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

Now let's simplify it even further:
1. PA
2. Either SWR meter 1 or SWR meter 2.
3. 50 ohm load.



You would then have either:

PA--+SWR meter1+50 ohm dummy load
(same Z as PA)

1 2


In which case, this would be equivalent to re-normalizing the
Smith, and as you change the Z of both the PA and SWR meter, you will
absolutely measure a different SWR (you will measure the reflected
power at point 2), going into 50 Ohms believe it or not. Please get
MIMP, it's a free program from Motorola.


Or:

PA--+SWR meter1+50 ohm dummy load
(50 Ohm meter)

1 2


In which case, you would NOT measure the reflected power at point 1
(from the discontinuity from the PA to 50 ohms, because this is before
the meter.
This is the case you guys are talking about, WHICH I TOTALLY AGREE
WITH, as the source Z shouldn't affect the SWR meter in this case.

The point is that it depends on where you measure it!




As Pozar - and countless others - correctly state, rho is a function of
both the Zl that you're measuring, and the system reference impedance
Zo. You cannot either measure or calculate rho without first assuming a
value for Zo; not ever.

Since you have to choose a value of Zo *before* you can calibrate the
rho-meter, it's not surprising that two differently-calibrated meters
read differently when connected to the same physical load impedance.
It's also not surprising that "re-normalizing" the Zo, either on the
Smith chart or in a computer program, will predict exactly what *both*
meters should read. You only need to know Zl and the two assumed values
of Zo for which the respective meters were calibrated.


The point is: Zo doesn't have to be 50 ohms necessarily.


The same would be true even if the terminating impedance Zl was not 50
ohms as assumed above. The measured values would alter, but the ability
to predict the readings of both meters would not.

In that respect, you've only been stating the obvious all the time.


And i've been told i was wrong from the start! But this is typical
of a NG, isn't it.



The other part, the apparent belief that a rho- or "SWR" meter will
change its readings according to the actual, physical source impedance
from which it's being fed, remains simply incorrect. The meter is only
affected by the impedance it sees at the downstream (output) side.


agreed, but if the meter is the same Z as the PA, you will indeed
measure the discontinuity going to 50 Ohms.


Slick