J -
Bill Bowers has spent a lot of time evaluating coil construction techniques.
I don't know if any of his articles from The Lowdown are available on-line.
If not, they should be! From memory, here are a few of his conclusions:
-Many of the rules for MF and HF coils do not apply at LF. Most of the
losses are simply due to the RF resistance of the wire. The best Q at LF
will generally result from creating the desired inductance with the least
amount of wire. To that end, coils with a diameter of (2.5 x length) give
the best Q. This is contrary to MF/HF practice!
The best wire is Litz, with hundreds of strands of fine gauge wire. But if
you stick with the 2.5xlength form factor, you can do very well with
plastic-insulated solid conductor wire.
Inter-winding capacitance is not a big issue, so basket-weaving does not
have the benefit that it does at high frequencies. If I recall correctly,
the same applies to turn spacing -- you want to achieve your inductance with
the minimum length of wire, and spacing the turns will cause you to use more
wire.
When all is said and done, there is no sense in quibbling about fractions of
an ohm when your ground loss is 20 ohms!
Anyway, if anyone can point Jay to Bill Bowers' articles, it would be
appreciated.
John Andrews
|
