[CQ-Contest] oxidized inner conductor when coax carries DC power
hs0zcw at gmail.com
Mon Oct 31 09:31:58 EDT 2016
Franki, I am happy to see a topic of importance to ham radio, and thus to
contesters, on this venue.
I have had lots of weird tarnish, corrosion, and coax deterioration over
the years...... first in damp Florida and now in damp Thailand.
Here is what I have witnessed with my eyes.....
-coax shield shrinking...... good soldering to the PL259, good function for
5+ yrs, but then in use, screwed on, but with no vibration or removal, the
shields of five of six coax junctions inside the plug body apparently
WITHDREW from the contact with the plug shield attachment point, using only
solder for attachment. I could detect no reason, not the shrinkage of the
inner insulation, no where. The shield braid broke loose under the solder,
which remained looking good.
-I have found only two products to deliver a genuine water proof covering
for coax plugs outside. Heat shrink sleeving with heat activated glue
inside.... and a wire sealing goop sold by 3M, liberally applied in
layers. Electrical tape, no. I am not sure about Coax-Seal.
-If your inner insulator is also discolored, then the "plastic" black outer
jacket is source of black coating inside due to "contamination".... the
plastic migrating inside to make the coating. Read about "non
contaminating" coax jacket (which the 213 should have had). Use of
cheaper "contaminating" jacketed coax is good for about 5 years before the
contamination distorts the impedance (in places).
-Copper will oxidize but "always" needs a trigger of some kind, usually
water and or air. That voltage impressed on it may, may function the way
cathodic action can prevent rust on steel.... your case is a mystery to me.
-I have many cases of a coating of what reacts like tarnish to occur UNDER
AND IN CONTACT of a plug-socket pair. How the "oxidation" can occur
between two similar metals within where the two are in spring loaded
contact is another mystery, but I had lots of examples. My big Yaesu costs
more with its gold plated sockets and those I have no problems.
I have more examples but got tired. 73, Charly
On Mon, Oct 31, 2016 at 5:51 PM, 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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