Variable Capacitor Motor Control
I have been doing some research on loop antennas, and used the internet to find out information. The antennas are small metal loops, using cooper half inch or three quarter inch type of tubing which can be purchased at Lowe’s, Home Depot, or other pluming sources. They are narrow band and require soldering, the fittings and are very high Q antennas. They usually use a capacitor which only uses the stator plates, in series, which may be welded or soldered, but do not use the rotor plates, because there is a loss in the moving bearing on the capacitors. They are trying to keep the losses, to a very small amount. The antenna has a very high voltage at the capacitors.
I found several circuits for building remote motor control units. I tried several, and was not happy with how they worked. I found the circuit below, which I built, and it worked all right. The circuit pulses the motor and also has a variable resistor (trimmer) which you can use to reduce the motor speed, thereby slowing down the tuning of the capacitor. I used a 12 volt geared motor, and found that 4-5 volts gave me a slow enough speed to rotate the capacitor which was coupled and insulated, to the gear motor.
The circuit can be used in a remote tuner arrangement as well.