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Dielectric for Tuning Capacitors
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 |
#2
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Dielectric for Tuning Capacitors
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 |
#3
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Dielectric for Tuning Capacitors
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? |
#4
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Dielectric for Tuning Capacitors
On 10/31/2015 4:33 AM, John S wrote:
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? I don't know what you are talking about. If you think I am a troll, why did you respond? I am asking you if you believe what you wrote initially that the high voltage does not make the dielectric useful, or if you believe what you wrote subsequently that the high voltage issue *is* important. It's not that important to me either way. I know what *I* think (and have been consistent about it) and I am pretty sure I am correct. I just don't know why you say I am wrong, then say I am right. If you think my answer is provocative, please don't respond. If you wish to discuss this then why not respond without the drama? -- Rick |
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