Here's what I came up with for the remote-controlled capacitor for my magnetic loop antenna.
The full view:
We have a thick dowel rod serving as the main mast. At the bottom is mounted a wooden plate with a geared motor attached to it:
The motor shaft is inserted into a long hollow rod made of ABS plasic. Two self-tapping screws are screwed into opposite sides of the plastic rod to grip the motor shaft.
At the top of the main mast the butterfly capacitor is mounted on a protruding wooden plank:
The shaft of the variable capacitor goes through a hole in the plank and is inserted into the other end of the long plastic rod (driven by the motor). A single self-tapping screw presses the variable capacitor shaft against the inside of the plastic rod, thus gripping it.
The protruding wooden plank has a small round curve cut into it where it meets the main mast, and the main mast has a small notch cut into it to accomodate the plank's curved indentation. Copious wood glue holds the plank in place. Some additional right-angle brackets or similar would be good to prevent the plank from collapsing.
Above the capacitor, a large plastic hook is screwed to the main support mast, which will bear the weight of the loop conductor (to avoid placing strain on the capacitor).
The motor is designed to run off of 3V, but it runs fine off of a 1.5V AAA cell with enough torque to turn the motor, and at a leisurely speed (rated for 9 RPM at 3V). Probably the low speed is sufficient for tuning without resorting to PWM, simply by using a double-pole switch to turn on or off the supply voltage in both polarities.
There is a bit of eccentricity on the plastic rod's rotation but it doesn't impact the operation at all. There is some backlash when reversing motor direction. I don't care about this because I'm not looking for repeatable absolute positioning, only for relative positioning (to increase or decrease capacitance until SWR is acceptable).
So why did I do it this way?
1. The capacitor is at the top, far away from the lossy ground or the metal tripod that will support the main mast. Keeping the capacitor away from nearby lossy or conductive materials is important to preserve balance, reduce losses, and ensure the antenna is tunable to a low SWR.
2. The motor and its voltage-control lines are far away from the capacitor. One report I read indicated that the mere presence of a disconnected wire hanging near the capacitor was sufficient to make the loop untunable (no low SWR could be attained). In that case, removing the wire fixed the problem. Again: we need to keep conductive materials away from the capacitor.
3. The feed loop will be at the bottom of the mast and the bottom of the loop, opposite the capacitor, which eliminates the need to run the feedline through the middle of the loop as would be needed if the capacitor were at the bottom and the feed loop at the top. Again, loop balance is improved by keeping the loop interior free of wires.
Next step is to come up with a low-loss mechanical connection to conect the butterfly cap to the loop conductor. A mechanical connection will introduce resistive loss, but will allow swapping out loop conductors easily to allow trying out differently-sized loops. The MFJ small loop tuners use a mechanical (wing nut) connection to allow swappable loop radiators, and they seem to work okay, so the losses may not be insurmountable. For the loop conductor I still think copper foil tape is the most practical material.
Looking at the finished motorized capacitor assembly, it looks pretty simple to build, but it was slow going for me, to make sure everything lined up reasonably well. I'm pretty happy with the result, and feel like this time around, I might at last have a good chance at making a good loop.
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