So, here’s my mostly plastic worm gear for tuning a butterfly capacitor in a small transmitting loop. If you have a 3D printer, you can make one of these for under $10.
Magnetic loop antennas are getting more popular, and are easy to build. These come in two flavors. One is the wideband active loop for receiving, like the ones I use in my loop array. The other flavor are small transmitting loops, which I also build. You need to tune these with a butterfly or vacuum capacitor as they are narrow band devices. You also need some way to remotely tune.
In the pictures above, you can see my latest build of a motorized plastic worm gear. The parts are in the lower frame, and the finished build above. Except for the motor, axles and bearings, I assembled everything with 3D printed plastic parts.
The small stepper motor (upper left) turns a worm screw, which in turn rotates the larger worm gear assembly. I designed this to provide a 25:1 reduction. Combined with my stepper motor’s 64:1 gearing, I can get more than 25,000 tuning steps across the 90° arc of a butterfly capacitor.
I used an OpenSCAD parametric model to design the gears, as described previously. To get the best quality, you should split your worm screw in half and print each separately flat on the print bed. Then, you carefully glue both halves together. Cheap skate board bearings and a inch steel rod are used for the main axis.
Plastic Worm Gear Assembly
I fabricated all of the plastic parts with PETG filament. Plastic couplers are attached to the gears with Superglue. You can see the gears held to the axles with brass threaded inserts and 2 or 3 mm screws.
And, speaking of screws, you can see that I printed some M3 and M4 screws and nuts to hold the structural parts together. Yes, that’s right. I printed my own screws. This works well even down to M3 sizing.
I will use a straight fiberglass rod, attached to the top of the main gear, for turning the tuning capacitor in increments of less than a picofarad.
You can buy the 28BYJ-48 stepper motor for $4 complete with driver board, and the bearings are around 30¢ each. All the rest is a few bucks worth of plastic filament, and your ingenuity.