RadioBanter - Dielectric for Tuning Capacitors

"rickman" wrote in message
...
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.

Try this ...

https://en.wikipedia.org/wiki/Electric_susceptibility