dave.harper wrote:


-ex- wrote:

Q in excess of 1000 is readily achievable. 200-300 is a starting point
on a decent dx set.


So what's a good inductance to DC resistance ratio for an inductor on
an xtal set? The one I wound is about 500uH, and I get a resistance of
3.2 Ohms.

Using the formula Q=2*pi*f*L/R, I get a Q for my coil of 981 (@1MHz).
But based on its performance, I KNOW it's not that good. I'm picking
up a couple stations at night, but just barely.

Dave

For BCB work the 'standard' is in the 220-240uh range for tuning with a
~365-400 pf cap. There's an (almost) infinite number of combinations
you can use if you want to split the band into segments which sometimes
has an advantage. But switches and tapped coils can also be Q-killers
once you get into the Q stratosphere.

In practice the coil Q is determined primarily by the form dielectric,
wire size, wire spacing, diameter/length ratio/neary coupling effects,
etc. R is far enough down the list that its generally not even
considered. When you do a DC measurement of coil R thats not
representative of the skin effects and true RF resistance, thats why the
textbook formula doesn't pan out.

If you want to make a fairly nice coil without getting into the expense
of litz, check out spider-web coils and rook coils. When done with say
16-18 ga wire, and diameters in the 4" range you can get a pretty nice
coil. With 166-strand litz (30-35c/ft) you'll note an improvement but
by that time its time to start thinking about a good hi-q ceramic
capacitor and circuit loading concerns.

The Rap-n-Tap forum is where to get some good info.
http://www.midnightscience.com/rapntap/ "Best coil" is a common topic!

-Bill