Hello All:
I, too, have been playing around with this idea.? Some time ago, I bought a couple of tin (yes, real tin) plates at a hobby shop.? They were about 4X6 inches in size, and I soldered a wire to a corner of each plate, and then put them face to face with a piece of plastic wrap between them.? I bored a hole through them on one corner and attached them together with a nylon bolt and nut so I could slide them across each other.? I attached this variable capacitor to my spider-web-coil crystal set and was amazed to find it very selective.? I imagine you could do a better job using double-sided PCB material, and cut it to shape using a Dremel tool.? Then you could make multiple-plate caps very nearly like the real thing.
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So, I looked up the formula for capacitance and plate area in the ARRL handbook, and confirmed that it was in the right range.? Also, the dialectric constant of various materials is in a table near this formula.? For those of you who don't have the ARRL handbook, I would recommend it very highly.?
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I have often thought that if I ever get back into programming (Nah!, I'm retired, and gardening is a lot more fun than programming.? That was for when I was young, driven, egoistic and dumb) I would try to build a designer's Swiss army knife by combining all the formulas in the ARRL handbook.? I'm sure someone has already done this.
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Good luck with your project!? Humm, now if we had a machinist with us, mebbe they could turn the little slots in the shaft to accept the plates made by the sheet metal worker....
Later,
Barry
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