This is a repost with a more relevant subject line. Recent
discussions of feedline losses on a kt34 and recurring
threads reminded me of this oldie, which I first posted
about four years ago.
A good thing to do every year or so is to measure your
feedline losses. If you have an antenna analyzer that reads
high values of swr with reasonable accuracy, the measurement
can be done from the comfort of your radio room, without
disconnecting the antenna.
Attach the analyzer to the feedline in question, with the antenna
still connected. Tune the analyzer's frequency until you see the
highest peak in indicated swr on the meter. There will usually
be several high peaks. Pick the highest one near the high end of
the analyzer's range.
Plug the peak observed swr into the following equation:
LOSS = 10LOG((SWR+1)/(SWR-1))
For example, if you see a peak swr of 8 at 24 mhz
LOSS = 10LOG(9/7) = 1.09 dB at 24 MHz
Then all you have to do is consult a coax loss chart to see if
the measured loss is reasonable for the type and length of
coax being used, and whether it would be cost effective to put up
new/different coax.
The reason this method works is that almost all antennas present
a high value of impedance when operated outside their design
range. This high impedance is a close approximation of an open
circuit, and the equation above is valid. This idea was
presented in QST a few years ago, but the author presented a
graphical, rather than mathematical, solution. I like the mathematical
solution better, because you don't have to go back and dig up the QST
article to find the graph.
Some people distrust the accuracy of high value swr readings
obtained on inexpensive analyzers, and prefer to take
repeated measurements over several years to detect trends,
rather than believing the absolute numbers. That's ok too.
Whenever I have used the above method to measure brand-new coax,
the results have always agreed closely with expected losses.
Dave Hachadorian, K6LL
Yuma, AZ
K6LL@juno.com
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