One month ago I discussed the construction of the 40 meter reversible Moxon. With the advent of NEC5 it became possible to develop accurate models of complicated antennas like this one. These include tapered elements, mid-span capacitance hats, and close spacing of elements. NEC2 and NEC4 cannot properly model this antenna without a great deal of effort with segmentation or to calibrate the incorrect calculations.
Well, that's the theory. Others have had success using NEC5 to develop models that closely match measurements of built antennas, and my initial experiments were promising. It is no trivial matter to build and test an antenna of this size and then use real-world measurements to revise and try again. Trial-and-error may be an acceptable strategy for small yagis and wire antennas but not for rotatable 40 meter yagis.
For those that prefer one article that encompasses the theory, construction, testing and performance of this unusual antenna, all I can say is "sorry". I am writing this up as I go and each stage from conception to on-air use is an interesting subject for a blog article. This blog has always been about the journey, not the destination.
So, now that we've covered the conceptualization of the reversible Moxon, the model and most recently its construction. we proceed to the next step: raising it onto the tower. The unique attributes of the antenna complicate the process, especially for side mounting on a guyed tower. For those with a self-supporting tower and no obstructions within the footprint of the antenna (43' × 37'; 13 × 11 m), raising it is a simple matter: assemble it around the tower and pull it straight up. It is not so straight forward with a guyed tower.
Moving fast
As I stated in the construction article, time was of the essence. The yagi was sitting in a hay field and the hay had entered its rapid growth phase. One way or another I had to get it out of there. I intended to raise it earlier in the month but, as usual, higher priority jobs got in the way. Finally I the antenna was completed and ready to go on the tower. Then it became unseasonably cold, windy and rainy.
I pointed a finger at the calendar on a day with no rain forecast and decide that would be -- must be -- the day. I contacted friends and soon had a crew ready to come over for the big event. They were Alan VE3KAE, Dave VE3KG and Vlad VE3JM. If you're a contester the last two calls may be familiar.
I rigged the tram line and other mechanisms on the tower and antennas one day in advance. I say "antennas" because the XM240 had to come down first.
Lowering the XM240
Luckily these antennas have short booms: 22' for the Cushcraft and 20' for the Moxon. There was a gap of just the right width between a tree and guy anchor (for the 140' tower off to the right in the picture below). There also was a tree perfectly centred in the gap to anchor the tram line winch. We chose manual power since we had enough people and gravity helped for taking down the XM240. It can be difficult to speedily communicate with a driver of a vehicle that can easily damage a fragile antenna that at some points must be moved inches, or less.
Lowering a yagi is not without drama. You are never quite sure whether you have the CoG (Centre of gravity) where you think it is, and with a 75 lb antenna you may not be able to easily make adjust the harness after it is unclamped from the mast. The XM240 is pretty well centred on the boom-to-mast clamp so I wasn't too concerned. However, the lever arm and rope I attached to the boom caused a slight tilt that we needed to closely monitor. The capacitance hats are fragile and it is far too easy for the long elements to snag a guy. Good communication between tower and ground is needed to accomplish the necessary choreography.
The lever arm is used to tilt the elements up as needed to clear the guys. Since I was on the tower near the antenna I had the best perspective to guide the crew handling the haul rope, the lever arm and the winch (tram line tension). With a series of rapid commands back and forth we were able to guide the yagi past the guys without any drama. It helped that one of my friend's set me up with a VOX-actuated HT clipped to my harness.
Rigging the Moxon
Unlike a typical yagi, the Moxon elements are joined at the tips. That imposed constraints on the rigging and operation of the tram line. There was also the challenge of guiding the antenna between the guys directly below the tower side mount and the TH6 above. The tri-bander was not so much higher that the upward tilt of the long 40 meter elements would not reach it. The many clamps and traps on the TH6 can and did snag the Moxon elements.
The major consideration for rigging the Moxon was whether to run the tram line over or under the rear capacitance hats. I decided to run it above the tips since that would help to keep the antenna oriented upward, which was needed to clear the guys. That is, the tips or the connecting fibreglass rod could rises no higher than the tram line. I used a ½" rope rather than steel cable for the tram line despite its greater elasticity since it was less likely to abrade the antenna. Some contact was inevitable.
The choice seemed reasonable and it worked as intended with respect to that one issue. Unfortunately it turned out to be a poor choice because there were greater concerns that I failed to account for in my rush to get the antenna out of the hay field. It was only after it was attached to the mast that I realized how seriously wrong my choice had been.
First, I'll note that it took 3 attempts to successfully pull the Moxon up the tram line. On the first try it failed to clear the guys. We added the lever and rope so that ground crew could steer the antenna. The second attempt failed because the weight of the rope on the lever tilted the antenna too far upward. Finally we got it more or less right. Well, right enough to guide the Moxon where we wanted it to go.
I took the picture at right when the forward capacitance hats reached the tower. You can see the tram line, haul rope and the rope dangling from the boom lever. After taking the picture I opened the tips and guided them around the tower. Pulling resumed after I reconnected the tips.
It was at this point that trouble arose. The angle of the antenna caused a collision of the forward hats and element tips with the TH6. There was no provision for tipping the boom more level to clear the elements of the higher antenna. The lever was still out of arm's reach. It is normal for the trammed yagi to increase its vertical tilt near the top of the tram line due to catenary physics: the proximity of the pulley carrying the weight increases the downward angle of the tram line at the tower anchor. I could have reduced this using steel instead of rope, however that entailed the risk that I described above.
A further increase of tram line tension helped enough that I could jiggle the ropes to encourage the Moxon past each clamp and trap on the longest TH6 elements. Clearance was easily completed when my arm could finally reach the boom and lever.
We slacked the tram line and the Moxon's boom rested against the mast. The haul rope was tied off when the boom was centred on the mast clamp. The u-bolts and saddles were then installed. That's when the more serious problem was discovered.
It was when I couldn't level the antenna with the lever arm no matter how hard I tried. I looked behind me and discovered the mistake. Have a good look at the picture above. Perhaps you will see the problem without me telling you, especially since I gave a strong hint earlier in the article. Now that you, hopefully, see it, I'll continue.
When the boom is rotated the rear tips strike the tram line. My attempt to rotate the boom pushed the inside ends of the capacitance hats downward. This occurred due to another mistake: I forgot to torque the nuts of the u-bolts on the element clamps. They were intentionally left snug but not fully tightened so that I could align the tips once it was cleared the ground. The antenna had been abandonned in the hay field for so long that I forgot.
Although I couldn't fix that problem, it didn't overly worry me. The mechanical risk of failure is low and the impact on antenna performance would be negligible. In the former case the fibreglass and aluminum are flexible enough to survive. In the latter case, performance of an antenna is largely determined by where current is highest. The attitude of the tilted capacitance hats has the same loading effect and will still cancel the fields with each side of each hat and pretty well with the hats at the other end of the elements. The distance between hat tips is critical and that remained unchanged.
My big worry was removal of the tram line. I could either let it go from the top or pull the full 200' length up and over the capacitance hats. I judged that the risk of damage was far greater by releasing the top of the tram line since the rope's weight would whiplash the rope tip as it wrapped around the fibreglass at high velocity. I opted to pull the rope up and over instead. That worked well enough in that no more damage resulted.A broken fibreglass rod is cheap to replace but expensive in time and effort. Since I expect to pull the antenna down in the fall to inspect and change the antenna according to how well it performs during summer storms, I may leave the curved hats as they are until then. However it's an interesting problem that had me considering a repair. I think I know how to do it. Perhaps it'll be the subject of a future article!
Finishing off
I decided to use the custom boom-to-mast clamp that came with the XM240 rather than I one I selected. It was more robust, already installed and it was aligned with the rotator direction indicator. I can make another for either the Moxon or XM240 when it comes down in the fall.
I kept my friends longer than planned so we quit for the day. Operating the winch and hauling up a 105 lb yagi is tiring work. All of us needed a break.
The next day I climbed the tower to test the antenna and connect the rotation loop. The coax to each element feed point are equal length should I ever be motivated to stack it with the 3-element yagi on top of the tower. Their lengths have no effect on Moxon performance. I made them longer than required so I coiled and taped the excess to the boom for the lift.
Notice the very slight droop of the boom. That 20' long schedule 80 2-⅜" OD 6061-T6 pipe is strong. It was worth the weight penalty. I also happen to find it amusing that the elements are trussed but not the boom.
All of the coax connectors were weatherproofed but more could be done for the feed point enclosures, relay keying wires and securing the cables to the boom.. Only one wire is needed to operate the reversing relays since the return path is via the boom and coax and tower. In my station I bond the grounds for DC, RF and ground rods.
Here you can see how I mounted the small box with the coax switch. I was careful to label all 3 boxes and the boom to indicate the driven element and reflector for normal orientation (relays using their NC contacts) to ensure the correct connections are made.
Before weatherproofing I measured the SWR by connecting the normal driven element to the analyzer. Compare it to the NEC5 model. That's pretty good agreement! Even though there are interactions with the guys and the TH6 above it, the wavelength is long and the TH6 coupling small since they are pointed the same direction.
Perhaps more important than the low SWR was the frequency where it was lowest. That agrees with the NEC5 model within 10 or 15 kHz. That raises my confidence that the design frequency range was achieved in practice. That success is a testament to the accuracy of NEC5 modelling a antenna featuring stepped diameter tubes, capacitance hats and close coupling between their tips. Neither NEC2 nor NEC4 are up to the challenge.
Performance verification will have to wait. During installation a wire must have been knocked loose so that the rotator turns in one direction but not the other. That left it locked west. I operated in the CQ WPX CW contest with it in that condition. It worked well but I don't yet know how well in comparison to the 3-element yagi. There was no difficulty transmitting with a kilowatt. The SWR barely changed with the Moxon pointed west and the TH6 pointed south. That was encouraging.
I had no time available to trace wires to patch a path back to the shack for the reversing feature. That will also require a software change to my custom antenna selection software. Due to enforced rest after routine surgery this week I likely won't get to it until at least mid-June.
Aftermath
The urgency to clear the hay field prevented completion of this antenna project. At least the antenna is on the tower and working, which is a great relief. Until it is complete and its performance properly assessed I am delaying the discussion of how I built and tested the switch boxes. An article covering performance and electrical construction will eventually appear, though I can't say when.
On a more practical note, I lost my 17 meter antenna. The XM240 is near resonance on that band so I've been using it that way since I don't have any WARC band antennas. The 18.1 MHz SWR on the Moxon is near infinite. After testing various antennas with the antenna analyzer I chose the 80 meter inverted vee as my 17 meter antenna. Its SWR is 1.5 across the 17 meter band. It works well enough that I've already logged one DXpedition. The pile up was small so its performance is difficult to assess at this time.
This is an opportunity to reflect on the lessons learned, from design through to raising of the reversible Moxon:
- NEC5 rocks! Although it can be slow due to the greater number of segments it typically requires, its ability to accurately model complicated antennas like this one is remarkable. It integrates nicely with EZNEC, which also supplements NEC5 with additional features such as insulated wires which NEC5 alone does not support. NEC5 is well worth the licensing fee.
- Double check all fasteners. While this is obvious it is easy to forget when one is in a hurry.
- Ground crew get rightfully annoyed when you ask the seemingly impossible or at least improbable. It is easy to forget the strain they're under while holding a massive antenna in position while I fiddle with the mounting hardware. You should have heard the language when I asked them to raise the antenna exactly ½" so that I could drive in the last u-bolt.
- Label and record every control wire in the station, no matter how unimportant. I have countless runs of Cat5 and other cable for the dozens of control lines, including antenna switches, stack switches, antenna mode switches, rotators and more. I'm pretty good about labelling the cables and recording the details but there are gaps. Hence the need to do wire tracing. I'm now taking the opportunity to revise and expand my records.
- Running the tram line under the rear capacitance hats would have been the better option. I failed to think through the entire process from lifting the yagi, riding the tram line, fitting the antenna to its mount and then, critically, removing the rigging. Counting on luck is no excuse for taking short cuts.
Despite the frustrations and mistakes the antenna works. I'll know more about how well it works after I repair the rotator and complete the reversing feature.
Photo Credits: Other than those I took on the tower, pictures in this article are by VE3KAE and VE3KG.