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

Gerry Treas K8GT k8gt at mi.rr.com
Wed Nov 9 14:37:56 EST 2016

Something that hasn't been mentioned in this thread that has been 
mentioned in posts in the past is that there are 50 Ohm and 75 Ohm N 
connectors.  The center pin of the 75 Ohm N connector is slightly 
smaller in diameter than the 50 Ohm.  If the male N connector on the 
cable is the 75 Ohm type mating with a 50 Ohm female the connection will 
be slightly looser.  Add in the DC current as well as the RF, and even 
without an intermittent, the connection will be a little more 
resistive.  Add in the DC current and things get interesting.

The shield connection is the same between the two types of N connector

Check to see if the cable N connector might be a 75 Ohm versione.

73, Gerry, K8GT

On 08-Nov-16 17:56, W5PR wrote:
> I think the 12 V would only be relevant when there is a dissimilar metals problem.  In other words copper against aluminum.
> Chuck W5PR
> Sent from my iPad
>> On Nov 7, 2016, at 4:16 AM, Franki ON5ZO <on5zo at telenet.be> wrote:
>> A lot of useful replies about the black inner conductor and the green mush inside the connector. Many of which sent directly to me.
>> Some people think that it water ingress even though I'm pretty sure it isn't. But if you ask advice, you must be open to views that don't match yours. So I looked for evidence.
>> I took a one meter stretch that I cut off before soldering a new plug a while ago. It's only the inner conductor that got black. The braid (shield) is pristine: gold-yellow, not a sign of contamination. Even the inside of the braid that touches the dielectric.
>> This end of the cable was never exposed to the elements. It sat in an outdoor cabinet. Although the inner conductor turned back, there was no green on the N-plug's pin.
>> I only found the green 'powder' (like you sometimes find on battery terminals) on the other side of the cable, where it mated an N-style jumper that goes to the RX loop's feedpoint. I inspected both connectors (male and female). The green was only around both center pins, where they mated. There is no sign of green on the shield side of the plugs. I also can't find any sign of real water sipping through. Moisture can be the case though. With lots of oxygen because there is plenty of air space inside an N-plug, especially the female type.
>> I unwrapped the sealing tape one of the jumper's N-plugs. No sign of water in any form. I took apart the plug (nut and grommet type). The plug is only green inside the shell, where it is exposed to air. By this I mean the air in the void between pin and shell inside the N-connectors when M+F get screwed together. The lower part of the pin (part that slides over the coax inner conductor and is soldered), where the white 'dielectric extension' spacer cap slides over the pin, was not green. This part of the pin is covered by this spacer and is not exposed to air.
>> The sleeve says 'RG 213 / UBX' made in Germany'. I can't find a manufacturer. I bought this 50m run at a hamfest. I seem to remember it was pretty cheap. I bought it with 'RX only' in mind so I didn't actually think of losses and power handling.
>> FWIW a sweep with the dummy from 1.5 MHz to 50 MHz shows a flat SWR graph.
>> The main question however, remains unanswered: is there a link with the fact that this coax carried +12V DC, or not?
>> 73
>> Franki ON5ZO
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