Egad.

Given only a line's characteristic impedance and the load impedance, I
can tell you exactly what the SWR is for a lossless line. For a lossy
line, I only need to know, in addition, the line's length and the amount
of loss per unit length. In no case do I need to know the source impedance.

If, as you insist, the source impedance affects the SWR on the line,
please provide an equation that gives the SWR on the line, with source
impedance being one of the variables. It's such a simple thing, surely
such an equation appears in one of the several references you cite. I
did notice that SWR doesn't appear in any of the titles or the quoted
passages, though, so you may have to dig a little.

And if Cecil's work leads to the conclusion that the source impedance
impacts the line's SWR, then it's wrong.

It is, for those who are interested, very easy to see intuitively why
the source impedance doesn't affect the SWR. Consider the situation that
occurs when the source is first turned on. A forward voltage wave
travels down the line toward the load. A reflected wave, whose magnitude
and phase are determined by the reflection coefficient at the load end
of the line, returns. If we stop time just as the reflected wave is
returning, we can calculate the SWR, so far, on the line, solely from
the ratio of the forward and reflected waves -- it's the interference
between these waves that create the standing waves. Turning time back on
again, the returning wave reflects off the source (assuming a source
mismatch), producing another forward wave. Let's watch this wave as it
travels toward the load, reflects, and returns. Exactly the same
proportion of this wave is reflected as for the original forward wave.
So, when this new forward wave reflects and its reflected wave returns,
we've got a total of two forward waves and two reflected waves. The
forward/reflected ratio of the second pair is exactly the same as the
forward/reflected ratio of the first pair -- it's the reflection
coefficient at the load end. So the ratio of the total forward wave to
the total reverse wave is the same for the first pair, the second pair,
and the sum of the two pairs. In other words, the second pair of waves
hasn't changed the SWR from what we calculated from the original pair of
waves. You can continue this observation for each forward-reverse pair,
and see that the SWR never changes (at least when observed when each
reflected wave just returns) from the original value. And the original
value was determined only by the load mismatch, not the source. The
source mismatch determines how big the total forward and reflected waves
end up being when all the reflections have died out to a negligible
value. But it has nothing to do with the forward/reverse ratio, which
determines the SWR.

Roy Lewallen, W7EL

Richard Clark wrote:
On Tue, 12 Aug 2003 02:48:49 -0700, Roy Lewallen
wrote:


Almost correct.

The transmitter output impedance has no effect whatsoever on the line's SWR.

Roy Lewallen, W7EL



Hi Roy,

Entirely incorrect.

Transmitter output impedance that does not conform to transmission
line Z, when presented with a mismatched load through that line, adds
mismatch uncertainty in the form of an indeterminate SWR and
indeterminate Power to the load.

This has already been demonstrated twice. This has long been
documented with NBS/NIST references going back 4 decades. There is
nothing mysterious about it at all, and it conforms to the rather
simple principles of wave interference so poorly presented by Cecil in
months past.

The authoritative site:
http://www.boulder.nist.gov/div813/index.html

Direct reference:
"Juroshek, J. R.; A Direct Calibration Method for Measuring
Equivalent Source Mismatch; Microwave J., pp. 106-118;
October 1997

Obscure references:
http://www.boulder.nist.gov/div813/r...00S_n2nNet.pdf
"With vector measurements of the generator and meter reflection
coefficients Ãg and Ãm, respectively, the power of the incident
signal am can be related to the power of the source."

http://www.boulder.nist.gov/div813/r...FRad_ARFTG.pdf
which describes radiometer calibration (perhaps too exotic for this
group)
"tests are based on two assumptions. First, the network responds
linearly to our signal ( no power compression), and second, the
radiometer is sufficiently isolated from the source impedance."
...
"One of the assumptions made in deriving eq. (2) was that the
output from the radiometer is not dependent on the source
impedance. In the construction of the radiometer, two isolators
are inserted at the input of the radiometer to isolate the
radiometer from the source."
...
"The mismatch uncertainty depends strongly on the poorly known
correlation between uncertainties in the measurements of different
reflection coefficients, and so we use the maximum of the
uncertainties obtained by assuming either complete correlation or
no correlation whatsoever."

"Forthcoming Paper: Influence of Impedance Mismatch Effects on
Measurements of Unloaded Q Factors of Transmission Mode Dielectric
Resonators"
IEEE Transaction on Applied Superconductivity

"Analysis of Interconnection Network and Mismatch in the
Nose-to-Nose Calibration
Automatic RF Techniques Group , June 15-16, 2000 , Boston, MA -
June 01, 2000
"We analyze the input networks of the samplers used in the
nose-to-nose calibration method. Our model demonstrates that the
required input network conditions are satisfied in this method and
shows the interconnection errors are limited to measurement
uncertainties of input reflection coefficients and adapter
S-parameters utilized during the calibration procedure. Further,
the input network model fully includes the effects of mismatch
reflections, and we use the model to reconcile nose-to-nose
waveform correction methods with traditional signal power
measurement techniques."

As I mentioned, obscure references. However, given the impetus of
their discussion is long known (and that I have already provided the
original references they rely on), NIST presumes the investigators
already have that basis of knowledge.

73's
Richard Clark, KB7QHC