Good post Dave, but PLEASE, use a SUBJECT LINE
that relates the the TEXT for ease in filing and/or deletion.
de Tom N4KG
On Thu, 12 Nov 1998 02:27:51 +0000 k6ll@juno.com writes:
>
>Speaking of recurring threads, let me dust off this
>oldie but goodie.
>
>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.
>
>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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