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.
--
73, Cecil http://www.qsl.net/w5dxp