Pardon the extensive pasting, but it will allow a clearer post, and save a
lot of click time. / RF

"Ian White, G3SEK" wrote :
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.

Richard Fry wrote:
Not that I said it did in my part of the thread, but nevertheless the
above statement is not strictly true. In the case where the source Z
of the tx PA does not match its load Z (which is typical), power
reflected from the load mismatch will at least partly be re-reflected
from the PA -- which then contributes to the power sensed by a
"wattmeter" in the output path.

"Ian White, G3SEK" wrote :
Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line
terminated by some arbitrary load at the far end) would depend
on the internal impedance of the device that's doing the
measuring - and that is not true, either in transmission-line
theory or in the real world. It is a function only of the line and
the load. etc

Richard Fry wrote:
How, then, do you explain the "ghost image" that can occur* in
analog(ue) TV transmission systems arising from reflections
at/near the antenna end of the station's transmission line?

*with sufficient round-trip propagation time in the transmission line

Ian White wrote:
Yes, that is a true observation, just as true as the one I made... so
now you have *two* different things to explain!

The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

This reads to me as though you know they are there, but choose
to ignore them...?

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere.

We agree on the subject of conflict resolution, but apparently not on
the location of the error.

If the multiple-reflection theory is extrapolated to infinite time, so
that it calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state
theory can be used, it will always get you there much more quickly.

This is true only to the extent that all the power ever generated by the
transmitter eventually either is radiated by the antenna or is dissipated by
losses somewhere.

For simplicity, let's assume a tx with a source impedance of zero ohms feeds
a lossless transmission line of uniform impedance throughout its length to a
mismatch at the far end. The mismatch reflects a percentage of the incident
power back down the line to the tx, and continues to do so as long as the
transmitter generates power. The tx will re-reflect the reflected power
back to the far end -- in this case all of the reflected power it ever sees,
in fact. To this easily-seen, real-world reality you agreed above ("Yes,
that is a true observation, ...").

The re-reflections combine with the power generated by the tx at that
instant to create a vector sum at the sample point used by the meter. The
typical tx meter is a frequency-domain device, and cannot by itself separate
the RF output of the transmitter from re-/reflections of it. That requires
a time-domain device. So the magnitude of the transmission line samples
driving the tx RF metering circuits during normal operation under these
conditions become a function of both the source impedance and the
load impedance.

The defense rests.

RF