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October 30th 15, 01:26 PM
posted to rec.radio.amateur.antenna
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Dielectric for Tuning Capacitors
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 |
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