[50mhz] Replacing coax

Sun Mar 6 10:43:43 EST 2011

Many of us use 75 Ohm cable and the CATV industry has been a source for 
decades.
Even Times woke up to the fact and has a LMR-400-75.

Carl
KM1H

----- Original Message ----- 
From: "Bill VanAlstyne W5WVO" <w5wvo at cybermesa.net>
To: "John Geiger" <aa5jg at fidmail.com>; "50mhz" <50mhz at mailman.qth.net>
Sent: Sunday, March 06, 2011 5:51 AM
Subject: Re: [50mhz] Replacing coax

> 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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