On Tue, 20 Mar 2007 22:57:48 GMT, Walter Maxwell wrote:

On Tue, 20 Mar 2007 22:06:35 GMT, Owen Duffy wrote:

I don't recall seeing experimental
results to convincingly demonstrate that the PA is a linear source,
though I have seen those that suggest otherwise. If the source cannot be
proven to be sufficiently close to a linear source, then the basis for
arguing the implicit conjugate match dissolves.

Owen, despite our previous discussion, I have explained many times that even though the PA source upstream of
the tank circuit is non-linear (and no one's saying it isn't), the energy storage in the tank makes the output
of the tank a linear source, no matter what the shape of the current wave form may be at the input. The output
of the tank is proved linear because the voltage/current ratio at the output is non-varying and the shape of
the voltage and current wave forms are essentially sine waves. Consequently, the output circuit can be
represented by a Thevenin source that supports both a conjugate match and the maximum power transfer theorem.

Are you now denying that the output of a PA with the routine Q of 10 to 12 is not substantially a sine wave?
If you agree that it is a sine wave, then why are you arguing that there is no basis for a conjugate match?

However, none of the responses above respond to the issue of why the reflected power does not cause heating of
the amp, which is what my treatise was all about.

Walt, W2DU

In the fourth line in the first paragraph above the word 'time' was inadvertantly omitted. It should have read
....the output is non-time varying and the shape...

Sorry about that,

Walt, W2DU

"Owen Duffy"
No one has demonstrated that using equivalent impedances etc
is not a valid analysis of the steady state behaviour.
_________

A reflection is a reflection. The reflection of a ~steady-state r-f source
may produce a different perceived/effective result than if that source
includes transients (modulation), but such does not negate the existence of
reverse/reflected power in the steady-state case.

Decades of experience with analog broadcast TV transmission systems
demonstrate that the reflected power from a mismatch at the transmit antenna
produces an amplitude variation (ripple) and other effects across the r-f
and demodulated video channel bandwidths that are directly related to the
magnitude of the antenna mismatch and the round-trip propagation time of the
transmission line between the tx and the antenna (period = 1 cycle per ~491
feet of air-dielectric line).

This is evident not only from accurate measurements made via a highly
directional r-f coupler sampling forward power at the tx end of the
transmission line, but also from results seen on the screen of TV sets
viewing those transmissions. I suspect, Owen, that you would agree that
this example originates from a "practical" system.

The r-f power supplied even by a CW source is subject to the same amount of
reflected power for a given antenna mismatch, which will have an appropriate
effect on system performance. Whether or not that reflected
power/performance effect is important (or even recognized as existent) is
the issue at hand.

RF

"Walter Maxwell" wrote
... I have explained many times that even though the PA source
upstream of the tank circuit is non-linear (and no one's saying it isn't),
the energy storage in the tank makes the output of the tank a linear
source, no matter what the shape of the current wave form may be
at the input. The output of the tank is proved linear because the
voltage/current ratio at the output is non-varying and the shape of
the voltage and current wave forms are essentially sine waves.
Consequently, the output circuit can be represented by a Thevenin
source that supports both a conjugate match and the maximum
power transfer theorem.
______________

If this statement about the tank circuit being ~ a linear source is valid,
does that mean that any load-reflected power that appears across the output
terminals of the tx stops at the tank circuit, and never sees the
non-linear, non-matching Z of the active PA?

And if so, would that also mean that such a tx would not be prone to
producing r-f intermodulation components when external signals are fed back
into the tx from co-sited r-f systems?

Yet experience shows that this is not the case for ~closely spaced
interfering signals. The only mitigation for this for a PA with a tank
circuit is the rejection of that tank circuit to those off-freq, external
signals, and to the resulting IM products generated by mixing with the main
tx signal in the active (and non-linear) PA stage of that tx.

And the tank has VERY low rejection to load reflections of the signal
bandwidth to which it is tuned.

Also to be considered are the modern broadband (88-108MHz) FM broadcast
transmitters, which have no tank circuits, but except for some designs
incorporating balanced 3 dB hybrid combiners are affected by load
reflections about the same as a tx with a tuned tank circuit.

RF

"Richard Fry wrote
(period = 1 cycle per ~491
feet of air-dielectric line).
_____________________

Sorry, make that 1 cycle == per MHz of bandwidth ==,
per ~491 feet of transmission line.

RF

On Tue, 20 Mar 2007 19:19:26 -0500, "Richard Fry" wrote:

"Walter Maxwell" wrote
... I have explained many times that even though the PA source
upstream of the tank circuit is non-linear (and no one's saying it isn't),
the energy storage in the tank makes the output of the tank a linear
source, no matter what the shape of the current wave form may be
at the input. The output of the tank is proved linear because the
voltage/current ratio at the output is non-varying and the shape of
the voltage and current wave forms are essentially sine waves.
Consequently, the output circuit can be represented by a Thevenin
source that supports both a conjugate match and the maximum
power transfer theorem.
______________

If this statement about the tank circuit being ~ a linear source is valid,
does that mean that any load-reflected power that appears across the output
terminals of the tx stops at the tank circuit, and never sees the
non-linear, non-matching Z of the active PA?

Richard, my earlier treatise considers only tube-type PA's with pi-network output coupling circuits used in
the Amateur Service, such as the Kenwood TS-830S on which my measurements were made. It was not intended to
consider PA's used in the tv service. Sorry, I didn't make this distinction earlier.

And if so, would that also mean that such a tx would not be prone to
producing r-f intermodulation components when external signals are fed back
into the tx from co-sited r-f systems?

This issue is irrelevant, because the signals arriving from a co-sited system would not be coherent with the
local source signals, while load-reflected signals are coherent. The destructive and constructive interference
that occurs at the output of a correctly loaded and tuned PA requires coherence of the source and reflected
waves to achieve the total re-reflection of the reflected waves back into the direction toward the load.

Yet experience shows that this is not the case for ~closely spaced
interfering signals. The only mitigation for this for a PA with a tank
circuit is the rejection of that tank circuit to those off-freq, external
signals, and to the resulting IM products generated by mixing with the main
tx signal in the active (and non-linear) PA stage of that tx.

Again, Richard, this condition is irrelevant to the re-reflection of the waves reflected by the load, because
the relevant signals are not coherent.

And the tank has VERY low rejection to load reflections of the signal
bandwidth to which it is tuned.

This may be true for PAs with bandwidths wider than those occurring in ham tx. However, the destructive and
constructive interference between the reflected and source waves in a correctly loaded and tuned ham tx
results in total re-reflection of the reflected waves.

Also to be considered are the modern broadband (88-108MHz) FM broadcast
transmitters, which have no tank circuits, but except for some designs
incorporating balanced 3 dB hybrid combiners are affected by load
reflections about the same as a tx with a tuned tank circuit.

And still further, Richard, the FM transmitters you refer to above are not in the same category as those used
in tube rigs used by hams.

Incidentally, Richard, have you really reviewed the report of my TS-830S experiment?

Walt

Cecil Moore wrote:
Richard Harrison wrote:
Terman says on page 96 of his 1955 opus:
"The reflected wave is identical with the incident wave except that it
is traveling toward the generator."

Gene needs to tell us how the TV modulation that
causes ghosting makes its predictable round trips
to the source and back without the aid of the
reverse traveling wave.

Cecil,

It is interesting that you can be so precise at times and so sloppy at
other times. I very carefully limited my discussion to steady state
conditions, which is what everyone is already talking about in this
case. You then conveniently inject modulation into the mix, completely
ignoring what I said.

Do the math and show us how my comment is in error. Add the two
traveling waves and see if you get the summation to be precisely a
standing wave plus a residual forward traveling wave. Go back and reread
your quoted references and try to figure out if anything I said is
different from your gurus.

73,
Gene
W4SZ

"Walter Maxwell" wrote
(RF): And if so, would that also mean that such a tx would not be prone
to producing r-f intermodulation components when external signals
are fed back into the tx from co-sited r-f systems?

This issue is irrelevant, because the signals arriving from a co-sited
system would not be coherent with the local source signals, while load-
reflected signals are coherent. The destructive and constructive
interference that occurs at the output of a correctly loaded and tuned
PA requires coherence of the source and reflected waves to achieve
the total re-reflection of the reflected waves back into the direction
toward the load.

But even for coherent reflections, if the PA tank circuit has very low loss
for incident power (which it does), why does it not have ~ equally low loss
for load reflections of that power? Such would mean that load reflections
would pass through the tank to appear at the output element of the PA, where
they can add to its normal power dissipation.

Also, does not the result of combining the incident and reflected waves in
the tx depend in large part on the r-f phase of the reflection there
relative to the r-f phase of the incident wave? And the r-f phase of the
reflection is governed mostly by the number of electrical wavelengths of
transmission line between the load reflection and the plane of
interest/concern -- which is independent of how the tx has been
tuned/loaded.

If the ham transmitter designs that your paper applies to produce a total
re-reflection of reverse power seen at their output tank circuits, then
there would be no particular need for "VSWR foldback" circuits to protect
them. Yet I believe these circuits are fairly common in ham transmitters,
aren't they? They certainly are universal in modern AM/FM/TV broadcast
transmitters, and are the result of early field experience where PA tubes,
tx output networks, and the transmission line between the tx and the antenna
could arc over and/or melt when reflected power was sufficiently high.

RF

On Mar 20, 3:43 pm, Cecil Moore wrote:
I understand what happens to the direction and
momentum in the reflected wave when it encounters
an impedance discontinuity at some distance from
the source, e.g. a Z0-match.

What happens to the direction and momentum in the
reflected wave when it encounters a non-dissipative
resistance at the source?

For some years now, you have been arguing the reality of 'reverse
power'. 'Reverse power' has served you well in that it appears
to offer reasonable explanation for some phenomena:
- 'forward power' minus 'reverse power' yields transferred power
- circulators
- TV ghosting
- dissipation of pulses in generators

But there are some challenges to the premise of 'reverse power':
- where does the 'reverse power' go?
- why does the change in dissipation of a generator when 'reverse
power' changes depend more on the design of the generator than
on the magnitude of the 'reverse power'?

In an attempt to resolve these, you have apparently done extensive
studies in optics looking for an explanation based on constructive
and destructive interference but are still left with the question you
posed above and others, like the one below from another of your
posts:

All one has to do to calculate the reflected power
dissipated in the source is to understand the constructive
and destructive interference occurring at the source
output terminal. THIS IS EASIER SAID THAN DONE. [emphasis mine]

Like myself, others have encountered difficulties with the premise
of 'reverse power'. But we have taken a different path to
enlightenment than yours; we have given up on the premise that
'reverse power' represents something that is real. To do this, we
have had to find alternative explanations to all the phenomena
listed above, but once this was done, life was good.

I would suggest that you try trodding this path. Make a list of
phenomena that you think are explained by 'reverse power'. For
each phenomena, explore the possibility of alternative explanations
that do not require 'reverse power'. When you have an explanation
for each, test the explanations against each other to ensure they
are self-consistent, then take the body of non-'reverse power'
explanations and compare it the body of 'reverse power' explanations.
Which is more complete? Which violates fewer fundamentals?

You have believed in 'reverse power' for so long that you will
probably find this path difficult. Make a conscious effort when
thinking about circulators, for example, not to give up because
it does not explain ghosting. Work out the solution to ghosting
later. Similarly, when working on steady-state examples, do not
confuse yourself with transients. Do those later. And when
exploring a phenomena using a hypothetical generator, do not
simply give up because it does not accurately model a real
transmitter. Much can be learned from the simplifications of
ideal voltage and current sources.

Those who have already trodden this path are, I am quite sure,
willing to assist you in finding the solutions, if you are willing
to learn, rather than tossing distractions into the discussion.
Save the other phenomena that trouble you for a later discussion.
Keep the discussion on track.

You can not lose if you take this path. In the best ending, you
end up with a coherent explanation for all the phenomena and can
give up on your search for solutions to the troubling issues posed
by 'reverse power' and the vanishing of the energy. But even if
you do not change your view you will have a better appreciation of
the alternative explanations and should be better able to partake
in debates on their correctness.

You could start by providing a list of phenomena for which you
think the reality of 'reverse power' is the only viable explanation
and offer a willingness to learn about alternative explanations.

....Keith

Keith wrote:
"For some years now, you have been arguing the reality of "reverse
power".

For good reason. You feed a transmission line into an open circuit at
its far end, and the power arriving at the open has no where to go but
to return towards its generator. What happens at the generator upon
arrival of the power reflected from the mismatched load depends on the
vector values of incident and reflected waves as well as the impedance
of the generator.

Searching the net for "reflected r-f power" returned over 25,000
examples. Belief in reverse power is obviously common.

Best regards, Richard Harrison, KB5WZI

On Mar 21, 12:08 pm, (Richard Harrison)
wrote:
Keith wrote:

"For some years now, you have been arguing the reality of "reverse
power".

For good reason. You feed a transmission line into an open circuit at
its far end, and the power arriving at the open has no where to go but
to return towards its generator. What happens at the generator upon
arrival of the power reflected from the mismatched load depends on the
vector values of incident and reflected waves as well as the impedance
of the generator.

Searching the net for "reflected r-f power" returned over 25,000
examples. Belief in reverse power is obviously common.

Best regards, Richard Harrison, KB5WZI

You are certainly correct; many people believe in reflected power,
though I've
always found that to be a poor basis for my own beliefs.

You have also provided the classic example where the numerology works
and 'reverse power' offers a tidy explanation. I am sure this neat
example is
the basis for many people's belief.

What drove me to look at alternate explanations for these kinds of
examples
was that the 'reverse power' explanation fails miserably when the
power
gets back to the generator. Having another explanation for this
classic
example lets one let go of 'reverse power' which solves the challenges
at the
generator end. When 'reverse power' is not real, the question of where
it
goes becomes irrelevant.

....Keith