Owen Duffy wrote:
"Jeff" wrote in
e.com:

If you used a TDR, for example, to look at the set-up you would see 2
points of discontinuity, firstly at the 100 ohm source to 50 ohm cable
interface, and secondly at the cable to 200 ohm load. BOTH of these
discontinuities add to the overall mismatch as seen by the 100 ohm
load.

Your TDR does not work in the steady state frequency domain space, and is
misleading you.

In the steady state, the (complex) ratio of forward voltage to reflected
voltage is determined solely by the load impedance and characteristic
impedance of the line.

In crude terms, during establishement of steady state, you can view that
a load end reflected wave which is then partially reflected at a
mismatched source end, will reach the load end and be reflected in the
same ratio as the earlier passes. The subsequent round trips as steady
state is approached do not change the (complex) ratio of forward voltage
to reflected voltage in the steady state.

I know you have support here for the assertion that source end mismatch
affects VSWR in the steady state, but you won't find it in reputable text
books.

Owen and Cecil are right: the source (transmitter) has no effect
whatever on the VSWR on the line.

That isn't just an assertion - it is part of the bedrock transmission
line theory. Owen referred to "reputable textbooks", one of which would
surely be 'Theory and Problems of Transmission Lines' by R A Chipman
[1]. This book gains a lot of its reputation from its very complete
mathematical development of the theory, showing all the detailed
working.

Chipman treats standing wave patterns in two different ways: first by
assuming the final steady-state conditions, and then in much more detail
by considering multiple reflections between the load and the source.
Given a sufficient number of reflections, the multiple-reflection model
converges on exactly the same results as the steady-state analysis -
just as it does in the physical world.

VSWR on the line is determined by the ratio |Vmax|/|Vmin|. The complex
impedance that the source sees at the input terminals of the line is the
ratio V/I at that point (where V and I are both vector quantities which
include phase information). An alternative way of calculating either
VSWR or Zin is through the ratio Vforward/Vreflected (again vector
quantities).

All of these approaches are alternative pathways through the same body
of theory. They are all consistent with one another, and there is no
contradiction between any of them.

You will notice that all these standing wave relationships involve
ratios. Chipman's detailed analysis confirms that these ratios are
determined EXCLUSIVELY by the properties of the line and the load -
never the source.

The source properties do determine the magnitudes of all of the
individual voltages and currents - but when you change the source
properties (output voltage and/or impedance) all the individual voltages
and currents on the line and at the load are changed by the same factor.
So when you take the ratio, the source properties cancel right out
again.

All this confirms that, if you sweat out the math in all the different
levels of detail that Chipman did, the source (transmitter) still has no
effect whatever on the VSWR on the line.




[1] Out of print, but well worth searching for: ISBN 0-07-010747-5.
The web bookstores currently have eight copies on offer, at a range of
prices.



--

73 from Ian GM3SEK