And another example in point:

From the input to a TV transmit antenna system, tx disconnected, I have
personally measured the far-end antenna system reflections of a 2T sin²
video pulse (0.25 µs H.A.D.) modulated onto a TV channel carrier, and
detected by a vestigial sideband demodulator tuned to that TV channel. A
high-directivity directional coupler at the input to the main line, a
display device, calibrated attenuators, and the time difference between the
incident and reflected pulse enable accurate measurement of the reflection
coefficient of the antenna system.

This was a common practice after a new antenna system installation to
measure and optimize the far-end match for the best quality radiated signal,
and was pioneered by RCA Broadcast Eqpt Div, my employer at the time. More
elegant means are used these days.

When this test shows a 5% pulse return 2 µs after the incident pulse time
(for example), then the same pulse passed through the tx also shows nearly
exactly the same reflection % and time separation -- assuming there is
enough RF delay in the system for the reflection to be resolved in the
demodulated waveform.

As the directional coupler driving the normal demodulator at the TV station
is looking at forward power only, it is clear that the reflection from the
far end of the antenna system has been re-reflected from the TV tx output
stage, and NOT absorbed by it in its "conjugate impedance."

RF

On Sun, 5 Sep 2004 05:14:46 -0500, (Richard
Harrison) wrote:

Mendenhall is wrong.

Hi Richard,

But this gets curiouser and curiouser (as Alice through the Looking
Glass would offer).

Cited as an example of the "NOT 50 Ohm" society (and one of its
leading proponents) we find that Geoff Mendenhall's notable
achievement in 1968 was building a 400W FM amplifier. Truly a
hands-on achievement. Now if we simply review the historical archive
and ask Geoff himself what the Z of his design was, we find by his own
hand:
RF Output Impedance: 50 Ohms

Let's see, no technical argument, and sources that are
self-contradicting. Whatchagonnado? Punt? :-)

73's
Richard Clark, KB7QHC

Richard Clark wrote
"Richard Fry" wrote:
if transmitters have a 50 ohm source impedance,

It seems you find controversy where there is none. :-)
I would again suggest you read what I wrote, and
point out what exactly your contention is with IT.
_________________

OK. Earlier you wrote, "To date in this matter, I have yet to see any
concrete value of source Z offered from those of the NOT 50 Ohms camp.
Further, I have yet to see any of them offer any experimental confirmation
of their assertion (made rather simple by the exhibition of uncertainty)."

Our controversy is illustrated by my posts with an opposite conclusion,
beginning last night and continuing this morning.

As for experimental evidence, I report some in my post here of a few minutes
ago about making refl coeff measurements of TV transmit antenna systems.

Mendenhall's paper also has experimental evidence of this. I will email it
to you.

I trust my contention is now clear to you.

RF

"Richard Clark" wrote
But this gets curiouser and curiouser (as Alice through the Looking
Glass would offer).

Cited as an example of the "NOT 50 Ohm" society (and one of its
leading proponents) we find that Geoff Mendenhall's notable
achievement in 1968 was building a 400W FM amplifier. Truly a
hands-on achievement. Now if we simply review the historical archive
and ask Geoff himself what the Z of his design was, we find by his own
hand:
RF Output Impedance: 50 Ohms

Let's see, no technical argument, and sources that are
self-contradicting. Whatchagonnado? Punt? :-)

73's
Richard Clark, KB7QHC

On Sun, 5 Sep 2004 11:05:24 -0500, "Richard Fry"
wrote:

I trust my contention is now clear to you.

Hi OM,

Actually no. Your reference, Mendenhall, specifically writes about
his design:
"It was necessary to determine the plate
load impedance (formula) = 1000 Ohms
where Emin min drop across the tube in saturation
I1 ac plate current.

"Since this Zp was to be coupled into a
Z output of 50 Ohm, a impedance transformation
of 20:1 was needed."

Fairly straightforward stuff there. Geoff's own notes are the model
of economy and directly to the nut of the issue. His notes are also
straight from old school first principles. As often happens, the more
elaborate the discussion of rather simple matters belies the inference
of hucksterism.

Seems like ALL my arguments, references, citations, data offered and
so on are congruent with Geoff's own description of amplifiers. I
certainly need no further testimony from him as I am perfectly capable
of finding his own work and offering it here. Thanx anyway, but no
thanx.

It would also serve you better to read more and write less, after all
I SPECIFICALLY mandated a discussion of transistor amateur amplifiers.
When I allowed this divergence to tubes, Mendenhall himself proved
that NOTHING changes in the physics of sources.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote
Now if we simply review the historical archive
and ask Geoff himself what the Z of his design was,
we find by his own hand:
RF Output Impedance: 50 Ohms

Let's see, no technical argument, and sources that are
self-contradicting. Whatchagonnado? Punt? :-)
________________

You assume he refers to the source impedance of/at output of the amplifier.
More likely he is following convention and stating the load impedance that
the amplifier was designed to work into.

The source impedance of most transmitters is not published even today. If
it was, probably we wouldn't be having all of this confusion about it, and
its effects.

RF

Brian Reay wrote:
"---dying to hear at what frequencies directional couplers suddenly
begin to exist."

It isn`t sudden.

They sure work at audio frequencies. In telephones, they are used to
prevent the user`s voice from overpowering the distant party`s voice in
the user`s ear. They are called hybrids.

Hybrids are also used to couple a 2-wire circuit which simultaneously
carries both directions of transmission with a 4-wire circuit consisting
of a transmit pair and a receive pair.

Best regards, Richard Harrison, KB5WZI

"Richard Clark" wrote
I SPECIFICALLY mandated a discussion of transistor
amateur amplifiers.
____________________

At least there appears to be an acknowledgement that some RF amplifiers do
not have a source impedance that is the conjugate of their load impedance.
So progress has been made.

RF

On Sun, 05 Sep 2004 16:21:27 GMT, Richard Clark
wrote:

On Sun, 5 Sep 2004 11:05:24 -0500, "Richard Fry"
wrote:

I trust my contention is now clear to you.

Hi OM,

Actually no. Your reference, Mendenhall, specifically writes about
his design:

Hi All,

I would add further, Mendenhall's notes of his design, as the model of
clarity, include references, one of which is particularly notable and
estimable within this group:
"Treman [sic], F.E.; "Electronic and Radio Engineering";
Mc Graw - Hill Book Co.; 1955"
the same publication I've had since the same date that Geoff built his
transmitter. Geoff's attachments also include the data sheets from
Eimac which show quite plainly that ALL of his formulas and
computations are congruent with ALL sources of information in his
references.

Another reference:
"Goodman, Byron (Ed.); 'The Radio Amateur's Handbook';
American Radio Relay League; Newton, Conn.;
1966"
(I used to have that publication, back then, too)

I also vaguely note some inference of peculiar intermodulation
products that would be produced by a transmitter with 50 Ohm output
characteristic - in that I may be mistaken because when the verbiage
gets particularly dense to explain simple matters, I must admit my own
filters kick in. However, Mendenhall's work was not simply that of an
amateur's project, nor was it a school term paper, nor was it the
speculation of an engineering sales pitch.

The report I am drawing upon was Geoff's own Type Acceptance
application to the FCC which included all the technical specifications
of spurs, intermodulation products, stability, efficiency (80%), class
of operation, modulation, out-of-band responses.... I don't think I
need go much further. :-)

For those who wish to read the COMPLETE story of how to build a rig,
how to specify it, how to measure it, and to note how it exactly
conforms to conventional wisdom; then visit:
http://www.techatl.com/wrek/docs/gnm_0011.htm
where you will find all of one page of theory, and 40 odd pages of
reality:
The WREK 425 Watt RF power amplifier, also known as the
"Goat-Mitter" was designed by Geoffrey N. Mendenhall (dubbed the
Goatman by WREK announcer, Ed Esserman) and constructed entirely
with hand tools by Geoff and the WREK staff in August of 1968.

73's
Richard Clark, KB7QHC

On Sun, 5 Sep 2004 11:25:02 -0500, "Richard Fry"
wrote:

You assume

Hi OM,

That is called a punt.

73's
Richard Clark, KB7QHC