Insulated antenna elements

Pete,

Did you catch the series of messages
on insulated antenna elements during
Jan. 24-30 or so? You were asking
if 12% shortening was reasonable.
According to Bob Haviland's note
(copied below), the answer is, "Yes."
(He must not have had a chance to read
the list in the last couple of days
to respond to you for himself.)

I notice that the THNN wire has
an inner dielectric of PVC. I have
some indications from what I read somewhere
that PVC characteristics vary quite
a bit with frequency since it has a
polar molecule that is affected by
the signals on the wire. By contrast,
polyethelene has a neutral molecule
and its characteristics are very well
behaved as frequency increases.
(I know just enough about this to be dangerous.)

I have a project to determine the
primary RLGC constants of telephone
cables up to at least 30 MHz.
Almost all of them use are twisted-pair
with polyethelene insulation.
The most common type of "drop wire,"
the last connection from the pole to the house,
is not twisted and uses PVC insulation.
I'll be curious what you learn.

Ron McConnell, w2iol
rcmcc_at_cc.bellcore.com

+++++++++++++
>To: nec-list_at_ee.ubc.ca
>Subject: Antennas with dielectric coatings

>See my article "Insulated Antennas" in Communications Quarterly, V3 N1,
>Winter 1993 for a graphical method of determining the effect of
>insulation. This is based on Richmond, NASA Report CR-2396, available in
>depository libraries.
>The general effect of insulation is to lower the resonant frequency,
>reduce the drive resistance and decrease the SWR bandwidth. Very
>approximately, insulation thickness equal to 1.5 times the wire radius
>reduces the resonant frequency of a dipole by 10 percent.
>For any accuracy, the Thin-Wire program or NEC-4 should be used. I
>believe the routine is also in NEC-3.
>bobh, W4MB, bobh_at_enterprise.america.com
Received on Mon Feb 05 1996 - 20:48:00 EST