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.. 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.

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.

73, Roger, W7WKB

Roy Lewallen wrote:
I think it might be useful to say a little more about standing waves.

Imagine a single lossless transmission line with a sine wave source at
one end and a load at the other. Begin with a load equal to the line's
Z0. Make a graph of the magnitude of the current or voltage as a
function of distance from the source. With the Z0 load, the magnitude
will be the same all along the line so your graph will be a straight
line. This is a "flat" line, with no standing wave. A probe sitting at
one spot would show the instantaneous voltage or current amplitude
going up and down in a sinusoidal manner. A probe a bit farther down
the line would look the same, but delayed; there's a phase difference
between the voltages or currents at the two points. The phase
difference is equal to the line's physical length in degrees divided
by the velocity factor.

Now change the load so the line is slightly mismatched. A standing
wave will appear -- the graph of amplitude vs distance won't be flat
any longer, but will have a ripple added to its previous constant
value. (The VSWR is, by definition, the ratio of the highest to the
lowest values of the voltage envelope on a line long enough to have a
full maximum and minimum. The current SWR is the same.) The maxima and
minima of the ripple don't move, hence the name "standing wave". If we
look at the instantaneous voltage or current at a single point, it
will go up and down in step with the source as before. If we also look
at the second point, it'll also go up and down as before, and there
will be a phase angle between the two. But there are two interesting
differences from the flat line: One is that the amplitudes at the two
points are now unequal unless they're an integral number of half
electrical wavelengths apart (or a few other special cases). The other
is that the phase shift isn't the same as before. There's still a
phase shift between the two points, but it's no longer equal to the
electrical length of the line between the points. We'll find that
either the voltage has shifted more and the current less, or vice
versa depending on the load and which points we've chosen. But at
every point the current and voltage still have phase angles which
change with position along the line. That is to say, the voltage or
current at one point is delayed compared to the voltage or current at
the other.

As the mismatch gets more extreme (i.e., the SWR increases), the
magnitudes at the two points get more different, and the phase
deviates farther from the electrical length of line between them.
(This is why you can't expect phased array "delay lines" to provide a
delay equal to the lines' electrical lengths when they're not
terminated with Z0.)

At the most extreme case of mismatch -- an open, short, or purely
reactive load, resulting in an infinite SWR -- the amplitude of the
standing wave along the line goes from zero to twice the value it had
when the line was flat. And a really interesting thing happens to the
phase of the voltages and currents on the line. Remember how as the
mismatch got worse, the voltage and current phase difference between
two points got farther and farther away from the electrical line
length between them? Well, when the SWR is infinite, it's gotten to
the point where the voltage or current phase remains the same for a
distance of a half electrical wavelength, then abruptly changes 180
degrees, repeating every half electrical wavelength. Some antennas
behave in some (and only some) ways like transmission lines, and
you'll find that modeling programs report just this behavior of the
phase of the current along a straight wire antenna.

The standing wave and all the characteristics of the voltage and
current (e.g., how their magnitude and phase varies with position
along the line) follow directly from an analysis of forward and
reflected traveling waves on the line. The voltage or current at any
point is simply the sum of the two waves at that point, and they have
the properties I've just described.

I hope this helps in clarifying the meanings of traveling and standing
waves, voltage and current along a transmission line. I'm sure there
are lots of good graphical illustrations available -- but some bad
ones too. Hopefully keeping this explanation in mind when you look at
the nice graphics displays will help you sort the bad ones from the good.

Roy Lewallen, W7EL