Dr. Slick wrote:
"Ian White, G3SEK" wrote in message
...


You would then have either:

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

1 2



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.



Nope. you are just changing where the reflections actually happen,
either before or after the SWR meter. In one case you measure it, in
another you don't. As simple as that.

No, it's even simpler. A good SWR meter only *senses* the conditions on
the line - it does not significantly disturb them. You seem to believe
that calibrating an SWR meter for a different Zo "changes where the
reflections actually happen" - it doesn't have to, and if at all
possible it should avoid doing so.

You're now going to claim that an SWR meter contains a section of
transmission line. In such cases you'd be right about the reflections,
but that is a feature of one particular type of meter, and one
particular type of construction - it isn't a fundamental requirement. As
a counter-example, the Bruene bridge meter is not based on a
transmission line, can be made as physically small and non-intrusive as
technology allows, and can be re-calibrated to any Zo by simply tweaking
a capacitor or a pot.

To repeat: a good SWR meter senses only the impedance at its *output*.
And the source impedance has no effect on the readings of |rho| or SWR.



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.


Oh REALLY? I'd like to see this program you have written. I doubt
you could give it to us

Oh come on, did you really imagine I was lying?

www.ifwtech.co.uk/g3sek/netcalc.htm
www.marsport.demon.co.uk/scalc.htm
www.marsport.demon.co.uk/sekpmk.htm

These are the ones that come immediately to mind that involve impedance
operations (there are also many others, in other areas). NETcalc is
entirely mine; Scalc is a collaboration; and the programs in the BASIC
software collection are sometimes mine, sometimes improved versions of
very ancient programs by other people. Even when someone else wrote the
original code, I've been 'under the hood' and take technical
responsibility for every line.

NETCALC in particular is a 'Swiss Army Knife' calculator for complex
impedances, which I wrote almost 20 years ago because I needed one.
There still isn't anything else that does quite the same job. It
includes Z0 transformations, and impedance transformation by loss-less
transmission lines, but it doesn't include line losses (never got around
to it) or complex Z0 (something I learned about here, from Reg).
However, if anyone wants to upgrade it...

The BASIC source code for all the programs is there to download (except
Scalc). Some of it isn't elegant, and all of it shows its age, but its
age also ensures that there aren't many technical errors - these
programs have been around for long enough to have been thoroughly
debugged by large numbers of users.

We haven't been maintaining these programs much in recent years, as
others like Reg and VE3ERP are now filling the small BASIC/DOS program
niche very well.


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

Ian, G3SEK wrote:
"You are now going to claim that an SWR meter contains a section of
transmission line."

At VHF where the wattmeter may affect SWR, it is customary to make the
meter plus its cable extension into 1/2-wave so that its insertion and
removal nave minimum effect.

Best regards, Richard Harrison, KB5WZI