Modelling antenna interactions is difficult. It is worth the effort since it is far more difficult to resolve problems after the antennas are on the tower. When I suspect a deleterious interaction, I take the time to do the modelling rather than cry afterward. When my suspicion proves to be wrong I don't regret having done the modelling work. I always learn from it.
On the right is a photo copied from an earlier article so that you don't have to flip back and forth to understand the problem I am currently addressing. The antennas on the tower from bottom to top are:
- XM240 at 21.5 m (72'), rotatable
- TH6 at 24 m (80'), south
- 80 meter inverted vee, apex 31 m (102')
- 10 meter 5-el at 33 m (108'), northeast
- 40 meter 3-el at 43.3 m (142'), rotatable
- 10 meter 5-el at 46.3 m (152'), rotatable
There are also 4 sets of guys that, though broken up into non-resonant segments on all HF bands, interact with the yagis.
It is a nightmare to fully characterize. Luckily the situation can be simplified with a few mostly reliable rules-of-thumb (heuristics):
- Down pointing guys and wire antennas below a yagi, especially if non-resonant or resonant on other bands, have minimal interactions with yagis above them.
- When stacking yagis vertically, they have little interaction with those for higher bands, except at very small separation. The reverse isn't true unless the separation is at least one (the longest) boom length.
- Interactions from guys, wires, towers and other antennas increase the closer they come to the tips of the antenna elements.
Heuristics provide guidance, not guarantees. Modelling are measurements are better options in critical cases. Relying on SWR deviations alone as your guide is not recommended since that typically only occurs when the interaction is so severe that the yagi antenna pattern is seriously degraded; that is, of gain, F/B and SWR, the latter is usually the last to show a measurable change as the magnitude of the interaction increases. Since measuring gain and F/B is exceedingly difficult and rarely done with usable accuracy in an amateur station, models can provide valuable insights.
After I rearranged yagis a year ago, I discovered that I had a problem. When the XM240 was turned west the SWR rose to about 2 where it had before been very close to 1. In that direction the TH6 and XM240 are at right angles -- the TH6 is fixed south.
Yagi side resonance is a type of interaction that I previously described and that I failed to take into account when I installed the yagis in their present positions. The TH6 is resonant on or near the 40 meter band when seen as a short yagi with large capacitance hats.
Rather than attempt to model the TH6 -- trapped yagis can be difficult to model -- I developed a simple EZNEC model for a 3-element 20 meter yagi of similar dimensions. The antenna does indeed resonate near the 40 meter band. Its feed point resistance is low, which is typical of a short, loaded dipole.
The resonant frequency is about 100 kHz higher (7.6 MHz) with NEC5. The boom must be included in the model and the source placed on the boom. The traps and tubing schedule of the TH6 will have a different resonant frequency due to the traps operating as coils on 20 meters that inductively load the "capacitance hats" (reflector and director). There is very little current on the other elements, including the driven element, so it is acceptable to simplify the interaction model by excluding them.
There is a secondary resonance on 10 meters that may cause an interaction with the 10 meter yagi higher on the tower. However, they are much farther apart and in any case the TH6 is (obviously) resonant on 10 meter in its ordinary application. The boom of the yagis are always at 45° (135° by the yagis' forward directions) to each other since both are fixed. In this article I won't be investigating that potential interaction.
Like the TH6, the XM240 is difficult to model with NEC2. It is easier with NEC5 to make an accurate model but I won't bother. There is no need since I will replace the XM240 with a rotatable and reversible 40 meter Moxon, and I have a NEC5 model for it. Most of it built, but I may have to leave it until the warmer spring weather. That's only a few months away although it would be nice to have it for the late winter contests.
It is easy to combine both antennas in the same EZNEC model and then arrange their separation and orientation to investigate interactions. My hope is to find a simple way to reduce the 40 meter interaction that I know is present due to the measured SWR sensitivity when the TH6 and XM240 are at right angles. The same will occur with the Moxon.
My intent is to estimate the magnitude of the interaction without striving for high accuracy. The modelled 20 meter yagi is just a proxy for the TH6 side resonance. There is no utility in a precise calculation that lacks real-world accuracy.
In my modelling exercise I did the following, which I'll discuss in this article:
- Excite the 40 meter Moxon with the 20 meter yagi 2.5 meters above it, pointed in the same direction and with the 20 meter yagi turned 90°
- Excite the 20 meter yagi, with the same conditions as in the previous point
- Repeat all of the above with the 20 meter yagi 5 meters above the 40 meter Moxon
While many other scenarios are possible, these are the ones of interest to me. I have limited space to raise the TH6 higher due to interaction (electrical and mechanical) with the next higher set of guys and interaction with the 10 meter yagi further up the tower. I am focussed on the 40 meter yagi's performance; the TH6 is mostly limited to multiplier hunting in the Caribbean, southeast US, and South America, so problems that are not too severe are tolerable.
The yagis are very close to each with respect to their size, yet that's the actual case on my tower as the opening picture shows. 2.5 meters (~8') is the measured distance. In these EZNEC plots of antenna currents only the 40 meter Moxon is being excited. NEC5 was used for these plots and will be used throughout the analysis since NEC2 cannot accurately model the Moxon.
Notice the difference when the antennas are at right angles. With both pointing in the same direction (or in opposite directions with 180° rotation), there is almost no induced current on the 20 meter yagi. But "almost" isn't none, and that will be addressed.
Due to the side resonance of the 20 meter yagi near 40 meters (~7.6 MHz, as discussed above) the induced current is quite large. The interaction impacts performance of the 40 meter Moxon.
What may be less expected is that the 20 meter yagi is also affected. The above plots are with the 20 meter yagi excited. The interaction is negligible when pointed in the same direction, but more pronounced when they're at right angles. Current induced on the forward Moxon element in the first case and on both in the latter. The asymmetry is due to the fields largely cancelling towards the rear per the yagi's design (F/B). I expected more current to be induced on the rear element because the antennas are so close and the Moxon elements are inside the 20 meter yagi parasites. This highlights the value of models over expectations (or guesses).
When isolated from each other so that their performances are unaffected, the SWR curves are much better than these. For the 20 meter yagi, the SWR is 1 (with a matching network) at 14.175 MHz and is less than 1.5 at the band edges. For the Moxon, the SWR is ~1 between 7.05 and 7.075 MHz, and is less than 1.5 across the 40 meter band.
The worst case SWR conditions are as depicted in the EZNEC plots above. I was surprised by the high SWR on the 20 meter yagi because I don't see this with the TH6 and XM240. The 20 meter interaction when the XM240 is turned 90° (to the west) doesn't significantly change the 20 meter SWR.
The XM240 is coil loaded and the Moxon has large capacitance hats. Those hats are long and are likely responsible for the interaction with the antennas at right angles -- I don't have an accurate XM240 model and I am not motivated to build one just to test this. This model strongly suggests that I will have a problem with the TH6 when the Moxon replaces the XM240.
When the yagis are pointed in the same direction there is less impact on the SWR. For the 20 meter yagi the minimum SWR slides down to 14.1 MHz and rises to 1.7 at 14.35 MHz. For the 40 meter Moxon, the minimum SWR occurs at 7.05 MHz and rises to 1.5 at 7.3 MHz. These relatively small deviations are acceptable, and that is my experience with the TH6 and XM240.
So much for the SWR, what about yagi gain and F/B? Sticking with my original premise that too much detail is uninformative, I have summarized the performance in tables with a minimum of data points calculated from the models. The yagis in isolation (no interaction) are the base line. These figures are calculated over EZNEC medium ground, not free space, so there is ground reflection gain. The Moxon is up 21.5 meters and the 20 meter yagi is up 24 meters.
2.5 meters separation: 14.0 14.1 14.2 14.3 Mhz
Baseline Gain: 13.3 13.4 13.4 13.5 dbi
24 m F/B: 18.4 21.6 22.8 20.7 db
Forward Gain: 13.0 13.0 13.1 13.2 dbi
F/B: 15.8 18.3 20.3 29.1 db
Side Gain: 13.1 13.1 13.2 13.3 dbi
F/B: 16.9 19.1 23.0 25.6 db
7.0 7.1 7.2 Mhz
Baseline Gain: 11.3 10.7 10.3 dbi
21.5 m F/B: 20.9 15.5 10.4 db
Forward Gain: 11.2 10.6 10.2 dbi
F/B: 21.9 14.5 9.6 db
F/B: 22.5 10.8 6.3 db
Other than the SWR, gain and F/B of the 20 meter yagi are affected very little. This is one of those interaction cases where SWR changes before gain and F/B are greatly affected. On 40 meters the effect is greater. That said, it is relatively modest, with up to 1 db gain and 4 db F/B lost when the 20 meter yagi is turned 90°.
In the next modelling exercise we increase the separation to 5 meters by lifting the 20 meter yagi higher. That's about the maximum I can achieve on the tower, and I will likely not separate them by more than 4 meters to avoid the interaction risks described earlier.
5 meters separation: 14.0 14.1 14.2 14.3 Mhz
Forward Gain: 13.3 13.3 13.4 13.5 dbi
F/B: 19.0 24.0 27.9 22.9 db
Side Gain: 13.3 13.3 13.4 13.5 dbi
F/B: 18.4 23.7 28.4 22.8 db
7.0 7.1 7.2 Mhz
Forward Gain: 11.2 10.7 10.2 dbi
F/B: 21.4 15.1 10.0 db
F/B: 23.8 13.6 9.2 db
The difference is not large but it is an improvement. Perhaps more important is that the SWR is much better. On 20 meters the minimum SWR shifts back to 14.175 MHz when in the forward (same) direction and 14.150 MHz when turned to the side. The SWR is never higher than 1.5 from 14.0 to 14.35 MHz, which is the same when the yagi is by itself (no interactions).
On 40 meters, the minimum SWR shifts to 7.075 MHz when in the forward (same) direction and 7.05 MHz when turned to the side. As for the 20 meter yagi, the SWR stays below 1.5 from 7.0 to 7.3 MHz. Again, that's a significant improvement.
Although I did not test for interactions with a similar proxy model for 10 and 15 meters on the TH6, I did an SWR sweep on the Moxon up to 30 MHz. Its third harmonic is, as expected, well above the 15 meter band at 26.35 MHz. I expect the interactions to be minimal especially for separations on the order of 5 meters. I am sufficiently confident that I will avoid the time and effort for the additional modelling. In practice I've not noticed the SWR or performance change on 10 and 15 meters as the XM240 is turned, understanding that without making careful measurements I could be wrong.
Just for fun I excited the Moxon on 20 meters to see what would happen. It actually works! The antennas are apparently close enough to behave as couple resonators, a popular technique for feeding a trap-free multi-band dipole or yagi or to increase the bandwidth of a mono-band yagi. The gain and F/B are very close to that of the 20 meter yagi on its own.
Unfortunately there is one difficulty: the SWR is extremely high. On 20 meters the 40 meter Moxon driven element is a 1λ dipole. The impedance at the feed point is therefore high because the current is low and the voltage high, just like an EFHW (end-fed half wave antenna). A transformer would be needed to bring the impedance down to 50 Ω. It's interesting, but without any useful application.
After all of this analysis, what will I do? The first step is to raise the TH6 a few feet. That should ameliorate the SWR increase when the XM240 is 90° to the TH6. Based on this modelling exercise, the increased separation will help even more when the XM240 is replaced by the Moxon. Lifting the TH6 and its side mount bracket isn't too difficult so I may go ahead and do it this winter
While the antenna configuration described in this article will not be the same as in other stations, it is likely that many readers have side mounted yagis or wire antennas that are close enough to the top of the tower to interact with the rotating yagi(s). Watching the SWR while rotating the top yagi(s) will determine whether you have a problem. My philosophy is that it is better to know than not know, whether or not one chooses to deal with it.
The Moxon is half built in the snow covered hay field. I'll continue to work on it as time and the weather allow. My hope is to put it into service within a few months. Then we'll see whether the interactions are as found in this modelling exercise.
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