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.

On 11/2/2015 3:42 PM, Dave Platt wrote:
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.

Yes, it *could* be a problem, but most transmitting loops have rather
high voltages on them if much power is used. So the spacing needs to be
fairly large making the precision of movement a lot less.

The use of plastic material would help both with maintaining sufficient
resistance to arcing and a higher capacitance for a given spacing. The
concern is the lack of stability with temperature of most dielectric
material. However, I did a first order analysis and found the capacitor
has a sensitivity to the tempco of expansion of the material and the
loop has a slightly higher sensitivity, order (n) and order (n ln(n))
respectively. A dielectric material with the right tempco of Er would
largely offset the two effects in the base antenna components reducing
the resulting resonant frequency shift to less than 100 Hz for nearly
any range of temperature you might reasonably expect to see. Ceramic
materials can be tailored by mixing different compounds so it is not
unreasonable to find something like this.

--

Rick