[amsat-bb] Finding a short in Coax to Antenna

Robert Bruninga bruninga at usna.edu
Wed Sep 6 17:16:55 UTC 2017

I used an MFJ Antenna Analyzer to quickly find the short in a 120’ run of
coax to the roof.

Just tune for the lowest frequency where the meter dips to 0 Ohms.  Then
calculate the quarterwave distance using the velocity factor for the coax.

>From downstairs it was around 2.1 MHz or a very long distance.  Suspecting
it might be right at the Antenna, I avoided any ambiguity in trying to
measure at the antenna end by adding a ten foot length of coax onto the
analyzer and then measured the dip with that 10’ shorted.  Turns out it was
31 MHz.

Then used that 10’ connected to the roof end of the coax and got the dip at
exactly the same 31 MHz showing the short was right at the connector.
Having the 10’ length of test cable made the measurement much more friendly
at 31 MHz compared to maybe 900 MHz which it might have been 2 inches from
the analyzer…  Which the analyzer couldn’t do anyway…


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