I hate to allow that 3% figure to stand without some challenge.
And Roy is not the source of the NEC-4 corrections, those are from the
collective authors of the licensed NEC-4, which the professional grade
EZNEC can run as an engine if you have licensed NEC-4. When you
license NEC-4 you legally agree (one signs a document which has to be
mailed in) to share any extensions and improvements to the process
with the collective licensees.
What may be at issue here on VF of insulated wire is accurate
identification of the material that insulates a wire. Some insulation
is actually two materials, one as the insulation per se, and another
for resistance to chemicals and abrasion. These kinds sometimes have a
"clear-coated" appearance. Also in some wire the surface seems to have
been "treated" with something, and if you cut a cross-section and
magnify it a bit, you can see the color of red or white change a bit
between the surface and the wire. What does THAT do to a predictable
VF. What is it when it gradually varies from a pure substance at the
wire to a combination at the surface?
Also, I don't know how many people have actually put up a test case on
a wide variety of insulated wires, with special attention paid to
keeping ground characteristics constant underneath. Just having the
ground get wet between measurements can invalidate the measurements if
one is trying to pick nits between a 1 and 3 percent VF measurement.
I'm sure that the manufacturers of insulated electrical wire are only
interested in the use and characteristics for electrical wiring (it's
use in 99.9999% of the cases) and have not invested any of their
business capital in evaluating the wire for use in an antenna,
particularly not in controlling manufacture to keep such
characteristic within a guaranteed spec. Its verifiable suitability as
electrical wire IS at issue.
The one exception to that might be that rather nice stuff sold by
Davis RF. But even they would not commit to its VF characteristics
last time I asked.
Given that manufacturers often market their name on wire manufactured
under outsourcing, from some number of different sources, even wire
bought under a single brand name may be different for VF from one
spool/lot to the next. Like paint you'd have to get enough from the
same lot/spool to do the whole job with some extra and MEASURE VF to
I don't intend to ridicule anyone's use of 3% as a broad
approximation, but I did not seen even one rigorous treatise on the
subject with various insulations when I went searching for one a
couple years back. Not meaning to put words in Bill's mouth, but if he
had found such a confidence inspiring treatise I doubt he would have
made his post.
My advice to anyone is to cut such wire to the uninsulated length and
then prune to frequency while measuring it. If you are trying to plan
something that requires 0.5% accuracy where prune & measure is
impractical, You might get lucky with 3%, then again you might not.
Can you GUARANTEE the composition and thickness of the insulation?
Think the federal government has inspectors verifying that all
electrical wire is properly labeled for VF? We're lucky enough that
house wiring will defend against 600 volts AC. Or are we?
It just wasn't made for what we're doing with it and manufacturers
The real trick probably is to invent some way to measure in those
situations where the books don't hand one a ready-made technique for
----- Original Message -----
From: "Ulrich Weiss" <firstname.lastname@example.org>
To: "Andreja Todorovic" <email@example.com>
Sent: Tuesday, June 22, 2004 5:49 AM
Subject: Re: [Antennaware] Re: Antennaware Digest, Vol 16, Issue 3
> hello Andreja,
> from your message to Bill, W4ZV, I can see that you have been
> with the resonant frequencies of insulated vs. bare wires...
> do I understand you correctly that - when using 2,5mm sq. copper
> PVC insulation - you have to input 3,85 MHz if you want to get prope
> results at 3,725 MHz??? that means your correction factor for that
> insulated wire is 3,3%...
> this value coincides very well with the empiric values that I have
> a series of experiments with dipoles and loops last summer...
> meanwhile EZNEC 4.0 offers an option for insulated wires taking the
> insulating material and its thickness into account... unfortunately
> results of this new option do not coincide with our values yielding
> correction of only abt 1,5% for a 2,5 mm sq. wire with
PVC-insulation of 0,5
> I plan to provide W7EL with my experimental data and hope that many
> hams world-wide will do the same to enable Roy to improve his
> greetings from Germany
..... snip .....
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