Bushing Protection

Often it's the simple things that can ruin your day. In a station as large and complex as mine, I expect things to go wrong, but I tend to worry more about towers, antennas, cables and complex electronics.

Immediately before the start of January's NAQP CW contest (North American QSO Party), I ran into one of these small and unforeseen problems. The TH6 on the rotatable side mount wouldn't turn. At a moderate height (22 meters) and 130° coverage from VE7 (west) to the east coast (south-southeast), it sees a lot of use working stations within North America on the high bands and for hunting multipliers in South America and the Caribbean.

NAQP was about to begin so there was no time to diagnose and resolve the fault. The tri-bander would therefore be fixed west and I had to use the 5-element mono-band yagis at twice the height to cover south and southwest directions. That's far from ideal because they are too high (40+ meters) for short paths. My usual strategy is to have the high yagis point to the more distant west coast and use the TH6 for the shorter distance stations in the US south, southwest and midwest.

It's impossible to know how the problem affected my score. I doubt it was significant. I came in last of our highly talented 5-member team, but there were other reasons to account for that. It is by design that I have enough antennas to be flexible when problems like this arise.

What happened? The 50-year old controller for the Ham-M rotator, which I plan to replace soon, was working properly. There appeared to be no cable faults per resistance tests. When attempting to rotate the antenna the direction indicator bounced a few degrees, as if the motor was struggling against resistance. Nothing obvious could be seen by looking at the side mount system and antenna out the window with binoculars. 

Hy-Gain Ham and Tailtwister rotators have low torque motors. The reduction gears multiply the torque so that they can turn moderate size arrays. Without their strong wedge brakes, the wind induced torque on large yagis can easily overturn the motor. Indeed, it is possible to stop these rotators with a strong hand grip. I've done it myself, just to prove to myself that it can be done.

The motor run capacitor, a common maintenance issue, is mounted at the rotator, with weather protection, so it could not be tested. Many hams mount the capacitor at the rotator to save on cable (two wires are eliminated) and reduced torque due to the resistance of those long wires. Most motor capacitors are rated for winter temperatures, and so is mine, which is not a Hy-Gain part. 

I didn't suspect that was the problem since the rotator behaved normally the day before and these capacitors typically deteriorate gradually rather than suddenly. They will last for many years. When they weaken or fail, they are inexpensive and available from MFJ or any local electric motor shop.

I could not climb the tower to check the capacitor or anything else because the contest was about to start and a couple of days of light rain followed by freezing rain and a temperature drop covered everything, including the tower, in a layer of ice. 

Even as I operated in the contest, I continued to think about it. I had a hypothesis. To test it I had to do nothing but wait. Two days later the weather warmed to a few degrees above freezing. This time when I tried the rotator, as I had done periodically during the contest and after, it turned. There was no hesitation and rotation speed was normal.

Perhaps you can guess what happened. By all means follow the link provided at the top of this article to the one describing the rotatable side mount. You can inspect the mechanical design from the pictures.

What almost certainly happened is the following. Rain water ran into the gap between the mast and bushing. Then it froze. Bare metal surfaces are often not the same as the air temperature. In the dark or under clouds, the metal temperature is colder than the air. When in bright sunshine, the metal temperature is hotter, sometimes much hotter. Putting your hand on the body of a car that's been out in the Sun will remind you of that. Conversely, as the old tales warn, don't lick a metal post in wintertime!

The reason is the low specific heat of most metals. They hold little heat and that heat is easy to lose (by infrared radiation) and to gain (by sunlight, torch or other source of radiant heat). Water on the mast and bushing will freeze quite readily.

Ordinarily this isn't a problem since it simply runs out the bottom of the open bushing before it can freeze. The ambient conditions cooled and froze the water quicker than it could exit out the bottom. The gap between the 1.5" (IPS Schedule 40) galvanized mast and 2" (IPS Schedule 80) 6061-T6 bushing is only 0.019" (0.5 mm). That's a good fit for a bushing but not when faced with freezing rain.

The weather warmed to a balmy 5° C under bright sunshine this weekend. Once I had 3Y0J in the log I climbed the 150' tower with the material I had prepared in advance. Adding a weather cap to the bushing was one of several jobs I did during that climb. As you see it's still deep winter. The trail of indentations on the snow is the from the tread of my snowshoes.

The solution to the problem is simple. I cut two strips of thick flexible plastic to fit around the mast pipe (found in my junk box) and compressed them with a hose clamp. Little if any water can pass this seal to enter the bushing.

There is no easy way to test the seal and I see no reason to do so. Whatever wind driven water does get past the seal will be a fraction of what an open bushing is subject to. I consider the problem solved.

While this may seem a trivial matter to justify its own article, I can only respond that it is often the simple things that can incapacitate your station. That can be devastating during a major contest effort, a public emergency response or when one of the rarest DXCC entities comes on the air for a few short days. 

Details matter. Don't overlook or dismiss small vulnerabilities in your station. You should have a backup plan at the very least.