lu6etj wrote in
:

....
For example: do you (*) recognize Roy Lewallen late example in "Food
for tought" assuming (or conceding that) as not representing a real
rig but a simple constant voltage source in series with a resistor, at
least? to give some credit to his ideas Until now, I could not
know...

In that article, Roy says "My commercial amateur HF transceiver is
probably typical of modern rigs in that it produces a constant forward
average power into varying load impedances—provided the impedance isn’t
extreme enough to cause the rig to severely cut back its power output."

Assuming that "constant forward average power" means 'as would be
indicated on a directional wattmeter calibrated for Z=50+j0', if that is
true for all load impedances that Pf is constant (within limits), then it
is evidence that Zs=50+j0 (within those limits).

He goes on to say "It turns out that a linear model of my transmitter
(without a transmatch) over its non-shutdown range is very simple—it’s
just a voltage source in series with a resistance." Subject to the
conditions I stated in the previous paragraph, that is correct, but that
whilst that model can be used to determine behaviour of the external
load, there are limits to the inferences that can be drawn about the
internals of the transmitter, including as mentioned in earlier posts,
internal dissipation and efficiency. Roy acknowledges that in the next
paragraph. Only a mischief maker would represent Roy as meaning
otherwise.

From my own experience, I don't agree that HF ham rigs typically produce
constant Pf into varying loads. Walt's transmitter measurements that we
are discussing do not show constant Pf, though the change is fairly
small. But this is a practical measurement project for yourself, don't be
put off by the attempts to discredit measurements with anything but
traceable calibration.

(*) I do not know how clearly denote plural in "do you"

I am not the expert that others are on English language, and we speak a
version of English closer to the English here... but "you" is plural and
singular (but if followed by a verb, it is treated as plural eg "you are
correct"), and you could say to a group "do you agree", though some
people may say "do you all agree" or "do y'all agree", though those might
be seen as asking each member of the group rather than collectively.

Owen