Roger wrote:
Roy Lewallen wrote:

That's only partially true. Both the traveling waves and the total
voltage and current have not only magnitude but also phase. A
directional coupler can measure both the magnitude and phase of the
traveling waves (but some directional detectors like a Bird wattmeter
indirectly measure only the amplitude). Traveling wave measurements at
different points along a lossless line will have the same magnitude,
but different phases. So the voltages or currents at those points
aren't the same.

Roy Lewallen, W7EL
This last paragraph gets to the heart of the issue. One concept of a
transmission line is that the traveling wave is always in phase in the
sense that the power contained in the wave is the envelope that is
properly considered. In this concept, the voltage and current are
always in phase, MUST be in phase. This power wave may be split as at a
reflection point, but the components will never be out of phase because
the power calculation would be incorrect.if it was out of phase.

Sorry, I can't make much sense out of that. Voltage and current don't
have to be in phase in order for power (energy flow) to be present. But
I'd rather not introduce power into the discussion. It's not necessary
in explaining what I've presented, and brings the opportunity for a
whole new level of misunderstanding and folklore.

The second concept of a transmission line allows the traveling wave to
have voltage out of phase with the current. Here the power can be all
stored in either the current (magnetic) field or the voltage field,
depending upon the phase of the traveling wave. The character of the
wave changes (so to speak) depending upon location and phase.

The ratio of V to I of the traveling wave is the Z0 of the line. In a
lossless line, this is a pure resistance, so the V and I of traveling
waves are in phase.

If the transmission line is terminated with a resistance, the
constantly-in-phase traveling wave concept provides the theoretical
basis for calculation of the reflection coefficient.

The reflection coefficient can easily be calculated regardless of
whether or not the load impedance is resistive or reactive. The load
impedance doesn't have any effect on the relationship between V and I
traveling waves going the same direction; it affects only the amplitude
and phase relationship between waves going in opposite directions.

I think that consideration of the conditions at the end of a
transmission line are a good place to examine as we try to get some
experimental guidance.
If the transmission line is shorted (or open), it is hard to visualize
how the voltage (or current) could flow to the short (or open) and then
just disappear. Does the wave cancel (or disappear) at the
intersection (open end)? Do the waves pass through each other, so we
see only the vector sum? Do the waves "pile up" at the open end, but
not at the short?

The voltage and current at any point along a line, including the ends,
equals the sum of the forward and reflected voltage or current waves at
that point. At a short circuit, the voltage is zero. This means that the
sum of the forward and reverse waves at that point is zero, which in
turn means that the two are equal in magnitude and out of phase.
(Another way of saying this is that the voltage reflection coefficient
is -1 at that point.) At an open circuit, the current is zero. I'll
leave it as an exercise to the reader to figure out what this means
about the relationship between the forward and reverse traveling current
waves.

The constantly-in-phase traveling wave concept requires the
difficult-to-believe observation that a directional ammeter placed very
near the end of an open transmission line will read the same current as
if it were placed at the source end. Perhaps someone can perform that
experiment some day, but I can not imagine how it can be done without
placing a load on the line, thus invalidating the initial assumptions.

73, Roger, W7WKB

Sounds like a good experiment for you to do. Please post your results
here so others can learn from them.

Roy Lewallen, W7EL