[TowerTalk] Measuring coax loss
David Hachadorian
K6LL at adelphia.net
Sun Dec 26 13:33:23 EST 2004
Here's an easy procedure to make a good approximation
of line loss, without disconnecting the coax up on the
tower:
1. Connect an antenna analyzer to the feedline in the
shack.
2. With the analyzer in swr mode, vary the frequency,
observing that the swr goes through maxima and minima.
Pick one of the maxima, in the vicinity of 25 - 30 MHz.
Note the swr and frequency. At that point, the antenna
up on the tower presents such an extreme impedance,
that you are essentially measuring the swr on an
unterminated line.
3. Plug the observed max swr into the following
equation:
loss (dB) = 10 log [(swr+1)/(swr-1)]
For example, if you measure a max swr of 9, at 26 MHz:
loss = 10 log(10/8) = 0.97 dB at 26 MHz.
4. Consult a coax loss chart to see how 0.97 dB at 26
MHz compares to the specifications for new cable of
that type and length at 26 Mhz.
The main limitation on this process is the ability of a
cheap analyzer to accurately measure high values of
swr. That limitation is mitigated by staying at the
high end of the frequency range, where the swr maxima
have lower values.
Dave Hachadorian, K6LL
Yuma, AZ
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