Richard Fry wrote:
"Ian White, G3SEK"wrote:
Richard Fry wrote:
"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.

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.

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
____________

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

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.

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

However, when you have finally done it your way, and accounted correctly
for all the reflections and re-reflections, we can predict the outcome
with complete confidence:

1. If you sum the successive reflections correctly to infinity, and
calculate the V/I ratio and phase at the station end of the line, then
the final result will be identical to the impedance given by the
steady-state transmission-line theory. It has to be, because that single
value is the reality.

2. Somewhere in your calculations, any value that you assume for the RF
source impedance is going to cancel right out of your calculations. The
correct mathematical result *must* be independent of that value -
because, again, that's the reality.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek