K7ITM wrote:
On Apr 28, 5:20 am, Bill M wrote:
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.htmlis 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

OK, I was curious. Was I way off-base, or is it reasonable to think
that you can get a 10MHz Qu considerably higher than 250 (and possibly
up in the stratosphere above 1000)? I'm not going to spend the time,
effort and money to build a seriously large coil as the theory
suggests I'd need, but I did wind a somewhat smaller one...

I wound 15 turns of #10 AWG (2.55mm) bare copper with about 2.25
inches ID and 3 inches long. I resonated it with 3 * 12pF C0G
capacitors; it resonates at 9.088MHz. I coupled an output to an
analyzer through 1pF tapped one turn up from the "cold" end, and
loosely coupled an input from the analyzer's source using a one-turn
loop spaced away from the coil. My back-of-the-envelope calculation
says such a coil with air insulation should have a Qu around 740 at
9MHz. In a tank circuit, the finite Q of the capacitors will lower
the tank Q below that value. What I actually measure is a 3dB
bandwidth of 15.87kHz, for a tank Q of 572. OK, so that's a bit lower
than I might have expected. BUT--this coil is wound on a length of
black ABS drain pipe, which is an absolutely terrible thing to use as
a coil form if you're trying to get the highest possible Q. (My plan
was originally to take the coil off the form after I wound it, but it
had too much of a mind of its own about what shape it was going to
assume. I've wound some smaller self-supporting coils, but this one
didn't work that way.)

I'm convinced by this little 'speriment that I could build an LC tank
resonant at 10MHz with a tank Q above 1000 with no trouble--and
probably _well_ above 1000 if I used really high Q capacitors and just
enough low-loss solid insulation to keep the coil turns properly
spaced. I don't need one at the moment, but if I ever do, I sure
won't be afraid to try it.

Cheers,
Tom

Ok, I admit to lowballing my practical estimates. I don't want to be
argumentative about it but it the steps of increase become incrementally
more difficult to obtain as you approach higher levels of Q. Never any
harm in trying to make the best possible coil for the application.

Good luck with your projects!

-Bill