Dish Rotators
A dish needs to be moved in elevation and azimuth to track satellites such as AO-40. Old TV rotators are fine for BBQ grill dishes and small yagis, but the bigger the antenna, to more power is required, of course. 5' and larger TVRO dishes typically use linear tracker arms for elevation. Azimuth is a bit more complicated. It is best to radially balance and counter balance the dish if you want the rotators to survive occasional storms. Most big dishes are parked looking upwards. A big dish at 0 degrees elevation develops a great amount of twisting torque in a windstorm. In snowy areas, like my QTH, a dish parked looking upwards can crush the mount in a heavy snowfall. A solid dish can hold many gallons of water in a rainstorm. As a result, I park my solid 14' dish at about 50 degrees elevation facing downwind, which lets the water run out and won't accumilate much snow. One time I had it at 60 degrees, and got showered with about 10 gallons of water after a storm. Another consideration is avoid wrapping the coax around the azimuth shaft in a windstorm. I put stops on my 14' dish after it tore up some expensive coax in a storm. The stops limit azimuth rotation to about 300 degrees, fine for AO-40 and EME.
I recently got a sprocket and chain set from a local Honda Motorcycle Shop. The set was made for125 cc "Enduro" motorcycles, so will definitely last forever! The large sprocket had 45 teeth, and the small sprocket had 12 teeth. The chain came in a "120 link" lenght and was called a "520" chain. It is roughly equivalent to #50 chain, with about 1/2" spacing between the teeth, but the teeth are narrower so the chain is narrower than the #50 industrial chain. The 45 tooth large sprocket has a 5" hole in the center which provides plenty of clearance for the rotating top cap of the dish mount. The large sprocket is held to the top cap by attaching some 2" square aluminum tubing to the top cap and large sprocket. The small sprocket has a slightly less than 3/4" center hole with splined teeth. A piece of 3/4" threaded rod is just slightly larger than the openining in this small sprocket, so I put a 3/4" nut on one end of the threaded rod, tapped the small sprocket onto the rod, and placed a locking nut on top of the small sprocket. It will never turn or loosen. A pair of 3/4" two bolt flange bearings complete the assembly.
AC motors are used on the various Yaesu, Ham-M and TV antenna rotators. DC motors are used in the dish linear arms and gearmotors. The AC motors have the advantage of non-sparking while running. I have not not noticed any spark noise from my DC motors on dishes which are looking away from the spark source. I do notice extraneous pulses occuring when either the AC or DC motors first turn on, which make the analog potentiometer voltages jump. Long cables on a Yaesu rotator will result in picking up of commonmode spark pulses. Putting the motor lines in one cable and the analog meter lines in another shielded cable, with separation, should reduce the problem. Be sure to attach the shield at only one end, preferrably the rig end.
Gearmotor
is a motor and a gear unit integrated into a single package. AC gearmotors are typically a single set speed, but a DC gearmotor usually has its speed set by the voltage level. I like the azimuth and elevation axis speed to be about 1/dish size in feet RPM for my dishes. A 10' dish should move its elevation and azimuth axes at 1/10 RPM. If you have a 48 tooth sprocket on your top cap, and a 12 tooth sprocket on the gear motor, this is a 4 to 1 gear reduction, so you would need a 2/5 RPM gearmotor. If the 12 tooth sprocket is fed by an 8" pulley with a V Belt connecting it to a 2" pulley on the gearmotor you wind up with a 16:1 ratio, and a 1.6 RPM gearmotor would be needed. Often the sprockets and pulley sizes are set by the gearmotor on hand. Generally, the slower the gearmotor, the more expensive. A 1/30 HP gearmotor is fine for up to a 16' dish. The gearmotor must be able to run in both directions. A DC gearmotor can be reversed by simply reversing the polarity of the the leads.The DC gearmotor is easy to limit switch protect by placing a limit switch at each end of travel with a properly polarized diode across each limit switch. This way the motor travels to the end and stops, but when the voltage is reversed, the diode conducts and the motor starts up in the opposite direction. Reversing an AC gearmotor is not quite so simple, but possible.
I recently visited Surplus Center in Lincoln, Nebraska (800-488-3407). This is an excellent source of industrial chain, small sprockets, and flange bearings at reasonable prices. I also visited McMaster-Carr in Chicago, Illinois, a very large supply house, but was stone walled by the front receptionist, 3rd class. Their prices are usually very high as well. Use them only as a last resort. Another source of gearmotors is Grainger http://www.nbkbearings.com .Industrial chain and sprockets tend to rust in outdoor usage, so local Motorcycle, Bicycle, and GoCart stores are your best local source. Motorcycle parts are expensive but built to take abuse.
A pair of Linear Tracker Arms
can be used to move a medium sized dish in azimuth. Each arm is able to move about 100 degrees, so the pair operates against a common idler arm to provide about 200 degrees of azimuth rotation. 200 degrees is fine for AO-40, but marginal for EME. THe two tracker arms are wired in parallel so that they will open and close together. The arms must each be equipted with limit switches as they may not run at the same speed. A third tracker arm moves the dish in elevation. There is a picture of my three Tracker arm systme in the section showing my various diish projects.
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