On Sat, 12 Aug 2006 15:54:24 GMT, Cecil Moore wrote:

Walter Maxwell wrote:
So I ask you, Cecil, why would you want a bugcatcher self-resonant at 4 MHz for
operation at 4.0 MHz, even if you used only 2/3 of it as a loading coil. Looking
just to heat the coil instead of radiating the energy into space?

Sorry I wasn't explicit, Walt. I use only 2/3 of the coil and
chop the other 1/3 off and discard it. That ensures that the
VF of the coil of 2/3 length is the same as the VF of the
whole coil at the frequency of operation.

The alternate approach would be to extend the windings on a
75m bugcatcher coil until self-resonance was reached at 4 MHz.
The VF could then be calculated and the extra windings
removed.

The purpose of the two above exercises is to determine the
VF of the coil *at the frequency of operation*. The VF of
large real-world loading coils changes with frequency. Knowing
the self-resonant frequency of a 75m bugcatcher coil is
6.6 MHz doesn't (necessarily) yield the correct VF at 4 MHz.

But Cecil, I thought this thread was about chokes to prevent common-mode
currents from flowing on the feed line.

Now yer talking about loading coils for mobile whip antennas. As I understand
the issue, one is 180° from the other. For the choke you want a high resistance,
which is what you get at the self-resonant frequency. But for the loading coil
you want the lowest series resistance possible, which you don't get when
anywhere near the self-resonant frequency.

Like I said above, the Hustler 80m loading coil achieved 'low swr' by making the
coil self resonant slightly above 4 MHz, with a series resistance of 31 ohms.
Now you are suggesting a bugcatcher coil self-resonant at 6.6 MHz, which means
yer coil is going to give you a nice low swr, but yer losing half of yer power
in the coil because of the high series resistance you can't avoid. Yer also
losing yer mind.

Walt