On 10/30/2015 2:29 AM, 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 (εr) 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 εr 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 εr 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.


Here is a PDF you might find useful:

http://tinyurl.com/oge6436

On 10/30/2015 12:29 AM, 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 (εr) 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 εr 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 εr 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.

Air and vacuum are self-healing in case of arcing.

Paul, KD7HB

On 10/30/2015 2:20 PM, Paul Drahn wrote:
On 10/30/2015 12:29 AM, 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 (εr) 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 εr 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 εr 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.

Air and vacuum are self-healing in case of arcing.

Yes, the air or vacuum survives mostly intact, but not so much the other
components. What's your point? How about preventing arcing by having a
higher dielectric withstand voltage?

--

Rick

In message , rickman
writes
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 (0 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 0 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 0 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.


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.

Brian GM4DIJ
--
Brian Howie

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

"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