David,
Thank you for explaining this in all the detail. It provides another useful
method of finding the problem.
Critters (teeth marks and holes) and lightning (uneven, pitted/fluffed up
and/or slightly charred jacket) aside, the fault is almost always at the splice
– either a failure of the connection or the connectors.
73
Bob KQ2M
From: David Gilbert
Sent: Friday, November 08, 2019 7:00 PM
To: Towertalk
Subject: [TowerTalk] The hunt for a feedline fault - update
To repeat the situation here ... I have roughly 150 feet of 1/2 inch
Heliax from the tower to just outside the shack, a good quality LF4-50A
connector to make the transition to about 20 feet of BuryFlex coax for
the rest of the run, and a high power fault somewhere in the line. The
procedure I described earlier told me that the fault was probably 23
feet from the shack end of the feedline, which strongly pointed to a bad
job by me installing that connector.
Before doing a bunch of digging, I decided to repeat the procedure
except from the tower end of the Heliax. The readings were a bit less
consistent (no doubt due to the much longer length to the fault), but
they ranged between 145 feet and 151 feet, with an average of 148 feet
... again pointing to the same location.
So I dug up that feedline starting at the house and working until I
found the splice between the BuryFlex and the Heliax, which indeed was
just over 20 feet from the shack end of the BuryFlex. As soon as I
removed the outer vinyl tape and the thick stretchy silicone inner tape
the connection came apart in my hands. The interior of the Heliax
connector was literally as black and sooty as the chimney of my wood
burning fireplace. It looked awful.
So the fault has been found and the location was as predicted. Given
that I had made eight connections like that and totally messed up one of
them, it seems my QC defect level for that particular task is about
125,000 PPM. Gotta do better.
By the way, I was wrong when I said earlier that TLW was telling me I
had a dead short. The graph only made it look like that, and a more
careful look at the axis dimensions said that the dips for each
frequency were always about 28 ohms resistive at the low power of my
analyzer ... which makes more sense than a short for a fault caused by a
high power RF arc over. I find that even more impressive (the data, not
my error), since it means that the location of even non-absolute
anomalies (neither open nor short) could potentially be found by this
method.
Hope this is useful to somebody.
73,
Dave AB7E
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