On Jun 21, 5:40 pm, K1TTT wrote:
On Jun 21, 1:03 am, Keith Dysart wrote:

You might consider the following experiment:

- a sinusoidal generator that will produce 50W in to 50 ohms
- attach one half wavelength of transmission line
- leave the far end of the transmission line open

After the line settles, a directional "wattmeter" anywhere on
the line will indicate 50W forward and 50W reflected.
The generator will not be putting any energy in to the line
since the line input appears as an open circuit at the
generator output.

Insert a circulator (with the reflected port terminated in
50 ohms) between the generator and the line. The circulator
termination will be dissipating 50W and the generator will
now be delivering 50W in to the circulator.

1. Please explain how inserting the circulator did not change
the circuit conditions. The generator went from delivering
0W to delivering 50W.

2. Where do you think the 50W being dissipated in the circulator
termination resistor is coming from? The line? Or the
generator (which is now outputting 50W)?

well, its coming from the generator via the far end of the line of
course.

That is the classic answer from the “reflected power” model, but
consider the following...

We construct two experiments similar to the above.
The first one:
- a sinusoidal generator that will produce 50W in to 50 ohms
- attach one half wavelength of transmission line
- leave the far end of the transmission line open
- the generator is constructed in the Thevenin style with a
voltage source and 50 ohm resistor

The second experiment:
- same as above except there is a circulator between the
generator and the line and the circulator is terminated
with 50 ohms

Examine experiment 1. After the line settles, a directional
wattmeter indicates 50W “forward power” and 50W “reflected
power”. The current in the voltage source and source resistor
is 0 so no energy is dissipated in the source resistor and
the voltage source is delivering no energy to the system.
The “reflected power” is not being dissipated in the source
resistor or voltage source so where is it going. Cecil will
offer some explanation where the “reflected energy”
is “redistributed” in the other direction as the “forward
energy”, which “must” be happening since it is not dissipated
in the source and it can not be reflected because there is
no impedance discontinuity.

Examine experiment 2. After the line settles, a directional
wattmeter indicates 50W “forward power” and 50W “reflected
power”. But with a circulator, the voltage source is
delivering 100W, the source resistor is dissipating 50W
and the circulator resistor is also dissipating 50W.
You offer that the 50W being dissipated in the circulator
is the 50W of “reflected power” coming from the line.
Presumably then, the 50W of “forward power” is coming from
the generator. This agrees numerically since the voltage
source is providing 100W and the source resistor is
dissipating 50W.

In both experiments, conditions on the line are identical.
There is 50W of “forward and reflected power” indicated.
There is no energy flowing past the end of the line since
it is open circuited.
Because the line is one-half wavelength long, the line
presents an open circuit to the generator (exp 1) or
circulator (exp 2).

In both experiments, the line is presented with a 50 ohm
source impedance, either from the generator or from the
circulator.

What puzzles me then, is how the “reflected power” knows
that in experiment 1, it should stay out of the generator
so that it is not dissipated in the source resistor
but in experiment 2, it should enter the circulator so
that it can be dissipated in the circulator load resistor.

Can you explain how the “reflected power” “knows”?

....Keith