On Apr 27, 8:02*pm, Bill M wrote:
K7ITM wrote:

To put some numbers on what Paul suggested:

If I want an unloaded coil Q of 2000 at 10MHz,

I stopped here.

I'd expect to need a....

No way you'll ever see 2000 Q at 10 Mcs. Can't happen. Some guys are
approaching 2000 at 1 MC BCB but there's lots of expensive hoops to jump
through to reach that point. *Simply cannot be had at 10 Mcs. *200-300
on a good SW coil is about all that can be achieved.
....
Interesting comment. In the filters I build for test fixtures, I use
air-core coils that are about 1" diameter and 1" long, and they give
me Qu in excess of 300 at 10MHz. For what I do, I don't need Qu up in
the thousands, and don't have room for really big coils, but can you
give me a reason I shouldn't expect Qu to scale linearly with size up
to the point where radiation losses become significant?

Can you tell me why I should think that the inductance calculator at
http://hamwaves.com/antennas/inductance.html is not giving me accurate
results when I put in, say, D=130mm, n=20, l=260mm, d=7mm, and
f=10MHz? It agrees with other independent ways I have to estimate the
Qu of the ~1 inch coils I build, and those coils measure within
engineering tolerance of the estimates.

When you go to very large coils like this, you have to be careful
about the self-resonance becoming too low, but in the example above,
it's (barely) OK at 10MHz. (Actually, you better be careful about the
self-resonance of any coil...) Fewer turns of larger diameter
"wire" (tubing: cheaper, and easier to work with) can yield about the
same Qu and a considerably higher self-resonance.

Cheers,
Tom