No. 45: What Should I Expect from a Yagi? 
With commercial antenna makers spreading all manner of numbers around, it is difficult for the antenna buyer--or even the prospective antenna builder--to know exactly what to expect from a Yagi or similar beam. So let's take some time to survey the field and give some estimates based on many years of designing and analyzing Yagis. But first, a little terminology.
I shall specify the gain of the Yagi designs in terms of free-space values that are average across the first MHz of 10 meters. The gain numbers will be in dBi, that is, dB over an isotropic source. Gain values in dBd, that is, gain over a dipole, are 2.15 dB lower than those given in dBi. Gain over ground varies according to antenna height, but at a height of 1 wavelength above average ground the gain values (whether in dBi or dBd, will be about 5.5-dB higher than the free-space value to account for both ground reflections and ground losses. By using free-space values throughout, you can get a comparative idea of the advantages of one design over another. So if one design has a gain advantage of 3.2 dB over another in free-space, then it will have the same advantage if we specify the values in dBd or specify the values over ground--or both. However, remember not to mix fruit: we are talking only of Yagi and similar designs based on horizontal half wavelength elements.
Unfortunately, some advertising does not give you a clear picture of the conditions for the gain specification, so that you do not know if they are citing inferior gains over ground or superior gains in free-space. Some makers only express the gain in dB, so that you do not know what the reference standard is. There is also the problem of stating only the peak value so that you have no idea of the gain across the operating passband of the antenna. Then there are antenna makers who are completely honest, even if that honesty seems to make their products inferior to those of makers who rely on one or another form of vagueness, ambiguity, or downright unsupportable claims.
Notes:
Let's notice some trends. First, the gain does not always go up with the number of elements, but it does go up with the boom length. The only exception to this trend is when we design a very wide-bandwidth beam, such as starred entries on the list. Then we may require a longer boom to achieve the bandwidth and the 50-Ohm impedance.
Second, adding an extra element on a given boom length does not usually increase gain. However, it does allow us to set the feedpoint impedance and to achieve greater control over the operating characteristics. For example, most of the Yagis with 3 or more elements show an upward gain trend across the band. However, the OWA design can center the gain curve and the front-to-back curve together so as to have more even performance across the band. As well, the OWA design allows a direct 50-Ohm feedpoint impedance and no need for a matching network.
You can generalize on these rough expectations on other bands in a very simple manner. Find the ratio of the band names, for example, 20 meters divided by 10 meters equals 2. Then multiply the boom length of the 10-meter beam of your choice by that ratio. For example, a 20-meter equivalent of larger a long-boom 24' 5-element Yagi design, will have about 10.2 dBi gain if the boom is 48' long. These ratios are significant not only in the evaluation of monoband Yagis for other bands, but as well when we try to develop some reasonable expectations of more complex designs.
The performance figures do not take into account non-electrical factors in a given beam's design. Commercial and home-built monoband designs may range from flimsy to greatly overweight, relative to the wind and ice loads at a desired operating location. The durability of both hardware and plastic fittings is an important question for both buyers and builders to address. As we look at booms that exceed about 24', we must also consider the needs for preventing excessive boom sag and stress.
Remember that all of these Yagi designs are monoband antennas. Hence, the designers were able to optimize performance. Next, we shall see what happens when we cannot fully optimize performance.
The question that faces us is how we can develop reasonable expectations from a tri-band design. One approach is to pose some basic questions.
1. How long is the boom for each band, counting from the rear-most element to the forward-most element for that band?
2. How many elements are active on each band?
3. Are any elements in a given band loaded? This question can be tricky. For example, a 10-meter trap will not load the element on 10 meters, but it will form an inductive load on 15 or 20 meters. An element shortened by loading has slightly less gain potential than a full size element.
4. Do any of the elements--especially directors--perform special functions that do not contribute significantly to gain? You may have difficulty determining the answer to this question, since non-gain element functions are difficult to determine without further analysis.
Now let's apply the questions to our two subject antennas. The 10-element tri-bander at left, marked A, has a 3 element Yagi on 20 meters that occupies most of the boom. The 20-meter section of a tri-bander--if the elements are not loaded--tends to come closest to full size performance. If the boom length for the 3 elements approaches 16', then it will yield about 7 dBi of gain--about the same as the 10-meter monoband Yagi on an 8' boom. There are also 3 15-meter elements on a proportionately shorter boom. However, note that the forward director is behind the 20-meter director. This situation often reduces gain somewhat relative to placing it ahead of the 20-meter director. Hence, we can expect less than 7 dBi gain on this band.
On 10 meters, there are 4 elements, and the boom length for them is similar to that of a 4-element short-boom monoband Yagi. However, note that the two directors bracket the 15-meter and 20-meter directors. Some of the function of the first 10-meter director is to "capture" the 10-meter energy to prevent the 20-meter element from controlling it. Hence, its function is not identical to that of the first director in a monoband beam. The result is a slight reduction in our gain expectations from the 10-meter elements--perhaps a half-dB reduction to the 8-dB region.
In almost all tri-band designs, achieving a 20-dB front-to-back ratio is rare, with that value being a peak value. Values from 14 to 18 dB are more typical as averages.
Now let's turn our questions to the design marked B in Fig. 1. It uses 6 physical elements, but some of them--as indicated by the squares--serve multiple duty. As we examine the design band by band, we need to count the elements that are active, even if only part of the element is active.
The three longest elements--the two at each end and the driver with multiple sets of traps--form a 3-element Yagi for 20 meters on a boom that is longer than the one in our first design. The longer boom suggests higher gain, but the traps, especially in the driver, suggest some gain reduction at least due to element shortening. The net result is a gain value of about 7 dBi (free-space), since the boom length is still less than that of a long-boom 3-element monoband design.
On 15 meters, we also have 3 elements. The reflector is the inner portion of the longest elements. Next comes driver, which terminates at the outer driver traps but passes through the inner traps. The director is the element behind the forward-most trapped element. The overall boom length is proportionately longer than for the first array and longer than for 20 meters. However, since the driver is still trapped for 10-meter use, the gain will not reach peak long-boom 15-meter levels, and once more falls in the 7 dB range.
On 10 meters, we have 4 elements: the independent reflector, the inner portion of the driver, an independent director, and the inner portion of the forward element. Although as a monoband Yagi, these elements might form a good 4-element beam with a fairly long boom, the interactions with the other elements--especially those for 20 meters--will reduce the gain to about 8 dBi average. However, those same interactions have two other effects. First, they tend to make performance show sharp changes across the 10-meter passband. Second, they tend to reduce the operation bandwidth to about 800 kHz.
The two tri-band designs turn out to be very comparable to each other, with the trapped design in B having a slight advantage in gain on 15 meters. However, gain is not everything. The placement of elements in a tri-bander must be a compromise between adequate gain over a sufficient bandwidth and the front-to-back ratio that can be achieved. Most current designs sacrifice some front-to-back ratio for gain, so realistic values tend to range from 14 to 18 dB. The exact value may vary from one band to the next. Our cursory analysis does not give us good clues to front-to-back performance.
The estimates of performance for the trapped design presume very well designed low-loss traps, and my presumption is correct for some makers--but perhaps not for all. As well, traps tend to suffer environmental effects more severely than simple elements. They use a combination of materials, some of which may age faster than others in the seasonal cycles. Most have weep holes to drain accumulated humidity. The weep holes are also entries for bug nests and atmospheric particulates, either of which can cause eventual harm or lower performance. Since access to the insides of most commercial traps is difficult, routine maintenance and inspection become difficult. However, routine periodic maintenance is a key to the continued performance of any antenna, whether a monoband Yagi or a multi-band array.
These notes are not designed to give definitive analyses of any particular monoband or tri-band array. Instead, they aim to give you a starting point in evaluating antennas that you might contemplate buying or building. In ads, thinking about commercial offerings, consider what the maker does not tell you. Then ask. If you do not get straight answers, factor that event into your evaluations along with the data that you do receive.
Updated 10-01-2004. © 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.
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