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

[Wes Stewart]

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 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.
>