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

[Grant Saviers]

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

On 12/25/2016 0:50 AM, Guy Olinger wrote:
> 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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