[TowerTalk] The hunt for a feedline fault - update

[Bob Shohet, KQ2M]

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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