How Much Power gets Lost due to SWR?

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Reflected power is not necessarily lost power

My Radio has reduced power because my antenna has an S.W.R. of 2.5 : 1
If I use an antenna tuner in the shack to keep my transmitter happy, how much power will I lose due to the high S.W.R?
The answer to this question is “it depends”
Many amateurs assume that with this situation if they were using a 100 watt transmitter they would have a reflected power in the vicinity of 20 watts this would be a loss of an additional 1dB.

Additional Loss Due to S.W.R

This Graph from The ARRL Handbook

  PDF Here Figure-1
If I am using a reasonable length of good quality coax cable with low loss, a very small loss actually occurs. On the other hand, if I have a high value of loss on my cable when matched the losses can be very high.
A 30 meter length of RG213 at 10 MHz has a matched loss of approximately 0.6dB
When an antenna tuner is used in the shack to compensate for this mismatch the standing waves on the line are not cancelled but a conjugate match is formed keeping my radio happy.
The additional loss due to the 2.5:1 S.W.R will be (from the Graph) about another 0.2 dB not 1 dB as many would think. In fact to get an additional loss of 1 dB (again from the Graph) the S.W.R would have to be in the vicinity of 5:1.
So the message is if you are using a good quality transmission line of reasonable length the matched losses will be small and losses due to S.W.R. will also be small. So do not be concerned about using an antenna tuner in order to keep your radio happy with antennas having moderately high standing waves.

The difference in signal strength at the receiving end will in most cases be imperceptible compared with that of an antenna having an S.W.R of 1:1.