No. 54: An Orientation to Reasonable Yagi Expectations

L. B. Cebik, W4RNL

Sometimes, purchasing a Yagi can be as daunting a task as building one. The advertisements use a bewildering array of incompatible terms and specifications that often result in more confusion than information. In this episode, I shall introduce some of the most important ideas to consider when purchasing a Yagi (or similar) directive array. I cannot single-handedly change the ways of advertisers, but I can alert you to a few things to examine carefully along the way.

1. The Physical Fundamentals: Even before you look at the specification sheets from various antenna makers, you should think about the physical requirements of a good, solid antenna installation. Fig. 1 shows some, but not all, or them.

The upper part of the graphic shows the bare outlines of a strong installation, included a tower (with guys omitted), a rotator, feedline, rotator cable, anchorage, and ground rods. All of these items must make a coherent package so that the system will withstand the stresses placed on it by the antenna at the top. If you are aging as fast as I am, or if you live in an area with a regular threat of very severe weather, then you should consider the added expense of a system that let's you lower the tower or the antenna in a storm or when periodic maintenance is due.

The antenna itself presents a number of physical concerns. We find 3 main construction philosophies in the world of Yagis. Some makers, like Force 12, use the willow theory of construction to produce light but flexible antennas. The American standard theory underlies most U.S. Yagis and uses fairly standard components, such as aluminum tubing with 1/16" walls. Most European-made Yagis subscribe to the oak theory, using aluminum with much thicker walls. In the end, their antennas tend to be 1.5 to 2 times heavier than U.S. beams with the same number of elements and wind-survival rating.

Beams present the wind with a certain area. Rotators usually carry two important ratings: the maximum antenna weight and the wind area. We need to match the latter figure with the antenna, since it is a measure of the maximum rotating stress that a rotator will withstand. Antenna themselves have a wind survival rating--usually in miles per hour. If you expect winds higher than the antenna rating, then be able to lower the antenna in a windstorm or check with the maker for the same antenna built to a higher wind survival rating.

The lower part of Fig. 1 suggests a few construction details to examine. How do the elements connect to the boom (and how does the boom connect to the mast)? Is all hardware stainless steel? Do clamps and U-bolts hold the elements and the boom without deforming them? Are all non-conductive (plastic) materials UV-protected for long life (or will a fixture become brittle and break in a few years)?

HF elements normally use a collection of tapered diameter tubing sections. The graphic shows 3 general ways in which makers secure the sections together. All three methods have proven track records if the hardware is non-rusting and strong. If a connection requires a special tool, be certain that the maker supplies the tool.

There are a few matters that the sketches cannot show. Be sure that the antenna fits inside your property. Find the distance from the mast to the tip of the longest element: that is the antenna turning radius. Be sure that it is well inside your property line with enough to spare to satisfy your insurance agent. If you antenna and tower fall, will they land completely on your property? Also be sure to adhere to any applicable laws, regulations, or covenants governing antennas in your area.

2. The Electrical Properties: The radiation properties of Yagis are designed to confuse you to the point of buying into whatever a given maker tells you. Consider the forward gain number. Some makers use a free-space value in dBi, based on computer modeling of the antenna. This figure avoids the variations in gain that occur over ground. Some makers cite values at a certain height in feet or meters above ground. Of course, this figure changes height as measured in wavelengths as you change the operating band. Still other use a figure based on the antenna 1 wavelength above ground. This value changes in feet or meters depending on the operating frequency.

Still other makers give gain figures in dBd, where the gain in dBd is 2.15 dB lower than the gain in dBi. When you combine these practices with the variations that I just cited, the gain values for seemingly similar Yagi designs can look very different. There is no solution to this morass except to perform calculations galore until you work through the specifications and come out with compatible figures for each candidate in your selection process.

To give you some guidance, I have compiled some (but not all) of the 10-meter Yagi models in my collection from 2 to 8 elements. Table 1 gives the free-space specifications at 28.5 MHz for each design. The gain is not dependent on the number of elements alone, but also on the total boom length. Therefore, a short-boom (8') 3-element Yagi may shows a little over 7 dBi free-space gain, while a long-boom (12') version with the same number of elements may have about 8 dBi gain. However, once we reach the long-boom size for any given number of elements, we may see one of two phenomena. First, the operating bandwidth of the antenna may be narrower. Many 10-meter antennas are rated only between 28 and 28.8 MHz, while versions with one more element in the same general boom length may be rated for the entire 1st MHz of the band. Second, we may find a lower feedpoint impedance. With good matching systems, impedances in the 20-25-Ohm range are fine, but below that level, losses begin to increase.

Except for the 2-element reflector-driver Yagis, most commercial monoband Yagis manage to achieve at least 20-dB front-to-back ratio over most of the band. Do not let peak values, such as the nearly 37 dB value of one 5-element model, fool you. Very high values like these only occur at a specific frequency, and there are usually quartering rear sidelobes with a more normal value close to 20 dB.

Table 2 shows the performance of the same beams 1 wavelength above ground, about 35' at 10 meters. Compare the gain values to those for the free-space models. As the beam gets longer and has more forward gain, the amount of increase provided by the ground reflections with a moderate height gets smaller. As well, the TO angle, that is, the elevation angle of maximum radiation gets a bit lower. As you raise the antenna height, these differences tend to diminish until they virtually disappear.

The final recommendation that I would make is to insist on seeing either the antenna patterns (such as in Fig. 2) or the gain, front-to-back, and impedance curves across the entire 10-meter band. In the figure, both antennas have about the same boom length. However, the 5-element antenna shows a relatively mediocre front-to-back ratio in the upper part of the band. If we had room for the graphs, we would also find that the 5-element antenna changes gain by about 1 dB across the band, while the 6-element design changes gain by only 0.2 dB. In terms of even performance across the band, the slightly higher element population of the 6-element design is superior, even though its peak gain does not match the value achieved by the 5-element beam at 29 MHz. You cannot reach your own conclusions about the antenna performance without the data for the antenna across the band. Do not be fooled by citations of peak values. As far as I am concerned, if a maker will not reveal all of the information about the performance of his antenna, he has lost me as a customer.

These all-too-brief notes provide the starting point for what you should consider when deciding on which commercially made monoband Yagi to purchase. Once you start learning about what to consider, you will think of equally important points to ponder and questions to ask. Note that I specified monoband Yagis. If you want to consider multi-band Yagis, you will enter a very different ballpark.

Updated 01-01-2007. © L. B. Cebik, W4RNL. Data may be used for personal purposes, but may not be reproduced for publication in print or any other medium without permission of the author.

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