[TowerTalk] Double Coax Runs to Reduce Loss

[Jim Thomson]

Date: Thu, 22 Dec 2016 22:08:18 -0500
From: Guy Olinger <k2av at contesting.com>
To: Jim Brown <jim at audiosystemsgroup.com>
Cc: towertalk reflector <towertalk at contesting.com>
Subject: Re: [TowerTalk] Double Coax Runs to Reduce Loss


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.

##  say what ?    IF  you had used an insulator on each end of the dipole, it would not
make any difference  what type of wire or rope you used between the outboard end of the insulators
.....and any tree.   Now if you didnt use an insulator..and just wrapped the THNN  around a 
tree branch, then all bets are off.  The peak V  at the ends of a dipole is just wicked..and will
arc right through any insulation to the tree.   A buddy tried wrapping the ends of his  4 x elevated 
radials... to  tree branches... and  set one of the trees on fire late one night.   So in went the 6 inch long
serrated insulators...with wire or rope from insulator to tree.  End of problem. 




**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.

##  what band, 80 or 160 ?  Elevated 65’  radials would be for a 80m GP, not
160m.   One ohm per foot of RF resistance at 1830 khz...  but extremely low DC
resistance ?   The only way I can see that measurement happening is IF  the far
ends were not cleaned up... where u connected to the test gear.   How many radials
are  you talking about ?   The only other way that could happen is if the   UV degraded THNN insulation
some how got into the terminating lugs, which is very unlikely. 



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?

###  It doesnt.  RF instead gets forced into the underlying clean cu.... provided
you have terminated the ends correctly.   Like using crimp lugs, then solder as  well, then
machine screw the lugs with SS machine screws to a drilled and tapped   AL plate.    



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.

##  did u install insulators on the ends of each radial ?   If not, then the trees
probably became part of the ends of each radial.   Your test gear would be indicating 
erroneous results. 

73, Guy K2AV

  ##  Later... Jim   VE7RF