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

[Joe Giacobello, K2XX]

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

> Grant Saviers <mailto:grants2 at pacbell.net>
> Monday, December 26, 2016 11:58 AM
> Guy,
>
> Please inform us of the details of the antenna and how the 
> measurements were performed.
>
> A little google research provides a lot of data about PVC degradation, 
> a exhaustive survey article at 
> http://www.sciencedirect.com/science/article/pii/S1658365514000880 
> This and other web sources show the complexity of the processes and 
> the huge variability of PVC compositions.  So perhaps there are 
> chemical end products that could cause severe losses at RF, but the 
> process/chemistry challenges my imagination given the thickness of the 
> insulation.   Free HCl is produced when PVC degrades which may account 
> for the damage to the copper surface.  This will occur on bare Cu 
> anyway, the severity depending on the free chlorides, sulfur, etc 
> compounds in your local atmosphere, which are very variable across the 
> USA.
>
> OTOH, nothing in the research would hold me back from buried THHN or 
> other insulated copper wire radials.  No UV, no high temperatures. 
> Bare buried aluminum wire is not a good idea due to the likely 
> corrosion issues.
>
> Regarding stripping THHN to get bare Cu wire for elevated radials or 
> antennas, why bother when aluminum electric fence wire 12.5 ga is much 
> cheaper and stronger than 14ga Cu ?? (the yield strength data is not 
> readily available, but it appears the Al will also have less stretch, 
> extrapolating from the ultimate tensile strength data I've found).  
> I've had 8x by 125' radials up 10' in mixed forest, had 2" branches 
> fall on them and they are all still as new. 
> https://www.amazon.com/Field-Guardian-2-Guage-Aluminum-1000-Feet/dp/B004423ZFM/ref=sr_1_7?ie=UTF8&qid=1482693026&sr=8-7&keywords=aluminum+fence+wire 
>
>
> or 9ga for even more strength 
> http://www.homedepot.com/p/Field-Guardian-1000-ft-9-Gauge-Aluminum-Wire-AF9000/204620699 
>
>
> Grant KZ1W
>
>
>
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> Guy Olinger <mailto:k2av at contesting.com>
> Sunday, December 25, 2016 3:50 AM
> First of all, and most important, a joyful holiday season and a prosperous
> New Year to all of you.
>
> Now down to microscopic issues that will probably have no bearing on World
> Peace...
>
> Let's be clear that I stand by my prior statements against using 
> unstripped
> THHN at RF, respectfully, others' contrary statements notwithstanding.
>
> We have careful measurements. We can't just walk away from measurements. A
> measurement is a measurement, not an opinion. We're stuck with
> measurements.
>
> The effect in one case, losses from deteriorated insulation on elevated
> THHN radials were the same as if one had placed a 15 ohm resistor between
> the coax center conductor and the vertical wire. ***The owner was unaware
> and thought everything was fine.*** How this came to light is an involved
> story.
>
> Someone with a low band dipole in the air using unstripped THHN may be
> paying quite a penalty, especially if it's been up a long time. It would
> have developed very slowly, very sneaky. Not like having a branch drop on
> your antenna and having the SWR suddenly go bonkers.
>
> I find the defense for using UN-stripped THHN outdoors and for RF
> intriguing. Even more intriguing, stuck with the prospect that THHN might
> be bad for us, some propose going to a different (less common, more
> expensive?) THH-something variant hoping for a better insulation lifetime,
> while admitting the new THH-whatever will probably go down from UV as 
> well,
> just later. ??????? You're hoping for what advantage from the insulation?
>
> With the single exception being some posters to this reflector, everyone I
> know locally or have corresponded with, or talked with on the phone on 
> this
> subject, they all bought a spool of THHN from a Home Depot/Lowes/etc for
> outdoor antenna wire because it COST LESS, maybe half the price of 
> same AWG
> from sources that sold it as bare wire.
>
> Likewise if they didn't strip it, the single reason they did not strip it
> was because it appeared to be a lot of work. Some tried to strip it 
> but had
> the problem of the knife digging the copper. They had never seen a
> description of "the method".
>
> It turns out that it is possible to strip 250 feet of THHN in the time it
> takes to walk the length of the wire, if you use the method. The limiting
> factor is the distance you have available to stretch it out before you
> strip it. Everyone who has seen it done, later says it's obvious once
> you've seen it. Once they have seen it done they all strip the THHN 
> and put
> up the solid bare wire.
>
> It's impossible to NOT take some hit with still-insulated THHN vs.
> stripped. If nothing else, they're out for the dielectric loss of the
> insulation.
>
> Then there are potential gradually increasing losses as the UV 
> deteriorates
> the material, with clearly proven examples of severe end-stage losses with
> the UV deterioration.
>
> Happy Holidays everyone, and back to the egg nog.
>
> 73, Guy K2AV
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