This has been a good example of a common pitfall in modeling. The error
made in this case was to attempt to apply an unsuitable model (a voltage
source in series with a resistance) to a system to be modeled (a
transmitter). As the OP showed, the attempt leads to an impossible
result. The classic example of this is the "proof" that a bumblebee
can't fly, based on a flawed model and immediately shown to be false by
simply observing that they do, indeed, fly. Yet we see people falling
into this trap daily, not only in modeling electrical circuits, but also
in modeling such diverse processes as human behavior, economic systems,
and roulette wheel numbers.

Unfortunately, the bad results of applying unsuitable models aren't
always so obvious as they were here. So it's always wise to check to see
if the model fits before putting faith in the results.

Roy Lewallen, W7EL

Good to see that everyone agrees that a generator with a resistor in series is
an unsuitable model for an RF transmitter.The easy part of the work is done!

Now the more difficult part. As, by the Thevenin theorem, any complex circuit
comprising resistors, voltage generators and current generators is equivalent to
a generator with a resistor in series, evidently the transmitter model must
comprise elements other than just resistors, voltage generators and current
generators.

Can one suggest how such a model looks like? (even a plain one, that does not
take into account second- or superior-order effects).

73

Tomy I0JX - Rome, Italy

On Jun 8, 6:52 am, "Antonio Vernucci" wrote:
Good to see that everyone agrees that a generator with a resistor in series is
an unsuitable model for an RF transmitter.The easy part of the work is done!

Now the more difficult part. As, by the Thevenin theorem, any complex circuit
comprising resistors, voltage generators and current generators is equivalent to
a generator with a resistor in series, evidently the transmitter model must
comprise elements other than just resistors, voltage generators and current
generators.

Can one suggest how such a model looks like? (even a plain one, that does not
take into account second- or superior-order effects).

73

Tomy I0JX - Rome, Italy

I believe you have mis-stated the Thevenin theorem. First, it applies
only to linear circuits. Fine -- over some narrow range at least, a
transmitter does indeed look like a linear circuit. But more
importantly, it describes ONLY what you observe at an external pair of
terminals, with no other connections, NOT what goes on inside the
"black box" containing those elements you mentioned.

A very simple example is a voltage source (a perfect battery) and two
resistors in series across the battery; the external terminals for
this example will be at opposite ends of one of the resistors. Let's
say the battery is 2 volts and each resistor is 2 ohms. That will
look like a Thevenin equivalent 1V in series with 1 ohm. Note that it
also looks like a one amp source in parallel with a one ohm resistor.
But it does not behave INTERNALLY like either of those. Consider also
the same internal circuit, except drop the voltage source to 1V and
add a 1/2A current source across the output terminals, polarity so
that there's no drop across the resistor between the voltage and the
current source (with no external load). Now figure the internal
dissipation for each of those two cases, with no load, with a 1 ohm
load, and with a short-circuit load.

Cheers,
Tom