Because I spend a lot of time going down the road in my mobile, I do work HF from my mobile using a push up fiberglass mast assembly populated with antenna built in my workshop.
For a time I utilized a simple hitch insert that I built in my workshop using aluminum for the main components.
After using it as such for some time, I decided to motorize it using a gearmotor assembly along with chain and sprockets purchased locally.
I use a wireless controller mounted in the cargo bay of my mobile to control the gearhead motor and in what direction I want the mast assembly with antennas to turn through the use a the keyfob.
Now I have been asked by others that would like to build a unit similar to the one shown here that I built, however the problem most guys have is that they don't have the capability of building it from aluminum because of the tig welding required.
There is no reason why you couldn't build the main assembly from thin wall steel tubing, the trade-off making the unit a little heavier in weight, but not excessive.
As far as the sprockets, I used a ratio of 4-1, for gearing with the large sprocket being 4" in diameter, and the small sprocket a 1" diameter. Both sprockets use size 35 roller chain that is readily available everywhere.
The 12 volt gearhead motor is a 3 rpm unit that I sourced on line, and are easily found for a nominal cost.
So if you want to build a similar unit, and would like more info as to the particulars, drop me an email.