Tom Bruhns wrote:

Robert Baer wrote in message ...
...
A high Q resonant circuit can be rather small.
For example, i made a tunable LC with a Q approaching 1000, and it was
not the size of a garbage can (resonant cavity); it was about 5 inches
tall and about 3 inches in diameter.

Based on an earlier P.Burridge thread, I'd say that's NOT small for
him. Of course, you didn't mention the frequency (I'd guess around
10MHz), but in the earlier thread, I was suggesting that he use a coil
at 18MHz or so with a Qu around 100, and he didn't seem to like even
the rather small size that one could make such a coil. I did it on,
um either a .68" OD or .80" OD powdered iron toroid, and that was
apparently too big. I also suggested a multi-pole filter which could
give the same effective filtering, and could use three small SMT
inductors. I gathered even that was too big. And I suppose coaxial
ceramic resonators for one-off projects at 18MHz aren't very
practical...

On one extreme, one uses standard LC parts and get fair Qs in small
size.
On the other extreme, one makes a ersonant cavity to get very high Qs
at the expense of size.
In between there is something that can be called either a "shielded
inductor" or a "resonant cavity with slow wave structure".

There seems to be a popular misconception that a helical resonator
gives better Q than an unshielded coil and capacitor. One of the key
nice things about helical resonators is that they are well
shielded...there's extremely little external field. That lets you
stack several of them side-by-side, with appropriately chosen coupling
apertures between the cavities, to make a nice, compact multi-pole
filter. But let's not assign a quality that isn't the the same
coil WITHOUT the shield will have a higher Qu, so long as it's not so
huge that radiation is a significant loss mechanism, and as Reg
suggests, that's BIG for most of the tanks we think about. In the
older editions of "Reference Data for Radio Engineers," e.g. the fifth
edition, there are some design nomographs for helical resonators in
the Transmission Lines chapter. They will give you the Qu. If you
find the Qu of the coil in air (see the same book, Fundamentals of
Networks chapter, or use Reg's coil program or WAIRCOIL), you'll see
that the coil's Qu is higher. And if you look also in the Fund. of
Networks chapter, you'll find a graph for the decrease of inductance
of a coil when shielded, and you'll find that that almost exactly
accounts for the Q lowering: same effective series resistance, but
lower inductance, gives lower Q. Is it significant? Well, I think
for a typical helical resonator, it's a 15% to 25% lowering.

Mainly I want to dispell the notion that a helical resonator is
something magic that _raises_ the Q of a given coil, because it's not.
It does have some very nice properties, but that just isn't one of
them.

Early helical resonator reference: W. W. Macalpine and R. O.
Schildknecht, "Coaxial Resonators with Helical Inner Conductor," Proc.
of the IRE, Dec. 1959 -- almost 45 years ago now.

Cheers,
Tom

Yes, but the emphasis was on small size, and a helical resonator
allows a goodly shrinkage of volume wihout a corresponding loss large of
Q.
But if the frequency is low enough, the ferite core method, if
properly wound, then becomes a "preferred" solution for small size and
high Q.
Maybe his requirements are not too realistic?