[50mhz] Replacing coax

Sun Mar 6 05:51:16 EST 2011

Hi John,

RG-11 is 75-ohm coax, so perhaps you're using it feed a dipole or similar 
dipole-like 75-ohm antenna, or perhaps through a 4:1 balun to feed a folded 
dipole (300 ohms). Just curious about that, as you don't see RG-11 around 
much these days. Your antenna tuner would of course match it to your 50-ohm 
radio just fine.

The biggest worry for me about any coax -- and this is from personal 
experience -- is not the coax, but rather the PL-259 connectors on either 
end of it. (Of course, if you're using N connectors, that's something else 
altogether.) But PL-259s, when you put them on yourself with the usual ham 
Neanderthal tools (a knife and an under-powered  soldering iron), they can 
appear to work OK on receive, but when you apply RF transmit voltage across 
them, they fail, sometimes intermittently. I had a problem once where my 
transmitted signal failed only on SSB voice peaks. Whenever this happened, 
the SWR on my voice peaks would go sky-high, the transmitter would 
automatically shut down to protect itself, and my RF output would 
momentarily drop to near zero. As soon as the RF voltage dropped slightly 
(after maybe 50-100 milliseconds), everything was OK again. Sounded exactly 
like an intermittent mic connector or element, as everyone on the air was 
quick to tell me. Except it wasn't.

After much substitution and metering, I finally eliminated the mic circuit 
as being the culprit. I then came up with the failure model described above, 
strictly in my head, as the only remaining possibility likely to produce the 
exact symptoms I was seeing. I started swapping coax sections, but nothing 
changed. I was about to give up and send the radio back to the manufacturer, 
when I realized there was one piece of coax I hadn't swapped out -- a short 
6-inch RG-8X jumper behind the radio, between the transmitter and the 
antenna tuner. I swapped it out, and bingo, that was it.

I had made that jumper myself, of course. And rather poorly, it seems.

Well, the point is, PL-259s are easy to screw up, especially (in my 
experience) when using the smaller size coaxes with those adapter collars. 
So unless you've become a bona fide EXPERT at installing PL-259s, I would 
regard replacing any working transmission line with a new that you made 
yourself as a potential source of failure. If what you have has been working 
for 11 years, I think you can assume that the PL-259s are good. I would NOT 
swap it out until some sign of deterioration or failure starts to appear.

Sorry for the long story, but I thought it might be entertaining as well as 
useful.

Another thing mentioned by one of your other responders I can equally vouch 
for, based on my work as field technician years ago. When you go into a 
basically functional electronic system and start poking around trying to fix 
some very obscure, trivial glitch, all kinds of system components that had 
been working fine for years now start to fail, having been disturbed by you 
in the process of trying to fix something else. This is the true origin of 
"If it ain't broke, don't fix it." There's actually a reason for that old 
aphorism.

Bill W5WVO

-----Original Message----- 
From: John Geiger
Sent: Tuesday, March 01, 2011 16:39
To: 50mhz ; 6meter at yahoogroups.com ; 'VHF REFLECTOR' ; DX-IS at yahoogroups.com
Subject: [50mhz] Replacing coax

I went out this morning and did a visual inspection on my coax runs, and 
they all look in very good condition. No cracking on the jacket at all, and 
the jacket still looks as good as when new.  One run of RG-11 I have is 12 
years old, but still looks great.  I don't recognize any signal degradation 
on the coax runs either, nor changes in SWR.  Now I don't have any complex 
test equipment to test the coax runs with, just a SWR/power meter built into 
my antenna tuner.

Given that the jacket on the coax looks ok, and I haven't noticed any change 
in antenna performance, is it safe to assume that the coax is still good, or 
is there an age limit for coax after which it starts to deteriorate from the 
inside?  Is there a rule of thumb on how often coax should be replaced?

73s John AA5JG
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