wrote:

John S wrote:
If Maxwell can have a Demon, then so can I.

There is a 100W RF transmitter attached to an antenna, through an
ammeter, whose terminal impedance is 50+j0 at an arbitrary frequency,
Fo. The transmitter has a drive control, that is, the amount of power
can be adjusted by my Demon.

I have an RF voltmeter also attached to the antenna/transmitter junction.

The goal is to measure the power into the antenna as frequency is varied.

At Fo, my Demon adjusts the drive level to 50V at the antenna terminal.
The power into the antenna is 50W. So far, so good.

As we sweep the frequency, my Demon adjusts the drive control to supply
50V at the terminal. So, let's plot just 3 points:

R X Z Amps Pwr
37.34 -48.86 61.49 0.813 24.68
50.56 0.06 50.56 0.989 49.44
68.44 48.44 83.84 0.596 24.34

My Demon is only watching the antenna's terminal voltage, adjusting the
drive to maintain 50V and has no idea about R and X. I have included R,
X and the resulting calculations (Z, Amps, Pwr) from a simulated antenna
for those who wish to verify that the the resulting calculations are
correct.

While the load current has changed from .8 amps through .99 amps to .6
amps, the voltage has remained constant.

If you the only thing you know is that the terminal voltage set my Demon
has maintained the voltage at 50V and you can read the current, what
conclusion could you draw about the source impedance? Well, since the
voltage has remained unchanged while the current varied, it would appear
that, for all practical purposes, the source impedance is zero because
the dynamic source impedance is dV/dI (0/dI = 0) at the antenna terminals.

I invite comments about this thought experiment.

Thanks folks, and I promise no bickering and name-calling from me.

Yes, the source impendance of a regulated voltage source can be
concidered to be zero over it's linear range, i.e. it does not go
into limiting.

That the voltage is AC and the load is complex is irrelevant.

I am afraid that the "demon" described does not necessarily represent a
zero source impedance generator. To specify a constant voltage
gererator you do not have to invent a "demon" at all. You just have to
specify an AC voltage generator with whch is either ideal (a theoretical
construct) or has an arbitrarily low source impedance so that the
voltage is constant to any degree of precision you require. This is
quite possible to make in practice, and would be said to have negligible
source impedance in the above circuit.


But the experiment as specified could also be implemented with a
generator with as high a source impedance as desired but which had an
automatic level control operating at loop bandwidth much lower than its
working frequency.

To get a zero impedance generator you have to specify that the demon
works at the operating frequency.


--
Roger Hayter