Water in a connector can do that. Looks fine on ohm meter but RF sees the very
very high dielectric constant of water and shunts to ground. This first got me
when some water got into a Type N barrel, a thin film of water coated the
internal insulator disc. Flushed it out with high test alcohol and function
was restored.
-Mike-
From: Patrick Greenlee <patrick_g@windstream.net>
To: towertalk@contesting.com
Sent: Monday, December 15, 2014 8:09 AM
Subject: [TowerTalk] Strange coax failure
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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