I'm in the habit of using insulated stranded all-copper conductors for
HF wire antennas. Usually U.S. standard #14 Type THHN "house wire"
because of it's easy availability everywhere at low cost. However
insulated wires pose an annoying problem, their specific velocity
factors are not published and vary all over creation depending on a
whole collection of variables.

I've tried to nail down the Vf of my usual #14 THHN by cutting the
lengths of 20M dipoles to one or another of the usual equations. I put
together the antennas, hoist them to various operatng heights and nip
the coax feedline to it's minimum possible length so that I can find
the resonant point with an antenna analyzer while I'm on the ground
directly under the feedpoint. A process which I believe should lead me
to a "correction factor" from which the Vf can be determined.

Modeling a 20M dipole at 35 feet indicates a fairly sharp null at the
resonant point. But that's not what I get when I build the antenna and
try to measure the frequency of it's resonant point with an antenna
analyzer. I get a much flatter SWR curve from the analyzer than I do
from modeling probably because of feedline losses and because of the
low analyzer frequency resolution (MFJ-259B). To the point where the
antenna appears to be resonant over a range of maybe 200Khz. Which in
reality it can't be.

From a practical standpoint this scenario isn't any problem in the case
of a simple dipole. I "cut long", put the antenna up, sweep it with the
analyzer, find what seems to be the center of resonace, do the quickie
numbers, trim it and take it to the airwaves.

The problem comes when trying to accurately model complex, fussy wire
antennas like hex beams when the Vf of the wire is unknown. A one
percent error in conductor length at 14 Mhz is 140 Khz which makes
decent modeling just about useless.

So two questions in this regard: Is there a way to measure the Vf of a
wire without having to resort to using 2" Heliax to feed a dipole and
without a lab full of HP and GR test equipment? Second, assuming the
Vf becomes known how does one handle it during the modeling process?
Model the antenna wire lengths at an upward-shifted frequency based on
the Vf?

Thanks,

w3rv