On Apr 9, 12:29*pm, Cecil Moore wrote:
Keith Dysart wrote:
Thus I strongly suggest that Vg, Ig, Pg, represent reality. The
others are a convenient alternative view for the purposes of
solving problems.

Of course they represent *net* reality but we are trying
to determine what is happening at a component wave level.
Defining the component waves out of existence is an un-
acceptable substitute for ascertaining what is happening
in reality.

Typically we see Vg split into Vf and Vr, but why stop at two.
Why not 3, or 4?

Because two is what a directional wattmeter reads. The
two superposed waves, forward and reverse, can be
easily distinguished from one another. Two superposed
coherent forward waves cannot be distinguished from
each other. That's why we stop at two - because it is
foolish to go any farther.

You sometimes use three. Discussions of ghosts have at
least three. Not so foolish, methinks.

There is power coming from the transmission line. Looking at Pg(t),
some of the time energy flows into the line, later in the cycle
it flows out. The energy transfer would be exactly the same if the
transmission line was replaced by a lumped circuit element. And
we don't need Pf and Pr for an inductor.

OTOH, the distributed network model is a superset of
the lumped circuit model so the inadequate lumped
circuit model might confuse people. Hint: changing
models to make waves disappear from existence doesn't
make the waves disappear.

The model is not inaccurate when the question is framed
with the model, as you do for Fig 1-1.

The lumped circuit model is adequate for lumped circuits.
It is inadequate for a lot of distributed network problems.
If the lumped circuit model worked for everything, we
wouldn't ever need the distributed network model.

True, but the question at hand, based on Fig 1-1 is lumpy.

I suggest that you take your circuit and apply distributed
network modeling techniques to it including reflection
coefficients and forward and reflected voltages, currents,
and powers at all points in the circuit. Note that the
reflections are *same-cycle* reflections. If the lumped
circuit model analysis differs from the distributed network
model analysis, the lumped circuit analysis is wrong.

Ummm. It was your circuit.

It goes up because the impedance presented by the transmission
changes when the reflection returns. This change in impedance
alters the circuit conditions and the power in the various
elements change. Depending on the details of the circuit,
these powers may go up, or they may go down when the reflection
arrives.

That is true, but the impedance is *VIRTUAL*, i.e. not an
impedor, and is therefore only an *EFFECT* of superposition.
We are once again left wondering about the *CAUSE* of the
virtual impedance, i.e. the details of the superposition
process. Ignoring those details will not solve the problem.

Actually, the transmission line input impedance is quite real,
formed from distributed capacitance and inductance. Like most two
terminal circuits, it can be reduced to simpler form.

...Keith