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Re: [TowerTalk] Double Coax Runs to Reduce Loss

To: Jim Thomson <jim.thom@telus.net>, towertalk@contesting.com
Subject: Re: [TowerTalk] Double Coax Runs to Reduce Loss
From: Grant Saviers <grants2@pacbell.net>
Date: Fri, 23 Dec 2016 10:01:48 -0800
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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