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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