On Apr 16, 8:42*am, spamhog wrote:
Tim, Tom, Jimmie, John, thanks you all! This thing might work, There's
no way around the need to get my feet wet and build/find some cavity.

Filippo N1JPR

Further to my comment about how big a coaxial resonator must be to get
high Q: to get Qu of 1000 at 50MHz will require an air-dielectric
line almost 1.5 inches OD. A quarter wavelength is about 60 inches;
you can shorten that some by loading the open end with capacitance,
but expect it will be at least half that long. Compare that with a
coil of 8 turns of #10 AWG copper wire 1.25 inches ID, spaced 2 wire
diameters center-to-center (less than 2 inches total length) that will
also give you a Qu over 1000.

I think if I had an overwhelming urge to try a design using a cavity
(or coaxial cavity) resonator, I'd do it at about 1GHz, or maybe even
500MHz, where I can make a resonator with high Qu in a practical size,
and where I can't wind a coil that's big enough to get really high Q
and also have a high self resonant frequency. For example, at 1GHz, a
quarter wave air dielectric cavity will be about 3 inches long, and if
I make it from "3/4-inch" copper pipe (properly cleaned), I can get Qu
well over 2000; it will be very tough to make a coil that has a self-
resonance above 1GHz that has a Qu even close to 1000 (short of using
superconducting wire!).

There are very practical reasons that you don't see cavity resonators
at low frequencies: you can do better with smaller, cheaper parts.

Cheers,
Tom