# KL7AJ’s forward and reverse power challenge

Recently on QRZ, KL7AJ opened a thread recommending his own slide presentation entitled “SWR meters make you stupid”.

After more than 100 posts, one of the participants tried to understand this diagram for the presentation.

Now there may have been some discussion at the meeting where this was presented, giving details that are missing from the slides.

Lets assume that all transmission lines are 50Ω lossless line, and that the calibration of the wattmeters is wrt 50+j0Ω.

If we start with a literal interpretation of what is presented, starting at the load which determines the VSWR seen by wattmeter #2 in this case, since VSWR=4 we can calculate rho=0.6, rho^2=0.36 and that Pfwd is Prfl/rho^2=36/0.36=100W.

Now we have Pfwd and Prfl, we can calculate P=Pfwd-Prfl=100-36=64W.

Looking at wattmeter #1, P=Pfwd-Prfl=100-0=100W.

So, there is 100W into the TUNER and 64W out, it has a loss of 1.9dB.

That is not an extraordinary loss for a TUNER, it is not so ridiculous as to be an obvious mistake… but it may not be what the author had in mind.

It is also possible that the stated Prfl=36W at wattmeter #2 was a mistake? An exercise for the reader is to calculate Prfl and Pfwd at wattmeter #2 if the TUNER was lossless, is it a problem that Pfwd is greater than the transmitter output power?