PTFE, virgin white Teflon sheet, has relative dielectric constant of
2.1. Very stable even with temperature. And very low loss, so good that
it is typically used for microwave insulators, next to polystyrene
(Rexolite). You won't heat it in a home microwave oven, especially with
a glass of water next to it. I have tested it in intense 15 kV RF fields
at 90 MHz, in a W.T.LaRose dielectric preheater, with no noticeable
heating. Nylon and Delrin would be melting in the same field.
However, one thing to be aware of, is when stacking films or sheets of
dielectric between plates, you may have tiny air gaps in between the
materials, even slight. Think of the capacitor then as a series of small
capacitors with a total voltage applied at the ends. The voltage
division across the capacitors will be in Cair+Cptfe+Cair. Since the
PTFE is thick and has Er=2.1 and the air gaps are thin and have Er=1,
you have the potential to develop some high voltages in those tiny air
spaces. What happens is that it can make corona in there, and eventually
eat the dielectric. Going to thinner Teflon (as voltage rating is high
per mil of thickness as Gerald said) would exagerate the problem as the
air gap would start approaching the Teflon thickness. If a single air
space is 0.01 inches wide and the Teflon is the same (with no air gap on
its other side, pressed tight against conductor), then the total voltage
would be divided so that ~33% is across the Teflon and ~66% across the
single air gap. This is because the Cptfe is 2.1 time Cair, or the
reactance is about half. The air space may have microsparking over time,
that can cause noise, ozone, and damage the dielectric and the conductor
surface.
A way around this is to have the electrodes tightly bonded to the
dielectric, the way commercial capacitors are done. Unfortunately this
doesn't work in variable meat-slicer style capacitors. As long as your
air space on either side of the Teflon sheet is large, then the
ionization should be minimal in the air space. Just something you should
watch out for in homemade RF capacitors.
Let us hear how it works out, Carel,
73
John
K5PRO
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