I've just recently (re)subscribed to this list, so I don't have the whole
thread, but this paper might be of interest:
http://k6mhe.com/n7ws/Plating.pdf
Wes N7WS
On 12/27/2016 1:30 PM, jimlux wrote:
On 12/27/16 12:13 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.
This is actually a fairly well understood problem, but non-trivial to analyze
- you see it all the time when analyzing plating on waveguides or PWBs for
instance: gold is good, copper is good, the nickle flash, not so good a
conductor.
You also see it when analyzing loss with conformal coatings on a PWB
https://www.microwaves101.com/encyclopedias/rf-sheet-resistance-in-multilayer-media
https://www.sonnetsoftware.com/support/downloads/publications/MicCondLoss_Mar03.pdf
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.
This too is a problem, that has been extensively studied. if you google
"electromagnetic model of multilayer media" you'll probably turn stuff up.
It's also been studied in the context of geophysical prospecting.
The real question is whether it makes a difference - you have to do some
sample cases, and you might find it's no big deal either way.
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