On Mon, 27 Apr 2009 16:34:23 -0700 (PDT), K7ITM wrote:

If I want an unloaded coil Q of 2000 at 10MHz, I'd expect to need a
coil about 5 inches (13cm) diameter and 10 inches (26cm) long--or
similar. That assumes no loss to radiation. If I shield it to make a
helical resonator, the Q will actually be lowered slightly, though
such a large coil may have enough loss to radiation that it would be a
wash between turning it into a helical resonator or leaving it in free
air. Of course, with the shielding, it's less susceptible to changes
in the environment around it. Anyway, the shield for a helical
resonator should be a couple times the diameter of the coil, so it's a
rather large arrangement anyway!

1/4 wave filters used in repeater duplexers have quite high unloaded-Q
and are available to at least down to 50 MHz. Of course these are
long, but in order to reduce total shape, these might be bent into
U-shape.

To go into even lower frequencies, the resonator can be bent into a
helix. At least the design nomogram in old ARRL handbooks seem to
indicate that with sufficient size very high unloaded-Q could be
achieved (up to several thousands). Of course the helix itself and the
inside of the resonator may have to be silver plated.

Paul OH3LWR