[TowerTalk] UV and WX deterioration of THHN insulation, and effects at RF.

[jimlux]

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 at 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.




>>