"Ian White, G3SEK" wrote in message ...

Input S11 of a system will certainly never change. But the SWR
is absolutely dependant on the source impedance.

No! SWR, S11, return loss, rho, Y-parameters, Z-parameters, etc, etc are
all different derived functions of the same two variables: an arbitrary
complex impedance, and the system reference impedance Z0 (a constant
which may or may not be defined as complex).


Correct, but it doesn't have to be 50 Ohms.


Only those two variables are involved, so all of these functions are
locked together. If one variable changes, all the derived functions
change too. Either all change, or none change; nothing else is logically
possible.


How about a 50 Ohm resistor, which is always 50 Ohms (impedance
doesn't change), fed with 20 ohms? Or 75?

Cecil is correct in saying that the SWR meter would then have to
be designed for 20 or 75 ohms, but that is beside the point.

SWR doesn't have to be strictly 50 ohms, and will involve TWO
impedances. If your source doesn't match your reference impedance
(normalized center of Smith Chart), then you won't be measuring the
reflected power coming right off the source.

And because most PA are not 50 ohms output, and most SWR meters
are 50 Ohms, there is problem.


As Roy says, the equations relating any one of these parameters to any
other are all well known. NONE of them ever involves source impedance.




Assuming the source impedance is 50 ohms, which it usually isn't
with most PAs.



If you had a network
analyzer calibrated for 20 Ohms, you would certainly have reflected
power and high VSWR going into 50 Ohms, and a 1:1 SWR going into 20
Ohms.

This would be the same as re-normalizing the Smith Chart for 20
Ohms in the center. You certainly can do this in MIMP.

I don't blame anyone for believing it's a 50-Ohm-only world!

No argument about any of that... but it's a totally separate point that
has no relevance whatever to your earlier statements about source
impedance.


it's very relevant if you consider the port on a network analyzer
to be 50 ohms or not... It should be, but your PA may be quite far
off.


Slick