Dear Tom K7ITM:

Thank you. Thank you. Thank you.

Let us put away forever the idea that a normal transmitter fits the
passive, linear model of an ideal voltage source in series with an actual
resistor.

The active device(s) in an amplifier (tube, BJT, FET, ...) needs to see
a certain impedance (at a given frequency) in order to have desirable
performance characteristics. Too many have extrapolated from that
information (found in data sheets for the active devices) the conclusion
that the active device has an internal Z that is the complex-conjugate of
the load Z.

The wording found in another string of messages communicates the right
idea. A normal transmitter "wants to see" a certain Z. That Z is most
often 50 ohms.

RIP 73, Mac N8TT

P.S. An IEEE paper explored this issue in the last year.
--
J. McLaughlin; Michigan, USA
Home:
"K7ITM" wrote in message
...
On Jun 6, 2:12 pm, "Antonio Vernucci" wrote:
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.

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%.

Pertinent comments are welcome.

73

Tony I0JX - Rome, Italy

Simple: a transmitter is not an ideal voltage source with a resistor
in series.

I'm playing with a switching power supply design that delivers about a
kilowatt at 100 volts. The output is designed specifically to have a
negative resistance, so the output voltage increases as the current
drawn increases. The output dynamic impedance is about -1 ohms
(adjustable, actually). The linear model is a 100 volt battery in
series with -1 ohms. With an 11 ohm load, I get 10 amps load current,
with the battery thus delivering 1000 watts, the load dissipating 1100
watts, and the -1 ohm resistance dissipating -100 watts. Which shows
the absurdity of thinking of a dynamic output resistance being
anything like a real resistance. In my switching supply, I can adjust
the dynamic output resistance between a small negative value and a
rather larger positive value, with very little change in efficiency.

Although transmitters MAY have dynamic output resistances similar to
the recommended load resistance, that's not a necessary condition, and
has little to do directly with efficiency.

Cheers,
Tom