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.

--

Rick

On Fri, 30 Oct 2015 03:29:26 -0400, 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 (?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.

Me thinks you are overlooking the very high voltages involved.

On 10/30/2015 9:07 AM, Jeff wrote:

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.

Me thinks you are overlooking the very high voltages involved.


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.

--

Rick

On 10/30/2015 10:46 AM, rickman wrote:
On 10/30/2015 9:07 AM, Jeff wrote:

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.

Me thinks you are overlooking the very high voltages involved.


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.


No, it is the increase in capacitance 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.

Yes.

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.


Are you planning to operate this antenna over a wide range of temperatures?


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.

Sure. Have you mathematically analyzed any of your proposed scenarios?
That might help.

On 10/30/2015 12:01 PM, John S wrote:
On 10/30/2015 10:46 AM, rickman wrote:
On 10/30/2015 9:07 AM, Jeff wrote:

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.

Me thinks you are overlooking the very high voltages involved.


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.


No, it is the increase in capacitance that makes the dielectric useful.

That sounds rather argumentative. I explain this in the next paragraph
which you seem to be agreeing with. So which is it?

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.

Yes.

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.


Are you planning to operate this antenna over a wide range of temperatures?


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.

Sure. Have you mathematically analyzed any of your proposed scenarios?
That might help.



--

Rick

rickman wrote:

On 10/30/2015 12:01 PM, John S wrote:
On 10/30/2015 10:46 AM, rickman wrote:
On 10/30/2015 9:07 AM, Jeff wrote:

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.

Me thinks you are overlooking the very high voltages involved.


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.


No, it is the increase in capacitance that makes the dielectric useful.

That sounds rather argumentative. I explain this in the next paragraph
which you seem to be agreeing with. So which is it?

It's clearly both, as an insulator with a higher breakdown voltage than
air would enable the plates to be closer and thus smaller for a given
capacitance, as well as more fitting into a given length, even if the
dielectric constant was the same as air.



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.

Yes.

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.


Are you planning to operate this antenna over a wide range of temperatures?


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.

Sure. Have you mathematically analyzed any of your proposed scenarios?
That might help.



--
Roger Hayter

On 10/30/2015 11:40 AM, rickman wrote:
On 10/30/2015 12:01 PM, John S wrote:
On 10/30/2015 10:46 AM, rickman wrote:
On 10/30/2015 9:07 AM, Jeff wrote:

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.

Me thinks you are overlooking the very high voltages involved.


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.


No, it is the increase in capacitance that makes the dielectric useful.

That sounds rather argumentative. I explain this in the next paragraph
which you seem to be agreeing with. So which is it?

Your posts are beginning to make me think you are a troll. Every person
who responds gets a provocative answer from you. If you already have in
mind the answer you want, why ask?

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

On 10/31/2015 6:01 AM, Jeff wrote:

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.


I think the change of Er with temperature is the least of your worries
when talking about a tuning capacitor for a magnetic loop. The change of
capacitance due to mechanical changes in the plates is likely to be at
least an order of magnitude greater than the dielectric changes.

I wonder about that. I know metals have a high tempco of expansion and
this will make changes in the capacitance. I haven't analyzed it to see
how significant that would be. So let me give it a try.

Expansion of the air gapped metal capacitor will have two opposing
effects. Enlargement of the plate surface area will increase the area
and so increase capacitance by the square of the tempco of the metal.
Enlargement of the spacing will decrease capacitance directly by the
tempco of the metal assuming the spacers are the same material. The net
effect will be to increase the capacitance in direct proportion to the
tempco of the metal.

Using a dielectric would reduce the effect of the plate spacing on
capacitance to a very small value since most of the capacitance will be
due to the material and much less to any air gap remaining. So it would
return the overall effect on the capacitance to the square of the
expansion tempco. Does that sound right?

In other words, it would be good if the dielectric had an effect that
was opposite to the effect of the metal tempco. I'll meed to consider
that as I search for materials.

--

Rick

rickman wrote:
On 10/31/2015 6:01 AM, Jeff wrote:

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.


I think the change of Er with temperature is the least of your worries
when talking about a tuning capacitor for a magnetic loop. The change of
capacitance due to mechanical changes in the plates is likely to be at
least an order of magnitude greater than the dielectric changes.

I wonder about that. I know metals have a high tempco of expansion and
this will make changes in the capacitance. I haven't analyzed it to see
how significant that would be. So let me give it a try.

Expansion of the air gapped metal capacitor will have two opposing
effects. Enlargement of the plate surface area will increase the area
and so increase capacitance by the square of the tempco of the metal.
Enlargement of the spacing will decrease capacitance directly by the
tempco of the metal assuming the spacers are the same material. The net
effect will be to increase the capacitance in direct proportion to the
tempco of the metal.

Using a dielectric would reduce the effect of the plate spacing on
capacitance to a very small value since most of the capacitance will be
due to the material and much less to any air gap remaining. So it would
return the overall effect on the capacitance to the square of the
expansion tempco. Does that sound right?

In other words, it would be good if the dielectric had an effect that
was opposite to the effect of the metal tempco. I'll meed to consider
that as I search for materials.

An aluminum plate 6 inches square at 75 F heated to 200 F changes
dimensions by 0.0092 inches.

For plate glass the change is 0.0031 inches.

I will leave it to you to calculate how much that will change capacitance.


http://www.engineeringtoolbox.com/li...on-d_1379.html


--
Jim Pennino