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@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@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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