On Fri, 30 Oct 2015 03:29:26 -0400, rickman wrote:

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 (?r) with temperature will cause 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 ?r with temperature varying many percent over a 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 ?r change with temperature as low as 10 ppm/°C. These materials
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.

Me thinks you are overlooking the very high voltages involved.