(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

Tom Bruhns wrote:
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.

That's true for a one-port load but not usually true for a two-port
impedance discontinuity in the transmission line.

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

S11 is the (physical) reflection coefficient when a2 equals zero. When
a2 is not zero, the physical reflection coefficient, S11, will not usually
equal the measured reflection coefficient, the square root of Pref/Pfwd.
--
73, Cecil http://www.qsl.net/w5dxp



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(Tom Bruhns) wrote in message om...
(Dr. Slick) wrote in message . com...

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.


Of course. We have no phase information. Only the magnitude of
the voltage reflection coefficient.


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

Cheers,
Tom


I define the S11 as the complex impedance, which never changes,
but S11 can also refer to the magnitude of the reflection coefficient
at a particular phase, AT A PARTICULAR NORMALIZED REFERENCE IMPEDANCE.
If you define the reflection coefficient as having phase
information, then yes, they are the same, but only at a particular
reference impedance.

Change the reference impedance, and you will have a new reflection
coefficient, but the complex impedance will still be measured to be
the same.

You can do this is Motorola's Impedance Matching Program (MIMP).

What this all comes down to is that your 50 Ohm SWR meters only
measure reflected powers after it, not before, so any mismatch from PA
to reference impedance (50 ohm coax from PA to meter) is not measured.


Slick