Wednesday, April 9, 2014

Choosing a High-bands Yagi (Part 1)

Big antennas are serious business. They are costly, require large (and expensive) towers and rotators, and even with all this effort they are more likely to fall to weather events than their smaller brethren.

Are they worth it? Although my 2014 objective is to a small 20-15-10 yagi (and possibly 17-12) for a small tower I intend to get the most out of whatever my choice of yagi will be. That means comparing small yagis to the best, regardless of size. Only then can we know what we're getting, or not getting, in the way of performance. Comparisons among small yagis or single-element antennas (dipoles, verticals, etc.) do not address my quest for maximum performance.

To begin, we must normalize on height. It will not do to compare a small yagi at a low height to a large yagi at a great height. They must be compared under similar conditions. Since my tower will not be more than 15 meters high that is my benchmark height. I also find it useful to do comparisons in free space to remove environmental variables.

Small, rotatable yagis come in several varieties. All are on my list of candidates.
  • Standard elements lengths (with or without traps, used in tri-banders) on a short boom, typically ~4 meters (0.2λ) long (e.g. TA33, TH3, A3S)
  • Fewer elements on a short boom, but where that boom length is optimized to the smaller number of elements (e.g. TA32, TH2, MA-5B)
  • Wire elements on a non-conducting frame/spreaders (e.g. Spiderbeam, Hexbeam)
Since I enjoy designing antennas you might wonder why I don't choose to build one. While I can do this it can be costly in terms of time. Time to order aluminum and other material, time to design strong and effective multi-band elements, time for construction, and, finally, time to adjust the antenna to match the design. Multi-band antennas have a large number of variables and require much fussing. Single band and even multi-element wire antennas are less time consuming.

What I can do is a careful evaluation of commercial products and make a reasonable choice from among them. Some are quite good, while others are less so. Software modelling takes out much of the guesswork, so that one does not have to go by reputation or (worse) marketing literature.

To establish a basis for comparison I designed an optimum 3-element yagi for 20 meters. The boom of this antenna is 0.35λ (7.5 meters, or 24 ft.). By optimum I mean with regard to gain, F/B and bandwidth. In free space this antenna has 9 dbi gain, and it holds close to this over a wide bandwidth. The design is based on an NBS (National Bureau of Standards), and was extensively modelled by W2PV in Yagi Antenna Design, 1986.

Optimization is achieved by selection of element lengths (resonant frequency) and element spacing. Elements are constructed from aluminum tubing. So is the boom but the models ignore exclude the metal boom for simplicity. That factor can be compensated with element length adjustment later in the design process.

More gain (9.8 dbi) and F/B (>30 db) over a narrower frequency range can be achieved by selecting element resonant frequencies that are tighter together. However, I do not consider a design optimum if it maximizes one performance measure at the expense of others. Such a design looks good on paper but is deficient in actual use.

The EZNEC view of the antenna is above right (element #1 is the reflector), and the modelled performance is summarized in the following chart. In free space this antenna has a maximum gain of 9.1 dbi. 

This antenna reaches its maximum gain of 13.8 dbi at 14.350 MHz. At 14.000 MHz it is 13.4 dbi, which shows how broadband this antenna's performance can be. F/B (front-to-back) doesn't fare as well although it is still quite good. F/B peaks at better than 27 db around 13.950 MHz. There is no easy way to bring the frequencies of best gain and F/B closer together without severely compromising performance. The 2:1 SWR bandwidth is 250 kHz using a beta match, and is easy to match with a rig's ATU across the entire band.

As a further comparison, the gain of a rotatable dipole is 2.1 dbi in free space (F/B is 0 db) and a 2-element yagi is 6.9 dbi (with a narrow gain and F/B bandwidth). It may surprise you to learn that the 3-element yagi is only a little more than 1 S-unit better than the dipole and just 2.1 db better than the 2-element yagi. However the bandwidth of the optimized 3-element yagi is far better than the 2-element yagi for all performance metrics. As we'll see later, over real ground the 3-element yagi compares more favourably.

This is just Part 1 of a short series of articles. Originally I intended to do this in one article then realized it was too long and would be delayed since I have not yet done all the required work to reach a conclusion. In other words, I don't know where this exercise will take me. Of course I have a strong sense of how this will go, but there is considerable doubt. As of now I expect this will take 2, or at most 3, additional articles.

I will finish Part 1 with my reasoning for choosing the optimum 3-element yagi as my reference and the list of issues that need to be addressed by the evaluation.

I have a TH6DXX in my garage that is perfectly good. Unfortunately its wind load is too great a risk for the small, guyed tower I plan to put up this year. I want to keep the projected wind area below 4 ft². The TH6DXX wind load is high because it uses optimum element spacing on each band: 20, 15 and 10 meters. This cannot be achieved with 3 trapped elements, so there are more (actually 4 elements on 10 meters). The 24' boom (0.35λ on 20 meters) is itself a substantial wind load. However, apart from the traps and shorter elements the 3-element yagi reference I've described above, it is a good proxy for the TH6DXX which is an excellent multi-band antenna that has stood the test of time.

Speaking of traps and element length, let's finish off with the list of issues.
  • Traps: Elements must be multi-band to keep the element count low (3 maximum), so traps are used. These are parallel LC circuits that are typically integrated with the element structure. Traps have loss. However not all traps are equal in this respect. Traps also reduce element length since they act as inductive loads for lower-frequencies. Achievable gain is reduced as the element length is reduced.
  • Element configuration: Element in the traditional aluminum yagi are parallel to each other. This is not mandatory. They can be square sections (e.g. Moxon beam) or vee-shaped (e.g. Spiderbeam). Those bends reduce achievable gain, but they do other things well.
  • Wires vs. tubing: Although aluminum (or, more often, aluminum oxide) has more resistance than copper, the size of the aluminum tubing serves to lower the losses in a yagi to negligible levels (<0.1 db for typical HF yagis). Copper wire actually has more absolute loss when used in a yagi, as we've seen before. Wire gauge and insulation become considerations.
  • Resonant elements: Resonant elements can be used in order to avoid traps and to keep elements at full length. Unfortunately a straight piece of wire or tube does not resonate on more than one 20, 15 or 10 meters (unless very, very long). Therefore more elements are required. This introduces, cost, complexity and wind load.
  • Element spacing: Elements that are optimally spaced on one band are not optimally spaced on the other bands. With a frequency spread of 2:1 for a tri-bander this is an important concern. Acceptable compromises must be found.
In Part 2 I will probably discuss 2-element vs. 3-element multi-band yagis. There is more to their differences than gain.

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