2019-05-31 DIY Plastic Variable Capacitor

I hear others complain about the high cost of variable capacitors, especially the older air variables with metal plates. So I decided to DIY and make one. This is my first attempt so forgive the lack of elegance and unfinished look. I’m sure it could be improved on and made better with some effort. The supplies and tools I needed were:

Two metal food cans, cleaned and labels removed

One 1 liter soda bottle, cleaned and label removed

Wire, solder

Tin snips or heavy duty scissors or hacksaw with fine tooth metal blade

Utility knife and ruler for straight cuts

Patience 😉

See the photos.

One can will be used like it is, with a length of wire soldered to it. I cut the rim of the other can off with the tin snips and slit it along its length so it can be compressed and fit inside of the other can. It’s a good idea to be careful with the sharp metal edges. For the most part they will be covered when it is assembled.

The liter bottle must be cut so that there is a smooth sheet of curved plastic that fits around the inner can, with 1/8″ to 1/4″ or 3 to 6mm of overlap around the outside so the metal cans don’t touch and short. I used the tin snips to rough cut and the ruler and utility knife to get straight edges.

Before assembly the two wires should be soldered on. I arbitrarily chose locations, they should be close to the loopstick coil.

It assembled without any glue or tape, but it could be improved with a bit of thought and elbow grease. Also it could be improved by mounting the assembly on a flat piece of wood. I guess some way of sliding the cans along could be added. Maybe cutting holes and putting a wood dowel through the assembly would make it more stable. But as it is, the snug fit does the job.

I measured the minimum capacitance at 52 pF and maximum at 185 pF. That’s not bad. If this is used with a 225 microhenry loopstick, it will tune from 780 kHz to 1470 kHz. The capacitance depends on the size of the cans, the size of the plastic sheet, the dielectric constant of the plastic, and how far the cans are overlapped. So, YMMV. 👍

Instead of cutting the inner can, I thought about putting it on a deep groove in a heavy piece of metal and hammering the can into the groove. If it’s deep enough the can’s diameter would be smaller until it would fit inside if the other can. Another way to do this would be to get two pieces of plastic pipe that would fit inside each other.

Tuck Choy
A dual gang could be made by mounting 2 of the outer ‘stator’ cans to a flat surface, end to end. The 2 inner ‘rotor’ cans would have a dowel through both centers so they move in unison. All this could have been made more compact by cutting the cans and laying the sheets flat, possibly stacking more to get the full 365 pF or more. The soda bottle dielectric is very thick, reducing the capacitance – it could be thinner.

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