RadioBanter - View Single Post

December 20th 03, 04:15 AM

In article , (Tom
Bruhns) writes:

Dunno how accurate you need it to be "as wound", and dunno what sort
of form factor you want, but if an air-core coil will work for you,
here's one way: get about 18 inches of 18AWG wire, nice and straight,
and a 1/2"x13TPI bolt. Wind 11 turns of that wire into the threads of
the bolt, leaving 1" of wire on either end (or start with a longer
piece if you want longer leads), and unscrew the bolt. The result
should be a coil about 0.85 inches long and 0.424" ID, and 0.600uH.
When you get it mounted near other things, the inductance will go down
a bit, and you can adjust it down further by stretching it a bit. The
Q at 14MHz should be just over 200. Or you can use 10 turns of 16AWG,
done the same way, which will have a slightly larger ID, 0.434 inches,
and about 0.561uH nominal and very slightly higher Q.

In general, the L = (n*r)^2/(9*r+10*l) formula works well, where r and
l are radius and length in inches, n is number of turns, and the
inductance, L, is in uH. My numbers above come from a program that's
easy to use and that I've learned to trust from verified results.
It's convenient to use a bolt to get consistent spacing and diameter,
though for high stability, you'd want some sort of support. For
highest Q in a given volume, go for a coil with roughly equal length
and diameter, though that's not terribly critical.

No need for a computer program for using data in the July 1977
issue of Ham Radio magazine article "Self-Supporting Coils."

While inductance over 0.50 uHy is not on the chart of Figure 1, the
0.55 uHy value will be very close for 9 turns of #12 bare copper
(standard electrical wire with insulation stripped off) wound on a
paint roller ferrule's threaded end of a paint roller pole. That's the
same threading for all sorts of portable appliance items for the
home; about 5/8" inside diameter, 1 turn per 3/16" length. Q at
30 MHz will exceed 300.

Well before that article got into print I sat down with a calibrated
Boonton 190A Q Meter and got wound up on forms from 4-40 screw
to a standard 117 VAC light bulb base. By using the largest solid
wire that fits the threading, the coil will be quite self-supporting
without any core material. The screw thread can be unscrewed
easily and the finished coil diameter will increase slightly in so
doing. Form leads as desired with long-nose pliers; long leads
will also add some inductance. Measurement of inductance and Q
was done on a simple solder-it-in-place fixture on the 190A.

I can e-mail repro scans of the article to anyone interested (1.2 MB).
As the author I have the right to do that under "First Rights only"
compensation. :-)

Len Anderson
retired (from regular hours) electronic engineer person