|To:||"Towertalk Reflector" <firstname.lastname@example.org>|
|Subject:||Re: [TowerTalk] Measuring coax loss|
|From:||"David Hachadorian" <K6LL@adelphia.net>|
|Date:||Sun, 26 Dec 2004 11:33:23 -0700|
Here's an easy procedure to make a good approximation
of line loss, without disconnecting the coax up on the
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.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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