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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