[CQ-Contest] oxidized inner conductor when coax carries DC power

Gerry Treas K8GT k8gt at mi.rr.com
Wed Nov 2 00:24:54 EDT 2016

The only black copper oxide that I experienced was the shield on a 100' 
run of RG213 due to a squirrel or more likely a rabbit chewing a small 
hole in the jacket.  Had to cut 4-5 feet out of the middle of it.

Yes CoaxSeal is terrible to clean off everything.  The very best thing 
to clean it with was with Freon TF in the late '60s and in the '70s 
before it got outlawed.  It was used at TV stations to clean videotape 
residue from the rotating heads on 2" Quadruplex VTRs.  It would wash 
CoaxSeal off and be dry in about 5 seconds. Also good for cleaning oil 
based paint off paintbrushes.  Nowadays I use 91% Isopropyl Alcohol 
which does a moderately decent job of cleaning it off..

I quit CoaxSeal and went to 3M #2242 self vulcanizing rubber slicing 
tape available at most of the Home Improvement stores or major hardware 
stores at about $3.50 a roll..  Properly installed it is as good as 
CoaxSeal without the mess.  Unfortunately it is not UV resistant so 
requires a covering of PVC tape.  A wider but more expensive version is 
Scotch #130 which requires fewer wraps to cover the same length.

73, Gerry, K8GT

On 01-Nov-16 10:17, George Harlem wrote:
> H2O molecules are pretty small. I'm told that copper oxide eventually turns from green to black. I use Coax Seal, but it can be nasty to remove-- at least it seems to do its intended job.
> George W1EBI
>  From George's iPhone
>> On Oct 31, 2016, at 6:51 AM, Franki ON5ZO <on5zo at telenet.be> wrote:
>> Probably not the appropriate forum but there ought to be a technically skilled ham here? Sorry that this post isn’t about cheating and what defines ‘assistance’. Some contesters have real issues though.
>> A few weeks ago I was doing some relocating and rerouting of the coaxes outside. One RG-213 is used for my active RX loop. It needed a different plug on the antenna switching side so I cut the existing one off and prepared the cable for a new one. Much to my surprise I found the inner conductor black from corrosion. I cut off a few centimeters at a time, but after having cut off two meters, it still was black. I have been working with these things for ages and I can tell you: it is NOT water ingress. Everything is sealed properly. I was a pain to solder the new plug to the center conductor. I had to sand the black film off and even then the tin wouldn’t flow.
>> Last week I took the loop down and cut away the layers of tape that kept the feed point coax dry. And dry it was. However I noticed the N male-female junction had a green mush developed around the mating pins of the inner conductors.
>> I didn’t pay attention in chemistry class, but I’d label this as corrosion. Right?
>> Since I have never seen this before, and this is the only coax that ever carried DC around here, I assume the DC voltage is the culprit here?
>> * Can I avoid this?
>> * Does it hurt? I seem to remember something about DC and polarity that can eat your copper away?
>> * My coax shields are tied to a dedicated earth system. Does this relate to the corrosion in a good or bad way?
>> I’m thinking of ways to improve my 80/160 RX situation, and several scenarios involve DC over the coax. So I better know what I’m up against.
>> Thanks and 73
>> Franki ON5ZO / OQ5M
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