In message , rickman
writes
When people talk about tuning caps for transmitting loop antennas, they
always talk about air or vacuum capacitors. I was wondering why
dielectrics are never used. Someone in a Yahoo group mentioned that
the variation of dielectric constant (0 the tuning to drift out of the bandwidth when keyed. I guess this also
requires a poor dissipation factor (DF), or at least a poor DF relative
to the application.

I took a look at some potential materials and indeed, many have a
rather steep slope of 0 50°C range. But they make fixed capacitors that have low temperature
coefficients.

I looked up some materials for fixed capacitors and found dielectrics
with 0 also have a loss tangent less than 0.001, some much less. I'm
wondering if they would be practical to use for the dielectric in a
variable capacitor.


I've seen polythene dielectrics used in the variable capacitors used in
transistor radios. You could use PTFE film, but the big problem in
transmitting loops is the air breakdown between the plates and the
dielectric. There will be a very high electric field in there.

Brian GM4DIJ
--
Brian Howie