Yagi on a Swing - 40 Meter Moxon Repair

Things break. For most hams making repairs to antennas is not too big a deal since most antennas are of modest size or made of wires. Perhaps I have more problems than many other hams since I design and build my own antennas. Errors creep in despite the care I take. Also, I have a lot of antennas.

When the repair is to a 40 meter yagi the process is especially difficult. Earlier this year the reversible 40 meter Moxon developed a fault that disabled one of its directions. An antenna of this size is not easy to work with. It is large, heavy and extraordinary awkward. I and a few guest ops lived with it until time and weather permitted repair. It took preparation and scheduling.

Confident as to the cause of the fault I chose to repair it on the tower rather than lowering it to the ground. Tramming this antenna down and then back up is difficult and introduces additional risk to the antenna. Also there is the matter of the hay that precludes taking the yagi down until after the harvest later in July.  Doing the repair on the tower required bringing one of the two Moxon elements close to the tower to access the switching electronics.

An XM240 was on the same side mount that the Moxon now occupies, about halfway up the 150' tower. To repair one of the XM240 elements I put the yagi on a swing with the anchor near the lower 5-element yagi of the 10 meter stack. That is about 30' (10 meters) above the 40 meter antenna. With the swing cable holding the antenna's weight, it is unbolted from the rotator mast and pushed outward to bring the faulty element within reach of the tower. In this case, about 7' (2 m).

However, the Moxon weighs about 50% more than the modified XM240 (105 vs 70 lb) and the swing is in the opposite direction. I not only had to contend with the TH6 above the Moxon but also the support strut for the 80 meter inverted vee. Luckily the TH6 didn't interfere with the swing in the planned direction. However, I had to loosen the accessible end of the inverted vee (the other is on the far side of the hay field), disconnect the coax and temporarily move the support strut out of the way. The weight and tension of the inverted vee legs and support rope made the operation awkward. 

The swing cable is ⅛" aircraft cable instead of the rope I used for the XM240. Despite being thin it is well within its load capacity. You just have to be careful with torque on the small hardware for the cable clamps -- quality counts. For the greater weight of this antenna I wanted less elasticity in the support cable. The cable was tensioned with a large turnbuckle. A chain grips the sides of the boom-to-mast clamp, which is at the centre of gravity. A couple of the following photos show the setup; I didn't bother taking a close up. Once the antenna's weight is taken by the cable the 4 saddle clamps holding the clamp to the mast are removed.

The picture by Dave VE3KG is annotated to show the arrangement which is otherwise difficult to see in a photo. The faulty switch box is circled. That's me on the right and Vlad VE3JM on the left. The cable swings parallel to the TH6 driven element. The swing cable's anchor is above the next higher guy station. At that height the cable only swings a few feet so the guy doesn't get in the way.

We used old positioning lanyards in case of disasters (swing cable failure) and to direct and hold the antenna where we wanted it. I have several of these lanyards in my toolkit that I've inherited from other hams. Despite being in reasonably good condition they have clearly seen better times and should not be used for climbing. Never bet your life on questionable personal safety equipment. But they can have other uses so don't throw them away. Don't give them to others if you don't know how they'll use them.

While I held the yagi in position at the end of its swing, Vlad tied the end of the boom to the tower. Another strap holds the boom to the mast for added safety. We then rearranged ourselves on the tower so that I could work on the switch box while he kept watch on the rigging.

As I suspected the fault was a cold solder between the coax connector and relay. The open circuit does not affect operation of the element as a reflector so that's why the antenna continued to work in one direction. The wire is already lifted off the relay in the photo. The relay pin was in good condition so I removed the wire and put in a new one. 

The temptation to simply solder the wire back in place should be resisted since you can't be certain that solder-clogged surfaces are sufficiently clean for a reliable joint. The "dead bug" construction doesn't allow a firm mechanical connection since the relay pin is short, designed for PCB mounting, and crimping the large wire onto it is difficult.

If you haven't read the original construction article you may be surprised by the ad hoc construction. Well, I'm a ham not a craftsman. It works well enough, even if I risk the occasional repair. Others might prefer to do it differently. In any case, the antenna is experimental and I expected to have to deal with issues and try alternative construction techniques.

Speaking of issues, notice the pool of water under the reflector coil. I didn't think water could get in so I didn't drill weep holes -- there's more in there than condensation can account for. There is now a hole at each of the box's 4 bottom corners and the top screws are taped. It is possible that the other element is in the same condition. Since the swing only works in one direction that will have to wait. Performance of the Moxon doesn't appear to be noticably affected by the rainwater.

Another problem I noticed is that one of the 4 hose clamps securing the capacitance hat tips to the fibreglass rod that hold the adjacent tips at the proper distance is loose. The ring of tape should be just within the ⅜" aluminum tube. With these small hose clamps there is only a narrow adjustment range between being too loose and stripping the threads. The antenna has to come down for repair, which won't happen sooner than autumn. It isn't an urgent repair since the rods are quite long it is unlikely that the capacitance hat tip will pull out.

Now back to the repair of the faulty switch box. I chose to solder in the new wire on the tower. It's more difficult than removing the box and doing it on the ground, but it would take more time and there was more opportunity to make mistakes. These include coax waterproofing and removing then reinstalling the tabs from the box that are secured to the element halves with hose clamps. It's a tight fit.

To use my trusty old Weller soldering gun I pulled a 100' extension cord up the tower and plugged it into a portable power station at the tower base. It was also used to drill the weep holes. I like the power station since I can use my many AC tools anywhere on my 50 acre property rather than acquiring battery operated tools. The old wire that I removed from the switch box is seen next to the small roll of solder I used.

Despite all the fuss the repair was as simple as soldering in a new wire and drilling a few holes. With the antenna swung back into its proper orientation I connected the antenna analyzer and control cable while my ground crew (Dave) clicked the mouse to switch the Moxon in both directions. That went well so we reattached the antenna to the mast, reconnected the coax and loosened the swing cable. 

With that I let my friends leave and get on with their day. I cleared the tower of the rigging and reconnected the affected antennas several days later.

Vlad and I took pictures of each other while we were working and I wanted to include that in this article. The view is interesting in that we were only half way up the tower! It was warmer than our clothes suggest, but we needed to protect ourselves from the bugs while we were on the ground. Compare this to a similar pair of photos we took of each other 5 years ago.

That's one more check mark on my lengthy to-do list. There are many more. It seems that each deserves an article so expect more as I work my way through the list over the coming months. Stay safe while you work on your towers and antennas this summer.