Thursday, May 15, 2014

Choosing a High-bands Yagi (Part 5) - Comparisons and Wrap-up

To close this series about choosing a high-bands yagi I will do some comparisons, review some of the key trade-offs and then consider antenna height. Recall that the reason for this series is my need to choose a commercial small tri-band (or 5 band) yagi suited to a small tower, which is my plan for 2014. Hopefully others get some value from this exercise as well.

First, below is an index to the preceding 4 parts of the series. This will make it easier to follow the references in this final article in the series.
Now then, why choose a yagi at all? I know many hams that do so with hardly a thought; it is just what they believe they need to do to get on the air. This is clearly not necessary. In my own way I have proved over the past 16 months with wires alone. I have not only been effective in contests (QRP category) but have also worked close to 200 countries. It is likely that I will reach the 200 DXCC mark before I get around to putting up a yagi.

Everyone has their own reason to put up a yagi. At least there had better be a reason, and a good one at that, to undertake the expense, trouble and risk of erecting a tower, complete with antenna and rotator. My own reasons are clear enough:
  • Be more competitive in contests, or at least have more fun by working more stations and multipliers.
  • Work more DXCC countries, including those over long and difficult paths, and to have more success in pile-ups on the rare ones.
  • Less struggle to make contacts, even on a casual basis, especially on SSB or when conditions are less than favourable.
The critical need is effective radiated power (ERP) at the required elevation angles and directions. Apart from changes within the shack (transmitter power) this comes down to antenna gain and height. F/B and SWR match are nice but are secondary to my objectives. That is, I am ready and willing to sacrifice F/B and some SWR bandwidth to squeeze more gain from an antenna.

With that in mind let's recap the gain performance of the antennas in this series on 20, 15 and 10 meters. You can refer back to the articles listed above for more information about F/B, SWR and model details. The charts at right only show gain.

An "optimal" 3-element full-sized reference yagi is only shown for 20 meters, which was presented in Part 1. Similar performance can be expected on 15 and 10 meters for the same antenna scaled to those bands. The gain-optimized spider beam was also only modelled for 20 meters (3-el spider+ in the chart). It was presented in Part 4.

From an inspection of the adjacent free-space plots of gain we can deduce the following points:
  • All small tri-band yagis require a compromise in performance in comparison to a full-size yagi. In some cases the compromise is profound.
  • 2-element designs have maximum gain at the low end of the usable bandwidth while 3-element designs lean the other way. However, the introduction of traps skews this theoretical attribute due to equivalent series resistance (ESR) loss in the traps.
  • Traps reduce performance bandwidth on 15 and 20 meters. It is especially important with these antennas to tune the elements for the band segment(s) of interest.
  • With respect to maximum gain the difference among these antennas is not very large. This is misleading since for the 2-element and trapped yagis the gain curve has a sharp peak and can be quite poor elsewhere in the band.
  • Excellent gain and F/B are often present even on band segments where the SWR becomes problematical. This can be matched in the shack at the cost of increased transmission line loss. This should not be a concern in most installations if the SWR if less than 3 or so and RG-213 or better coax is used.
Let's also recap some important points about the design of these antennas:
  • Traps must be well designed to minimize loss due to ESR. Loss is greatest where the gain is highest since that is where radiation resistance is lowest.
  • Broadband SWR comes at the cost of gain. You can't have both, so you must choose. That gain and smaller SWR bandwidth requires a matching network (e.g. beta match or balun) to raise the impedance to 50 Ω. The matching network, when required, must work on all 3 bands, and with a common feed point. Most commercial products in this category already do so, saving us the trouble.
  • Rotatable wire yagis have a somewhat complex physical design that can make them difficult to install on towers that are guyed or that support other antennas.
Effect of height over ground

All the forgoing modelling and analysis has been done in free space. This is useful since it removes variables to do with antenna interactions with the environment. Once we introduce the ground a number of subtle differences creep into the picture. That is what we will now look at.

First, provided that any of these yagis is higher than 5 meters above ground there will be little impact on SWR, gain or F/B performance. While it is true that height affects gain and F/B versus elevation angle the overall gain and F/B performance are pretty much the same as in free space. This behaviour is common with directive antennas. If the downward radiation (as measured in free space) is small in comparison to the main lobe there is less interaction between the antenna's near field and ground. Get up high enough and even other houses and utilities in the vicinity can effectively "disappear".

For the height analysis I will in any case keep the antenna even farther above ground, at least 10 meters (½λ on 20 meters and 1λ on 10 meters), so we can comfortably assume that the preceding free-space antenna analyses are both valid and height invariant.

Since most of the modelled yagis have significant frequency-based performance variability I selected a frequency where the performance is representative or close to average for each antenna. On 20 meters this is typically in the range of 14.100 to 14.150 MHz. You can judge for yourself whether this choice is appropriate by reviewing the gain charts shown above.

On 40 meters I chose an elevation angle of 10° for the gain comparison since that is the empirical (experimentally measured) median angle for long and medium length DX paths on that band. On 20 meters the median is lower at about 5° -- the angle generally declines with increasing frequency. That will be the basis for the comparison.

The elevation angle of the main (or lowest) lobe is misleading since that is not the angle for the DX path. The ionosphere determines the path direction (azimuth and elevation) which then informs the optimum antenna design. It is never the other way around! If the main lobe is in a different direction you are wasting energy since it is not going where you need it.


It is no surprise that the 5° gain increases with height for all of these yagis, and indeed for any horizontally-polarized antenna. It is therefore also unsurprising that the performance differentials for the most part remain the same at a variety of heights. There are subtle height-dependent variations due to the different shapes of the main lobe, but we can ignore these since they are quite small.

The full-size 3-element reference yagi is the clear favourite. This is expected since all the tri-banders involve compromises that affect gain performance. The gain range among the candidate tri-banders is ~2 db, but, again, keep in mind that some of these antennas do less well over much of the bands of interest. As I said in Part 1, it is important to compare tri-banders against a superior standard rather than only against other "compromise" antennas; we need to look reality in the face, and know what we are, or aren't getting.

It is interesting that my gain-optimized 20 meters spider does quite well. The unmodified Spiderbeam is less impressive, even though it is most like the reference yagi in that the gain holds up pretty well across the band.

There is another quite interesting way we can interpret the above chart. That is to look at how much higher each antenna must be raised to equal the gain of the reference yagi at a lower height. The approximate round numbers for the range of heights plotted in the charts are as follows:
  • 2-element trap tri-bander: 6 meters
  • Spiderbeam (unmodified): 5 meters
  • 3-element trap tri-bander: 4 meters
  • Modified 3-element spider: 3 meters
For example, if the reference yagi is up 15 meters you would have to place a TH-3 (or equivalent product) at 19 meters height for similar gain performance at 5° elevation. It is a simple matter to calculate the equivalent height differential for any other pair of antennas in the list.

That's some food for thought should you ever have to decide between bigger antennas (e.g. stacked monobanders) versus a taller tower. If you consider what is involved it is almost always better to place a tri-bander on a taller tower. Up to a point. Once you start talking about serious height, say 30 meters or more, the objectives and comparison can be quite different. Even so it is no surprise that some smart hams who build competitive contest or DXing stations will often complement their antenna farms with a large tri-bander such as the TH-6 or TH-7 on one of their towers.

Conclusion

That's all I have to say on this topic. I still have to buy a 15 meters high tower and small yagi if I am to fulfill my objectives for 2014. When I'll do so is unpredictable since I am looking for used equipment that can be acquired locally. I won't spend a lot since I might only be using them for a year or two. My antenna choice may come down to what's available on the local used market rather than choosing the best.

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