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January 31st 16, 11:44 AM
posted to rec.radio.amateur.antenna
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Dielectric for Tuning Capacitors
"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 |
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