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

Ron Notarius W3WN wn3vaw at verizon.net
Tue Nov 1 11:37:00 EDT 2016


I've found over time that the easiest way to remove Coax Seal (or some of the more generic knock-offs) is to first wrap the intended connector(s) in electrical tape, then put the Coax Seal on top of that (completely covering the tape from end to end).  If done properly (famous last words), it seals just as well, and removing the stuff later comes down to peeling most of it off the tape, then removing the tape.

I've had hams tell me that that's overkill, especially when they're making a "permanent" juncture (coax from antenna meeting up with coax from shack, for example).  But as Murphy's Laws of Radio Communications will tell us, that almost guarantees that you'll have to undo the juncture at the most inconvenient time in the most inconvenient way.

73, ron w3wn

On 11/01/16, 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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