Ok, I'll try once more.

Look again at what I wrote earlier:

No, you won't see a change in the SWR at point 1 as you change the PA
impedance. All the fiddling you do with your Smith chart just won't
make
it happen. Sorry.

The SWR, voltage, current, impedance, power, reflection coefficient,
waves, or anything else don't change in response to your Smith chart
exercises.


To which you replied:

Absolutely incorrect. Time for you to review your Smith Chart
again.

Here's an experiment for you to try.

On your workbench, measure the characteristic impedance of a cable, or
connect it to a source or load and measure the SWR on it. Now go over to
your desk, take out your grease pencil, and change the reference
impedance of your Smith chart. Go back to the bench, check the SWR and
the cable Z0. Has it changed?

If the answer is yes, I'll admit having a serious shortcoming in my
knowledge of the power of the Smith chart.

If *you'll* review the Smith chart again, you'll find that *if* you set
the reference impedance to the Z0 of the transmission line you're
analyzing, then the SWR, impedances, and so forth that your read from
the Smith chart are correct. If you set the reference impedance to some
other value, an SWR read from the chart certainly isn't the SWR on the
transmission line.

Z0 in the equation you quote refers to the characteristic impedance of a
transmission line. If you change the impedance of the transmission line,
given the same load impedance Zl, you change the SWR, as the equation
indicates. (Changing the reflection coefficient changes the SWR on the
line.) But changing the reference value on your Smith chart doesn't
change the characteristic impedance of the cable.

Now let's see what you wrote this time:

Dr. Slick wrote:
. . .
When you change Zo, you change the normalized center of the
Smith, and therefore the Ref. Coeff. and SWR, looking into the same
load.

Yes. When you change the line's Z0, you should change the normalization
of the Smith chart. But changing the normalization of the Smith chart
doesn't magically change the cable dimensions to give it a
correspondingly new Z0. (Or does it? Something I'm missing here?)

The Smith Chart is an extremely powerful graphical RF tool, which
has become part of the basis for communicating in the RF world, as
well as a standard for displaying impedance on most RF measuring
devices.

You need to read up on this if you want to understand me.

Thank you, although I'm not an expert at using the Smith chart, I know
my way around the circle. But perhaps more study would reveal the
mechanism by which changing the chart reference causes spontaneous
redimensioning of the cable.

Even when the other person is correct too? I think it IS an ego
trip if you can't admit someone has a point, and are too scared to
discuss it further for fear of looking weak or exposing a lack of
knowledge about something.

All right, I was scared to admit that you have a point, that all you
have to do to change a line's SWR is to go over to the desk and change
your Smith chart. But I've overcome my fear now, and am exposing my
ignorance, or weakness as you say, for all to see.

I'd like to learn more about the mechanism, though. When you renormalize
your Smith chart, is the line's Z0 changed by a spontaneous change in
inner conductor diameter, outer conductor diameter, dielectric
permittivity, or some combination? Is it some sort of telekinesis, or
perhaps something to do with Chi? I never could quite get the hang of
Feng Shui, so maybe that's it. I can't find any reference to the
phenomenon you're claiming in my engineering texts -- perhaps an occult
or New Age bookstore would be more fruitful?

From Pozar's Microwave Engineering:

Reflection Coefficient = (Zl-Zo)/(Zl+Zo)

Where Zl is a purely real load impedance, and Zo is the
purely real characteristic impedance reference.

When you change Zo, you change the normalized center of the
Smith, and therefore the Ref. Coeff. and SWR, looking into the same
load.

Almost right, but a misplaced "therefore". The reflection coefficient
and SWR don't change because you change the Smith chart normalization.
They change because you've changed the cable's Z0. If you know how to
use a Smith chart correctly, you can then go and renormalize your Smith
chart to the new Z0 (as you've said), and from it you can read what the
new reflection coefficient and SWR are. Or you can forget the Smith
chart altogether and measure them, or calculate them from an equation.
But if you normalize your Smith chart to some value other than the
cable's Z0, you're no longer reading the transmission line impedances
and SWR from it.

It sure looks to me like you're confusing reality with what you read
from your arbitrarily normalized Smith chart. They're not the same. Just
like the reading on an SWR meter isn't the same as the SWR on a
transmission line of a different impedance. And actually for exactly the
same reason. Hm, maybe that provides even one more way to say it. Set up
your source, cable, and SWR meter like you have it in your earlier
posting. Replace the 50 ohm SWR meter with a 75 ohm SWR meter.
Renormalize your Smith chart for 75 ohms. Presto! The SWR meter reads
the same as the Smith chart! But y'know what? YOU DIDN'T CHANGE THE SWR
ON THE TRANSMISSION LINE. Put the 50 ohm SWR meter back in just on the
load side of the 75 ohm one. It reads just the same as it did before,
and it's reading the actual SWR on the 50 ohm line between it and the load.

Again, changing the Smith chart's normalization does not change a
cable's Z0 or SWR. Feng Shui and voodoo notwithstanding.

A Smith chart is simply (not to disparage in any way its ingeniousness
or utility) a polar plot of reflection coefficient on a special scale.
(The trick is, of course, generating the scale, an exercise in conformal
mapping I recall doing in fields class.) Check it out -- measure the
length of the vector from the center to any impedance point (with the
chart radius equalling one), and the angle from the main axis (zero ohms
being the positive direction), and you'll see you have the reflection
coefficient.

I can't figure out any more ways to say this.

I am always interested in learning. I don't use a Smith chart a great
deal, but I know Tom Bruhns, a regular poster here, does. I'll gladly
defer to him on issues of Smith chart use, and hope he'll feel free to
correct me on any errors I've made in the above discussion -- as he has
a number of times in the past, and which I've greatly appreciated. Walt
Maxwell, an occasional poster, is truly a Smith chart expert, and I'd
also welcome any corrections or amplifications he'd care to make. Or any
other knowledgeable Smith chart user.

Roy Lewallen, W7EL