On Jun 15, 6:36*pm, Roy Lewallen wrote:
Owen Duffy wrote:
lu6etj wrote in news:da3e5147-cad8-47f9-9784-
:

...
OK. Thank you very much. This clarify so much the issue to me. Please,
another question: On the same system-example, who does not agree with
the notion that the reflected power is never dissipated in Thevenin
Rs? (I am referring to habitual posters in these threads, of course)

Thevenin's theorem says nothing of what happens inside the source (eg
dissipation), or how the source may be implemented.
. . .

Cecil has used this fact as a convenient way of avoiding confrontation
with the illustrations given in my "food for thought" essays. However,
those models aren't claimed to be Thevenin equivalents of anything. They
are just simple models consisting of an ideal source and a perfect
resistance, as used in may circuit analysis textbooks to illustrate
basic electrical circuit operation. The dissipation in the resistance is
clearly not related to "reflected power", and the reflected power
"theories" being promoted here fail to explain the relationship between
the dissipation in the resistor and "reflected power". I contend that if
an analytical method fails to correctly predict the dissipation in such
a simple case, it can't be trusted to predict the dissipation in other
cases, and has underlying logical flaws. For all the fluff about
photons, optics, non-dissipative sources, and the like, I have yet to
see an equation that relates the dissipation in the resistance in one of
those painfully simple circuits to the "reflected power" in the
transmission line it's connected to.

Roy Lewallen, W7EL

obviously its not the 'reflected power'... that can be easily
disproved by showing that the length of the line changes the impedance
seen at the source terminals without changing the power that was
reflected from the load. since it is the impedance at the source
terminals that determines the performance of the amp the power that is
reflected is irrelevant.

however, it should be relatively easy to derive such a relation for a
simple thevenin source with a lossless line and a given load
impedance... just transform the impedance along the length of the line
back to the source then calculate the resulting current or voltage
from the source. that would give you the power dissipated in the
source. then you could also calculate the reflection coefficient and
separate the forward and reflected waves... and if you did it all
correctly and kept everything in terms of RL, Z0, and the length of
the line you could come up with a family of parametric curves relating
the power dissipated in the source resistance to the reflected power
over a range of load impedances for a given line length, or for
varying line length for a given load. obviously a purely academic
exercise that should be left for a rainy day.