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.
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