"K7ITM" wrote in news:1175469074.999185.17760
@d57g2000hsg.googlegroups.com:

On Apr 1, 2:38 pm, Owen Duffy wrote:
I am intrigued that many people have attempted to measure the
equivalent
source impedance of a transmitter with such varying results.

On the one hand is the assertion that a transmitter adjusted for
optimum
operation is comparable with a linear source, and the source impedance
must therefore be the conjugate of the load.

...

I have a lot of trouble with that one, especially the "must therefore"
part. What is "optimum operation"? Is it delivering the most power
to the load, or is it delivering the RATED power to the load, at some
particular efficiency and level of distortion? I'd claim it's the
latter.

Tom, I chose the word optimum for a reason, and I agree with you.

The design process does not find a drop dead maximum power in the way
that loading a source with a variable impedance finds a maximum power.
The rated power of an amplifier is a compromise, and dependent on the
available voltage and current, required linearity / IMD, active device
characteristics (eg saturation effects), dissipation limits (anode,
control grid etc), harmonic output, efficiency to name just a few. To
complicate crude experiments to determine maximum power output, the valve
is usually operated close to saturation, so small load changed result in
severly non-linear behavior. Further, apparent output impedance is
affected by the regulation of the DC supply, which is many transmitters
is better for short term current demands than sustained load.

For example, I have a Ameritron AL811H amplifier with 4 x 811A. The
operating point for SSB telephony is different to AM due to anode
dissipation limits. Some would suggest that when optimised for each of
the SSB telephony and AM operating points (ie different anode load
resistances) into a 50 ohm load, that the equivalent source impedance
*must* be 50 ohms, and that it happens without specific design
provisions.

Owen