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

[Herbert Schoenbohm]

With such a high amount of copper theft some AM radio stations in Puerto 
Rico choose to replace their radial field with inexpensive buried barbed 
wire. They would plow it into the ground with a Ditch Witch and 
vibrating plow hollow blade attachment.  The large spool of barbed wire 
would be mounted on the back.  I was told that his was a relatively fast 
process.  But I have no knowledge of how this would perform compared to 
the same amount of bare copper wire in the ground. At least the problem 
of cooper theft seems to have been abated.   At the AM radio station 
here in St. Croix 200 feet of 4 times AWG 4 was fed from the generator 
building to the transmitter location.  A few years ago some cooper 
thieves at night threw the main breaker and cut the wires at both ends.  
It appears from the tire tracks that the a truck just tied the wires to 
the trailer hitch and drove down the road pulling out the 3 phase feed 
from the conduit.  These wires most likely were coiled up and later at 
some remote location doused with gas to remove the insulation.

So ...back on the subject.....what about barbed wire for radials?

Herb, KV4FZ


On 12/26/2016 12:58 PM, Grant Saviers wrote:
> 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.
>>