Why THHN insulated Cu wire would abnormally degrade vs other Cu wires is
a bit of a mystery to me. Yes, the nylon exterior will severely degrade
after a couple of years in the sun from UV, but the PVC layer is the
same as every other PVC insulated wire and will slowly degrade also
unless a UV resistant grade which just takes longer. Re the "insulation
carrying RF current" of course it is in the RF field, just put the
correct numbers for thickness and dielectric constant into NEC/EZNEC to
see the effect. I use the thickness weighted average of the nylon and
PVC constants as a reasonable guess (3). Other TTer's can probably
provide better and more detailed ways to analyze the dielectric constant
and losses.
There are many specs that apply to electrical service. ASTM B49 for the
copper is constant for all insulation systems including none. Federal
A-A-59544 & 59551, UL83 and many more if you are willing to pay for
them. The Fed specs are free. Of course anything is possible for out
of spec alloys, China Cu over Al wires, 14 ga that is really 18ga, etc.
THHN is only approved for dry locations, but that is for electrical
service to 600v. Does that matter for radials? I think not. Just
consider the losses of earth burial, overwhelming compared to a few thou
of insulation.
Grant KZ1W
On 12/23/2016 6:22 AM, Jim Thomson wrote:
Date: Fri, 23 Dec 2016 07:54:06 -0600
From: Patrick Greenlee <patrick_g@windstream.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Double Coax Runs to Reduce Loss
The skin effect will not force all the current to flow in a corroded
surface layer with good copper below it. Current density will be
distributed proportional to the impedance presented by the different
depths. This includes the skin effect's redistribution of impedance.
If a simple application of irrefutable skin effect phenomena were
applied, the insulation over copper wire would be the current carrying
medium at RF freqs rendering insulated wire unusable in most applications.
Except as specifically noted above I mostly agree with the post
reproduced below.
It might be interesting to put a watt meter first at one end of the DIY
twisted pair and then at the other end and see what the efficiency is.
Why debate the number of angels dancing on the head of a pin when we can
use a loupe and count the little buggers?
Patrick
## I dont get it. As long as the wire is terminated correctly, any
deterioration of the
THNN insulation should not enter into the equation. Copper tubing, used in 144
mhz,
2 kw tube amps can turn GREEN..and or black,and zero redux in po....as long as
the tubing is terminated
correctly.
## IF the terminations are done right.... and the thnn insulation turns
crumbly, the RF current should travel along
the inner layers of cu wire, just below the top later of cu.
## It might be a case where the bad THNN insulation has somehow migrated into
the termination, dunno.
## f12 used 12 ga THNN wire...home depot stuff, to wind the coils in their ssb
– cw switch boxes. But
they used nema boxes, and the coils lasted a long time..so no UV. I initially
replaced em with 8 ga polyimide
magnet wire, then later on with silver plated 1 inch wide cu strap.
Jim VE7RF
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