Reg Edwards wrote:
Cecil, what formula do you use for the velocity factor of a coil of
diameter D, length L, and N number of turns, in metric units if its
convenient.

Reg, it's equation (32) from Dr. Corum's paper at:

http://www.ttr.com/TELSIKS2001-MASTER-1.pdf

There is a test in the preceeding paragraph to see if
that equation is appropriate for a particular coil.
Equation (32) is derived from empirical data collected
on coils that pass that test.

Just be sure the diameter, pitch, and wavelength are
all in meters and it will be metric.

I'll send you a .gif file of that page of Dr. Corum's
paper. The graph in Fig. 1 is for equation (32).

While you are at it, take a look at equation (47) for
the characteristic impedance of the coil and let us
know what you think.

Do you have a formula for the self-resonant frequency?

Here's what I have been doing lately:

1. Using as close as EZNEC can come to my 75m bugcatcher
coil stock, create enough turns for the modeled coil to
be self-resonant on 4 MHz. My 75m bugcatcher coil stock
is ~0.5 ft diameter and 48 turns per foot.

2. Delete enough turns to make it look like my real-
world bugcatcher coil. Use that coil for EZNEC modeling
at 4 MHz.

3. Assume the velocity factor didn't change appreciably
when deleting those turns.

4. Calculate the number of linear feet occupied by the
coil by dividing the length of the coil by the velocity
factor.

5. Calculate the percentage of a wavelength occupied by
the coil by dividing the results of (4.) above, by 246
feet, a wavelength at 4 MHz.

Of all the measurements and modeling so far, this is what
I have come up with as the most accurate estimate of the
percentage of a wavelength occupied by the coil.

And no, it is not 90 degrees minus the rest of the antenna.
The requirement for a purely resistive feedpoint impedance
is that the superposition of the forward and reflected voltages
have the same phase angle as the superposition of the forward
and reflected currents - nothing more.
--
73, Cecil http://www.qsl.net/w5dxp