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…
Bob, WB4APR
From N1GNN:
Remember that manufacturers published values for velocity factor are "nominal" and vary in "real life" from reality. It is not uncommon, for example, to see the velocity factor vary from one end of a 500' reel of coax to the other. Part of the problem is the inability of coax spooling machines to maintain the same tension on the cable when the reel is just being started (small winding radius) to when it is finished (large winding radius).
Presently building UHF phase matching with rg-142. Book value? .7, Actual retail price? .6.
Got this roll from from Skycraft, 50' for 24$. Probably why it's surplus.
Norm n3ykf
On Wed, Sep 6, 2017 at 1:16 PM, Robert Bruninga [email protected] wrote:
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…
Bob, WB4APR _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
participants (2)
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Norm n3ykf -
Robert Bruninga