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

[Joe Giacobello, K2XX]

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.
>
> 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 at 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 at 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
>