated so that the lower wire was 1/2 wl above ground.</p>
|
||||
<p>John Schultz, W2EEY, wrote in the November, 1968, <i>CQ</i> of the "Expanded Lazy-H Antenna." Bill Orr, W6SAI, recalled this antenna in one of his many columns during the 1980s. Another 15 years has gone by, so let's recall this effective array one more time. The key to expanding the Lazy-H is to increase both the horizontal and vertical dimensions by just a little bit.</p>
|
||||
<p>If we increase the wire lengths from 1 wl to 1.25 wl, we have stacked extended double Zepps in our Lazy-H. The effect is to give us a bit more gain per wire and a significant amount more from the pair. Then, if we increase the spacing from 1/2 wl to 5/8 wl, we achieve approximately the maximum stacked gain possible with two simple wires.</p>
|
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<p>Now, let's build one of these expanded Lazy-Hs for 10 meters.</p>
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<p><b>Fig. 1</b> shows the antenna outline. For 10 meters, a length of 44' per wire is satisfactory and not critical: 40' to 50' will work, but the pattern on 10 meter begins to split up as we lengthen the antenna too far beyond 1.25 WL. Vertical spacing between the two wires need not be too fussy, but the recommended 22' gives us not only 5/8 wl at 10 meters but a usable spacing at other frequencies.</p>
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<p>The recommended minimum of 1/2 wl at 10 meters is a bit low for optimal performance. I would recommend that a lower-wire height of about 44' be used, which places the top wire at 66' up. Lower heights will reduce the gain and elevate the TO angle from the figures I shall present as we think about this simple array.</p>
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<p>The mechanical beauty of the Lazy-H design is that it requires only two supports--although fairly tall ones. The electrical beauty of the antenna is that it provides excellent bi-directional performance from 10 meters through at least 17 meters, with good performance down to 30 meters. It can also be pressed into service on 40 meters without much difficulty.</p>
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<p><b>Fig. 2</b> shows the azimuth pattern of the antenna on 10 meters at an elevation angle of 8 degrees. The phase-fed array still retains the EDZ "ears." These ears are the beginnings of the multi-lobe pattern that emerges as the antenna wire length grows toward the 1.5 wl mark.</p>
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<p>On all bands below 10 meters, the length of the antenna is under the EDZ mark, so the pattern is bi-directional with single lobes each way. In fact, at 15 meters, the antenna becomes a standard Lazy-H: two 1 wl wires spaced 1/2 wl apart vertically.</p>
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<img src="lh-3.gif" width="640" height="480" border="2" vspace="5" hspace="9">
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<p>In <b>Fig. 3</b>, we see the 17-meter pattern at its elevation angle of maximum radiation of 13 degrees. As the tables below will show, the lobes become wider as we reduce frequency and narrower as we increase frequency.</p>
|
||||
<p>For a more systematic view of anticipated performance on all of the possible bands on which we might use this one Lazy-H, here is a table of modeled performance over average ground, with the lower wire 44' up. The table lists the usual gain and TO angle data, but also adds numbers for the vertical and horizontal beamwidths between the -3 dB (half-power) points. This date is useful in determining the azimuth coverage of the antenna in each direction and in estimating the elevation angles to catch the skip for varying circumstances.</p>
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<pre>Freq Max. Gain TO angle Vert. BW Hor. BW
|
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MHz dBi degrees degrees degrees
|
||||
28.5 15.1 8 9 31
|
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24.9 14.6 10 11 41
|
||||
21.2 12.5 11 12 52
|
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18.1 10.9 13 14 61
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14.15 9.0 17 18 73
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10.1 8.1 24 27 85
|
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7.15 6.4 33 44 99</pre>
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<p>By the time the antenna's operating frequency is lowered to 40 meters, the pattern becomes a broad oval with a fairly high TO angle, as shown in <b>Fig. 4</b>. However, sufficient radiation occurs at lower angles to make it usable for general purpose communications on that band.</p>
|
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<p>How good is the antenna's performance? I could use any number of comparators here, but the simplest would be a single 44' wire placed 66' up in height, the same height as the top wire of the array. The usefulness of this comparison is that it helps reveal something of the array's characteristics.</p>
|
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<pre>Freq Max. Gain TO angle Feedpoint Z
|
||||
MHz dBi degrees R +/- jX Ohms
|
||||
28.5 10.5 7 150 - j 695
|
||||
24.9 10.4 8 620 - j1700
|
||||
21.2 9.0 10 4200 + j 850
|
||||
18.1 8.6 12 835 + j1560
|
||||
14.15 7.7 15 190 + j 490
|
||||
10.1 7.6 20* 56 - j 105
|
||||
7.15 7.0* 29* 24 - j 600*</pre>
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<p>The starred gain entry for 40 meters indicates that the single wire at this frequency shows more gain than the array (by about 0.6 dB). In the TO angle column, the starred entries indicate that the single wire shows a significantly lower angle than shown for the array. Both phenomena are related. The array elevation angle of maximum radiation is a composite from radiation from both wires, with the lower wire radiation raising the angle of the final composite pattern. The difference is slight until the very lowest bands on which we might press this antenna into service. On 40 meters, the lower wire is just over 1/4 wl above ground, so that it raises the overall pattern angle of the array by a goodly amount and provides slightly less gain than the single wire that is about 1/2 wl up. As well, The high ratio of reactance to resistance in the feedpoint impedance suggests that there may be difficulty in obtaining a good low-loss match.</p>
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<p>From 20 meters on up, the Expanded Lazy-H shows good gain over a single wire. The benefits increase the higher one goes in frequency, up to the break-up of the pattern when the wires are longer than 1.25 wl. It is certainly possible to scale the antenna for maximum benefits at a lower frequency, but that lower frequency of maximum gain will become the highest frequency at which one can use the array and still have a bi-directional pattern with a single main lobe off each side of the wires.</p>
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<p>The single 44' wire also shows a wide variation in feedpoint impedance according to the length of the wire. The 10-meter value is typical for an EDZ. The 15-meter value is also typical, but of a 1 wl center-fed wire. Parallel feeders and a highly competent antenna tuner would be needed for this antenna. However, careful analysis of the impedance excursions along the chosen feedline can minimize the chances that the tuner antenna terminals will see either a resistance or a reactance value outside its range of adjustment.</p>
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<p>So far, I have given no figures for the feedpoint impedance of the Expanded Lazy-H. The two elements in Fig. 1 are fed in phase by the simple expedient of using equal lengths (11') of line to a center point to which we attach the parallel feeders going to the antenna tuner. There are two controllable variables that will affect the feedpoint impedance at the junction with the main line to the shack. One is the length of the lines, which we have set at 11' each. The other is the characteristic impedance and velocity factor of the phasing lines. I shall not here explore other phasing line lengths, but instead shall show some anticipated feedpoint impedances for each band using three different phasing lines. One will be a 450-Ohm, 0.95 VF line, typical of windowed vinyl-covered lines. Another will be 300-Ohm, 0.8 VF line, typical of good quality TV line. The third will be 600-Ohm, 1.0 VF line, which might be bought or built from wire and spacers.</p>
|
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<pre> Feedpoint Impedance (R +/- jX Ohms)
|
||||
Freq 450-Ohm 300-Ohm 600-Ohm
|
||||
MHz 0.95 VF 0.8 VF 1.0 VF
|
||||
28.5 65 + j 425 115 + j 570 105 + j 610
|
||||
24.9 17 + j 115* 11 + j 140* 30 + j 140
|
||||
21.2 22 - j 15* 10 + j 38* 40 - j 50
|
||||
18.1 45 - j 125 16 - j 26* 90 - j 230
|
||||
14.15 385 - j 395 75 - j 150 1050 - j 350
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||||
10.1 50 + j 105 40 + j 65 50 + j 155
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7.15 10 - j 95* 6 - j 80* 13 - j 90*</pre>
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<p>Starred entries represent very low resistive components to the feedpoint impedance which might present larger excursions along whatever line is chosen as the main feedline to the shack. Note that the starred entries are fewest with the 600-Ohm phasing line. Once more, it is worth noting that these numbers are derived for general guidance from models. Variations will emerge from the actual construction of the antenna and from conditions and clutter at the antenna site.</p>
|
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<p>One question that almost always emerges with respect to comparing the single wire and the array gain figures for 10 meters is this: how can the array deliver over 4.5 dB gain over the single wire? The answer is straightforward if we compare elevation patterns for the two antennas. <b>Fig. 5</b> tells the tale.</p>
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<img src="lh-5.gif" width="640" height="550" border="2" vspace="5" hspace="9">
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<p>Like any single-wire antenna, the EDZ at 66' on 10 meters shows an array of nearly equal-strength vertical lobes: 4 to be exact. In contrast, the upper lobes of the Expanded Lazy-H are suppressed leaving a single dominant lobe and a secondary lobe well over 4 dB weaker. All other lobes are down by 12 dB or more. The array tends to waste far less power at very high angles of radiation compared to the single wire. This comparative pattern, with variations, tends to hold true down through 20 meters.</p>
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<img src="lh-6.gif" width="640" height="550" border="2" vspace="5" hspace="9">
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<p>On 20, the effect is less pronounced but still easily measured, as shown in <b>Fig. 6</b>. The area enclosed by the upper lobes of the single wire at the top of the figure is distinctly greater by a considerable margin than the area enclosed by the upper lobe (barely discernable as a double lobe) of the array. The difference in area (assuming that the azimuth patterns are comparable, as they happen to be in this case) is a rough measure of the added power appearing in the lower lobes. In this case, that additional power shows up not only in the maximum gain, but as well in the vertical beamwidth. The phased feeding of vertically stacked horizontal wires has benefits hard to match in a typical flat-top wire array.</p>
|
||||
<p>Along side the benefits come some limitations. The Lazy-H requires a pair of tall supports and is suited to the antenna farm with more tall trees than money. It is possible to lay out more than one of these inexpensive antennas in order to cover additional regions along the horizon. It is likely that no special treatment will be needed to detune unused arrays to prevent them from altering the pattern of the array in use. Either leaving the shack end of the unused feedline open or shorting it will introduce to the wire feedpoints sufficient reactance to detune the wires. However, this is a facet of multiple array installation that the builder should keep in mind. Sometimes Murphy dictates that nothing will work to prevent interaction short of greater physical separation of the arrays.</p>
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<p>The Expanded Lazy-H is an outstanding bi-directional array for 10 meters in the design given here. Its performance holds up well down through 20 meters, and we can press it into service on lower bands. It takes up very little room horizontally in the yard, although a couple of optimally spaced tall trees certainly can aid the installation process. The wires for the elements and the phasing lines, as well as the feedline to the shack and the UV-resistant support ropes, are certainly inexpensive compared to the cost of a tower, rotator, coax, and commercial aluminum antenna. It is a design worth recollecting every 15 years or so just to make sure that we do not forget it.</p>
|
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<p>Can the extended Lazy-H be converted into a directional beam for some or all of the bands that it covers? To find out--at least in principle--see <a href="lhc.html">"<b>Curtains for the Extended Lazy-H</b>."</a></p>
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<p><i>Updated 4-5-1999; 04-15-2003. © L. B. Cebik, W4RNL. This item first appeared in</i> AntenneX<i>, Mar., 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="../radio.html"><img src="../images/return.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-multi-band'>HF Multi Band</a>, <a href='../topics.html#lazy-h'>Lazy H</a></p><br>
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<img src="../images/wirecad.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">
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<h1>Not Everything Good is New:<br>
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Curtains for the Extended Lazy-H</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<h4>The Basic Expanded/Extended Lazy-H</h4>A few years ago, I called attention to a wire array that receives notice only about every fifteen years--despite its excellent performance as a fixed bi-directional array of modest proportions. With two 44' long wires spaced 22' apart, the extended or expanded Lazy-H provides primary service on 20 through 10 meters, with quite adequate service on 30 and even 40 meters. From each element center, we run a parallel feedline--in phase--to a center position, which then becomes the primary feedpoint. We may use any feedline from 300 Ohms to 600 Ohms, although any figures shown in these notes will apply to 450-Ohm line with a 0.95 velocity factor. These notes also presume AWG #14 or #12 copper wire for the elements.
|
||||
<p>The base of the antenna ideally should be about 44' or more above ground for the lowest elevation angles of maximum radiation (take-off or TO angles). However, if necessity prevent the top wire from reaching 66', then a lower height and higher elevation angles are tolerable.</p>
|
||||
<p><a href="lh.html">"<b>The Expanded Lazy-H</b>,"</a> at this site provides a set of representative elevation and azimuth patterns for the antenna. The following table summarizes the performance when set at a 44'-66' height. The gain is the maximum gain at the TO angle. The Beamwidth is the angle between half-power or -3 dB points away from the bearing for maximum gain. The feedpoint impedance (Feed Z) is at the junction of the two 11' lengths of 450-Ohm parallel line.</p>
|
||||
<pre> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
||||
Extended Lazy-H Performance Potential
|
||||
|
||||
Freq. Gain TO angle Beamwidth Feed Z
|
||||
MHz dBi degrees degrees R+/-jX Ohms
|
||||
28.5 15.1 8 31 64 + j 425
|
||||
24.95 14.6 10 41 17 + j 115
|
||||
21.1 12.5 11 52 22 - j 18
|
||||
18.118 10.9 13 61 43 - j 125
|
||||
14.1 9.0 17 73 403 - j 395
|
||||
10.125 8.1 24 85 49 + j 105
|
||||
7.1 6.3 33 99 10 - j 100
|
||||
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</pre>
|
||||
<p>The impedances are all quite manageable for a wide-range balanced antenna tuner, with the possible exception of 40 meters and 12 meters. However, the impedance at the tuner will also be a function of the line length in wavelengths from the feedpoint to the tuner, so adjustment of the values to be matched is feasible.</p>
|
||||
<h4>Planar Reflectors</h4>My reason for returning to the extended/expanded Lazy-H grew out of an inquiry from Bill Burton, T88BA of Pelau. He had assembled the pieces for the antenna. However, he had also seen some commercial arrays that used a curtain-style reflector. Hence, he asked whether such a reflector was feasible for the big Lazy-H.
|
||||
<p>I have seen a number of schemes for multiple extended double Zepps of about 44' and even for the Lazy-H, all designed to use parasitic elements or phased elements to convert the bi-directional array into a directional beam. Their complexity could be daunting, often with the result that only the builder knew the correct adjustments for each band. On the other hand, a planar or curtain reflector is a broad-band passive addition that might be serviceable.</p>
|
||||
<p>In UHF work, a planar reflector generally exceeds the dimensions of the active element or array by perhaps 3/8 to 1/2 wavelength on all four sides. Such a curtain would be somewhat ungainly for the extended/expanded Lazy-H. So I somewhat arbitrarily selected a set of dimensions that seemed close to feasible for someone with sufficient land to erect the basic antenna at the ideal minimum height. The modeled screen is 50' wide by 30' high. This is only 6' wider than the element lengths and 8' larger vertically. In many ways, it is a minimalist planar reflector.</p>
|
||||
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||||
<img src="lhc-1.gif" width="528" height="445" border="2" vspace="5" hspace="9">
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||||
</center>
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||||
<p><b>Fig. 1</b> shows the parts of the array, although not to scale. The extended/expanded Lazy-H remains unchanged from independent use which results in its bi-directional characteristics. The screen is centered behind the active array. Although the modeled screen uses a grid-square assembly, it is possible to use a sequence of wires parallel to each other. For this horizontal array, the wires must also be horizontal.</p>
|
||||
<p>The 10' spacing was my initial trial spacing for the reflector--just a bit shy of 1/4 wavelength at 15 meters. After looking at numerous other spacings, I returned to my intuitive selection, since it provides approximately equal front-to-back ratios at the array limits, namely 10 meters at the high end and 30 meters at the low end.</p>
|
||||
<p>To see how the curtain or planar reflector changes array performance, examine the following table and compare various values to the ones for the array alone.</p>
|
||||
<pre> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
||||
Curtained Extended Lazy-H Performance Potential
|
||||
|
||||
Freq. Gain TO angle Beamwidth F-B Ratio Feed Z
|
||||
MHz dBi degrees degrees dB R+/-jX Ohms
|
||||
28.5 18.4 8 30 13.7 112 + j 430
|
||||
24.95 17.1 10 40 15.5 29 + j 125
|
||||
21.1 15.5 11 50 17.1 25 - j 3
|
||||
18.118 14.4 13 56 16.8 33 - j 100
|
||||
14.1 13.1 17 63 14.2 175 - j 460
|
||||
10.125 12.2 23 69 13.8 21 + j 140
|
||||
7.1 10.0* 30 75 8.3 2 - j 100
|
||||
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</pre>
|
||||
<p>The gain increase in the favored direction is between 3 and 4 dB, depending upon the band, with peak increases in the 17 and 20 meter bands. The gain improvement values are consistent with those for any casually designed 2-element driver-reflector Yagi. Peak front-to-back ratio occurs on 15 meters and decreases slowly above and below that band.</p>
|
||||
<p>The impedance values are interesting. No great change occurs in the reactance values. However, the resistive component shows an interesting pattern. On 15 meters, its value is about the same as the value for the array alone. Above 15 meters, where the spacing is greater than 1/4 wavelength, the resistive component of the impedance is higher than for the array alone. Below 15 meters, the resistive component is less than for the array alone, while the spacing is less than 1/4 wavelength.</p>
|
||||
<p><b>Fig. 2</b> through <b>Fig. 7</b> provide azimuth and elevation patterns for the array on each band. They require no individual comment. However, the trends in the rear lobe formations should be reasonably clear. In all cases, the patterns are well-behaved, with no spurious lobes--other than the emergent secondary lobes inherent to the array when the active element length approaches 1.25 wavelengths.</p>
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<p>Before leaving our patterns, we must take special note of 40 meters. The gain value for that band is starred. That star indicates that the pattern shows maximum gain in the reverse direction relative to all of the other bands. See <b>Fig. 8</b> for azimuth and elevation details.</p>
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<p>The reason for pattern reversal is simple. The horizontal dimension for the reflector (50') is 1/2 wavelength or more for all bands from 30 to 10 meters. However, on 40 meters, the reflector horizontal dimension is only about 3/8 wavelength. Hence, despite its vertical dimension, the screen acts like a director at less than 0.1 wavelength spacing. The very low resistive component of the feedpoint impedance reflects this condition. It is unlikely that one would be able to take advantage of the reverse pattern, given the potential difficulty in achieving a low-loss match.</p>
|
||||
<h4>Physical Realities</h4>The proof-of-principle exercise suggests that a minimalist planar reflector or curtain behind an expanded/extended Lazy-H will yield a competent directional beam with a noticeable improvement in gain and quite usable front-to-back characteristics. However, the requirements for the reflector are sufficiently challenging to make this array an antenna with a somewhat small niche.
|
||||
<p>Planar reflectors require both vertical and horizontal dimension for highest effectiveness. Too narrow a vertical dimension will degrade the array characteristics as much as too short a horizontal dimension. Since the reflector is untuned, it must be at least 1/2 wavelength at the lowest frequency used, with additional length up to about 1 wavelength wherever feasible. Vertically, we improve performance with height greater than those used here, although the vertical dimension will reach its limits of helpfulness more quickly than the horizontal dimension.</p>
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||||
<p>Assuming that we can erect vertical supports of the needed height, I recommend some form of halyard assembly to raise and lower the reflector screen. Even an extremely open "chicken-wire" screen reflector will show considerable aggregate wind resistance in violent storms. However, if one desired beaming in both broadside directions, then a pair of screens--with only one raised at a time--will yield a reversible beam. If the user has only one primary target region, lowering a single screen will return the array to its inherent bi-directional pattern, and that may suffice for other operations.</p>
|
||||
<p>For raising and lowering, we need an open-weave metal "fabric" that will resist snagging when it is crumpled on the ground. However, one might wind the screen around a ground-level cylinder instead of lowering it into a heap directly on the ground. One might even use a combination of horizontal wires and vertical ropes to create a more flexible screen for this purpose. The final product is suited to the skills of one who has experience with square-rigged sails, with the sail and spar arrangement inverted from those we find at sea. Of course, we shall invert another matter as well: we shall seek to slip the wind rather than catching it.</p>
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||||
<p>The curtained Lazy-H is a fixed position wire array of considerable mechanical size and requires equal mechanical ingenuity to implement. However, it holds promise of providing multi-band gain and front-to-back ratio so that the operator can use the upper HF band on which propagation is nearest to optimal for a desired path. The beamwidth on the upper bands is narrow enough to require careful siting. Although the values in the charts emphasize the amateur bands, the general arrangement may be suitable for short-wave listening in the intervening spectrum--and possibly even for an economical short-wave broadcast installation.</p>
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||||
<p>The curtained Lazy-H is certainly not an antenna for everyone. But it may be an antenna for someone.</p>
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<p><i>Updated 04-15-2003. © 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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<p><a href="../radio.html"><img src="../images/return.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-multi-band'>HF Multi Band</a>, <a href='../topics.html#lazy-h'>Lazy H</a>, <a href='../topics.html#planar-reflector'>Planar Reflector</a></p><br>
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<h1><img src="../images/wirecad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10"> Some ABCs of A-B-C: Notes on Triangles of Doublets <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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<p>On occasion, folks have asked me questions of this sort:</p>
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<blockquote>
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My all-band doublet doesn't seem to be doing the job. So what if I replace it with a longer one, a loop, or whatever. Or suppose that I add an antenna of a different kind to the existing doublet. What should I do?
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</blockquote>
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<p>In many cases, I recommend a second or third antenna of the same type. It struck me that perhaps some short background on why I make that recommendation on some occasions--but not all--might be useful to those who do not have much experience with all-band wire doublets.</p>
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<h4>The Single Doublet</h4>Every wire antenna has a length (in feet, meters, etc.) and a height. We shall use a center-fed doublet throughout these notes. Since our question does not directly involve elevation angles of radiation for the best DX, etc., I shall use a constant height of 50' above average ground for this discussion. Those constants will allow us to make direct comparisons. Anyone with lower or higher wires can read other notes at this site to make any adjustments in the comparisons.
|
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<p>A difference of height will make little or no difference in the azimuth patterns of a doublet as we move from band-to-band (as long as the antenna is not too close to the ground). The pattern is mostly a function of the antenna length in terms of wavelength at each operating frequency. Small changes of frequency do not materially affect the pattern, so we can use a single frequency on each band as a sample that holds true for the whole band.</p>
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<p>Let's start with a 135' center-fed doublet. 135' is about (and "about" is plenty good enough here) 1/2 wavelength long on the 80-75-meter band. At 40 meters it is 1 wavelength. On 20, it is 2 wavelengths, and on 10 it is 4 wavelengths.</p>
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<p>Any center-fed doublet will have only 2 lobes for any length in wavelengths up to and just beyond 1 wavelength--1 lobe on each side of the wire, broadside to the wire. When the antenna is 2 wavelengths long, there will be 4 lobes--2 on each side of the wires--and they will angle away from the wire leaving a null directly broadside to the wire. A 4 wavelength antenna will have 8 lobes--4 on each side--and the strongest ones will be angled further away from the broadside directions.</p>
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<p><b>Fig. 1</b> provides snapshots of the azimuth patterns that reflect the notes I just gave. In addition, it shows the 15-meter situation. The antenna is 3 wavelengths long, so we get 6 lobes. For all of the patterns, the antenna runs from left to right (or right to left) across the center line of the plots. So broadside is up and down on the patterns.</p>
|
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<p>The first thing we can do with these patterns is explain why a given doublet gives good results in certain directions on some bands but not on others. Assume that we set the wire in the U.S. so that broadside goes to Europe and to Australia. By the time we operate on 15 to 10 meters, our strongest lobes are no where near the headings for those two major target areas.</p>
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<p>So our first lesson is in wire antenna orientation. By knowing the antenna length, we can roughly determine the directions of lobes on our favorite bands and set up the wire to give the strongest performance in those directions by how we orient the wire. We cannot obtain a perfect setting on all bands, but we can (assuming that we do not need to move any supporting trees) obtain good settings on our favorite bands.</p>
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<p>Before we look further at the matter of direction, we have a few more preliminaries to note. For example, what band should I choose as the basic one for DX in my favorite directions? Here is where a small performance table may help for the bands illustrated in <b>Fig. 1</b>.</p>
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<pre> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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||||
135' Doublet Performance at 50' Up
|
||||
|
||||
Freq. MHz Max Gain dBi TO Angle deg
|
||||
3.75 3.2 30 (arbitrary)
|
||||
14.175 9.4 19
|
||||
21.225 9.8 13
|
||||
28.5 10.6 9
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</pre>
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<p>The TO or take-off angle is the elevation angle of maximum radiation. It is correct for 20-10 meters, but is not for 80 meters, where the height of the antenna is low enough to direct most radiation at very high angles. So I chose a reasonable short-skip angle to take the gain reading.</p>
|
||||
<p>Since the gain on 80 is so low and the radiation angle so high, 80 is not a good candidate for use in orienting the antenna for DX directions. The gain on 20, 15, and 10 are similar, so one of those bands is a better candidate. However, if we prefer local rag chewing, then the 80-meter broadside should aim at our target areas. Remember that the 40-meter pattern will also be a 2-lobe broadside affair, but the strength will be higher and the beamwidth narrower (since the added gain has to come from somewhere).</p>
|
||||
<p>There are, of course, a number of other amateur bands on which the antenna is not close to an integral number of wavelengths long. What happens when the antenna is X.5 wavelengths long, where X is any integer? The answer is a function of how lobes appear. They do not pop into existence, but grow and shrink as we change the length of the antenna (or as we raise and lower the operating frequency, which achieves the same change in antenna length when measured in terms of wavelengths). At X.5 wavelengths, the azimuth pattern will show two sets of lobes in approximately equal strength: the set for wavelength X, longer than which we now are, and the set for wavelength X+1, which we are approaching.</p>
|
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<p>On 17 meters, a wavelength is about 54' long and our 135' doublet is about 2.5 wavelengths long (give or take a little). (A wavelength is about 984/f feet long, where f is the frequency in MHz.) We shall have 4 lobes for the 2 wavelengths that we passed in length. We shall have 6 lobes for the 3 wavelengths that we are approaching. So the pattern will be composed of 10 lobes total. That sounds good, since we get lobes in so many different directions. However, for every lobe, there is also a null. So we have 10 blank directions and each lobe is narrower than its counterpart in a 2 wavelength or a 3 wavelength wire.</p>
|
||||
<p>This note on nulls gives us the second lesson concerning wire doublets and their azimuth patterns. Study the azimuth patterns twice. First, look at the lobes that tell you where the radiation is going. Second study the nulls that tell you where performance will be very weak. Only then should you make decisions about how to orient the antenna.</p>
|
||||
<p>What happens when a wire is X.25 or X.75 wavelengths long. Since lobes grow and shrink, the answer is almost obvious. At X.25 wavelengths, the wire is not long enough for strong lobes from the X+1 length, but will show some smaller lobes derived from that length. At X.75 wavelengths, the wire is too long to support full scale lobes from the X length, but will have smaller lobes (meaning lower gain) still present. In either case, as some lobes show low development, the larger ones are that much larger, since the radiated power is relatively constant (ignoring external variables) across all frequencies.</p>
|
||||
<p>Now suppose that you have studied the azimuth patterns and decide that you cannot place a strong lobe every where that you want to communicate. It is precisely here that folks immediate jump to thoughts of other antennas. But virtually every other horizontal wire antenna, whether straight or looped, has a pattern of lobes and nulls. Most of them are more complex to install. So what is a solution to our quandary?</p>
|
||||
<p>Now wire is relatively cheap. Antenna supports are not. So any solution that we come up with should have the desirable property of involving a minimum of new supports.</p>
|
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<h4>The 135' Doublet Triangle</h4>There is no perfect solution to the problem of working everywhere we want to work with an array of horizontal wire doublets. However, we can go a good distance toward that solution by adding only one more antenna support. We shall create a triangle of wire doublets, something like the idealized sketch in <b>Fig. 2</b>.
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<img src="wt-2.gif" width="453" height="296" border="2" vspace="5" hspace="9">
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<p>The drawing shows an equilateral triangle, but almost any shape will do. In fact, a better ideal is to angle each antenna so that its lobes--on your favorite bands--hit your favorite targets. Just give yourself a little separation at the wire ends--perhaps 10' on the end of a 135' doublet--to minimize interactions among the wires.</p>
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||||
<p>The sketch also shows two different means of feeding the antenna. You can run 3 feedlines, each the same length as the others, to a central point where you install a weather-proof relay box to switch among the wires. A single feedline runs to the shack, along with a relay power line and switching lines via an A-B-C switch to activate one of the 3 antennas at a time.</p>
|
||||
<p>The alternative is to run three lines, again, all the same length, to the shack for use with a manual A-B-C switch at that location. Since the lines will be parallel transmission lines, follow the usual precautions about keeping them free and clear of anything that might disrupt their balance.</p>
|
||||
<p>The point of using equal-length lines in each case is so that you can switch between antennas and determine by ear the strongest signal. If they are not the same length, you will have to do some rapid re-adjustment of the tuner settings for each switch position. With identical antennas and feedlines, you should be able to pick out the strongest one and then only do a final tweaking of the settings on the tuner.</p>
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<img src="wt-3.gif" width="414" height="235" border="2" vspace="5" hspace="9">
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<p>As <b>Fig. 3</b> shows, you may not always have a good choice as to exactly where you position the supports for the antenna. Fitting the triangle to available yard space is an eternal amateur antenna problem. However, as we shall see, we can effect some improvement on our operation, even if we cannot perfect it.</p>
|
||||
<p>Notice that we were able to add two antennas for the cost of wire and with only one extra support. We may well trade any remaining imperfections in the system for that major simplification of structure.</p>
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||||
<p>Now the question is simply this: what do we get for our pains? The easiest way to show what we get is by overlaying azimuth patterns for each antenna in one massive plot for each band covered in <b>Fig. 1</b>. We shall note both the advantages that we accrue from the new arrangement and the remaining problems that we could not solve.</p>
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<img src="wt-4.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p><b>Fig. 4</b> shows the three antenna patterns at the same 30-degree angle that we used in <b>Fig. 1</b>. It is immediately apparent that we can cover more of the horizon with our signal (and reception) than with a single antenna. However, since the antenna is close to 1/2 wavelength, there is some interaction between the antennas so that the inactive ones act as reflectors. The 2-dB difference should not affect performance too much. You will find the same phenomenon on 40-meters, where the wires are all nearly 1 wavelength.</p>
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||||
<img src="wt-5.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>The 20-meter situation appears in <b>Fig. 5</b>. We still have some nulls, but count the major directions that we can cover, letting overlapping lobes count as 1. We have 6 directions, not just the 4 that a single wire would give us. As well, the overlaps are not perfect, so that a signal that is on the fringe of one lobe may be centered in overlapping one. Of course, by carefully planning of your triangle, you can minimize the overlap and spread the area of coverage. The patterns shown simply use our equilateral triangle as their basis.</p>
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<img src="wt-6.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>In <b>Fig. 6</b>, we find 15-meter quite well covered by the 3 antennas and their strongest lobes. Indeed, 15 meters is a band that really benefits from a triangle of 135' doublets.</p>
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||||
<img src="wt-7.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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||||
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||||
<p>If you look at <b>Fig. 5</b> and <b>Fig. 7</b>, you may get the impression that when an antenna is an even number of wavelengths, it leaves more nulls than when it is an odd number of wavelengths, as in the 15-meter case. In general, this is a correct conclusion, although as we further increase the antenna length, the number of lobes becomes high enough to make it difficult to tell the difference. 10-meters is a good band for which to redesign the triangle to place a lobe in the direction that you want it.</p>
|
||||
<h4>The 88' Doublet Triangle</h4>If you lack yard space for 135' doublets in a triangle, you might try 88' doublets. Here is a performance table for a single doublet on the same bands that we surveyed for the longer doublet.
|
||||
<pre> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
||||
|
||||
88' Doublet Performance at 50' Up
|
||||
|
||||
Freq. MHz Max Gain dBi TO Angle deg
|
||||
3.75 2.8 30 (arbitrary)
|
||||
14.175 10.1 19
|
||||
21.225 9.1 13
|
||||
28.5 10.3 9
|
||||
|
||||
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</pre>
|
||||
<p>The performance figures for each band are not very much different than for the 135' doublet, but the patterns are considerably different. Remember that the 88' doublet is only about 1/3 wavelength at 3.75 MHz. <b>Fig. 8</b> shows the 88' doublet azimuth patterns when we place the antenna 50' above ground.</p>
|
||||
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|
||||
<img src="wt-8.gif" width="400" height="400" border="2" vspace="5" hspace="9">
|
||||
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|
||||
<p>The 80-meter pattern is similar to the one for the longer wire. However, the 20-meter pattern shows the typical "ears" of an extended double Zepp, since that is exactly what the antenna is at 14 MHz. It is 1.25 wavelengths, which means that the 2 wavelength lobes are just beginning to emerge. On 15 meters, the wire is 2 wavelengths long and shows the same sort of pattern that the 135' doublet showed on 20 meters. The 10-meter 88' doublet pattern is an example of a 10-lobe pattern for a 2.5 wavelength antenna.</p>
|
||||
<p>Besides taking less space, the triangle of 88' doublets also shows less interaction among the wires. Hence, we can use somewhat smaller separations of the wire ends in making the triangle. However, in exchange for spatial economy, we shall encounter differences in the ability of the triangle to fill in the nulls on the single-wire patterns.</p>
|
||||
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|
||||
<img src="wt-9.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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||||
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||||
<p>On 80 meters, as shown in <b>Fig. 9</b>, the absence of pattern distortions created by interactions among the wires yields almost complete horizon coverage. However, remember that this pattern is at an elevation angle of 30 degrees, and most radiation is upward. We can improve long-haul performance of the triangle by "merely" raising the supports to the 90-100 foot level.</p>
|
||||
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|
||||
<img src="wt-10.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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||||
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||||
<p><b>Fig. 10</b> shows us the 20-meter combined patterns. The azimuth plot provides a good model for a hex symbol to embroider for good luck. More importantly, it shows some nulls that may call for careful design of the triangle to ensure the desired target-area coverage.</p>
|
||||
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|
||||
<img src="wt-11.gif" width="376" height="345" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Like its counterpart 20-meter pattern, the 15-meter patterns in <b>Fig. 11</b> add up to fairly complete coverage, but with nulls and overlapping lobes. Hence, one might wish to design the triangle to spread the lobes a bit. However, you will discover that with 4 lobes per wire, every spread in one direction increases an overlap somewhere else.</p>
|
||||
<center>
|
||||
<img src="wt-12.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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||||
</center>
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||||
<p>The situation grows both better and worse on 10 meters, as shown in <b>Fig. 12</b>. The null areas are wider, but often not as deep, since minor 10-meter lobes fill the null at about 1.5 S-units lower strength. Once more, designer triangle formation seems the order of the day.</p>
|
||||
<p>It would be difficult in a general discussion to provide samples of designer triangles, since each would prove useful for only one region of the U.S.--and likely be useless outside the U.S. However, one can experiment most easily with altering the triangle orientations with modeling software. There are inexpensive packages, and even some free MININEC programs. It may pay to master them well enough to go with your self-study geography lessons in order to give yourself the bast chance of placing doublet supports at the correct locations.</p>
|
||||
<p>In the end, there are no perfect solutions. However, at a cost of one extra support and a bale of wire--plus feedlines and an A-B-C switch--you cannot get much better horizontal coverage much more cheaply than with a triangle of doublets.</p>
|
||||
<p>An additional caution or two: The doublet lengths used here place the lowest band at 80 meters. You can use a 70' doublet if you wish to cover 40-10 meters. In that case, the 80-meter and the 20-meter patterns shown in <b>Fig. 1</b> become the patterns for 40 and 10 meters, respectively. As well, you can use a 44' doublet in place of the 88' version used in these notes. The same adjustments then apply to the patterns in <b>Fig. 8</b>. In both cases, the performance data in the tables would apply with band adjustments to an antenna at about 25' above ground.</p>
|
||||
<h4>Xs and Ls</h4>Some folks ask what happens when we have only two doublets. They envision crossing or end-to-end arrangements like those in <b>Fig. 13</b>. However, they often have in mind to use something other than a right angle.
|
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<img src="wt-13.gif" width="474" height="255" border="2" vspace="5" hspace="9">
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<p>The first thing that happens is a requirement for either 1 or 2 more support posts. If you have plenty of Douglas Firs handy, a 4-post system is no problem. But if you have to construct or erect your own supports, then the support work either matches or exceeds the work required by a triangle. The L-configuration is like the triangle, but only lacks 1 wire and its associate feedline. Of course, you can now get away with an A-B switch, rather than having to figure out how to make an A-B-C switch.</p>
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||||
<p>Let's put up 2 135' doublets that cross at the center (with a separation to keep the wires apart) and see what we can achieve with only 2 antennas on each of our sampling bands.</p>
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<p><b>Fig. 14</b> shows the 3.75-MHz results. Since the deepest null is now only about -3 dB or about 1/2 S-unit, it is likely that performance will be satisfactory within the height limitations that we discussed earlier.</p>
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<img src="wt-15.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>On 20 meters, as shown in <b>Fig. 15</b>, we have deep nulls and considerable lobe overlapping. Hence, for this band, adjusting one antenna by at least 25-30 degrees off a right angle will likely produce better coverage. For an individual antenna, the lobes are only about 35 degrees each side of a broadside tangent line relative to the wire, so some angling to enhance coverage seems in order.</p>
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<p><b>Fig. 16</b> gives us the 15-meter story. Once more, the main lobes heavily overlap, but each is about 40 degrees off the line tangential to the antenna wire. Finding a compromise angle for both 20 and 15 meters will require some thought, especially when we add in the need to be aiming at communications target areas.</p>
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<img src="wt-17.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>The most thorough coverage occurs on 10 meters, due to the multiplicity of lobes. See <b>Fig. 17</b>. Despite the gaps or nulls that remain on each of the bands, the level of coverage with just two 135' doublets is significantly greater than with a single doublet. Conclusion: if you cannot swing 3 antennas, at least try for two.</p>
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<p>The 88' doublet does not fare quite as well in a 2-wire system as the 135' doublet. Indeed, the 88' doublet seems best suited to a triangular environment.</p>
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<img src="wt-18.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>80 meters appears in <b>Fig. 18</b>. As is evident, there is no significant difference between the 135' and 88' 80-meter situation with respect to coverage.</p>
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<img src="wt-19.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>The 20-meter patterns are in <b>Fig. 19</b>. The extended double Zepp patterns simply give us two different bi-directional options. Hence, careful broadside aiming of the wires seems the order of the day.</p>
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<img src="wt-20.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>If we wish to target areas on 20 meters, they will be broadside to the antenna. However, on 15 meters, as shown in <b>Fig. 20</b>, we cannot target the same areas, since the lobes on that band angle away from broadside by about 35 degrees. Unlike the situation with a triangle of doublets, the 2-wire system of 88' doublets appears to force us to declare that either 20 or 15 meters is our favorite band, but not both--at least not into the same parts of the world.</p>
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<p><b>Fig. 21</b> gives us the 10-meter picture. At 2.5 wavelengths, the antenna yields fairly solid coverage, although there likely is room for wire aiming on this band.</p>
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<p>However we construct the doublets, the triangle provides superior coverage and more versatility than a crossed or L-ed doublet system. Obviously, 2 wires are better than 1, but 3 is significantly better than 2 without requiring any further supports.</p>
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<p>I have omitted construction details, since they are so variable with the circumstances of the individual builder. As well, I have omitted inverted Vees, which will tend to change the situation on the lower bands more than on the higher, since the patterns will be broader ovals.</p>
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||||
<p>Nevertheless, the switched doublet triangle offers flexibility that takes advantage of the strongest lobes--especially on the upper bands--in the antenna pattern. A switched-doublet system is cheaper than a rotator and tower system, and repair costs are reduced usually to the cost of antenna wire and possibly some parallel transmission line. However, the performance of a wire--when you can place one of its main lobes on the desired station--can be surprisingly good.</p>
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<p>Nothing here is intended to compare the doublet with other wire antennas--or even non-wire antennas. These notes are intended only to show some possibilities that we often overlook when thinking about wire doublets.</p>
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<p>A love triangle is usually a disaster. However, a triangle of doublets is often a happy marriage of economy and performance.</p>
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<p><i>Updated 05-15-2003. © 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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<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#doublet'>Doublet</a>, <a href='../topics.html#hf-multi-band'>HF Multi Band</a></p><br>
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<h1><img src="../images/wirecad.gif" align="middle" width="125" height="62" border="2" vspace="5" hspace="10"> Resurrecting the Y-Doublet <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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<p>After presenting some notes on triangles of doublets of various lengths, I received more than one message recalling an old Y-configuration from the 30s and 40s. The basic scheme was designed for a given band and consisted of three 1/4 wavelength wires at 120-degree angles coming together at a center point. There, according to recollections, the old timers used a 3-wire twisted feedline to the shack. At any one time, the operator hooked up two of the 3 wires to the antenna tuner (or, in more remote past times, to the rig output terminals). The result was a steerable doublet.</p>
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<p>Essentially, the operator was selecting the pair of feed wires that created a doublet, with the third antenna wire relatively inert. We normally think of a doublet as linear, but bending it by 30 degrees does not especially harm its performance. So that part of the system is quite sound.</p>
|
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<p>More foreign to current practice is the twisted feedline. In the 1930s, many hams commonly feed their dipoles with low-impedance parallel line. A 72-Ohm transmitting parallel line used to be available, but apparently the high power version is no longer made. 72-Ohm parallel lines made from round wires are not feasible with open-wire construction, since the required center-to-center spacing would require contact between the wires. However, by using a carefully calculated thickness of an insulating material with a known dielectric constant on each wire, the desired impedance is achievable.</p>
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<p>Amateur practice tended to rely on two factors. First, the resulting parallel line resembled ordinary line cord, sometimes called zip cord. Second, properly configured antenna tuners or even amplifier output circuits were capable of handling a fairly wide range on impedances. Therefore, amateurs used to simply twist pairs of insulated wires together to form a low impedance parallel feedline. The wires might be line cord or they might be other insulated wires twisted and taped together.</p>
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<p>Adding a third wire to the set and leaving it disconnected from the RF source was relatively harmless. If the wire was equally spaced from the other two hot wires, it would have negligible current on it. The antenna wire would be at essentially right angles to the main pattern and hence induce a minimum of coupled antenna current into the inert feeder. Since the currents on the other two feeder wires would be equal in magnitude but opposite in phase, any induced currents in the third wire would cancel, leaving no current in the third wire.</p>
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<p>Overall, the system effects a space savings over three doublets in a triangle. With good solid AWG #12 wire for the elements, one can use only three corner supports and let the triangle of antenna wires support the center assembly. (One can always add a center support, if convenient or necessary.) With that promise and the potential for having a steerable doublet, the idea is worth further exploration.</p>
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<h4>The Steerable Y-Doublet Array</h4>Let's begin by looking at the antenna wires and their potential performance. We shall look more closely at the feed system later on. The basic configuration of the Y-doublet appears in <b>Fig. 1</b>.
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<img src="wy-1.gif" width="462" height="401" border="2" vspace="5" hspace="9">
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<p>We shall use as our test array a Y cut for 3.6 MHz. My free-space model used 67' legs for initial checks. Hence, ignoring the necessary insulating end ropes to the support trees or posts, we get a triangle about 116' on a side and capable of fitting within a rectangular back yard that is about 101' by 116'. The figure shows the three feed wires, of which we shall use only two at a time. For modeling, that means terminating each leg short of the exact center point. Then we connect a short wire between 2 of the 3 wires. I used a separation between inner leg ends of about 3' so that I could use a 3-segment wire for the source and use segment lengths of about 1' on the antenna wire legs.</p>
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<img src="wy-2.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p><b>Fig.</b> 2 shows the overlaid free-space patterns for the Y-doublet at 3.6 MHz using different pairs of legs to form each of 3 doublets. The patterns indeed promise full horizon coverage as we switch pairs of feedlines at the shack end of the feeder lines.</p>
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<p>I also checked the antenna's performance on each of the bands above 80 meters. All of the traditional ham bands (40, 20, 15, and 10 meters) yielded very high feedpoint impedances. Since we are working with a low-impedance feedline, I set these bands aside as not especially feasible for use with the system. (We shall review this decision before we are finished.) However, 30, 17, and 12 meters showed feedpoint impedances sufficiently low to potentially allow use of the antenna on these bands using the low-impedance feeder system employed in first half of the 20th century. The following table shows the free space performance potential of the array.</p>
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<pre> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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Y-Doublet Modeled Performance: Free Space
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|
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Freq. Gain Feed Z
|
||||
MHz dBi R+/-jX Ohms
|
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3.6 1.70 59 - j 6
|
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10.125 5.16 106 - j 375
|
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18.118 4.62 134 - j 103
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24.94 4.92 171 - j 291
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</pre>
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<p>The free-space patterns are generally only applicable for a real horizontal antenna over ground if the height is at least 1 wavelength. 80-meter doublets at 270' or more are rare. Therefore, I remodeled the antenna at a 50' height to reflect a more realistic scenario. At that height, the maximum gain of the antenna has an elevation angle that is nearly straight up. So I chose for that band an angle of 34 degrees to reflect typical skip angles. The resulting 3.6-MHz patterns, shown in <b>Fig. 3</b>, are a good bit more oval than their free-pace counterparts.</p>
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<img src="wy-3.gif" width="376" height="345" border="2" vspace="5" hspace="9">
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<p>On the upper bands, I used the take-off (TO) angle for gathering potential performance data. The antenna promises performance as shown in the following table, with the leg-length adjusted to 66.5' to bring the array close to resonance at 3.6 MHz. (Wire doublets tend to vary their feedpoint impedances with height in noticeable ways when the doublet is less than 1 wavelength above ground.)</p>
|
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<pre> . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
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Y-Doublet Modeled Performance: 50' Above Average Ground
|
||||
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||||
Freq. Gain TO Angle Feed Z
|
||||
MHz dBi degrees R+/-jX Ohms
|
||||
3.6 3.43 34* 56 + j 8
|
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10.125 9.98 26 110 - j 424
|
||||
18.118 9.59 14 135 - j 153
|
||||
24.94 10.23 11 182 - j 365
|
||||
* 80-meter elevation angle arbitrary.
|
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</pre>
|
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<p>The patterns on the upper bands are not ovals by any means. <b>Fig. 4</b> shows these patterns, but only one pattern per band for clarity. As we increase frequency, we find two especially interesting pattern properties. First, as the legs become longer in terms of wavelengths, the patterns develop growing side "wings." Eventually, by 12 meters, the main lobe has split into two forward lobes. Second, as we increase frequency, the array becomes more directional, with a growing differential between the forward and the rearward gain.</p>
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<img src="wy-4.gif" width="600" height="220" border="2" vspace="5" hspace="9">
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<p>Still, the patterns may be usable for general amateur operations. The question left is why we get reasonably low impedances at 30, 17, and 12 meters. <b>Fig. 5</b> shows part of the reason.</p>
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<img src="wy-5.gif" width="500" height="340" border="2" vspace="5" hspace="9">
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<p>The graphics display the relative current magnitude distribution along the doublet for each of the 4 bands. On 80 meters, we have a somewhat typical dipole current distribution, with the current peak at the feedpoint. On the other bands, we approximate a 3/2-, 5/2-, and 7/2 wavelength doublet current distribution. Each of these configurations places a current peak at the doublet center, resulting in a relatively low feedpoint impedance.</p>
|
||||
<p>We should also note that the unused wire shows a flat current line. The current on it models out (in its perfectly spaced geometry) at about 4 orders of magnitude less current than on the active wires. That is, if the current at the source is 1.0, then the current on the inert wire shows a value of 0.0001 or 1E-4 or less.</p>
|
||||
<h4>The Feeder Question</h4>The original system was designed for use with a twisted trio of feedline wires, in other words, a twisted pair plus one. <b>Fig. 6</b> shows the general hook-up, but without any poor attempt on my part to sketch a braid of 3 wires.
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<img src="wy-6.gif" width="275" height="255" border="2" vspace="5" hspace="9">
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<p>There are several questions about the feasibility of using such a system in modern times. The first quandary is whether we can build such a feeder system.</p>
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||||
<p>Modern insulated wire tends to use higher quality (lower loss) insulation than did the wire of yore. I would steer away from line cord, but modern wires use plastics with better RF characteristics, even if the only intend use is carrying DC. Since the system is designed for a low characteristic impedance, but with considerable SWR on the higher bands, I would recommend a heavy gauge wire, perhaps #12 or so. The actual characteristic impedance will depend on the thickness of the wire, the dielectric constant of the insulation material, and how tightly we hold the wires together. Consequently, I can give no exact figures.</p>
|
||||
<p>However, you can make up lengths of a proposed feedline and check the impedance in a number of ways with a variety of dummy loads and a low-level signal source. Any one of the current crop of antenna analyzers will give you a fairly accurate reading. Given the relatively high dielectric constant of the insulating material, expect to find a significant velocity factor, something in the 0.6 to 0.7 region.</p>
|
||||
<p>The next inquiry has to do with the effective inertness of the unused 3rd feeder wire. I re-created the model of the Y-doublet using parallel feedlines. Since twining the leads is not feasible in a physical model, I simply dropped the three leads straight down from the 50' level to 1' above ground. At that point, I connected two of the feeder ends with a 3-segment source wire. Again, all wires used a 1' segment length.</p>
|
||||
<p>The resulting feedpoint impedances are not accurate to the low-Z feeder system. However, that was not the point of the tests using the feeders with something over 800 Ohms as the characteristic impedance. The question was whether the unused antenna and feeder wires would remain inert relative to the active wires.</p>
|
||||
<p>As one measure, the following performance table shows the effects of the added copper losses of the physically modeled feedlines.</p>
|
||||
<pre>. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
|
||||
Y-Doublet Modeled Performance: 50' With Feeders
|
||||
|
||||
Freq. Gain TO Angle
|
||||
MHz dBi degrees
|
||||
3.6 3.35 34
|
||||
10.125 9.74 26
|
||||
18.118 9.60 14
|
||||
24.94 9.85 11
|
||||
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .</pre>
|
||||
<p>Gain remains virtually unchanged. So, too, do the patterns, and the outlines shown in <b>Fig. 3</b> and <b>Fig. 4</b> remain valid for the reconfigured model.</p>
|
||||
<p>A second test is to check the current distribution along both the unused antenna wire and the ostensibly inert feeder. I actually performed two tests, one with the unused feeder simply left open and another with the feeder extended 1 foot to touch the ground. The 12-meter current distribution graphic in <b>Fig. 7</b> remains valid for both.</p>
|
||||
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|
||||
<img src="wy-7.gif" width="532" height="349" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Note that the current line on the unused feeder and antenna wires is flat. The relative current magnitude under either test condition on all of the bands remained less than 1E-4 relative to a source current of 1.0.</p>
|
||||
<p>The modeling test, of course, has limitations relative to an actual twisted trio of wires. In the test, the modeled wires are widely separated and perfectly spaced along the entire 49' feeder run. How well the twisted trio performs may turn out to be as much a careful-construction issue as any other kind of issue.</p>
|
||||
<p>However, the tests suggest an alternative feed system that just might open up the Y-doublet to use on all of the HF bands. <b>Fig. 8</b> tells the story.</p>
|
||||
<center>
|
||||
<img src="wy-8.gif" width="277" height="326" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The Y-doublet on the traditional upper ham bands, 40 through 10 meters, can show feedpoint impedances in the thousands of Ohms, with considerable reactance. Indeed, shrinking or expanding the basic 80-meter legs may prove useful in reducing the high reactance levels that accompany lengths that are close to even numbers of half wavelengths. Commonly, we try to select for a doublet a feedline characteristic impedance that is about the geometric mean between the feedpoint impedance extremes that we are likely to encounter. There is a practical limit to this effort, since lines above 600-800 Ohms are difficult to produce. Hence, 600-Ohm or so open wire becomes typical for such applications.</p>
|
||||
<p>We can create a trio of pairs by using circular spacers of the type shown in the figure. For HF work, Plexiglas or polycarbonate spacers should be satisfactory. We can cut a hole in the center of each to reduce the weight. The holes can actually be slots if we add bridge wires to hold the spacers in place. In essence, we are adapting techniques normally used to create caged elements and applying them to the feedline. Such lines might permit the use of the antenna on all bands with a wide-range antenna tuner and will go a long distance in maintaining something close to the modeled ideal geometry we used in the test cases.</p>
|
||||
<p>Of course, should you choose to work with a system of this order, you can replace the alligator and crocodile clips of yore with an in-shack switching system to change the orientation of the pattern on all bands.</p>
|
||||
<p>The resurrected Y-doublet as some potential of still being serviceable today. There are many variables beyond the limits of this initial feasibility check, so success is not assured. However, for some hams who are restricted to backyard wire systems but who wish some directional flexibility, the system may be worth a try.</p>
|
||||
<p>With the wide spaced feeder system, the system may also be adaptable to 102', 88', 67', and 44' doublet lengths discussed in other notes at this site and in mountains of other literature. However, as with all horizontal doublets, the rule of thumb that calls for the maximum feasible height remains in play for effective operation.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 05-16-2003. © 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#doublet'>Doublet</a>, <a href='../topics.html#hf-multi-band'>HF Multi Band</a></p><br>
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