I recently found a nice way to add a position-sensing potentiometer to a Radio Shack TV antenna rotator. These rotators are cheap, reasonably weatherproof, and sturdy enough to hold a couple of VHF/UHF beam antennas. However, they do not have any position-feedback circuitry. They use AC synchronous motors that run at a predictable speed, and the control box simply runs the motor for the amount of time needed to turn the antenna from where the controller thinks it is to where the controller wants it to be. Of course, over time, the error in position grows and grows, and in practice you often have to run outside to have a look at the antenna to see if it is really pointing where it is supposed to.
I have used a Radio Shack rotor for several years, but I finally got sick of never really being sure that the antennas were pointed in the right direction. The problem is worse with AO-40 S-band reception, since the dish antenna beamwidth is much smaller than that of a UHF or VHF Yagi beam. A pointing error of 5 degrees can cause a significant loss of signal strength with a 60cm dish, and more than 10 or so degrees of error pretty much wipes out the signal.
“Real” ham radio antenna rotors have a potentiomenter coupled to the drive train, so that there is continuous, unambiguous feedback of the antenna position. After much thought I devised a simple, cheap, and reliable way to add a feedback potentiometer to the Radio Shack rotor.
Have a look at the pictures (see bellow). They are worth a thousand words. Note that you have to chop a hole in the top of the case to provide room for the pot. I weatherproofed the case by epoxying a PCV pipe cap over the pot and the hole.
Here is a parts list:
Rotor: I used the Radio Shack one. I suspect that the Radio Shack rotator is actually identical to models sold by other companies.
Gear: From Small Parts, Inc. (www.smallparts.com) It is part #GD-4860, cost about $6. It is a 60-tooth 48-pitch plastic gear with a 1/4″ hole. Small Parts has a catalog full of cool stuff. Try to order some other stuff with the gear, to offset the shipping charge.
Potentiometer: A Bourns 3540-series 10-turn pot, resistance 2k ohms. (Any standard 10-turn model whould work). Mouser and Digi-Key carry them. The Bourns model is very accurate and well-made. I got it at a hamfest for three bucks. New, they run $12 or so. The actual resistance value depends on the rest of the control circuitry, but you should probably not make it more than 2K or so, to reduce noise pickup.
Metal plate: A bit of scrap aluminum about 1.5″ x 3″.
Nuts and Bolt: From Home Depot 🙂
Cap over pot: A 1″ PVC pipe cap, also from Home Depot.
When the rotor turns 360 degrees, the 10-turn pot turns about 9.3 turns. When putting everything together, make sure that the rotor can travel over its full 360-degree range without jamming the pot at the ends of its rotation.
Let me know if you have any questions, and if it actually works for you.
Doug Braun – doug (at) doughbraun . com
The original document was found at: http://www.dougbraun.com/rotor_mod.html