On simple test for the performance of any radial, and I have used this
method to detect broken buried copper at an AM station, is to run abut
20 watts into the antenna, then hold a good field strength meter loop
close to the ground and walk along the radials path. Even a diode
detector on a micro-ammeter you may have in the junk box with a coupling
loop attached may work if a good "FSM is not available.) Any
deterioration of the wire should show up immediately and they could
easily be compared to a solid bare copper radial for testing purposes.
If there is any appreciable deterioration in conductivity either laying
on the ground or other wise this should be noticeable on the FSM. This
method sure beats digging up or striping the insulation from 100's or
perhaps 1000's of feet of wire.
Herb, KV4FZ
On 12/27/2016 4:13 PM, Joe Giacobello, K2XX via TowerTalk wrote:
Guy, that certainly makes sense since formation of HCl is known to
occur, and it might indeed attack the copper's surface resulting in an
increase in the resistance of the "skin." Frankly, I have to admit
that I never heard of this problem, but what you describe is certainly
a plausible mechanism. Aside from the EDZ that I mentioned, I have
used THHN for radials on an 80/160M vertical that's been in use for
about six years or so now. I haven't noticed any deterioration in
performance on either band, but it might be hard to tell. Certainly,
it hasn't resulted in any change in the SWR that I routinely see.
Since the radials are not buried but just lying on the ground, I could
peel back a portion of the insulation on several and see if there's
any visible signs of corrosion. I may just try that.
73, Joe
K2XX
Guy Olinger K2AV <mailto:k2av.guy@gmail.com>
Monday, December 26, 2016 7:08 PM
I agree that the extensive loss is unlikely from dielectric loss in
the deteriorating insulation.
My best understanding of the process is that some depth of surface of
the copper is severely deteriorated from a byproduct of the
insulation breakdown, possibly because it is held against the copper
by the deteriorating insulation, and is never washed off and dried.
There is NO bright, shiny, smooth copper surface underneath the
deteriorating insulation of the wire in question. Then by skin effect
RF current is forced to transit the *deteriorated copper near and at
the "surface".
I would note that everything I can find on skin effect **assumes**
conductivity equal throughout the entire conductor cross section, and
that the insulator is absolutely not a conductor.
There is nothing at all that I have found about the behavior of skin
effect, if near and at the surface, the conductivity gradually
deteriorates to a very poor value at the actual surface, and if due
to etching, current is not following a straight line.
The identical problem is found, with an equal lack of clarity in
results, where in models ground material is monolithic and
invariable, when in fact the conductivity of the earth varies
extensively as one drops down from the surface, and that set of
variations varies widely with weather, etc.
73, Guy K2AV
Guy Olinger K2AV <mailto:k2av.guy@gmail.com>
Monday, December 26, 2016 6:46 PM
Please see my most recent post on this subject on this reflector:
http://lists.contesting.com/pipermail/towertalk/2016-December/154456.html
73, Guy
Joe Giacobello, K2XX <mailto:k2xx@swva.net>
Monday, December 26, 2016 2:34 PM
With all due respect to Guy and the other contributors to this
thread, I am mystified by any deterioration in RF performance that
can occur with antennas made from THHN house wire. Grant is quite
correct that the PVC insulation routinely generates small amounts of
HCl on exposure to the elements, i.e, moisture. But, it seems to me,
that the deterioration of the insulation can only affect the
dielectric loading and effectively shorten the electrical length of
the antenna. While I admit to not making any actual measurements of
RF performance, I have made many antennas from THHN, including ground
mounted radials, elevated radials, EDZs on 30, 40 and 80M and have
not detected any deterioration in performance with time.
As Grant implies and as I have been warned when I suggested using
THHN for quad elements, the manufacture of commonly available THHN is
pretty haphazard in that the composition of the insulation is a
potpourri of recycled PVC and, perhaps, other synthetics. I had no
way of checking that, but the quad aficionado who advised against it,
seemed to know what he was talking about. OTOH, the elevated radials
mentioned above were used on a 40M vertical, and the actual antenna
matched the EZNEC model perfectly with regard to resonant frequency
and SWR. (BTW, I also appreciate the recommendations for easily
available, heavy gauge aluminum wire. I recently reinstalled an 80M
EDZ using #14 THHN. I immediately regretted not having used a
heavier gauge, but it was what I had on hand. The next inevitable
re-installation will use one of the suggested aluminum sources.)
73 and the best for the season,
Joe
K2XX
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