K7ITM wrote:
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.

Self-capacitance (ultimately self-resonance) is always a contributing
factor that prevents coils from achieving their maximum theoretical Q.
Estimating coils whose Q is going to fall in the 200 range for other
reasons is relatively easy but that doesn't mean you can scale upwards
proportionately.

I build air coils in the 3 inch range along the lines of what is
pictured here, http://www.sparkbench.com/homebrew/grebe/cr18.html

They only *measure* in the 250 range. Silver-plated wire could
certainly improve a coil of this size but no way would you achieve
numbers like 2000.

-Bill