[TowerTalk] Double Coax Runs to Reduce Loss

[Grant Saviers]

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 at windstream.net>
> To: towertalk at 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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