I've been in Amateur Radio since 1974, and still find new and interesting things to do. I like to build, restore, and operate on the air. This is a blog of various info about my Ham Radio operations and activities, projects, and opinions. Visit www.WB4IUY.net for the lowdown at WB4IUY.
Sunday, July 15, 2012
Light Duty Rotator? No problem!
I was listening to a conversation on HF between two guys who each had spent well over $1000 for antenna rotators, each having less rotator load than myself. I joined into the conversation and both guys quickly told me how I was doing this wrong, why I was doing this wrong, and how my rotator would never hold up the way I have it loaded. So, I'm typing in my blog pretty much what I told them...
Suffice it to say I'm cheap when it comes to this hobby. I've been a ham since 1974, and most everything I've had, I've bought used, as junk or "fixer uppers", or built. There is only one rig in my shack I've bought new... Back in 1993, I had my doubts about using a light duty rotator to turn my array. I'd been picking up pieces of salvage or used Rohn 25 & other parts for a couple of years to build my tower, and only had about $300 total in my 105' tall tower setup. I had a big pile of aluminum I had collected from swap meets, salvage from another ham's hurricane damage, and stuff I'd accumulated from helping other folks with their tower setups (read $0 investment). The rotating mass consisted of a Cushcraft A3S, a 5el 6m Cushcraft yagi, 2ea. Cushcraft 17b3 long boom yagis for 2m, and a 15' tall Diamond vertical on top. The problem was, a rotator for this much stuff was gonna cost more than I had in everything else combined, and was way out of my budget.
After pondering it a bit, I remembered an experiment I had with an old TV antenna rotator many years ago. I discovered that most of the small rotators use a shaded pole motor (similar in design to that used in small fans and such). Shaded pole motors are neat in that they don't really overheat or fail when overloaded or stalled. I found that if I fed the small TV rotator with a long run of a small gauge rotator cable, there was sufficient voltage drop across the cable when the rotator was stalled to prevent the rotator from overheating. I visited a local radio shop and found a HD-73 rotator by Alliance. This is a very light duty rotator, but unlike some of the cheap TV antenna rotators, it had a feedback pot in the rotator housing for position indication on the control box. So, even if it was slow, the indicator would still display the direction accurately. Many TV antenna rotators have a separate motor that turns the indicator on the control unit and they easily get out of sync if the rotator stalls. A feedback pot is the only way to go...
I made a tower mount for it, installed a bearing around the vertical mast with a home made ice shield, and installed it. Once everything was stacked on top, it was time for the real test. In June of 1993, I found that it took about 75 seconds to turn from stop to stop. As of this writing, 19 years later, it still takes about 75 seconds to make the journey from south to south. It has survived a direct lightning strike that welded the bearings together. I've worn out the direction control switch on the unit and had to replace it. It has been stalled when I'd forget and leave the control box rotating in one direction or another (it is no longer 'spring return' to the OFF position), to the point where the thermal sensor in the control unit would trip. It has run flawlessly while horribly overloaded for 19 years.
I've learned that cheap can often be shoe-horned into operation and work pretty good, a little planning makes 'cheap' work a long time, and the HD-73 has got to be tough WAY beyond it's price point!