Lifting the Side Mounted TH6

One of the smaller tasks I set out for myself this year was to increase the separation between the south pointing TH6 tri-bander and the 40 meter XM240 on the rotatable side mount. Their proximity isn't a problem most of the time, but when they are 90° to each other (XM240 pointed west) the TH6 degrades the performance of the XM240. That's because, from the side, the TH6 behaves as a short 40 meter dipole with large capacitance hats.

In the model I developed, the separation was increased from its previous 2.5 meters (8') to 5 meters (16'). That tamed the interaction to my satisfaction. Unfortunately that isn't achievable since there are guys and an 80 meter inverted vee above the yagis. The reflector element tips of the TH6 would get close to touch them and degrade the tri-bander's performance. I opted for a smaller step of about 11' (3.4 m) to preserve the performance of both antennas as well as the situation allowed. 

Even simple tasks like this involve complications, both mechanical and electrical. It took twice as long as expected when the unforeseen occurred. In this article I'll take you through the process, including what went wrong, and finally the results of the move.

I installed a pulley about 20' (6 m) above the TH6. The extra distance was needed to make it possible to lift the yagi and its tower bracket at the same time. The TH6 centre of gravity, like many Hy-Gain yagis, is at the boom-to-mast clamp so that's where I tied the rope. The bracket is much lighter than the antenna so it doesn't unbalance the load by much. To keep it level for the lift I cinched the rope to the upper end of the mast.

On the right you can see the problem. Convenient at the time, I installed the tower bracket for the TH6 at the desired height with the lower strut under the top strut of the rotatable side mount for the XM240. The TH6 had originally been there so it was easy to install the bracket that way and simply the yagi several feet. However, that made it a hassle for climbing since, pointed south, the boom and truss were near the one climbing face of the tower. I've had to squeeze past it many times! It would have been better, but quite a lot more difficult to mount the yagi on the other side of the face

For this lift the challenge was different: to swing the yagi and bracket outward so that the lower strut cleared the XM240 upper bracket. It was easy to do by placing the pulley high on the tower. I used the same technique for the more complicated repair of the XM240 last year.

After the tower was rigged I asked a friend to come over later in the week to help with the lift. In between we had high winds that may have contributed to a problem I discovered too late.

The lower pulley takes the haul rope horizontally to the winch. The pulley is extended out from the tower so that the rope clears the boom of the XM240 which is below the TH6. The hand winch is attached to a convenient guy anchor. The tarp is for tick protection.

The rigging is straight forward. It is more than capable of handling a 5' lift of a load less than 100 lb. Yet when my friend cranked the winch nothing moved. When I unbolted the bracket the rope held the load but it wouldn't budge.

After searching for obvious problems and seeing none, I climbed up to the high pulley and saw what I really didn't want to see. The ½" polypropylene rope had jumped the sheave and jammed between it and the pulley housing. I didn't take a picture at the time so I took one later in the workshop (below right). It is amazing to think that the rope could squeeze into that small gap. Yet it did.

I have several of these cheap pulleys and I really shouldn't use them. This wasn't the first time they've given me trouble. I picked it since it was first my glance fell upon when I was gathering material for the job. Since this wasn't going to be a heavy lift I didn't give it a second thought.

With a bit of jiggling I was able to lower the load slightly and reattach the bracket to the tower. I climbed up to disassemble the pulley to free the rope (left picture). After a critical appraisal of both rope and pulley I put it back together with the rope properly riding the sheave. The second lift attempt went smoothly. In less than 30 seconds the bracket was swung outward and upward and ready for bolting to the tower in its new location. 

The girts on the tower are approximately 5' apart so that was the minimum distance it could be moved. I lowered the TH6 on the stub mast to where I wanted it. The next day I took a few pictures to show how close the reflector is to wires and cables.

I took these pictures the next day after pointing the TH6 south. We are looking down on the TH6 and XM240. To the northwest (left) the rear reflector is several feet from a 105' level guy and one leg of the 80 meter inverted vee. To the northeast (right) the other leg of the inverted vee if not as close but the guy (nearly invisible in the picture) is just a few feet from the opposite reflector tip. The south guy (not shown) is centred on the boom and some distance from the TH6. From previous modelling of interaction scenarios I am confident the south guy has negligible impact on the TH6.

Despite the proximity, I was surprised to note that the SWR on all 3 bands (20, 15, 10) was affected only a little. SWR measurements on the tower were not accurate since I couldn't stand far enough away to avoid detuning the antenna -- the jumper to the analyzer was too short. Body interaction was particularly bad on 15 meters; it's happened before with this antenna. On 20 meters the SWR at the band edges was a little high at 2, however back in the shack it was about 1.7 to 1.8, probably due to some loss in the long run of LDF5 Heliax.

The 15 meter SWR was very good when measured in the shack. The 10 meter SWR was about the same as measured on the tower, with the SWR 2 bandwidth ranging from 28.0 to 28.8 MHz. 

That's good enough. The TH6 isn't used heavily since it is primarily a "multiplier antenna" used in contests to work the multiplier rich and QSO poor Caribbean, South and Central America while the other, larger antennas can stay pointed to Europe and elsewhere.

But what of the XM240? Did this manoeuver help? That's difficult to measure with any confidence. What I can say for certain is that the SWR deviation when the antennas are at right angles is significantly improved. 

The deviation is now small enough that there is little need to re-tune the amps when the XM240 is rotated. I consider that a win. If modelling is any guide, gain and F/B are also better preserved. That, however, is very difficult to measure and confirm.

With this job out of the way I can proceed to finish the 40 meter Moxon and swap it for the XM240. I hope to get that job done in the near future. 

One final and slightly amusing note. After measuring the distance from the TH6 feed point and the Heliax connector I realized it was a perfect fit for the 20 meter yagi delay line that was retired from service last year. It had been hanging on a hook ever since. Further reducing the clutter, the small box for the L-network is also being reused in the 40 meter Moxon project. 

There are a few benefits to being a pack rat who rarely throws anything away.