dave.harper wrote:

Joel Kolstad wrote:


No, although for low Q resonators, it's often pretty close.

Generally speaking, most relatively simple AM receivers are really only
intended to pick up relatively strong, nearby transmitters. As such, you can
get away with an awful lot of "cut and try" when it comes to designing the
circuit (largely ignore matching impedances, Q's, etc.) and still obtain
acceptable results.


Thanks for the response Joel. So if I understand correctly, Q is
basically an indicator of how well the LC circuit resonates? Could you
think of Q as the inverse of a dampening coefficient?

If so, I guess ideally you'd get the best Q with an iron-core inductor,
thick windings, and as few windings as possible?

Dave

If I may jump in....
The case of crystal receivers is somewhat different than a "powered"
radio in that you want to keep things at as high a Q as possible to
match the high impedance of the circuit. High Q is desirable in any
case but moreso in a crystal set.

After having established a good high Q with the LC configuration you can
count on the circuit/antenna loading it down somewhat so then it becomes
a matter of selecting appropriate diodes, decoupling the ant, etc. so it
always helps to start out with as much as possible.

Textbook Q of more C/less L is dictated simply by resistance. In
practice, at least for BCB xtal radios, the dielectric of the coil, skin
effects of the wire, interwinding capacitance are the key players.
Thats why certain coil dimensions, use of litz wire and certain winding
techniques can generally be counted on for the highest Q.

The Q of a coil, and/or complete circuit, will have a curve of its own.
With BCB, what is good at 600kc may be better at 800kc and
(relatively) terrible at 1600kc.

I've had good success with ferrite toroids approaching Q=400, although
ferrites are by nature very unpredictable Q-wise. This is as good as
one can expect with something like a 3-4" diameter coil of #18 wire on a
good coil form. OTOH, the toroid stops there. That same 4" coil wound
with 660-strand litz can get up into the Q=800 stratosphere with a
basket-wind technique.

There's always a downside. A big, hi-Q coil needs to be kept well away
from ANYTHING or else the Q will take a nosedive and then lead
capacitance starts biting you from the backside.

I find xtal sets fascinating. I've been radioing for 35-40 years and
never gained a full appreciation for L, C and Q until I got into DXing
with xtal sets. Logged 105 BCB stations in the competition earlier this
year including two in Brazil!

-Bill