On Sun, 17 Aug 2003 07:25:45 -0500, W5DXP
wrote:

Richard Clark wrote:
Thank you. I see that for the 50 Ohm load that the line exhibits a
characteristic of 50 Ohms as described by its concentricity about the
reference.

This is no surprise given the cable is premium short run (length, not
reels) material specified out to 20 GHz (from Boeing's precision
electronics facilities). I have several hundred feet brand new.

Please plot your data on a Smith Chart. Since your data is
obviously nonlinear, I would suggest diagnosing the nonlinearity
problem before concluding anything. For instance, what is the
one tenth scale RF RMS voltage before it is rectified by the
diode?


Hi Cecil,

That is the whole point of Mismatch Uncertainty. What you call
nonlinearity is the NBS documented conclusion of the mismatched
conditions at both ends.

Feel free to investigate and enquire further about your concern over
nonlinearity.

73's
Richard Clark, KB7QHC

Roy Lewallen wrote in message ...

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?


Do you often change the impedances of your equipment by scribbling
on a pad of paper? Maybe you can tell us how you do this.



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.



PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy
load
1 2 3



Once again, if you placed an SWR meter of the same output
impedance of the PA at point 1, you will measure the SWR at point 1,
and it would be akin to re-normalizing the Smith. Change the PAs S22
(and the impedance of the SWR meter, and you will definitely change
the SWR at point 1. Do you agree with this?

I've already agreed with you that the SWR at point 3 won't change
in theory, as it is surrounded by 50 ohms. What is it about this
simple concept that you cannot seem to understand?




Thank you, although I'm not an expert at using the Smith chart, I know
my way around the circle.


It's obvious you don't. Time to read up on it, Roy.



I am always interested in learning. I don't use a Smith chart a great
deal,


Obviously.


Slick

Dr. Slick wrote:
Roy Lewallen wrote in message ...

. . .

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.




PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy
load
1 2 3



Once again, if you placed an SWR meter of the same output
impedance of the PA at point 1, you will measure the SWR at point 1,
and it would be akin to re-normalizing the Smith. Change the PAs S22
(and the impedance of the SWR meter, and you will definitely change
the SWR at point 1. Do you agree with this?

Absolutely not. The SWR meter will measure the SWR at point 1 only if
it's the same impedance as the *line*. If the PA has a different
impedance than the line, and the SWR meter has the same impedance as the
PA, it will not be reading the SWR on the line.

If you want to use a Smith chart to see what the SWR is on the line, you
must normalize it to the impedance of the line. Change the PA impedance
all you want. Change the Smith chart normalization all you want. Neither
will have any affect on the SWR on the line. Changing the meter
impedance won't have any effect on the SWR on the line, either, although
it'll change the reading on the meter so that it's no longer indicating
the SWR on the line.

I've already agreed with you that the SWR at point 3 won't change
in theory, as it is surrounded by 50 ohms. What is it about this
simple concept that you cannot seem to understand?

?

Thank you, although I'm not an expert at using the Smith chart, I know
my way around the circle.



It's obvious you don't. Time to read up on it, Roy.



I am always interested in learning. I don't use a Smith chart a great
deal,



Obviously.

I see you're reduced to insults now. Time for me to really leave this
thread. I'll leave you with your conception of how things work, hoping
only that the other readers aren't left with them also.

Roy Lewallen, W7EL

Dr. Slick wrote:

PA----+----50 ohm line----+SWR meter+----50 ohm line----+50 ohm dummy
load
1 2 3



Once again, if you placed an SWR meter of the same output impedance
of the PA at point 1, you will measure the SWR at point 1, and it would
be akin to re-normalizing the Smith. Change the PAs S22 (and the
impedance of the SWR meter, and you will definitely change the SWR at
point 1. Do you agree with this?

What exactly do you think you mean by "change the SWR at point 1"? All
you have done is to replace one SWR meter by a second one that has been
calibrated for a different system reference impedance.

You had to *choose* a value for the new reference impedance in order to
pre-calibrate that second meter. When you re-normalize the Smith chart,
you choose the *same* new value. So of course the SWR reading on the
second meter is going to be the same as you'd calculate by
re-normalizing the SWR1 circle on the Smith chart from Z1 to Z2.

If that's all you ever meant, we've spent a week homing-in on something
that is totally obvious.

The only other problem with your example is that by mentioning the PA's
S22 value, you seem to imply that it is relevant. It isn't, because
variations in the source impedance will not change the SWR reading on
either meter. Only the load impedance can change the SWR.

Now take away all the coax and move the 50-ohm load to the outputs of
the respective SWR meters. The indicated values of SWR1 and SWR2 will be
exactly the same as before.



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

Richard Clark wrote:
That is the whole point of Mismatch Uncertainty. What you call
nonlinearity is the NBS documented conclusion of the mismatched
conditions at both ends.

I suspect many of the irregularities would be greatly diminished
if you ran the experiment at 50 watts forward instead of 5.
--
73, Cecil http://www.qsl.net/w5dxp



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On Sun, 17 Aug 2003 15:01:44 -0500, W5DXP
wrote:

Richard Clark wrote:
Feel free to attempt it. You are one of the more qualified to do this
by having a variable transmission line like mine.

Unfortunately, my variable transmission line is Z0 = 388 ohms and I
wouldn't trust my homemade 388 ohm SWR meter for wattmeter accuracy.
The lengths of my T-line sections are also not super accurate.

Let me get straight what you are saying. As I understand it, you are
saying that if you chop off one foot of coax at a time when driving
a fixed amount of forward power into a 3:1 mismatched load, the SWR
will change from, e.g. 3.8:1 to 1.9:1. Is that correct?

Hi Cecil,

Your trepidation about the accuracy is well taken, but perhaps
overstated for a first pass approximation. I will leave you to judge
that, but I would point out, that if you can conspire to load both
ends to mismatch as I have described (or worse), you should see the
same symptoms. That your lines do not fall on cardinal points; I have
faith in that your craftsmanship is of high enough caliber that it
won't make any difference if you can replicate the same wavelength
interval (which means you will probably need to work this out above 20
MHz). Error is not inescapable, but your regression analysis should
reveal the same trend.

If you in fact do not know the characteristic source Z of your
transmitter, then forcing a known value imparts a basic maximum. In
other words, what ever unknown value in parallel with a known value
could never exceed that known value. A simple 50 Ohm load at the
transmitter output guarantees that the source could never exhibit any
higher value. This feeding your 388 Ohm line is by far and away more
severe than any of the reports or tests I have offered. With the far
end terminated in another 50 Ohm load, then you should see extremely
wide variation. Your regression analysis should reveal the same
pattern as you found with my data (what you call nonlinearity). You
may then compare your regressions' chi-squares.

Or simply measure the line loss in your standard configuration and
compare it to the severely mismatched arrangement I describe above
(both ends terminated in 50 Ohm loads).

Accuracy demands a lot of front work, but a first pass test such as I
describe should present a quick resolution to whether or not you want
to commit beyond to engage in that labor. The worst of this is how
much you can (or your transmitter can) tolerate working into these
deliberate mismatches.

73's
Richard Clark, KB7QHC

"W5DXP" wrote in message
...
How about looking at the voltage across the resistor? i.e. the difference
between the voltages at each end of the resistor?
--
73, Cecil http://www.qsl.net/w5dxp
That is what I mean, By looking at both ends at the same time with a dual
trace scope. I guess I could also go A - B. I have the stuff to do it with.
About 60 feet of 9913 that I can short the far end on, a dual trace 100 MHz
scope, and MFJ analizer as a signal source. The latter is also usefol for
finding exact 1/4 wavelength frequency, as I can't measure the coax length.
( Goes through several walls).

Tam/WB2TT

Richard Clark wrote:
Accuracy demands a lot of front work, but a first pass test such as I
describe should present a quick resolution to whether or not you want
to commit beyond to engage in that labor. The worst of this is how
much you can (or your transmitter can) tolerate working into these
deliberate mismatches.

My SGC-500 is speced to work into a 50 ohm SWR of 6:1.
--
73, Cecil http://www.qsl.net/w5dxp



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Richard,
The frequency is going to end up around 4 MHz, which is not too bad for ALT.
We will see what happens. I have a pair of 10X probes, which I can match. I
guess we both know I am going to end up measuring the loss of the coax

Tam/WB2TT

Roy Lewallen wrote in message ...

let me rephrase:

PA----+SWR meter1+----50 ohm line----+SWR meter2+----50 ohm
line----+50 ohm
(50 Ohms)
dummy

load
1 2 3


If you measure the SWR at point 1, you let Z1 be 50 ohms, and Zo
is the output impedance of the PA. SWR meter1 should be characterized
to Zo.

I've agreed that SWR meter2s SWR reading will not change as you
change PAs impedance, but the SWR at point 1 definitely will.

So, Does the SWR change as you change the source impedance? IT
DEPENDS ON WHERE YOU MEASURE IT!

From Pozar's Microwave Engineering (Pg. 606):

Reflection Coefficient looking into load = (Zl-Zo)/(Zl+Zo)

It should be easy to see from this well known equation that a
perfect 50 Ohm Z1 will definitely change SWR as you change Zo away
from 50 ohms.


Slick


BTW Roy, comparing someone to the intelligence of your adolescent
kid who waits for trains because you told him you are an engineer is
indeed an insult.