The reversible Moxon replaces the XM240 on the rotatable side mount. Despite the limited rotation, since it's reversible it covers 275 of the compass, from southeast to west (default direction) and from east to northwest (reverse feature enabled). The dead spots are between west to northwest and east to southeast, which from here are the least productive directions. The 3-element yagi at the top of the tower has full compass coverage, including over-rotation when needed.
The antenna default direction allows coverage of the Caribbean, South/Central America and almost all of the US. That leaves its big brother (the 3-element yagi) to focus on Europe and longer path DX where it shines. Even so, the lower antenna performs better in all directions before sunset and after sunrise, and during geomagnetic disturbances, when lower elevation angles encounter D-layer absorption. That's due to their respective heights, not their relative performance.
As previously noted, this antenna requires NEC5 for accurate modelling. Although it took a lot of segments to get sufficient accuracy, the built antenna tracks the model exceptionally well. It seems to be well within 0.5% of its design frequency -- about 25 kHz. That's even more impressive considering the close spacing of the capacitance hat tips (30 cm) which increases sensitivity to inaccurate modelling.
Another critical measurement is the inductance of the coil that makes an element the reflector. Careful tweaking of the design resulted in a value of 1.2 μH. Values between 1.1 and 1.3 μH would be acceptable, provided that the two coils are equal. Otherwise the frequency range will differ from the other direction, which is undesirable. The measured difference is a very close 25 kHz. That's the same as measured for the sanity check with the antenna on the ground.
The test setup at right was used to measure the inductance for both element switch boxes. To roughly account for the short leads from the analyzer, I set the inductance a little higher than 1.2 μH. The inductance of the internal leads and relays requires no compensation since it is present during use.
To test the opposite direction, a 50 Ω load is connected to the coax connector. The stray inductance of the leads and relay has an effect, however it is only a few ohms of inductive reactance. That is low enough to be considered negligible.
The DPDT relays behave oppositely in each switch box, with one connected to the element in its NO (normally open) position and the other to the coil. The relay coil is energized to test the switch box in its opposite direction.
You can see from the alligator clips to the 13.8 VDC power supply that I use a common ground (return) path for both RF and DC. I do the same for all my switching systems. Therefore only one conductor is required to energize the relay coil.
A stainless screw stud serves for the relay power. A 1N4007 diode in the centre box (see below) acts as the flyback (suppressor) diode for the relays in all three switch boxes. This is possible because to reverse direction all relays are energized.
One of the faults that I had to repair was the relay in one of the element switch boxes. I was surprised to find that the relay coil was essentially shorted: 5 Ω rather than the expected 340 Ω. I don't know how that happened since it worked fine on the ground. Repair of the relay isn't possible since it's a sealed unit. I've never had a problem with this line of Omron relays for RF switching. I tossed it into my NFG bin and replaced it with my last DPDT relay (I'll have to order more).
The other fault was in the centre switch box. The stud for the relay coil loosened, allowing the internal lug to touch the enclosure wall and short. It's a tricky bit of hardware since the DC path has to pass through the metal enclosure.
I discovered that the insulator was too high and could not be properly tightened. I replaced it with a plastic washer that has an inner rim to centre the screw within the hole. A thin layer of silicone caulk is used as a moisture seal.
Do you recognize the enclosure? I reused the one for the upper 20 meter matching network to compensate for a misbehaving gamma match. After I repaired the yagi's gamma match the network was removed. All it needed was a third coax connector and a relay stud.
The capacitance hat clamp hardware was tightened. I had forgotten to fully torque the nuts before its first raising in the spring. The clamps rotated when the tram line was drawn over them. While the yagi was on the ground I made a few other hardware adjustments to the antenna. It should now be far more robust.
The same crew from the earlier raising (and lowering) happily came over one more time -- it's good to have friends. Hopefully this is the last time we need to raise the Moxon for a long time to come!
The only rigging change was to run the tram line under the trailing capacitance hats. As usual there were steering issues to fit the elements between the guys immediately below its mount on the tower and the TH6 above. We used tag lines and a lever on the boom to tilt up the leading element tips. It took a lot of force on the rope connected to the end of the lever to rotate the boom of such a heavy antenna. I did the same for tramming the 3-element 40 meter yagi to clear the upper guys.
It was a bit of bother to get the boom attached. The antenna is too heavy to push it into position with one hand while driving the saddle clamps home with the other and reaching around the back to thread on the lock washers and nuts. I gave my friends what were to them very confusing directions on how to use the tag lines to bring the boom flush to the plate. But it got done just the same. The little HT's VOX feature was a perfect third hand.
With the antenna secure I measured the SWR in both directions (a friend in the shack operated the reversing switch). The SWR was near perfect but about 25 kHz different between directions. That agreed closely with my sanity check on the ground. You can look at the forward SWR curve in an earlier article and imagine the reverse direction curve shifted to the left a bit.
Unfortunately I could not easily grab screenshots on the tower at the time and then the connector was weather proofed. The above plots were taken in the shack when I was back on the ground. Different but still excellent. There are two likely causes. One is the ~80 meters of transmission line and several antenna switching relays between the antenna and shack. Although I use good and tested coax, mostly LDF5 in this case, some impedance deviation is normal. The other less likely but possible cause is that there is no CMC (common mode choke).
I debated whether to put one at each feed point, at the element switch boxes, or just one at the rotation loop. Since the SWR is fine at the latter point I'll probably put the CMC there. It isn't urgent.
Now we come to the important part: how it performs on the air. The overlaid azimuth plots at right, in 75 kHz steps from 7.0 to 7.3 MHz, set the expectation. Again, NEC5 is used in the model with the antenna at its actual height over EZNEC medium ground.
The free space gain peaks at close to 7 dbi at 7.0 MHz and gradually declines as we move up the band. F/B behaves the same. Gain, F/B and SWR are really quite good right across the band.
When I had the XM240 on the same rotatable side mount during DX contests I would often aim the 3-element yagi to Europe and use the XM240 to work the US, South America and other southerly directions. Since, like most yagis, the XM240 is uni-directional so it only covered west to southeast (~130°).
As already mentioned, the opening to Europe before sunset favours high elevation angles. With the reversing feature of the Moxon I can quickly switch directions to take advantage of its lower height in this circumstances. It also does well after sunrise towards the west while I point the 3-element yagi north for long haul contacts into east Asia.
Recent conditions turned the tables on my expectations. With so much geomagnetic activity, low elevation angles encountered high absorption. That meant the Moxon at its lower height outperformed the 3-element yagi on many DX paths, including the all important one to Europe. That shows the Moxon's value but it made comparisons difficult.
It is fair to say that the Moxon's gain is better than the XM240 with its coil-loaded element. That isn't a surprise since the latter is a high-Q antenna that also has a narrow SWR bandwidth. This is best I can do to assess gain on the air since it is very difficult to measure differences of a db or two, especially with the XM240 on the ground!
F/B is far easier to assess with the reversing feature. It is excellent on all paths within the CW segment and up into lower part of the phone segment. Once we reach 7.2 MHz the F/B becomes quite poor, possibly worse than the model predicts. I have more testing to do so this is not the final word. However, the excellent SWR means that the entire band can be worked without an ATU, even with my Acom 1200S solid state amplifier.
I have extra coax coiled up for the runs to each element that could be used to wind on a ferrite toroid to make a CMC. I used longer lengths of LMR400 for routing flexibility, not for a CMC; LMR400 is too stiff for that use. When I added the outer shield of the coax to the model it suggested that common mode may be present. A poor F/B (pattern distortion) is the most obvious consequence. However, the excellent F/B lower in the band is evidence that disfavours that explanation.
Another possible cause is the interaction due to the proximity of the capacitance hat tips to the guy wires in some directions. The distance isn't really very close except in comparison to the 40 meter wavelength. Again, that seems like another unsatisfactory explanation due the antenna's excellent F/B low in the band. The same can be said for the proximity of the TH6 above the Moxon, even after we lifted the TH6 higher earlier this year.
Clearly I have more testing to do. Even if it is the F/B is poor high in the band I am not too concerned. The reason is that as a contester I value gain and SWR more than F/B for the potential contacts to be made off the back. I can live with it. I will add a CMC later at the rotation loop (not at each element).
A picture further above shows another oddity: the rotator. That's a Hy-Gain Ham-M that I've owned it for 40 years -- I refurbished at least twice -- and it's at least 10 years older than that. It is under-powered for this large antenna. What saves the day is the symmetry of the reversible Moxon. High winds, even those that excite oscillations, place very little net torque on the mast.
The only real problem is the 105 lb antenna's momentum. You typically have to wait several seconds for the antenna to coast to a stop before it stops turning so that the brake can be safely engaged. If it becomes a problem I'll swap in a rotator with a higher overturning moment. But not this year since I don't have one at hand.
To summarize the antenna after a years long process is, to put it simply, I like it. I could live with the reversible Moxon as my only 40 meter antenna. It's that good. I am not surprised that many contesters stack 2 or 3 W6NL Moxons on 40 meters rather than take on the far greater challenge of a 3 or 4-element yagi despite the latter's better performance.
NEC5 made this antenna possible. I still marvel at how closely the model agreed with the built antenna. It took many modelling variations and tweaking to make a symmetric and reversible yagi that performed to my satisfaction. While few will ever buy or build an antenna of this size, I believe that I've demonstrated what is possible with enough ambition. Perhaps one or more readers will be incentivized to do the same.
And the XM240? I might yet raise it again on the 20/15 tower and point it south as a multiplier antenna (similar to the how the TH6 is being used). The Moxon's reversing feature makes that job less urgent. I'll mull the idea over the winter, and whether to stack the Moxon and the 3-element yagi.
I look forward to working you with the Moxon in the upcoming CQ WW SSB and CW contests.
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