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