"Antonio Vernucci" wrote in
:

Someone may regard the following question a bit OT, but as it deals
with impedances I have considered that the antenna newsgroup could be
the most appropriate one where to post it.

Let us regard a transmitter as an ideal RF generator with a resistance
in series. It is well known that, for maximum power transfer, the load
resistance must be equal to the generator resistance. Under such
conditions efficiency is 50% (half power dissipated in the generator,
half delivered to the load).

To achieve a higher efficiency, the load resistance should be made
higher than the generator resistance, although this would turn into a
lower power delivered to the load (the maximum power transfer
condition is now no longer met). This can be verified in practice: by
decreasing the antenna coupling in a transmitter, one obtains a higher
efficiency though with a lower output power.

That said, now the question.

But your statements are not true. The model you propose for a transmitter
does not apply in general. Whilst it would be possible to build a
transmitter like that, most transmitters that hams use are not built like
that.

So... it is a loaded question of a type, a question premised on a
falsehood.


Usually, when a transmitter is tuned for maximum output power,
efficiency results to be higher than 50% (typically 60% for class-B,
70% for class-C). This would seem to contradict the above cited fact
that, under maximum power transfer condition, efficiency is 50%.


If the equivalent source impedance is not important, ie it does not need
to be fixed by the design, there here is an analysis.

If you take the case of a grounded cathode triode in class C with a
steady signal, the conduction angle is usually somewhere around 120°. The
anode current waveform is a little like a truncated sine wave, but even
for the range of grid voltages where anode current is greater than zero,
the transfer characteristic is not exactly linear, and the wave will be
further distorted.

If the nature of the anode load is that it is some equivalent R at the
fundamental and zero impedance at all other frequencies, the power output
can be determined by finding the fundamental component of the anode
current waveform, squaring it, and multiplying it by R. The input power
is the average anode current multiplied by the DC supply voltage.
Efficiency is OutputPower/InputPower. By varying the grid bias, drive
voltage, load impedance and supply voltage for a given triode, different
efficiencies will be found, and the maximum could be well over 80%.

Nothing in this approach to design attempts to fix the equivalent source
impedance, the design is performed without regard to that characteristic.

Nevertheless, some argue that the output network performs magic and
achieves source matching naturally without designer intervention, and
does this irrespective of parameters like the dynamic anode resistance,
and the effects of feedback (such as cathode degeneration in grounded
grid amplifiers which in turns depends on the source impedance of the
exciter).

Owen