[TenTec] SWR VS Power Loss

[Michael Tope]

Dr. Gerald N. Johnson wrote:

>oes not compute! 1 KW forward and 1/2 KW reflected says 1.5/.5 = 3:1
>SWR, not 6:1. E.G. SWR = (forward + reflected) / (forward - reflected). 
>6:1 SWR is 71.45% reflected, 1.7145/0.2855..
>
>What that table means is rubbish.
>
>73, Jerry, K0CQ
>

Hey Jerry,

VSWR = (1 + |RHOv|)/(1-|RHOv|)

For VSWR = 6:1, |RHOv| = 0.71 (i.e. 1.71:/0.29)

Return Loss = 20*log(|RHOv|) = 20*log(0.71) = -3dB

I think you forgot to square the voltage reflection coefficient when 
calculating return loss (easy mistake to make). Ergo, I think the table 
values are numerically is correct, but the table is probably easily 
misapplied given the supporting material and the context. As Ken N6KB 
points out, the table is only very useful if you have an isolator at the 
transmitter output (or perhaps a big resistive attenuator pad).

Happy 4th!

73, Mike W4EF........................

P.S. Here is a good "cheatsheet" for checking calculations:

http://www.microwaves101.com/encyclopedia/calvswr.cfm