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Re: [TenTec] SWR VS Power Loss

To: geraldj@storm.weather.net, Discussion of Ten-Tec Equipment <tentec@contesting.com>
Subject: Re: [TenTec] SWR VS Power Loss
From: Michael Tope <W4EF@dellroy.com>
Reply-to: Discussion of Ten-Tec Equipment <tentec@contesting.com>
Date: Fri, 04 Jul 2008 15:37:31 -0700
List-post: <tentec@contesting.com">mailto:tentec@contesting.com>

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:


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