Not long ago a friend asked me about using his short boom 3-element tri-band yagi on 6 meter. Apparently the idea is more popular than I expected. But it makes sense: it's already there, it's rotatable and it's high on a tower, so maybe it's worth a shot. He wanted to try it to see if he could do better than with his wire dipole.
When loading any antenna on a band for which it was not designed there are two critical metrics that must be addressed:
Unless you're very lucky the SWR will be high. That has a strong impact on the efficiency of the antenna system. Elements far longer than that of a half-wave dipole can have peculiar patterns with multiple lobes and nulls. That can be a problem if the antenna is not rotatable.
I gave him my opinions solely based on my experience and thinking through the issues. Out of curiosity I followed up with a software analysis. It wasn't difficult to do since I have a EZNEC models of a tri-band dipole and small 3-element tri-band yagi in my files, and TLW easily analyzed efficiency and matching of the transmission line and tuner (matching network). The dipole is the driven element of the yagi without the beta match. The element model utilizes traps similar to that for common Hy-Gain antennas like the TH3.
It turns out that what I told my friend was partly right and partly wrong. While an educated guess is often good enough for ham work, a model and measurements will do far better for a modest amount of effort. Even experienced hams can be wrong so be very careful whose opinion you solicit. I don't mind being wrong since it's an opportunity to learn. Hence this article.
Above is my EZNEC model of a tri-band yagi fed on 50 MHz, showing the element currents. It is no surprise that the reflector and director currents are low since the spacing is almost double that on 10 meters (by wavelength) and the elements are not resonant on 6 meters. The low impact of the parasitic elements is apparent when they are removed from the model, since the pattern and impedance are little different (see below).
There are current "bumps" at the beta match and traps where there are impedance discontinuities. The traps have a net capacitive reactance at frequencies higher than their resonant frequencies. The beta match is essentially a dice roll since it is arbitrarily transforming a high SWR to a different but also high SWR
The azimuth pattern is pretty good. It's bidirectional with modest gain due to the narrowing of the lobes. The narrow lobes are due to the long length of the yagi driven element; as already noted, the parasitic elements have little effect so that the pattern of the tri-band dipole is almost the same. I wouldn't read too much into the higher free space gain of the dipole (red) since the difference is small and not easily predictable for the variety of commercial antennas of this type.
When I inspected the behaviour of the traps in EZNEC I was surprised to find that their loss was negligible even for realistic Q values of the constituent coil and capacitor. I expected the loss to be low but significant. I haven't yet looked into the details of what was to me an unexpected result. I have an idea how it comes about, but rather than present a guess let's move along.
Well, that was the good news. Now it's time for the bad news.
The SWR is very high, and that's bad. It is similarly high for the tri-band dipole, except that the R and X values are transformed to a high R and negative X by the beta match. I used a beta match in the model due to its use on Hy-Gain yagis and some other tri-band yagis. With other yagi matching networks the effect will be different but perhaps not so much in its behaviour on 50 MHz.
The SWR can be corrected in the shack using a tuner. The ATU in our transceivers cannot cope with an SWR of 30. In actuality, the mismatch loss on the coax is so high that the SWR in the shack will be far lower than 30. TLW is my tool of choice to model the effect of the transmission line.
Many hams use RG213 class coax in their HF antenna systems so that's what I started with. I further assumed 100' (30 m) of coax length, which is typical for a yagi on a small tower located a short distance from the house. The matched loss of RG213 is quite high at -1.6 db, and it balloons to -8.2 db due to the extreme mismatch. With LMR400 the loss is "only" around -6 db. Even with pricier LDF5 the loss is still -2.7 db.
Notice that the SWR of 5 in the shack is almost within reach of a transceiver's ATU, and it might even manage it depending on the R and X values. That will depend on the length of the coax.
It should be obvious that to reduce the loss the transformation network should be placed at the feed point. It will have to switched by relays (one at each port) so that it is bypassed on HF. With fixed C and L an L-network can be very efficient. TLW has the following to say based on average quality components.
To connect all of this to the real world, I measured the impedance of my TH6 on 50 MHz. Of course there are differences between a real 6-element tri-band yagi and the 3-element tri-band model, but they're both trapped and the driven element of the TH6 has a close resemblance to the model. That was my intention when I originally developed the model years ago.
The impedance was measured in the shack with a RigExpert AA-54, which you've probably seen in many other articles on the blog. I could have used a VNA for improved accuracy, however that really isn't needed for this exercise. The transmission line has a few short sections of RG213 and LMR400, so I mentally estimated how that translates as an extension to the LDF5-50A Heliax transmission line I'm using. I only needed the approximate SWR, not the precise impedance at the antenna feed point.
The measured SWR is 5, which is about 8.5 at the feed point. Although that seems significantly less than 30 for the model it really isn't. Very high SWR is acutely sensitive to small details of the load and transmission line. For example, when I substituted RG213 in TLW the result was nonsensical because the load would have to have a negative impedance. The reason is that you could never read an SWR as high as 5 since that requires a transmission line with loss better that of RG213.
Suffice to say that the SWR is high and the mismatch loss can be very high. Few hams in the situation described in this article are likely to be using LDF5 Heliax!
For hams looking for a simple and effective 6 meter antenna, a switched network on top of the tower is probably not what they are looking for. A dipole mounted above the yagi and an inexpensive remote coax switch (or separate feed line) are probably a preferable alternative.
A 2-element yagi is very small and has significantly better uni-directional gain. Hams of my acquaintance who get on 6 meters with their 20 to 6 meter hex beams seem to be satisfied with the performance. It isn't what I would choose for myself but few hams have the towers and property that I do.
The northern hemisphere's summer sporadic E season is now finished. For those planning to get active on the band in 2023 this article may motivate you to put up a resonant antenna rather than try to get by loading an existing HF antenna. The improvement will have a remarkable impact on your success.