Dr. Slick wrote:
"Ian White, G3SEK" wrote in message
...
Dr. Slick wrote:
As you might know, the input S11 or SWR will change when you go
from an antenna analyzer or network analyzer to measuring with the
actual full power PA and meter hooked up. This may be partly due to
the fact that the meter is usually not a perfect 50 ohm thru, and
partly due to the fact that the analyzers outputs are closer to 50 ohms
than the PA.

Sorry, that is exactly wrong. S11, SWR and the impedance itself, do
*not* change when you connect a different instrument to the same load.
All the changes you have described are due entirely to instrument
errors.

That's how the instrument errors are determined... by knowing for a fact
that, whatever all the different instruments may say, the impedance
they're trying to measure is the one thing that has *not* changed.


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

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).

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.

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.


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.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek