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Re: [TowerTalk] Strange coax failure

To: Patrick Greenlee <patrick_g@windstream.net>, towertalk@contesting.com
Subject: Re: [TowerTalk] Strange coax failure
From: Tom LeClerc W1TJL <wb1cby@gmail.com>
Reply-to: w1tjl@arrl.net
Date: Mon, 15 Dec 2014 11:20:12 -0500
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
I saw some coax a few years ago where the center conductor was not entirely in the middle of the insulator. Bad molding? It was RIGHT up near the shield in some spots and centered in others... If I remember correctly it was a foam dielectric...

Just a guess but I would assume that would cause some pretty weird reading depending on movement, frequency and power levels...

73, Tom W1TJL


On 12/15/2014 11:09 AM, Patrick Greenlee wrote:
Most all tower users have fairly extensive coax runs on which we depend for near trouble free service. Here is one strange tale...

I had been having fits chasing an antenna/feedline/connector problem that was common to all antennas/towers. I whipped up a quickie temporary 10 meter dipole and hung it with woodworking spring clamps from bungee cords clamped to gutters. I did use a nice balun at the center. Put Comet antenna meter on it and cut the length to resonate at 29 megs where it was dead flat and 50 ohms. Not having a piece of coax long enough to make it to the transceiver I used some jumpers joined with T's (didn't have enough barrel connectors handy) and reached the transceiver. The antenna meter attached for a quick test before hooking coax to rig and the needle went crazy and uninterpretable. I tried at the various junctions of jumpers until I got good readings thereby identifying the bad jumper. ( I had 6 each 6 ft jumpers and needed 3-4 for this lashup.)

Brief digression: I bought a 6 pak of 6 ft RG-58U jumpers with PL-259's for a "bargain" price on evilBay.

So, I visually inspected the suspect jumper and it looked brand new with its nice part number tag and "made in China" prominently displayed. I grabbed a VOM and checked the bad jumper finding continuity where and only where it was supposed to be and nowhere it wasn't. Connected the bad boy to a 50 ohm dummy load and other end to antenna meter. Really weird readings. Substitute another jumper and indications are perfect 50 ohms irrespective of frequency, as should be expected.

It isn't like the antenna meter is causing arcing given its low output. I haven't a clue as to what sort of anomalous defect could cause these indications. I have been using coax for over 50 years (first ham lisc in 1962 and CB before that plus a year and a half as field service engineer in marine electronics/communications) and maybe I have just been lucky but I have never seen anything like this before.

Anyone have an idea regarding what the defect might be? I'm going to cut the bad jumper in half and test the halves and then cut the bad piece in half and test those pieces. Whichever is bad will then be dissected carefully to see, if possible, what was physically wrong with the coax. This is a real head scratcher for me. The good news is I'm back on the air with a pan adapter full of signals again and loving it.

Patrick     NJ5G


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