Just pursuing some speculation about materials for tuning capacitors on
transmitting loop antennas. I was thinking of a capacitor formed by
fixing end plates on the two ends of the loop and adjusting by
stretching the entire loop to widen the gap. The main problem with this
is the limited high end of capacitance from the breakdown voltage
required, restricting the spacing which can be used to at least a few
mm. This in turn requires rather large plates. Then to get a
sufficient ratio of capacitance the gap must be widened to quite a few
inches.

One way to mitigate this is to use a dielectric material which will both
provide a higher permittivity as well as a higher breakdown voltage
allowing a smaller minimum plate gap and a much smaller plate size for
the same capacitance.

I found an example tuning capacitor using PTFE as a dielectric in a
trombone configuration. The owner says it works well. I'm not sure
where to get tubes of "virgin" PTFE and I'm not sure how stable this
material is over temperature and weather conditions.

Another material that appears to be a great dielectric is fused silica.
I can get that inexpensively in tube form in a variety of sizes and it
seems to have better properties than PTFE, especially the dielectric
coefficient temperature curve which is virtually flat from 25°C to 500
°C, so I assume it is also flat down to reasonable outdoor temps. It
does not absorb moisture and has a higher dielectric coefficient
allowing a smaller plate size than PTFE.

So I'm wondering why I can't find where anyone has used fused silica in
these capacitors. Maybe it would be a bit pricey in plate capacitors,
but it would be good and cheap in trombone style caps.

--

Rick