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<title>The Monoband Log-Cell Yagi Revisited Index</title>
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<h1><img src="../images/lpda.gif" align="left" width="150" height="83" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10">The Monoband Log-Cell Yagi Revisited</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Although the design had been known earlier, the monoband log-cell Yagi array was briefly popular in amateur literature in the late 1970s and early 1980s, largely through the work of Rhodes, K4EWG, Painter, W4BBP, and Zimmer, K4JZB. The purpose of this series is to contribute a little toward the re-evaluation of the log-cell Yagi, using NEC-4 as a means of analyzing various aspects of the design.</p>
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<p>In Part 1, I shall look briefly at a superior log-cell Yagi design, and then look at the performance characteristics of some pure Yagi designs that we might use as standards of comparison. In this way, we can begin to see more clearly where the log-cell Yagi fits into the amateur arsenal of antennas.</p>
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<p>In Part 2, we shall examine some basic principles behind the log cell itself, with especial attention to element phasing. One might also use LPDA principles to show how a log-cell works, but the basics of element phasing can make a number of facets of both Yagi and log-cell Yagi design somewhat clearer.</p>
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<p>In Part 3, we shall look at several (at least 4) practical 10-meter log-cell Yagi designs. I shall claim no great originality for any of the designs, although each has required considerable effort to optimize all of the operating characteristics, including gain, front-to-back ratio, and SWR bandwidth. All of the antenna designs will feature direct 50-Ohm feedpoint impedances.</p>
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<p>In the final section, Part 4, we shall examine the V-element question. Does bending half wavelength elements forward contribute anything useful to the performance of the log-cell Yagi? This question, of course, will involve us in a broader question of V-ing any half wavelength element.</p>
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<p><a href="logc1.html"><b>Part 1: An Introduction to the Log-Cell Yagi and Some Standards of Comparison</b></a></p>
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<p><a href="logc2.html"><b>Part 2: Element Phasing and Log-Cell Design</b></a></p>
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<p><a href="logc3.html"><b>Part 3: Some Practical Log-Cell Yagi Designs</b></a></p>
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<p><a href="logc4.html"><b>Part 4: Vee-ing the Log-Cell Yagi Elements</b></a></p>
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<p><i>Updated 7-14-2000. © L. B. Cebik, W4RNL. A version of these items appeared in</i> The National Contest Journal<i>. 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 Index</a></p>
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<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#lpda'>LPDA</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>The Monoband Log-Cell Yagi Revisited Part 1: An Introduction to the Log-Cell Yagi</title>
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<h1><img src="../images/lpda.gif" align="left" width="150" height="83" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10">The Monoband Log-Cell Yagi Revisited<br>
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Part 1: An Introduction to the Log-Cell Yagi and<br>
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Some Standards of Comparison</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Although the design had been known earlier, the monoband log-cell Yagi array was briefly popular in amateur literature in the late 1970s and early 1980s, largely through the work of Rhodes, K4EWG, Painter, W4BBP, and Zimmer, K4JZB.<sup>1</sup> Versions can be found in Orr and Cowan's <i>Beam Antenna Handbook</i>, and in the <i>ARRL Antenna Book</i>. In recent times, interest in the design has renewed.</p>
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<p><b>Fig. 1</b> shows the outline of a typical monoband log-cell Yagi. It consists of a log-cell driver consisting of 2 or more elements driven with a phasing line that reverses as it connects each element. The element set is fed at the forward-most position, much like a log-periodic dipole array (LPDA). To the driver cell are added a reflector (usually) and one or more directors.</p>
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<p>In <b>Fig. 2</b>, we see a common variant of the basic log-cell Yagi. In this case, favored especially by K4JZB, the elements are bent forward by about 40 degrees from linear each side of center.</p>
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<p>Since most of the articles on the log-cell Yagi appear before the advent of computer antenna modeling via MININEC and NEC, the claims for their performance are highly optimistic. One source reports a 6 element log-cell Yagi to have a gain of 16 dB, but it conveniently gives no reference standard. Most sources report gain to be greater than for Yagis of equivalent boom length, but these reports compare the log-cell Yagi with antennas deeveloped before computerized optimization of the Yagi design became commonplace. Perhaps only Rhodes and Painter stress operating bandwidth as a major advantage of the antenna design.</p>
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<p>With the renewed interest in the antenna, many potential users read the older claims as if they would stand up to modern scrutiny. However, to date, I have seen no re-evaluation of the log-cell Yagi design. Modern analytical tools, such as computer modeling, offer us a chance to better understand the antenna and to assess its place among monoband antennas used by amateurs.</p>
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<p>The purpose of this series is to contribute a little toward the re-evaluation of the log-cell Yagi, using NEC-4 as a means of analyzing various aspects of the design. Throughout, I shall uses 10-meters as a focal point, since this band is the widest of the upper HF amateur bands. In this introduction, I shall look briefly at a superior log-cell Yagi design, and then look at the performance characteristics of some pure Yagi designs that we might use as standards of comparison. In this way, we can begin to see more clearly where the log-cell Yagi fits into the amateur arsenal of antennas.</p>
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<p>In Part 2, we shall examine some basic principles behind the log cell itself, with especial attention to element phasing. One might also use LPDA principles to show how a log-cell works, but the basics of element phasing can make a number of facets of both Yagi and log-cell Yagi design somewhat clearer.</p>
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<p>In Part 3, we shall look at several (at least 4) practical 10-meter log-cell Yagi designs. I shall claim no great originality for any of the designs, although each has required considerable effort to optimize all of the operating characteristics, including gain, front-to-back ratio, and SWR bandwidth. All of the antenna designs will feature direct 50-Ohm feedpoint impedances.</p>
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<p>In the final section, Part 4, we shall examine the V-element question. Does bending half wavelength elements forward contribute anything useful to the performance of the log-cell Yagi? This question, of course, will involve us in a broader question of V-ing any half wavelength element.</p>
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<h3>A Real Log-Cell Yagi of Considerable Potential</h3>Let's begin with an advanced log-cell Yagi design using a 5-element log-cell plus a reflector and director. This 7-element array was extensively revised from a CB design sent to me by Alan Hughes, ZL3KR. The original had a free-space gain of about 9 dBi, but poor front-to-back ratio. In addition, the SWR and operating characteristics remained usable over only a very narrow portion of the spectrum.
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<img src="logc1-3.gif" width="394" height="359" border="2" vspace="5" hspace="9">
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<p>The basic dimensions of the refined model appear in <b>Fig. 3</b>. The material for the model is 1" diameter aluminum, although similar performance can be achieved with elements as small as 0.5" in diameter.</p>
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<p>The log-cell is designed as a true LPDA, with elements tapering in length and spacing as one moves forward toward the feedpoint. As one might expect, the reflector is the longest element of the entire set. However, the director is longer than the forward-most element of the log cell. Directors for log-cell Yagis must be cut for the operating frequency, while the forward element of the log cell will be resonant well above the highest operating frequency. The overall length of the antenna is about 14.6' or so, which would fit the antenna easily on a 15' boom.</p>
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<p><b>Fig. 4</b> provides a snapshot of antenna performance across all of 10 meters from 28.0 to 29.7 MHz. The highest free-space gain is at the upper end of the band, with the free- space gain at 28 MHz being just above 8 dBi. The first MHz of the band also shows a very high and stable front-to-back ratio of 30 dB or more, with the figure at 28.5 MHz exceeding 40 dB. The 50-Ohm SWR of this antenna remains well below 2:1 across the entire 10-meter band.</p>
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<p>Actually building this antenna would require element length adjustment if an element diameter tapering schedule is used. However, nothing in the design would require special construction except perhaps the 100-Ohm phasing line for the log cell. We shall return to this and other practical designs later in the series. First, let's consider whether the antenna is worth building. For that evaluation, we need some standards of comparison.</p>
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<h3>Some Standards of Comparison</h3>Since the days in which log-cell Yagis were claimed as higher gain, more compact beam designs than pure Yagis, the understanding of Yagi design has improved considerably. Lawson's <i>Yagi Antenna Design</i><sup>2</sup> has become the basic volume for modern Yagi design. In addition, there are several Yagi optimizing programs whose results correlate well with NEC models, assuring the builder of predictable results. Consequently, monoband Yagi designs as we approach the end of the century are quite different from those of 15 to 20 years ago.
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<p>Because the most common comparator for a log-cell Yagi is a pure monoband Yagi, perhaps it may be useful to examine some of the operating characteristics of several good Yagi designs. Let's begin with designs I refer to as medium-bandwidth arrays, because they hold their operating characteristics from 28 MHz up to 29 MHz--or close to it. We shall look at three designs in particular.</p>
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<p>First is a 3-element Yagi on an 8' boom (3-8). The actual overall length of the antenna is about 7.5'. The design is adapted from one of Dean Straw's (N6BV) design in the collection of antennas accompanying the program YA.<sup>3</sup> <b>Table 1</b> provides the modeled dimensions for this and the other two antennas in the medium bandwidth group.</p>
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<pre> Medium-Bandwidth 3- and 4-Element 10-Meter Yagi Dimensions
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1. 3-Element, Short-Boom (< 8') Yagi: 0.5" diameter aluminum elements
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Element Length (") distance from reflector (")
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Reflector 211.9 ------
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Driver 193.8 36.0
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Director 184.9 90.0
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2. 3-Element, Long-Boom (<12') Yagi: 0.5" diameter aluminum elements
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Element Length (") distance from reflector (")
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Reflector 206.3 ------
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Driver 197.0 62.4
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Director 185.3 134.5
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3. 4-Element, 13'-Boom Yagi: 0.5" diameter aluminum elements
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element length (") Distance from Reflector (")
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reflector 207.5 ------
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driver 195.9 35.8
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director 1 194.4 65.5
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director 2 182.2 152.2
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Table 1: Dimensions of medium-bandwidth Yagis used as standards of comparison</pre>
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<p>The second design is adapted from Brian Beezley's (K6STI) design in the samples accompanying AO.<sup>4</sup> This longer-boom design (3-12) is actually about 11.2' long and fits easily on a 12' boom. The third design (4-13), again adapted from an N6BV design, uses 4 elements in under 12.7' of length for an easy fit in a 13' boom.</p>
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<p>For this exercise, all designs use uniform diameter elements. All are modeled on NEC-4 in free space for the purposes of direct comparison.<sup>5</sup> The driven elements of the 3-element beams have been resonated so that SWR figures can be taken relative to the resonant impedance. Because the 4-element beam had a somewhat lower impedance, it has been equipped with a beta match, that is, a shorted transmission-line stub to effect a match compatible with 50Ω coax. The 3-element beams can be matched with a quarter wavelength matching section, or their drivers can be shortened for use with a beta match.</p>
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<p><b>Fig. 5</b> provides a sweep of the free-space gain of each beam design from 28.0 to 29.0 MHz. As one might expect, the 3-element, 8' boom model shows the lowest gain--just above 7 dBi. However, the gain is fairly constant across the selected portion of the band.</p>
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<p>The long-boom 3-element Yagi shows considerably higher gain, averaging nearly a full dB above the short-boom model. Because the boom length is close to the limit of stable operation for a considerable bandwidth, the curve shows greater changes with increasing frequency.</p>
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<p>In contrast, the 4-element Yagi shows only slightly higher gain than the 3-element long-boom model. However, the boom length is only about a foot greater than the long-boom 3-element Yagi. What the fourth element provides is more even gain across the selected bandwidth.</p>
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<p>In <b>Fig. 6</b>, we get a picture of the 180-degree front-to-back ratio of the antennas. Interestingly, the short-boom 3-element Yagi shows the highest peak front-to-back ratio and the highest average front-to-back ratio across the band, never falling below 20 dB. Comparatively, the long-boom 3-element Yagi shows a good peak front-to-back ratio, but the value falls below 20 dB between 28.7 and 28.8 MHz. The 4-element Yagi shows a much lower value of peak front-to-back ratio, but the overall curve is smooth and falls below 20 dB only at the lower edge of the band.</p>
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<p>From these two parameters alone, we can obtain an impression of the designs. The short-boom 3-element and the 4-element designs are conservative. However, the long-boom 3-element design is pressing the limits of what is possible for that number of elements and boom length. One might obtain even higher gain, but at the expense of an even narrower bandwidth for the operating characteristics.</p>
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<p>The impression is further deepened in the SWR curves in <b>Fig. 7</b>. The long-boom model shows under 2:1 SWR relative to the resonant impedance through 28.9 MHz. The short-boom 3-element Yagi easily achieves a 2:1 SWR bandwidth relative to the resonant impedance that is wider than the selected band portion. Despite the slight narrowing of the long-boom SWR bandwidth, the use of a beta match would likely permit a wider operating bandwidth at the 50-Ohm matched value. This is illustrated by the 4-element Yagi 50-Ohm bandwidth, which shows under 2:1 SWR across the band. The native bandwidth relative to the antenna's resonant impedance would be about 800 kHz. (However, there may be slight losses associated with operating a beta match well off its optimal values, despite the resulting good impedance match.)</p>
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<p>These three antennas are good designs of their types, despite the limitations of each. However, they are not adequate to cover the entirety of 10-meters. For that, we must turn to wide-band designs. The dimensions of two wide-band Yagi designs appear in <b>Table 2</b>.</p>
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<pre> Wide-Bandwidth 3- and 4-Element 10-Meter Yagi Dimensions
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1. 3-Element, Long-Boom (<12') Yagi: 1.0" diameter aluminum elements
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Element Length (") distance from reflector (")
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Reflector 214.0 ------
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Driver 195.6 74.5
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Director 176.0 134.5
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2. 4-Element, Short-Boom (8') Yagi: 0.5" diameter aluminum elements
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element length (") Distance from Reflector (")
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reflector 212.0 ------
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driver 205.0 40.5
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slaved driver 189.0 44.0
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director 181.0 96.0
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table 2: dimensions of wide-bandwidth Yagis used as standards of comparison</pre>
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<p>One design is a 3-element Yagi. This version was developed by Joe Reisert, W1JR, and is similar to a design published by Bill Orr, W6SAI, in Ham Radio many years ago.<sup>6</sup> The boom would be 12' long to hold an antenna whose inherent length is about 11.2' or so. The other design is my own, which fits on a 8' boom and uses 4 elements. The extra element is a second driver open-sleeve coupled to the first such that the two together cover all of 10-meters.</p>
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<p>As shown in <b>Fig. 8</b>, the gain curves of the two antennas are very similar, with the 4-element model having a slight edge at the lower end of the band, a function of the dual driver system. It is notable that in a 3-element design, wide-banding the gain requires a boom length similar to that of the higher-gain medium-bandwidth long-boom model--about 12'.</p>
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<p>The front-to-back curves in <b>Fig. 9</b> once more do not give one design a major edge over the other. The dip in value below 20 dB occurs at opposite ends of the band for the two designs--but might be made more coincident with slight redesign of element lengths and spacings. The 4-element model shows a very high front-to-back ratio peak around 29 MHz, where all 4 elements show the highest activity in terms of current magnitude.</p>
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<img src="logc1-10.gif" width="646" height="526" border="2" vspace="5" hspace="9">
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<p>The SWR curves for both wide-band antennas in <b>Fig. 10</b> are referenced to 50 Ohm without need for a matching system. The 3-element antenna easily achieves a 2:1 operating bandwidth that covers the entire band. The open-sleeve coupled drivers of the 4-element model allow superior performance in this department, with an SWR that never rises to 1.2:1 across the entire band.</p>
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<h3>Relevant Comparisons</h3>The standards of comparison we have just established will be used throughout this series in various ways. To illustrate how we may sensibly use them, we may return to the 7-element log-cell Yagi that we briefly described. The 4-element medium-bandwidth Yagi matches the log-cell Yagi in gain on a boom that is 1-2 feet shorter. However, that antenna is limited to only about 1 MHz of the band, while the log-cell Yagi provides coverage of the entire band. The gain of the log-cell Yagi is from 1 to 1.5 dB greater than either of the 2 wide-band beams discussed.
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<p>In front-to-back ratio, the log-cell Yagi is superior to all of the standard designs, with better than 30 dB until well past 29 MHz and better than 22 dB across all of the band. One of the areas in which well-designed log-cell Yagis excel is in front-to-back ratio. The log-cell Yagi also has a 50-Ohm SWR well under 2:1 across the entirety of the 10-meter band, matching both wide-band Yagis in that performance category.</p>
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<p>For a given boom length, then, a log-cell Yagi does not make its claim to fame at the end of the 20th century in the gain department. Advances in pure monoband Yagi design give the edge to the pure Yagi. What may have been true of 1980 Yagi designs is no longer true today.</p>
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<p>However, well-designed log-cell Yagis can achieve very wide operating bandwidths, not only with respect to SWR, but as well with respect to operating characteristics. In particular, the log-cell Yagi has the potential for very smooth front-to-back ratio curves at very high levels across a band as wide as 10 meters.</p>
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<p>There is, of course, a cost involved in achieving these goals: extra elements and their associated weight. In addition, the log cell requires careful design with considerable attention to the phasing line that interconnects the phased driven elements. To the subject of element phasing we shall turn in Part 2.</p>
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<h3>Notes</h3>
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<p>1. For information on various log-cell Yagi designs see the following items on this incomplete literature list:</p>
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<p>P. D. Rhodes, K4EWG, and J. R. Painter, W4BBP, "The Log-Yagi Array," <i>QST</i>, Dec, 1976. The main elements of this article are reprinted in The ARRL Antenna Book, 18th Ed., pp. 10-25 to 10-27.</p>
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<p>Robert F. Zimmer, K4JZB, "Development and Construction of 'V' Beam Antennas," CQ, Aug., 1983, pp. 28-32; and "Three Experimental Antennas for 15 Meters," <i>CQ</i>, Jan., 1983, pp. 44-45.</p>
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<p>W. I. Orr, W6SAI, and S. D. Cowan, W2LX, <i>Beam Antenna Handbook</i>, pp. 251-253. John J. Meyer, N5JM, "A Simple Log-Yag Array for 50 MHz," <i>Antenna Compendium</i>, Vol. 1, pp 62-63.</p>
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<p>Reference to log-cell Yagis is also made by L. A. Moxon, <i>HF Antennas for All Locations</i>, 2nd Ed., pp. 199-200, but the design shown is the Rhodes-Painter version in The ARRL Antenna Book.</p>
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<p>2. James L. Lawson, W2PV, <i>Yagi Antenna Design</i> (ARRL, 1986).</p>
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<p>3. YA is a Yagi Analysis program developed by Brian Beezley, K6STI, and accompanies recent editions of <i>The ARRL Antenna Book</i>.</p>
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<p>4. AO is a MININEC analysis and antenna optimizer program by K6STI that is no longer available.</p>
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<p>5. Two commercial implementations of NEC-4 are available: EZNEC Pro by Roy Lewallen, W7EL, P.O. Box 6658, Beaverton, OR 97007; and GNEC by Nittany Scientific, 1733 West 12600 South, Suite 420, Riverton, UT 84065. Use of NEC-4 requires licensure from the University of California. Fortunately, most of the analysis in this series can be replicated using more easily obtained versions of NEC-2.</p>
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<p>6. Joe, Reisert, W1JR, "Yagi/Uda Antenna Design: Part 1: A Different Approach," <i>Communications Quarterly</i>, Winter, 1998, pp. 49-59. Orr's version of the antenna appeared in his regular column for <i>Ham Radio</i>, May, 1990.</p>
|
||||
<p>Also see the <a href="../model/nec.html">Antenna Modeling Programs</a> page for more information about modeling software.</p>
|
||||
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|
||||
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<p><i>Updated 1-15-2000. © L. B. Cebik, W4RNL. A version of this item appeared in</i> The National Contest Journal<i>, Jan-Feb, 2000, 19-22. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
|
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<br>
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<a href="logc2.html"><img src="../images/fwdrd.gif" width="40" height="40"> <b>Go to Part 2</b></a>
|
||||
<p><a href="logc.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> <b>Go to Log-Cell Yagi Index</b></a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#lpda'>LPDA</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>The Monoband Log-Cell Yagi Revisited Part 2: Element Phasing and Log-Cell Design</title>
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<h1><img src="../images/lpda.gif" align="left" width="150" height="83" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10">The Monoband Log-Cell Yagi Revisited<br>
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Part 2: Element Phasing and Log-Cell Design</h1>
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<h3>L. B. Cebik, W4RNL</h3>
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<p>Monoband log-cell Yagis have been designed using anywhere from 2 to 5 elements in the log cell itself. They may or may not use a reflector, and the number of directors has ranged from 1 to 3 in the designs I have encountered. Some log-cell designs have been very casual, while others (such as the Rhodes-Painter array<sup>1</sup>) have adhered to rigorous LPDA design procedures. Since the key to a log-cell Yagi is the log cell itself, it may be useful for us to spend some time exploring some facets of its design.</p>
|
||||
<p>When the log cell has only 2 elements, one cannot distinguish it from a 2-element phased array. Indeed, one can build a successful beam by adding a director to a 2-element phased array--if the phased elements are properly designed. So let's begin with this simplified case and then proceed to more complex log cells.</p>
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|
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<h3>The Phasing of 2 Elements</h3>
|
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</center>
|
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<p>Element phasing refers to the relative current magnitude and phase of each element in an array of elements. The current magnitude and phase are ordinarily read at the center of elements in symmetrical arrays in which each element length is in the vicinity of 1/2 wavelength.</p>
|
||||
<p>By this accounting, a 2-element Yagi is a phased array, even though only the driven element is fed. The current magnitude and phase on the parasitic reflector is a function of coupled energy from the driver. We alter the current magnitude and phase on the rear element by varying the lengths of the elements and the spacing between them. For a simple 2-element driver-reflector Yagi, we have limited abilities to adjust the rear element relative current magnitude and phasing through modifying the antenna geometry itself. For example, the rear lobe gain of such arrays is rarely more than 12 dB below the main forward lobe.</p>
|
||||
<p>By some judicious alterations of geometry, we can change the rear element current magnitude and phase to improve the depth of the rear null. One of the most remarkable designs in this regard is the Moxon rectangle. Folding the elements toward each other at the ends results in a rear element current magnitude and phase for the element spacing that yields a very deep rear null--often better than 35 dB below the main forward lobe at the design frequency.</p>
|
||||
<p>As an alternative to the limitations of geometric means of altering the rear element relative current magnitude and phase, we can directly feed both elements of the array. Let's adopt the convention that the forward element will be set at a relative current value of 1.0 at a phase angle of zero degrees. With this constant, we may then focus on the current magnitude and phase angle of the rear element (always relative to the constant values of the forward element).</p>
|
||||
<p>The required current magnitude and phase on the rear element will depend upon several variables. First are the lengths of the elements. We may make them equal or unequal. Moreover, we may set the lengths close to resonance or distant from resonance. Each variation will show changes in either or both the magnitude and the phase on the rear element for a desired operating characteristic of the array. For example, if the elements, whether equal in length or unequal, show a feedpoint impedance close to resonance when only the forward element is fed, then the phase angles of equal length and unequal length element sets will be very close in value, although the current magnitudes will vary for a given spacing and operating condition.</p>
|
||||
<p>Second, element spacing will have a major affect on the required rear element current magnitude and phase for a desired operating characteristic. Third, the desired operating characteristic will also alter the current magnitude and phase for any set element lengths and spacing.</p>
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<img src="logc2-1.gif" width="400" height="289" border="2" vspace="5" hspace="9">
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<p>As a little experiment, let's look at what happens when we phase both elements of two different array pairs, shown in <b>Fig. 1</b>. At a spacing of about 0.125 wavelength, the unequal element pair makes up a very workable 2-element Yagi for 28.5 MHz, when only the forward element is fed. At the same spacing, the equal-length pair is close to resonant, but with a typical dipole pattern.</p>
|
||||
<p>Now let's set as our operating goal achieving a maximum rear null 180 degrees from the peak of the forward lobe. We can define the null as adequate if it exceeds -50 dB relative to the forward lobe. This value would exist only over a tiny bandwidth, but for study purposes, it is a goal that modeling programs, such as NEC-4, can easily show. We shall vary the distance between the elements in 0.05 wl increments. For each distance, we shall change the current magnitude and phase on the rear element until the desired null is achieved.</p>
|
||||
<pre> Phasing 2 Elements for Maximum Rear Null
|
||||
Equal vs. Unequal Element Lengths
|
||||
|
||||
Rear Element Current
|
||||
Spacing Spacing Magnitude Phase Free-Space Front-to-Back
|
||||
wl inches (relative) degrees Gain dBi Ratio dB
|
||||
|
||||
Designed for Maximum Rear Null:
|
||||
Equal-Length Elements (196.8" x2 at 28.5 MHz)
|
||||
0.05 20.7 1.035 163 6.55 >50
|
||||
0.1 41.4 1.07 145 6.46 >50
|
||||
0.15 62.1 1.09 125.5 6.18 >50
|
||||
0.2 82.8 1.09 106 5.76 >50
|
||||
0.25 103.5 1.07 87 5.14 >50
|
||||
0.3 124.2 1.045 69 4.26 >50
|
||||
0.35 144.9 1.02 51 2.72 >50
|
||||
0.4 165.7 1.00 34 0.31 >50
|
||||
|
||||
Unequal-Length Elements (192" forward, 208.1" rear at 28.5 MHz)
|
||||
0.05 20.7 0.925 163.3 6.57 >50
|
||||
0.1 41.4 0.945 145 6.45 >50
|
||||
0.15 62.1 0.955 126.0 6.19 >50
|
||||
0.2 82.8 0.95 106.7 5.77 >50
|
||||
0.25 103.5 0.94 88 5.16 >50
|
||||
0.3 124.2 0.92 69.5 4.21 >50
|
||||
0.35 144.9 0.90 51.8 2.73 >50
|
||||
0.4 165.7 0.88 34.5 0.28 >50
|
||||
|
||||
Note 1: All forward element currents set at a relative magnitude of 1.0 at 0° phase angle.
|
||||
|
||||
Note 2: All values of rear current relative magnitude and phase angle taken when the rear
|
||||
null passed -50 dB relative to the forward lobe.
|
||||
|
||||
Note 3: Elements are 1" diameter aluminum.
|
||||
|
||||
Table 1: Phasing 2 elements for maximum rear null using equal and unequal element
|
||||
lengths.</pre>
|
||||
<p><b>Table 1</b> shows the results for both element pairs. As predicted, the current phase for each step is virtually the same for both arrays, but the required current magnitude on the rear element is different according to whether the elements have the same or different lengths. Other element lengths we might have chosen would have resulted in other values.</p>
|
||||
<p>For each increase in spacing, the current magnitude changes very little with each array, but the required phase angle on the rear element shows a continuous decrease. In short, there is no single ideal spacing for achieving a deep rear null. Instead, for any spacing, there is a current magnitude and phase angle that will achieve the null.</p>
|
||||
<pre> Phasing 2 Elements for Maximum Forward Gain
|
||||
Equal vs. Unequal Element Lengths
|
||||
|
||||
Rear Element Current
|
||||
Spacing Spacing Magnitude Phase Free-Space Front-to-Back
|
||||
wl inches (relative) degrees Gain dBi Ratio dB
|
||||
|
||||
Designed for Maximum Rear Null:
|
||||
Equal-Length Elements (196.8" x2 at 28.5 MHz)
|
||||
0.05 20.7 1.02 173 7.32 7.64
|
||||
0.1 41.4 1.03 165 7.35 7.19
|
||||
0.15 62.1 1.02 158 7.23 6.90
|
||||
0.2 82.8 1.03 152 7.03 6.00
|
||||
0.25 103.5 1.03 147 6.76 5.03
|
||||
|
||||
Unequal-Length Elements (192" forward, 208.1" rear at 28.5 MHz)
|
||||
0.05 20.7 0.91 173 7.33 7.70
|
||||
0.1 41.4 0.92 166 7.36 7.22
|
||||
0.15 62.1 0.92 159 7.24 7.03
|
||||
0.2 82.8 0.92 150 7.04 6.59
|
||||
0.25 103.5 0.93 147 6.77 5.13
|
||||
|
||||
Note 1: All forward element currents set at a relative magnitude of 1.0 at 0° phase angle.
|
||||
|
||||
Note 2: All values of rear current relative magnitude and phase angle taken when the
|
||||
forward lobe reached a peak gain, beyond which gain fell off.
|
||||
|
||||
Note 3: Elements are 1" diameter aluminum.
|
||||
|
||||
Table 2: Phasing 2 elements for maximum forward gain using equal and unequal element
|
||||
lengths.</pre>
|
||||
<p>Much of antenna element phasing theory is devoted to the achievement of rearward nulls. Little attention has been given to achieving maximum gain from the array. Let's look at <b>Table 2</b> to see what the effects of changing space might have on the required rear element relative current magnitude and phase for this goal. For spacing from 0.05 through 0.25 wavelengths, the required current magnitude for each array remains relatively constant. However, the required phase angle decreases with increased spacing, but at far less than the rate for achieving a maximum rearward null. Maximum gain does not occur with the closest spacing, but in the vicinity of 0.1 wl. As one might expect, the front-to-back ratio of two elements becomes mediocre (at best) when the goal is maximum gain.</p>
|
||||
<p>The reason I have presented the table of values for maximum forward gain is simple: when designing an array with a pair of phased elements plus some further element--such as a director--the proper design procedure is to set the phased pair of elements for maximum forward gain. It will be the added element (or elements) that shapes the antenna's operating pattern to the desired specifications.</p>
|
||||
<center>
|
||||
<img src="logc2-2.gif" width="377" height="267" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Let's examine a test array consisting of a phased pair plus a director, as shown in <b>Fig. 2</b>. The phased portion of the array consists of unequal-length elements. In this design, a 50-Ohm phase line about 69.3" (for 0.66 VF line) provides the requisite current magnitude and phase transformation. (Although 50-Ohm parallel line is not possible using round conductors, parallel strips can be used, with the velocity factor adjusted back to 1.0. If the boom is RF transparent, then coaxial cable can also be used.) The design frequency for this test array is 28.5 MHz.</p>
|
||||
<center>
|
||||
<img src="logc2-3.gif" width="640" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 3</b> shows two things at once. One azimuth pattern show what happens if we omit the director. The phased pair is set for maximum gain--or very close to it. Adding the director increases gain, but even more significantly, the director increases the front-to-back ratio to a very respectable level. (Even in pure Yagi design, reflectors do not control the front-to-back ratio nearly so much as do the directors.)</p>
|
||||
<center>
|
||||
<img src="logc2-4.gif" width="647" height="524" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Let's look more closely at the performance of this antenna across the first MHz of 10 meters. <b>Fig. 4</b> graphs the gain across the band, with the 4-element Yagi presented as a comparator in Part 1 as a standard for comparison. Both antennas are about the same overall length--a bit over 12.5' long. The 3-element array (labeled "3-L 2-cell" on the graphs) shows a very steep gain curve, especially when compared to the stable 4-element Yagi curve. At the design center frequency (28.5 MHz), the 3-element array actually show slightly better gain.</p>
|
||||
<center>
|
||||
<img src="logc2-5.gif" width="646" height="524" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The front-to-back curves appear in <b>Fig. 5</b>. The 3-element array shows a very high peak value at the design frequency, but exceeds 20 dB for less than half of the bandwidth in the graph. The stability of the 4-element Yagi front-to-back ratio across the band is self- evident.</p>
|
||||
<center>
|
||||
<img src="logc2-6.gif" width="647" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The native feedpoint impedance of the 3-element array is about 15 + j23 Ohms. This value is amenable to a beta match using an open stub (instead of the usual shorted stub used when the reactance is capacitive). 2:1 SWR operation across all of the first MHz of 10 meters is not possible, as shown in <b>Fig. 6</b>.</p>
|
||||
<p>The narrow-band characteristics of this array illustrate in part what happens when 2- element phased pairs are operated too close to maximum gain. Nevertheless, scaled for any of the WARC band, this array might provide quite good performance with a minimum of elements.</p>
|
||||
<center>
|
||||
<h3>More Complex Log Cells</h3>
|
||||
</center>
|
||||
<p>Larger log cells are often designed exactly as one might design a full LPDA, except that the design will be for a single band and also be considerably shorter that an independent LPDA, as illustrated in <b>Fig. 7</b>. The design principles for LPDAs are fully described in The ARRL Antenna Book and in standard professional antenna compendia, so I shall not review them in detail here.<sup>2</sup> Most of the math can be passed through a computer design program, such as LPCAD by Roger Cox, WB0DGF.<sup>3</sup> To these resources, we can add only a few practical notes.</p>
|
||||
<center>
|
||||
<img src="logc2-7.gif" width="380" height="296" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>First, many LPDA and log-cell designers select too high a phase-line impedance to achieve maximum gain from the array. My experiences designing a monoband LPDA suggest strongly that the lowest practical phase-line impedance yields the highest gain and overall operating characteristics. This procedure may require careful rethinking of the mechanical aspects of the design, especially implementing a low impedance phase line with double-boom construction or other means.</p>
|
||||
<p>Second, the fatter the elements, the higher the cell gain and the wider the bandwidth for the desired operating characteristics. For monoband cells and LPDAs at 10 meters, elements should be at least 0.5" in diameter, with diameters up to 1" desirable.</p>
|
||||
<p>Third, the closer one attends to making the cell in accord with the LPDA principles in which both element lengths and spacings decrease together, the wider-band the resulting cell and array. One test of a good log cell--as we shall illustrate in more detail in Part 3--is that the feedpoint impedance of the log cell without added parasitic elements should not radically change from the feedpoint impedance with those elements in place.</p>
|
||||
<p>Even with these practical notes in mind, a good modeling program is a major aid to log-cell Yagi and LPDA design. Every cell design requires Tw<sup>2</sup> (Twisting and Tweaking), that is, final adjustment of element lengths and spacings, along with phase-line impedance value settings, to produce the desired operating characteristics of the antenna.</p>
|
||||
<center>
|
||||
<img src="logc2-8.gif" width="424" height="294" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>To illustrate this point, let's look briefly at an LPDA--a log cell without additional parasitic elements--for 10 meters. <b>Fig. 8</b> shows the outlines of the antenna, which is given in two versions, one with a 217" rear element, the other with a longer 218.9" element. The 75- Ohm phase line can be achieved with twin square booms or with facing aluminum strips. Although the basic dimensions emerged from LPDA calculations, the final dimensions are the result of considerable tweaking.</p>
|
||||
<p>Because this antenna sought to combine smooth curves of both acceptable gain and an adequate adequate front-to-back ratio, a ratio of about 0.90 was selected. That is, each element forward of the one to its rear is about 0.90 of its length. Moreover, the element spacing moving forward is also 0.90 of the spacing between the next elements rearward. As we shall see in Part 3, practical log cell design for log-cell Yagis employs a ratio closer to about 0.95.</p>
|
||||
<center>
|
||||
<img src="logc2-9.gif" width="646" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The gain across the entire span of 10-meters appears in <b>Fig. 9</b>, with the curve for the 4-element wide-band Yagi from Part 1 added for comparison. The LPDAs and the wide-band 4-element Yagi are both 8' long. Version 2 of the LPDA provides slightly higher gain than version 1. Both curves are more stable across the band than is the Yagi curve.</p>
|
||||
<center>
|
||||
<img src="logc2-10.gif" width="646" height="524" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Although version 1 of the LPDA has slightly less gain than version 2, the first version shows an overall better front-to-back profile across the band, with a very high peak at 28.5 MHz, as shown in <b>Fig. 10</b>. Both version of the LPDA exceed the Yagi in average front-to- back ratio across the band.</p>
|
||||
<center>
|
||||
<img src="logc2-11.gif" width="647" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>In <b>Fig. 11</b>, we have the 50-Ohm SWR curves for all three antennas, none of which requires a matching network. With a peak SWR value of 1.35:1, there is little to choose among the antennas in this department.</p>
|
||||
<p>A 4-element log-cell designed for 10-meters without parasitic elements is capable of better than 7 dBi free-space gain all across the band with excellent front-to-back ratio values and an easy direct coax match--all on an 8' boom. This becomes another standard of comparison for log-cell designs by giving us a new question for our list: what advantages do parasitic elements give us?</p>
|
||||
<p>A partial answer to that question showed up in the narrow-band, high-gain, high-front- to-back design we discussed earlier. We can add some gain and possibly improve the front- to-back ratio. We shall do that by designing our log cells to enhance gain rather than striving for a balance of operating characteristics. Parasitic elements will finish the job of tailoring the pattern.</p>
|
||||
<p>We shall encounter some practical designs that casually design the cell and some that design it very carefully. The results of each practice will show themselves in the resulting antenna performance. But all of that is for Part 3.</p>
|
||||
<center>
|
||||
<h3>Notes</h3>
|
||||
</center>
|
||||
<p>1. P. D. Rhodes, K4EWG, and J. R. Painter, W4BBP, "The Log-Yagi Array," <i>QST</i>, Dec, 1976. The main elements of this article are reprinted in <i>The ARRL Antenna Book</i>, 18th Ed., pp. 10-25 to 10-27.</p>
|
||||
<p>2. See <i>The ARRL Antenna Book</i>, 18th Ed., pp. 10-1 to 10-6, plus such professional references as Johnson and Jasik.</p>
|
||||
<p>3. LPCAD has been available at many world wide web archives, but availability may vary in this fast-changing medium. LPCAD 2.8 appeared in 2000.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 3-5-2000. © L. B. Cebik, W4RNL. A version of this item appeared in</i> The National Contest Journal<i>, Mar-Apr, 2000, 10-13. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
|
||||
<br></p>
|
||||
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|
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</center><br>
|
||||
<br>
|
||||
<a href="logc3.html"><img src="../images/fwdrd.gif" width="40" height="40"> <b>Go to Part 3</b></a>
|
||||
<p><a href="logc.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> <b>Go to Log-Cell Yagi Index</b></a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#lpda'>LPDA</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<h1><img src="../images/lpda.gif" align="left" width="150" height="83" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10">The Monoband Log-Cell Yagi Revisited<br>
|
||||
Part 3: Some Practical Log-Cell Yagi Designs</h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p>In this part of our visit to the log-cell Yagi, we shall look at some practical designs. The first two versions--using log cells of 2 and 3 elements, respectively--will involve casual designs, typical of those in some of the past literature. Then, we shall examine more complex designs using log cells with 4 and 5 elements, each carefully constructed on LPDA principles. In the process, we shall also look at a test we can perform to estimate the chances for a log cell Yagi performing to its fullest potential.</p>
|
||||
<p>Each of our design examples will use a reflector and a director in addition to the log-cell driver. Hence, the total element count will be two greater than the number of elements in the cell. As with all of the models in this series, the designs will be for 10 meters. Scaling to 20 meters in one direction and to 6 meters in the other direction are straightforward tasks.</p>
|
||||
<p>All models will use uniform diameter elements. Actual element lengths will have to be lengthened if a builder chooses a tapered diameter schedule. Additionally, the builder will have to devise a plan for implementing the phase line associated with each log cell. High impedance lines can be fabricated from round wires. Low impedance lines may require the use of flat aluminum strap or of a double square boom to effect a satisfactory phase line.</p>
|
||||
<center>
|
||||
<h3>Casual 4- and 5-Element Log-Cell Yagis</h3>
|
||||
</center>
|
||||
<p>Our initial models employ either 2 or 3 elements in the log cell, as illustrated in <b>Fig. 1</b>. Both models use 200-Ohm phase lines, with driver elements spaced a standard 2' apart. This spacing accords with a number of articles from the past, although the magic in its selection eludes me. The resulting 4-element log-cell Yagi is 96" (8') long, while the 5-element log-cell Yagi is 138" long (11.5'). Coincidentally, these two lengths coincide closely with the lengths of the medium-bandwidth Yagis introduced in Part 1 as comparators for log-cell Yagis. You should keep the graphs for those antennas handy as we examine the 2 new designs. Both of the antennas in <b>Fig. 1</b> use 1" diameter elements.</p>
|
||||
<center>
|
||||
<img src="logc3-1.gif" width="386" height="484" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Both log-cell Yagis exhibit very smooth gain curves over the first MHz of 10 meters, as demonstrated in the frequency sweep graph in <b>Fig. 2</b>. The 4-element antenna with only 2 elements in the cell has the lower gain level, as one might expect. It coincides roughly with the gain curve for the 8' 3-element Yagi of Part 1. The 5-element antenna provides only about a half dB of additional gain. In contrast, the 3-element Yagi of the same boom length in Part 1 provides an average free-space gain of about 8 dBi, another half dB greater than the log-cell Yagi with the same boom length.</p>
|
||||
<center>
|
||||
<img src="logc3-2.gif" width="647" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Fig. 3</b> shows that the two log-cell Yagi designs provide fairly mediocre front-to-back ratios. No where in the specified bandwidth does the front-to-back ratio of either antenna reach 18 dB. (In contrast, both Yagi designs exceed 20 dB front-to-back ratio for most of the first MHz of 10 meters.) Where the log-cell Yagis have an advantage is in the feedpoint impedance. Both designs, as illustrated in <b>Fig. 4</b>, provide less than 2:1 50-Ohm SWR from 28 to 29 MHz. By way of contrast, the two Yagi designs require a beta match or comparable network to yield similar results.</p>
|
||||
<center>
|
||||
<img src="logc3-3.gif" width="647" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<img src="logc3-4.gif" width="646" height="525" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The two log-cell Yagis, then, require extra elements to provide performance that fails to equal the performance of well-designed 3-element Yagis. One only skirts the issue by saying that the failure results from casual design, since that statement gives no clue of how to distinguish casual from careful design. However, there is a fairly simple modeling test we can perform as a measure of a log-cell Yagi's performance.</p>
|
||||
<p>If we extract the log-cell driver elements from the overall antenna, we may model them independently. In a well-designed log-cell driver, the array will show fairly high gain and a feedpoint impedance that does not depart radically from the values obtained when the driver is part of the total log-cell Yagi.</p>
|
||||
<p><b>Table 1</b> provides values for the 2- and 3-element log cells extracted from the antennas we have been examining. The check points at 28, 28.5, and 29 MHz for both cells show fairly low gain, with the 2-element cell especially low. (Although registered for reference, the low front-to-back ratios are of no concern in this test.) The feedpoint impedances of the cells are roughly one-fourth the values obtained for the complete antennas. We shall want to keep these figures in mind as we check more complex and more carefully designed log-cell drivers.</p>
|
||||
<pre> 2- and 3-Element Log Cell Independent Performance
|
||||
|
||||
Frequency (MHz) 28.0 28.5 29.0
|
||||
|
||||
2-Element Log Cell:
|
||||
|
||||
Free-Space Gain (dBi) 4.58 4.70 4.83
|
||||
|
||||
Front-to-Back Ratio (dB) 6.88 7.21 7.48
|
||||
|
||||
Feedpoint Impedance
|
||||
(R +/- jX Ohms) 13 + j 0 12 + j 5 11 + j11
|
||||
|
||||
3-Element Log Cell:
|
||||
|
||||
Free-Space Gain (dBi) 7.09 6.93 6.74
|
||||
|
||||
Front-to-Back Ratio (dB) 11.6 11.9 12.0
|
||||
|
||||
Feedpoint Impedance
|
||||
(R +/- jX Ohms) 11 - j22 9 - j 8 8 + j 3
|
||||
|
||||
|
||||
Table 1: 2- and 3-element log cell independent performance.</pre>
|
||||
<center>
|
||||
<h3>A 6-Element Log-Cell Yagi</h3>
|
||||
</center>The 6-element log-cell Yagi, with a 4-element log cell, shown in <b>Fig. 5</b>, is adapted and scaled from the Rhodes and Painter log-cell Yagi for 20 meters in The ARRL Antenna Book.<sup>1</sup> The log cell has been designed according to LPDA principles, using an element length and spacing ratio of approximately 0.95. This ratio, when applied to a pure LPDA, tends to produce more gain but a lesser front-to-back ratio than lower numbers, for example, the value of 0.90 used in the LPDA design examined in Part 2. The higher ratio value also produces a shorter cell for the same number of elements. The entire antenna, including the reflector and director, requires a 12.2' boom, nearly as long as the 4-element medium-bandwidth Yagi presented in Part 1 as a potential comparator.
|
||||
<center>
|
||||
<img src="logc3-5.gif" width="399" height="327" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>If we extract the log cell from the antenna, we obtain the check-point values recorded in <b>Table 2</b>. Note the relatively uniform gain across the entirety of 10 meters, as well as the 50-Ohm SWR values. According to our test, this log cell promises to form the basis of a good antenna that may be useful across all of 10 meters.</p>
|
||||
<pre> 4-Element Log Cell Independent Performance
|
||||
|
||||
Frequency (MHz) 28.0 28.5 29.0 29.5
|
||||
|
||||
Free-Space Gain (dBi) 7.24 7.47 7.47 7.29
|
||||
|
||||
Front-to-Back Ratio (dB) 17.7 14.0 12.8 13.1
|
||||
|
||||
Feedpoint Impedance
|
||||
(R +/- jX Ohms) 95 - j 2 39 - j11 39 + j12 75 + j 4
|
||||
|
||||
50-Ohm SWR 1.90 1.41 1.42 1.51
|
||||
|
||||
|
||||
Table 2: 4-element log cell independent performance.</pre>
|
||||
<p>Before we look at the modeled performance figures, we should note an additional dimension of this antenna. The phase line impedance is low (75 Ohms). In addition, if we use different element diameters, we obtain results that change to a degree that is greater than the changes we might expect in a Yagi using the same two element diameters. The effects of element diameter on the log cell driver (or on LPDAs) are significant. Therefore, the performance graphs for this antenna will record values for both 1/2" and 1" diameter elements.</p>
|
||||
<center>
|
||||
<img src="logc3-6.gif" width="645" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>Free-space gain figures appear in <b>Fig. 6</b>. The fatter element model not only shows a gain peak that is lower in frequency than the thinner version, but as well its peak gain values are higher. Moreover, the curve is flatter. The gain values rival those of the 3-element medium-bandwidth Yagi on a 12' boom, but do not match the values for the 4-element medium bandwidth Yagi on the 13' boom. Both of the Yagis, of course, only covered the first MHz of 10 meters.</p>
|
||||
<center>
|
||||
<img src="logc3-7.gif" width="644" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The front-to-back values are less radically different, as illustrated in <b>Fig. 7</b>. Essentially, the thinner version is capable of a higher peak front-to-back ration. However, both versions of the antenna exhibit better than 20 dB front-to-back ratio across the 28 to 29.7 MHz span.</p>
|
||||
<center>
|
||||
<img src="logc3-8.gif" width="646" height="524" border="2" vspace="5" hspace="9">
|
||||
</center>Both versions of the antenna exhibit acceptable SWR curves across all of 10-meters, as shown in <b>Fig. 8</b>.
|
||||
<center>
|
||||
<h3>A 7-Element Log-Cell Yagi</h3>
|
||||
</center>The bandwidth of 10 meters presses the 4-element log cell to its limits, although the 6-element log-cell Yagi does manage to cover the band with good gain, good front-to-back values, and a direct 50-Ohm feed system. We can improve upon the design by adding one more element to the log-cell to obtain the design shown in <b>Fig. 9</b>. The 5-element log cell for this antenna uses the same tapering ratio for elements in the log cell. However, using an additional element allows the longest element to be a bit longer and the shortest element to be a bit shorter. The cost is a longer boom, about 14.6' long in this case. The phase line is 100 Ohms.
|
||||
<center>
|
||||
<img src="logc3-9.gif" width="392" height="366" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p><b>Table 3</b> provides a look at the performance of the log cell independently of the entire antenna. Gain is even more uniform across the band than for the 4-element log cell, with acceptable 50-Ohm SWR figures. Once more, the front-to-back figures are unimportant in this context, since the parasitic elements will establish those values in the final antenna. In fact, the log cells used in these antennas are designed for gain rather than for a balance of operating characteristics, just as was the case for the 2-element cell in the 3-element array examined in Part 2.) We should expect the overall antenna to reflect the potentials of the log cell.</p>
|
||||
<pre> 5-Element Log Cell Independent Performance
|
||||
|
||||
Frequency (MHz) 28.0 28.5 29.0 29.5
|
||||
|
||||
Free-Space Gain (dBi) 7.31 7.38 7.42 7.43
|
||||
|
||||
Front-to-Back Ratio (dB) 12.0 12.4 13.4 15.2
|
||||
|
||||
Feedpoint Impedance
|
||||
(R +/- jX Ohms) 34 - j 6 46 + j14 80 - j 1 46 - j27
|
||||
|
||||
50-Ohm SWR 1.51 1.37 1.60 1.77
|
||||
|
||||
|
||||
Table 3: 5-element log cell independent performance.</pre>
|
||||
<p><b>Fig. 10</b> shows the free-space gain of two versions of the resulting log-cell Yagi, one using 1/2" diameter elements, the other using 1" diameter elements. For contrast, values are also shown for the 4-element wide-band Yagi, introduced in Part 1. We should expect lesser performance from this 8' boom Yagi. If you desire, you may substitute the values for the 8'-boom LPDA.</p>
|
||||
<center>
|
||||
<img src="logc3-10.gif" width="644" height="523" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The differences between the half-inch and 1-inch versions of the log-cell Yagi are even more dramatic than for the preceding model, with nearly 0.25 dB differential in gain in places across the band. Values for the half-inch model are similar to those for the 3-element 12-foot boom medium-bandwidth Yagi, but the log-cell Yagi covers the entire 10-meter band. The one-inch model shows only slightly less gain than the 4-element medium-bandwidth Yagi. For either model, the gain curve is very smooth, illustrating the benefit of the extra element in the log-cell.</p>
|
||||
<center>
|
||||
<img src="logc3-11.gif" width="644" height="527" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>One reason for adding the wide-band 4-element Yagi to the graphs is that it demonstrates the incremental improvement in front-to-back ratio provided by the 7-element log-cell Yagi all across the band, as shown in <b>Fig. 11</b>. Because no element length adjustments were made when changing element diameters, the half-inch model exhibits the superior curve, with a front-to-back ratio better than 30 dB up to 29.5 MHz. The 1" model, with a few added adjustments, can replicate the half-inch model curve, but with a slightly lower peak value. If you refer to the azimuth "snapshot" in Part 1 of this series, you will also learn that the rear quadrants show a very well-behaved rear lobe with no major quartering side lobes to falsify the impression left by the 180-degree front-to-back values.</p>
|
||||
<center>
|
||||
<img src="logc3-12.gif" width="645" height="525" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<p>The 50-Ohm SWR curves, shown in <b>Fig. 12</b>, demonstrate that the 7-element log-cell Yagi has a smoother curve than its 6-element counterpart. The curve for the version using 1" diameter elements is flatter, but does not dip quite so low as the curve for the half-inch version. However, adjustments to the exact phase-line characteristic impedance would likely permit either curve to bottom at close to 1:1 SWR. The phase-line characteristic impedance selected for the models represent a standard number, but actual construction would permit refinements.</p>
|
||||
<center>
|
||||
<h3>Summing Up So Far</h3>
|
||||
</center>The development of a log-cell Yagi requires careful attention to the design of the log-cell driver to obtain optimal results. Well-designed log-cell Yagis are capable of good gain, but their chief operating characteristics that fall into the range of excellence (when compared to other available designs) are the front-to-back ratio and the operating bandwidth. As the 6- and 7-element log-cell Yagis demonstrate, the antenna type is capable of well over 6% frequency coverage in a monoband design.
|
||||
<p>Designing a log-cell Yagi for gain as we cross into Y2K appears to be an exercise in futility. Although Yagi design in the late 1970s and early 1980s had yet to reap the benefits of computerized optimization, current Yagi design can provide as much or more gain for a given boom length than log-cell designs. The Yagis have the additional advantage of mechanical simplicity, since they do not require the precision construction of a phase line to interconnect the elements in the log cell driver.</p>
|
||||
<p>An interesting example of this point can be found by modeling the 5-element log-cell driven Yagi in Orr and Cowan.<sup>2</sup> The antenna uses a 2-element log-cell with a reflector and 2 more directors. This design on a 21' boom is capable of a peak free-space gain of about 9.5 dBi, with a very sharp peak in both the operating characteristics and the SWR curve. The rear lobes were acceptable but the front-to-back ratio exceeded 20 dB for only a narrow bandwidth.</p>
|
||||
<p>I had occasion to study 5- and 6-element 20-meter Yagis of existing design.<sup>3</sup> The boom lengths ranges from 45 to 55 feet, corresponding to 22- to 27-foot booms on 10 meters. All of the designs were capable of a free space gain of 10 dBi across all of 20 meters, with better than a 20 dB front-to-back ratio. Some, such as the NW3Z/WA3FET OWA 6-element design, were capable of exceptionally low 50-Ohm SWR values all across the band. In fact, the OWA design can be scaled readily for 10 meters and provide 1 MHz coverage on a 24' boom.<sup>4</sup></p>
|
||||
<p>As a gain enhancement, the log-cell driver technique has very limited utility amid current Yagi technology. Its chief merits involve operating bandwidth and front-to-back ratio. However, even here, its utility may be limited when the complexity and weight of the array are factored into antenna design and construction decisions. The medium-bandwidth Yagis described in Part 1 as comparators are fully adequate to provide full coverage of all of the upper HF bands except 10 meters. Only if weight is no concern and if extra front-to-back performance is a necessity on 20 or 15 meters would a log-cell Yagi such as the 6- and 7- element designs seem justified.</p>
|
||||
<p>The natural home of the log-cell Yagi in Y2K is at 10 meters and above, where the bandwidths are more than 3% or so of their center frequencies. However, as we increase frequency, the materials we use for antenna elements increase in diameter relative to a wavelength. So even at VHF, the fat elements of Yagis can provide a wider operating bandwidth that often precludes the need for log-cell technology.</p>
|
||||
<p>These notes are far from exhaustive, and my summary is based only on a few hundred models, the best of which have appeared in this series. Since antenna enthusiasts have an endless appetite for experimentation, it would not surprise me to see these analyses supplanted in the future by better and more ingenious log-cell Yagi designs.</p>
|
||||
<p>One perennial direction of experimentation that we have not examined is the effect of setting the antenna elements into a forward swept Vee. Perhaps we can overstay our welcome for one more part in this series, devoted to this one topic in order to discover whether "V" means "victory" or only half of a "virtual reality."</p>
|
||||
<center>
|
||||
<h3>Notes</h3>
|
||||
</center>
|
||||
<p>1. P. D. Rhodes, K4EWG, and J. R. Painter, W4BBP, "The Log-Yagi Array," <i>QST</i>, Dec, 1976. The main elements of this article are reprinted in The ARRL Antenna Book, 18th Ed., pp. 10-25 to 10-27.</p>
|
||||
<p>2. W. I. Orr, W6SAI, and S. D. Cowan, W2LX, <i>Beam Antenna Handbook</i>, pp. 251-253. For a 6-meter adaptation, see John J. Meyer, N5JM, "A Simple Log-Yag Array for 50 MHz," Antenna Compendium, Vol. 1, pp 62-63.</p>
|
||||
<p>3. See "Modeling 6 Long-Boom Yagis" at my web site, ...</p>
|
||||
<p>4. An model of a 10-meter version of the NW3Z/WA3FET OWA is reported in Cebik, "The OWA for 10, 6, and 2 Meters," <i>AntenneX</i>, August, 1999.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 5-7-2000. © L. B. Cebik, W4RNL. A version of this item appeared in</i> The National Contest Journal<i>, May-Jun, 2000, 10-13. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<a href="logc4.html"><img src="../images/fwdrd.gif" width="40" height="40"> <b>Go to Part 4</b></a>
|
||||
<p><a href="logc.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> <b>Go to Log-Cell Yagi Index</b></a></p>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#lpda'>LPDA</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
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<title>The Monoband Log-Cell Yagi Revisited Part 4: Vee-ing the Log-Cell Yagi Elements</title>
|
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||||
<h1><img src="../images/lpda.gif" align="left" width="150" height="83" border="2" vspace="5" hspace="10"><img src="../images/w4rnlwwr.gif" align="right" width="99" height="83" border="2" vspace="5" hspace="10">The Monoband Log-Cell Yagi Revisited<br>
|
||||
Part 4: Vee-ing the Log-Cell Yagi Elements</h1>
|
||||
</center>
|
||||
<center>
|
||||
<h3>L. B. Cebik, W4RNL</h3>
|
||||
</center><br>
|
||||
<br>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<p>One perennial design feature of log-cell Yagis has been the use of elements that form a forward Vee. Perhaps the chief proponent of this design features has been Zimmer, K4JZB, in his 1983 <i>CQ</i> articles on log-cell Yagis, although the idea reappears from time to time in related contexts.<sup>1</sup> For one 5-element version of the antenna, the text claims a 16 dB gain, although the frame of reference for the gain figure is not given.</p>
|
||||
<p>All of the designs we have explored in the first three parts of this series have used linear elements. Given the wide-spread repute of Vee-ed elements to improve gain, directivity, or other aspects of beam performance, it may be useful to explore the matter further. Since Vee-ed elements present no challenges to the limits of NEC, we may use this modeling software to develop some appropriate comparisons between various types of antennas using linear and Vee-ed elements.</p>
|
||||
<center>
|
||||
<h3>The Vee-ed Dipole</h3>
|
||||
</center>In order to understand the performance of Vee-ed beams, we should begin with the Vee-ed dipole, that is a dipole that is bent forward from linear by a certain number of degrees on each side of center. <b>Fig. 1</b> shows the general outline of the models used in this exercise. A standard 200" dipole length is used throughout, with 1" aluminum tubing as the material. The model uses a short, 3-segment, linear wire at the center of the antenna in order to provide the feedpoint segment with equal length segments on either side.
|
||||
<center>
|
||||
<img src="logc4-1.gif" width="352" height="264" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
Fig. 1 General outlines of linear and horizontally-Vee-ed dipoles.
|
||||
</center>
|
||||
<p>The degree of Vee-ing refers to the angle made on each side of the antenna relative to a line that would represent a linear element. Hence, 10 degrees of Vee-ing would bend each side of the dipole 10 degrees forward of the linear line. None of the angles used in this test presses any NEC limitation for accuracy of results.</p>
|
||||
<pre> Gain, Front-to-Side Ratio, and Impedance
|
||||
of Dipoles at Various Degrees of Vee-ing
|
||||
|
||||
Forward Angle Free-Space Front-to-Side Feedpoint Impedance
|
||||
Relative to a Gain (dBi) Ratio (dB) (R +/- jX Ohms)
|
||||
Linear Dipole
|
||||
(Degrees)
|
||||
|
||||
0 (linear) 2.15 > 30 77 + j18
|
||||
10 2.12 21 76 + j17
|
||||
20 2.02 15 70 + j15
|
||||
30 1.85 12 62 + j10
|
||||
40 1.62 9 50 + j 2
|
||||
50 1.37 7 37 - j 8
|
||||
|
||||
Note 1: The total length of the 1" diameter aluminum dipole element is 200" to yield a
|
||||
feedpoint impedance close to resonance at 28.5 MHz when each side is bent forward 40
|
||||
degrees from linear. See <i>Fig. 1</i> for the general outline of the test model.</pre>Table 1: Gain, front-to-side ratio, and impedance of dipoles at various degrees of Vee-ing.
|
||||
<p><b>Table 1</b> provides an indication of what occurs when a dipole element is vee-ed forward. The free-space gain of the antenna decreases for each level of Vee-ing. As well, the feedpoint impedance decreases. Perhaps most significantly, the front-to-side ratio also decreases. <b>Fig. 2</b> compares the free-space azimuth patterns of a linear and a 40-degree Vee-ed dipole and graphically illustrates the reduction in side rejection for the Vee-ed version.</p>
|
||||
<center>
|
||||
<img src="logc4-2.gif" width="640" height="480" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 2 Free-space azimuth patterns for linear and Vee-ed dipoles at 28.5 MHz.</p>
|
||||
</center>
|
||||
<p>When used as an inverted Vee antenna with the legs angled downward, the reduced side rejection is sometimes listed as an advantage, despite the reduction in broadside gain. However, when the dipole is Vee-ed horizontally, nothing is gained by way of directivity or other effect that might be useful in a multi-element beam antenna. Since all of the designs that we shall consider use the 1/2 wl dipole as their starting point, we should not have any expectations that Vee-ing the elements will yield added performance in any particular area.</p>
|
||||
<p>Perhaps what lies behind the idea that Vee-ing elements may yield added performance is the concept of the Vee-beam, a very old and simple antenna design. However, the Vee-beam is always many wavelengths long and produces many lobes and nulls. When the designer chooses the proper angle between the elements, the main lobes combine to form a single very strong bi-directional lobe set along the line bisecting the angle between wires. There will almost always be lesser lobes and nulls to the sides, that is, roughly broadside to the wires. If one terminates each of the far ends of the Vee with resistors to ground, then the Vee-beam develops a unidirectional pattern.</p>
|
||||
<p>However, the 1/2 wl dipole develops only a single lobe ar right angles to the wire, resulting in a bi-directional pattern. There are no lobes at angles away from broadside that may combine into a single stronger lobe. The dipole lobes can only be distorted from their shape when produced by a linear wire.</p>
|
||||
<center>
|
||||
<h3>2-Element Vee-ed Beams</h3>
|
||||
</center>
|
||||
<center>
|
||||
<img src="logc4-3.gif" width="360" height="313" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 3 General outlines of linear and horizontally-Vee-ed 2-element Yagis.</p>
|
||||
</center>
|
||||
<p>Rather than leave the subject with only the dipole as an indicator of the performance of Vee-ed antenna arrays, let's look at a few beam designs, beginning with 2 elements. Throughout, we shall bend the elements forward 40 degrees as a standard level of Vee-ing. <b>Fig. 3</b> shows the general outline of linear and Vee Yagis using a driver and reflector in each case. The driver length is 196" for both antennas, and the reflector is 210" long. Element spacing is 48". The Vee-ed version of the antenna shows a feedpoint impedance of 23 + j 4 Ohms at 28.5 MHz, close to resonance. When stretched to linear shape, the impedance rises to 36 + j30 Ohms.</p>
|
||||
<center>
|
||||
<img src="logc4-4.gif" width="640" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 4 Free-space azimuth patterns for linear and Vee-ed 2-element Yagis at 28.5 MHz.</p>
|
||||
</center>
|
||||
<p><b>Fig. 4</b> provides comparative free-space azimuth patterns for the two versions of the Yagi. The Vee-ed version has a free-space gain of only 5.45 dBi. compared to the linear version gain of 6.09 dBi. Both antennas have front-to-back ratios of about 10.8 dB, but the Vee shows far less side rejection than the linear antenna. This result is, of course, consistent with the results of our dipole test.</p>
|
||||
<p>Since our ultimate goals is to evaluate Vee-ed element use in log-cell Yagis, we may revise the outline in Fig. 3 to provide each element with a separate feed point. In this manner, we may directly control the relative current magnitude and phasing on each element. Let's try this experiment to see if Vee-elements promise any improved performance when independently phased.</p>
|
||||
<center>
|
||||
<img src="logc4-5.gif" width="640" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 5 Free-space azimuth patterns for linear and Vee-ed 2-element phased arrays at 28.5 MHz, using separate feeds for each element.</p>
|
||||
</center>
|
||||
<p>When independently phased for a maximum rear null, the Vee-ed version shows a free-space forward gain just below 5.9 dBi when the rear element is set at a relative current magnitude of 0.94 and a phase of 141 degrees (with the forward element set to a magnitude of 1.0 at a phase angle of zero degrees). For a maximum null to the rear, the comparable linear rear element must be set at a current magnitude of 0.98 with a phase angle of 139 degrees. Under these conditions, the linear phased array shows a forward gain of nearly 6.4 dBi. <b>Fig. 5</b> shows free-space azimuth patterns that illustrate the pattern differences. Besides the half-dB gain differential, the low side rejection of the Vee version is clearly evident.</p>
|
||||
<center>
|
||||
<img src="logc4-6.gif" width="640" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 6 Free-space azimuth patterns for a Vee-ed 2-element phased array at 28.5 MHz, using a phasing line between elements.</p>
|
||||
</center>
|
||||
<p>There are no simple means of obtaining the optimal phasing conditions for the Vee-ed phased array. The closest that I have come is the use of a 35-Ohm phasing line from one element to the next. Higher values of phase-line characteristic impedance yield lower performance figures. However, unlike available lines, the modeled line required a velocity factor of 1.0, with lesser values producing poorer results. <b>Fig. 6</b> shows the resulting free-space azimuth pattern, which has a forward gain of just over 5.6 dBi and a front-to-back ratio of just under 17 dB.</p>
|
||||
<p>All-in-all, we must account the results of our attempt to Vee 2-element arrays a disappointment. However, the results should not be surprising, since such arrays depend for their performance directly upon the dipoles that compose them.</p>
|
||||
<center>
|
||||
<h3>The Vee-ed Log-Cell Yagi</h3>
|
||||
</center>The results of our experiments with 2-element parasitic and phased arrays unfortunately do not bode well for the performance of Vee-ed log-cell Yagis. However, with a multi-element cell and additional parasitic elements, we cannot dismiss the possibility of superior Vee performance without suitable testing. Therefore, I have taken one of Zimmer's designs--a 5-element log-cell Yagi--and developed both linear and Vee-ed models. The general outline of the Vee-ed version appears in <b>Fig. 7</b>.
|
||||
<center>
|
||||
<img src="logc4-7.gif" width="392" height="339" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 7 General outline of a 5-element Vee-ed log-cell Yagi.</p>
|
||||
</center>
|
||||
<p>The reflector for each model is 211.5" long and placed 48" behind the 3-element log-cell. Working from the rear forward, the cell elements are 201', 198.8", and 196.6", each spaced 24" from the next. The director is placed 48" forward of the cell and is 187.6" long. The phase-line characteristic impedance producing the most usable results was 200 Ohms.</p>
|
||||
<center>
|
||||
<img src="logc4-8.gif" width="640" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 8 Free-space azimuth patterns for linear and Vee-ed 5-element log-cell Yagis at 28.5 MHz, using 200-Ohm phase lines between driver cell elements.</p>
|
||||
</center>
|
||||
<p><b>Fig. 8</b> shows free-space azimuth patterns for the linear and the Vee-ed versions of this antenna. The linear version is virtually identical to the 5-element log-cell Yagi examined in Part 3 of this series. Once more, the Vee-ed version of the antenna shows lower gain with a reduced front-to-side ratio.</p>
|
||||
<p>For the Vee-ed log-cell Yagi, the relative current magnitude and phasing on the three driven elements at 28.5 MHz with the 200-Ohm phasing line--from front to rear--was 0.87 at 15.9 degrees, 0.52 at 147.2 degrees, and 0.32 at 171.4 degrees. These values offer us one more experimental possibility. Suppose we separately feed each element of the log cell and optimize the current magnitude and phasing on each element. For example, if we set the forward element at a magnitude of 0.7 and a phase angle of 20 degrees, the middle element at 0.67 at 145 degrees, and the rear element at 0.4 at 169 degrees, we can increase both the gain and the front-to-back ratio of the array. The resulting free-space azimuth pattern appears in <b>Fig. 9</b>.</p>
|
||||
<center>
|
||||
<img src="logc4-9.gif" width="640" height="370" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 9 Free-space azimuth patterns for a Vee-ed 5-element log-cell Yagi at 28.5 MHz, using separate feeds for each element.</p>
|
||||
</center>
|
||||
<p>For a further comparison across the first MHz of 10 meters, we can plot the free- space gain values of the linear and 200-Ohm phase-line Vee array against the Vee array with separately fed driver elements. <b>Fig. 10</b> shows the results. The linear array exceeds the gain of the phase-line-fed Vee array by an average half dB. The hypothetical separately fed array has slightly more gain than the linear array.</p>
|
||||
<center>
|
||||
<img src="logc4-10.gif" width="647" height="528" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 10 Free-space gain from 28-29 MHz of log-cell Yagis: linear, Vee-ed with a phase line, and Vee-ed with separate feeds.</p>
|
||||
</center>
|
||||
<p>In <b>Fig. 11</b>, we can see the potential front-to-back values for each antenna, with the linear and phase-line-fed Vee-ed array having quite similar values. The hypothetical array using separately fed driver elements is potentially capable of considerably better front-to-back performance.</p>
|
||||
<center>
|
||||
<img src="logc4-11.gif" width="647" height="526" border="2" vspace="5" hspace="9">
|
||||
</center>
|
||||
<center>
|
||||
<p>Fig. 11 Front-to-back ratio from 28-29 MHz of log-cell Yagis: linear, Vee-ed with a phase line, and Vee-ed with separate feeds.</p>
|
||||
</center>
|
||||
<p>The difficulty with both the phase-line-fed Vee array and the alternative with separately fed drivers is feeding the system. The Vee-ed array with a phase line shows a tendency toward rapid feedpoint impedance changes, ranging from 50 Ohms at 28 MHz down to about 10 Ohms at 29 MHz. Indeed, experiments that varied the spacing of the reflector and the director failed to come up with a relatively constant feedpoint impedance for the first MHz of 10 meters. The smooth 50-Ohm direct feed obtained by the linear model (which was far from the best of the log-cell Yagis examined in Part 3) is wholly absent from the Vee-ed model. Hence, the Vee-ed model with a phasing line would be useful for only a narrow operating bandwidth.</p>
|
||||
<p>With separate feed for each driver element, the problem becomes insurmountable for the average amateur construction project. I know of no practical way to effect separate feeds for each element short of phasing networks for each element. The builder would also need the ability to measure currents and phase angles to a degree of precision beyond most ham shops.</p>
|
||||
<center>
|
||||
<h3>The Bottom Line</h3>
|
||||
</center>In the entire set of experiments reported here--plus a considerable number of other models--Vee-ing elements of 1/2 wl-based arrays has proven to be an exercise in futility. Throughout, the Vee-ed versions always exhibited lower gain and reduced side rejection relative to comparable arrays using linear elements. The comparative azimuth patterns shown in this final part of the series are truly representative of the total collection of Vee-ed models run.
|
||||
<p>Since each Vee-ed model shows its heritage in the Vee-ed dipole, we may take the performance of that basic antenna in comparison to a linear dipole as correctly indicative of the performance reduction likely to occur in any vee-ed array when set over and against a comparable array of linear elements. This note, of course, applies only to arrays based upon the 1/2 wl dipole. As we noted at the very beginning, multi wavelength Vee-beams are another matter entirely.</p>
|
||||
<p>The myth of the Vee-ed element array of 1/2 wl elements has perhaps persisted too long in amateur circles. I hope these notes help dispel it to some degree. More to the point, if a monoband log-cell Yagi is the design of choice to meet a given set of operating needs, then the best of the linear element log-cell Yagis examined in Part 3 will likely always be a better selection than a Vee-ed counterpart.</p>
|
||||
<center>
|
||||
<h3>Notes</h3>
|
||||
</center>
|
||||
<p>1. Robert F. Zimmer, K4JZB, "Development and Construction of 'V' Beam Antennas," <i>CQ</i>, Aug., 1983, pp. 28-32; and "Three Experimental Antennas for 15 Meters," <i>CQ</i>, Jan., 1983, pp. 44-45.</p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center>
|
||||
<p><i>Updated 7-14-2000. © L. B. Cebik, W4RNL. A version of this item appeared in The National Contest Journal, Jul-Aug, 2000, 11-14. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.</i><br>
|
||||
<br></p>
|
||||
<center>
|
||||
<img src="../images/colorbar.gif" align="middle" width="540" height="4">
|
||||
</center><br>
|
||||
<br>
|
||||
<a href="logc.html"><img src="../images/return.gif" align="bottom" width="40" height="40"> <b>Go to Log-Cell Yagi Index</b></a>
|
||||
<p class="topics-footer">Topics: <a href='../topics.html#hf-upper'>HF Upper</a>, <a href='../topics.html#lpda'>LPDA</a>, <a href='../topics.html#yagi'>Yagi</a></p><br>
|
||||
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|
||||
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|
||||
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