On 10/30/2015 12:18 PM, Jeff wrote:

I would have thought that glass was a good candidate and in plentiful
supply in various thicknesses, and would withstand very high voltages.
The Er is in the range 5 to 10 depending on the actual type.

It is the high voltages that makes the dielectric useful. The plates in
these capacitors have to be widely separated and the use of dielectric
allows this spacing to be reduced, that's one dimension. The Er
increases the capacitance which allows the capacitor to be reduced in
the other two dimensions.

The problem is the change in Er with temperature which will cause the
resonance of the antenna to change, potentially outside the bandwidth if
the Q is high enough.

I'm not sure how low the loss tangent would need to be to minimize self
heating to a point that higher Er changes with temperature won't matter.
Even if self heating is not a problem, larger Er changes will
temperature would mean you could not retune the capacitor to the same
value with environmental temperature changes and so the tuning would not
be repeatable. Possibly this could be compensated for by measuring the
temperature and calibrating for temperature.


Glass is used as a dielectric in high quality low loss RF capacitors so
I suspect that it would be usable in a home-made one.

Doesn't necessarily follow. The loss tangent of glass is low to very
low so it won't heat up much in use. But the important part is the
change in Er with temperature as I explain. In fixed value caps changes
in capacitance of a few percent are usually not a problem. But in this
application tuning of the circuit may be very critical and require a
much higher degree of stability.

I am also looking at alumina ceramics. The properties vary with
composition, but there are composites with very high stability numbers.
They usually are in a materials data sheet rather than in a product
offered for sale. Seems a lot of ceramics are custom items.

--

Rick