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## [TowerTalk] Easy measurement of feedline losses

 To: [TowerTalk] Easy measurement of feedline losses k6ll@juno.com (k6ll@juno.com) Thu, 12 Nov 1998 16:00:48 +0000
 ```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 ___________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com/getjuno.html or call Juno at (800) 654-JUNO [654-5866] -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: owner-towertalk@contesting.com Search: http://www.contesting.com/km9p/search.htm ```
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