RadioBanter - View Single Post - Length of Coax Affecting Incident Power to Meter?

August 13th 03, 06:23 PM

(Dr. Slick) wrote in message . com...

On second thought, i believe we are all wrong to equate S11 with
SWR!

(Of course they are not equal, just related by a formula [which is
usually stated slightly incorrectly])

If your reference impedance for measuring S11 is not the line
characteristic impedance, it's true that SWR = (1+|S11|)/(1-|S11)
won't give you the right answer. But then you haven't really measured
the reflection coefficient on the line, if you've used the wrong
reference impedance, since reflection coefficient is _defined_ as
reverse voltage divided by forward voltage. All this is right at the
heart of why I've been telling you that you should make sure your SWR
meter is calibrated to the impedance you think it is, and in
particular, to the line impedance (or close to that) if you really
want to know the SWR on the line. On the other hand, knowing measured
S11 and the reference impedance for it, and the line characteristic
impedance, you can determine the SWR on that line. But your "SWR"
meter isn't really an S11 meter; at best it's a |S11| meter.

This also brings up another point: do YOU define S11 to be the same
as reflection coefficient?

Cheers,
Tom