I'm working on a project that will require 16 variable high-voltage
transmitting capacitors with a maximum value of 200pF. I think it's
worth reminding ourselves that a coaxial capacitor can be made cheaply
using materials that are quite readily available. For the two
"plates," I used 0.75-in OD aluminum tubing that was left over from an
antenna rebuild project and "3/4-in copper tubing" left behind by a
building contractor. (Note: the "3/4-in copper tubing" has an OD of
0.875-in and an ID of 0.785-in.) For the dielectric, I purchased
0.5-mm x 20-cm PTFE sheets from eBay, which should provide 30-kV of
breakdown strength. This combination of dimensions results in the
aluminum tube sliding smoothly but firmly inside the single-layer PTFE
"sleeve" when placed inside the copper tube.
A capacitor made with these materials delivers 250-pF of variable
capacitance for next to nothing.
73 -- Brian/K1LI
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Topband Reflector
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