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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? |
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 |
Dielectric for Tuning Capacitors
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 |
Dielectric for Tuning Capacitors
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 |
Dielectric for Tuning Capacitors
rickman wrote:
On 10/31/2015 7:39 PM, wrote: 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 Did you have a point? It should be obvious, but since it is not, any change in capacitance due to thermal expansion is going to be miniscule for a capacitor large enough to withstand kilovolts. -- Jim Pennino |
Dielectric for Tuning Capacitors
On 11/1/2015 12:05 PM, wrote:
rickman wrote: On 10/31/2015 7:39 PM, wrote: 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 Did you have a point? It should be obvious, but since it is not, any change in capacitance due to thermal expansion is going to be miniscule for a capacitor large enough to withstand kilovolts. I don't know what you consider to be "miniscule". I also don't see how the voltage matters. I believe I have already posted that without considering fringe effects, but only the first order effects of plate area and spacing, the end result is a linear change in capacitance with temperature according to the temperature coefficient. When I run the numbers I get around 370 PPM for a 30°F rise with aluminum or closer to 277 PPM for the same rise with copper (I haven't seen a copper tuning capacitor though). It appears the temperature effect on the loop inductance is larger at 413 PPM for aluminum and 309 PPM for copper. These numbers may not be spot on because I used a handy calculator for the inductance which may not have considered the diameter of the conductor, most loops are wide material. These two effects augment to feed the equation for resonant frequency which uses the square root of the product resulting in 391 PPM for an all aluminum system or 340 PPM for a system with a copper loop and aluminum cap. This is enough to impact the tuning of a high Q antenna to give more than a 3 dB drop over the course of a day. Over the course of a year some locations will see a change of 50°C or a three fold greater change. That would easily be enough to disrupt an auto-tuner and require recalibration. Do you get significantly different numbers? If a dielectric were chosen with a slight negative temperature coefficient, it could offset the natural drift of the antenna tuning bringing it closer to zero. -- Rick |
Dielectric for Tuning Capacitors
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 |
Dielectric for Tuning Capacitors
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. |
Dielectric for Tuning Capacitors
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 |
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