[amsat-bb] Finding a short in Coax to Antenna
normanlizeth at gmail.com
Wed Sep 6 17:41:03 UTC 2017
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.
On Wed, Sep 6, 2017 at 1:16 PM, Robert Bruninga <bruninga at usna.edu> 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
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