Dr. Slick wrote:
"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,
When you talk about "the Z of the PA", you mean the output impedance.
When you talk about "the Z of the SWR meter", you mean the system
reference impedance Zo for which the meter is calibrated. These are two
completely different physical quantities. It is totally unrealistic to
talk about varying those two quantities together, keeping them
numerically equal like they were on some kind of twin-gang pot.
However, it's not completely beyond the reach of imagination, so let's
imagine for the sake of argument that somehow you could do it...
you will
absolutely measure a different SWR (you will measure the reflected
power at point 2), going into 50 Ohms believe it or not.
Yes, you will indeed see varying values of |rho|/SWR on the meter - but
the point you're missing is this: although your imaginary experiment
involves varying two different values called "Z" at the same time, only
*one* of them is having any effect!
The changes you see on the meter are *entirely* due to what you're doing
to the meter itself - changing the Zo for which the meter is calibrated.
Changing the output Z of the PA is having *no* effect on the values of
|rho|. It never does.
You can easily demonstrate this in Real Life (tm) by doing what hams do
every day - adjust the transmitter, which will change its output Z and
its output power; but don't tinker with the calibration Zo of the meter
(which is normally factory-adjusted for life). What you will find is
that the "forward" and "reflected" readings change, but their ratio
|rho| does not (within the limits of accuracy of the meter).
This is effectively the same experiment that I suggested this morning.
Please get
MIMP, it's a free program from Motorola.
Look, I've *written* programs like that, so I know exactly what's going
on under the hood. What we're witnessing here is a demonstration that
such programs can only give answers, not basic understanding. You have
created some construct that seems to explain the program output for you;
but it will not survive real-life experiments.
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.
No, the source Z will not affect the |rho|/SWR reading in *any* case.
Go try it - not on the computer, but on the bench.
--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek