K7ITM wrote:
The question that comes first to my mind is, "Why do you care?"
Certainly an antenna does not need to be resonant to work well. I
can
imagine you'd like a reasonably low indicated SWR, just so your
transmitter has a reasonable load to drive.

If you really want to know what's going on at the antenna feedpoint,
you'll need to back the effects of the feedline out of your antenna
analyzer readings, or use an analyzer that does it for you. If you
have a reasonable estimate of the feedline loss and know its
electrical
length (easy to find if you put a short at the end of the line and
look
at the resulting impedances read on the analyzer), then you should be
able to translate your analyzer readings to actual feedpoint
impedance.
Do you have the feedline properly decoupled from the antenna so it's
not a significant part of the radiating system? If not, there seems
little reason to bother making the measurements.

I'd expect half-wave dipole resonance to result in lowest SWR on a
50-ohm feedline, but it won't be a very sharp minimum. So is it
worth
worrying about?

I agree 100%, it's not worth worrying about - if all I wanted is to get
a 20M dipole running on the air. But that's not my point. I'm trying to
use dipoles to determine the velocity factors of insulated wires used
for the radiators.

Another 'speriment to try: build a fairly wide-spaced two wire
transmission line from your wire. Short it at one end, open at the
other, and look for quarter-wave resonance; or short both ends and
look
for half-wave resonance. Measure the resonant frequency, which will
be
a pretty sharp resonance (much sharper than the dipole). Remove the
insulation and see how much the resonance changes. Try for various
spacings to see what effect the spacing has. (Expect that close
spacings will show more effect than wide.)

Now we're cookin', I like it, this approach has definite appeal and I
need to explore it for several reasons. First because of the sharp
nulls, it takes out the coax and it's a simple sort of "bench test" I
can do at ground level. I'll build three identical shorted 20M or 30M
close-spaced quarter wave lines. One with bare #14 stranded wire, one
with the insulated #14 THHN wire I usually use for quick & dirty
dipoles and one with The Wireman's very flexible #544 or #546 #14
insulated PVC jacketed wire I'd use to build a hex wire beam or a quad.


Then I'd use a grid dip meter to find the resonant frequencies of all
three. I'd use an HF rcvr with a digital freq display to listen to the
GDO rather than trust the GDO dial calibration and resolution. Yes?

Which leads into another head-scratcher I've had in the past. I've had
a bad time coupling a GDO to quad elements because it takes a couple
turns of wire near the GDO coil to get enough coupling between the quad
element and the GDO. Which in turn means that I've shortened the
element length and the result is wrong.

What's your suggestion on a method to accurately measure the resonant
frequencies of the quarter-wave lines in this exercise?

Thanks,

Cheers,
Tom

w3rv