[TowerTalk] Measuring coax loss

[David Hachadorian]

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