Figuring Additional RF Power Loss

Regarding the program, “What Does Your SWR Cost You?”, that I gave at the January 2013 LETARC meeting, here is the process for figuring the additional loss in an antenna system due to mismatch between the antenna and feedline.

First, find the published loss for your feedline at the frequency of interest.  You can use manufacturer’s data, or general data from a source such as the ARRL Antenna Book.

Then, get an SWR reading at the input, (transmitter end), of the feedline.  Use chart, “SWR@FeedpointChart”, to determine the SWR at the antenna input.  Plot your measured SWR on the bottom edge of the chart, trace up to a point in the curved lines that reflects the published feedline loss you found in the first step.  Trace to the left to find the SWR at the antenna feedpoint.  This is a rough approximation, but quite accurate enough for our purposes at HF frequencies.

Then, using the chart, “AdditionalLossChart”, find again the published loss on the bottom of the chart, and trace upward to the point in the curved lines that reflects the SWR at the  antenna, found in the previous step.  Trace to the left from that point and find the additional loss due to this SWR.

Add this additional loss figure to the published loss figure for your feedline.  This should closely approximate your total loss in the antenna system.

The charts can be found here:

http://letarc.org/main/wp-content/uploads/2013/01/SWR@FeedpointChart.pdf

http://letarc.org/main/wp-content/uploads/2013/01/AdditionalLossChart.pdf

The QST article, on which the program was based, is “What Does Your SWR Cost You?”, by Stan Gibilisco, W1GV.  It is in the January 1979 edition.

Tom Wilbeck – N5KGN

 

 

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