Guy,
Ok .... this is the first I've heard someone say NOT to use THHN. And I am
specifically addressing for antenna material - not for transmission lines.
Of course it is designed for standard house wiring, we're all aware of
that.. and I'd hope also all aware it has not been tested and thus is not
normally specified for RF use ... but that goes for - aluminum tubing, even
bare stranded wire isn't "specified" for RF ... is it - by the manufacturer?
So I had been planning on 4000' of #14 THHN insulated as buried radials ...
what say you on this?
I realize your example is not buried - but I'm curious if you have a thought
on this.
Also skin effect isn't going to "push" current where the impedance won't
allow current to go - I know you know this and suspect the sentence was not
quite meant as written - or maybe I just read it wrong ... if indeed the
surface becomes resistive doesn't that simply mean the RF then flows through
the layer below that resistance? It is afterall going to seek the path of
least resistance. I'm not sure why wire would get resistive at RF unless
that corrosion continues to the point that all of the copper has transformed
in to high resistance.... I can understand a connection doing so - if the
mating surfaces corrode and thus there is no common 'path of least
resistance' between them and the connection goes bad... but wire is wire
isn't it?
Gary
K9RX
-----Original Message-----
From: Guy Olinger
Sent: Thursday, December 22, 2016 10:08 PM
To: Jim Brown
Cc: towertalk reflector
Subject: Re: [TowerTalk] Double Coax Runs to Reduce Loss
Be careful to never, ever use THHN (with insulation NOT stripped off)
outdoors.
Also personally, I would never use it for RF transmission line. Do not
confuse Z0 with efficiency. What part of the Z0 is actually loss? Do not
assume that the characteristics of THHN have been engineered in any way
whatsoever for RF.
THHN is manufactured for 60 Hz indoor power uses to comply with the US
National Electrical Code. All the manufacturer's efforts are designed to be
compliant with the code with the least possible cost of manufacture.
There is ZERO testing of THHN for RF usage. Why should there be? So why
should we expect different batches and different manufacturers to all the
the same at RF. "Well, I tested mine and it was fine". Except you can't
extrapolate it. If yours was fine, you were lucky. Get in a supply of #12
teflon sleeve and #14 double polyimide wire. Use teflon sleeved #14 double
polyimide for RF and winding on cores. Wind it, install it, forget it.
Also, using THHN to keep dipole ends from shorting to trees did NOT work. A
length of #11 teflon sleeve over the otherwise bare solid #12 did work, has
worked, and will continue to work.
What does seem to be a very practical *outdoor* use of THHN for ham use, is
solid #14 or #12 THHN bought by the 500 foot spool, and **then stripped**
for bare #14 and bare #12. This results in bare copper wire at last check
something like half the cost of already bare copper bought from the online
"copper stores". 500 foot spools of THHN are manufactured and shipped in
extreme bulk, economies of scale in full operation, all to our advantage.
**Measured and verified,** THHN used for elevated 65' radials became
severely deteriorated from approximately 3 years of UV exposure. Apparently
corrosive artifacts of the deterioration etched the copper surface into a
weird "crumbly" surface, rendering the copper resistive at RF, roughly an
ohm per foot at 1.83 MHz, while DC testing with an ohmmeter showed the
usual low R for copper wire.
An interesting demonstration of skin effect. What happens when skin effect
forces RF current into a deteriorated area of poorer conductivity? What if
the conductor surface gradually blends into an insulator?
RF measurement designed for measuring radial ground induction was 85 ohms
per 65' deteriorated THHN radial. Same measurement after replacing with NEW
THHN with insulation STRIPPED was 18 ohms. 15-20 ohms was expected and
normal for that length with the area's typical ground characteristics. 67
of the original 85 ohms was from deterioration. 85/4 = 21.25. 18/4 = 4.5.
The antenna DID have to be retuned afterward.
A duplicate experiment with an identical second set of radials on the same
site produced values within 5% of the first set.
The business of twisted pair THHN in the shack for anti-RFI 12 VDC high
current transmission is splendiferous. Yes, that is a real word you can
look up :>)
73, Guy K2AV
On Thu, Dec 22, 2016 at 2:52 PM, Jim Brown <jim@audiosystemsgroup.com>
wrote:
Hi Ed,
No, not twisted pair for chokes. But I have recommended parallel enameled
wires and THHN (ordinary house wire) wound on #31 toroids connected as
two-wire transmission line. You're correct that closely spaced enameled
wire yields Zo about 50 ohms; closely spaced THHN yields Zo in the range
of
85 ohms - 100 ohms. Jerry Sevick, W2FMI, noted this result in late
editions
of his book, and my measurements confirm that within measurement accuracy.
73, Jim K9YC
On Thu,12/22/2016 11:22 AM, Edward Mccann wrote:
Re twisted pair, I seem to remember you suggesting at one time that using
twisted pair enameled wire on FT-240 #31 mix provided about 50 ohms on
the
CMC Choke.
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