Roy Lewallen wrote:
Roger wrote:
Hi Roy,

Could I add this observation? Both traveling waves and standing
waves can be measured. A single volt meter or ammeter will measure
the standing wave which is the sum of the traveling waves..

This isn't quite correct. A standing wave is the result of isn't the
sum of traveling waves. It's a description of the envelope of the
current distribution that sum produces. The sum of the traveling wave
voltages is the total voltage. The sum of the traveling wave currents
is the total current. It's relatively to measure the total voltage or
current at any point and, if you measure them at enough points you can
use the measurements to draw a graph of the standing wave.
Yes, you have said it better than I.

A DIRECTIONAL volt meter or ammeter will measure only the traveling
wave within the design direction, but can not distinguish between
components from multiple reflections that might combine.

Yes.

A directional voltmeter or ammeter will measure the same voltage or
current no matter where it is placed in the transmission line under
steady state conditions, assuming no resistive losses in the
transmission line.

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.

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.

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.

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 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