In article ,
Brian Howie 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.

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.

I've seen at least one or two small-transmitting-loop designs, in
which the tuning capacitor was a motor- or manually-driven "trombone"
variety, with one or two sets of nested metal tubes that are slid into
or out of one another to vary the capacitance.

Ir I recall correctly, one such design recommended the use of PFTE
film, the other suggested Kapton. You *could* use an air dielectric,
but keeping the two nested tubes from touching and shorting out would
be a mechanically-difficult problem.