Cecil, W5DXP wrote:
"Neither the instantaneous magnitude nor instantaneous phase is
constant.'

To keep it simple, phase difference between forward and reflected waves
is locked. Velocity and distance to any point (P) on a transmission line
don`t change. So, pick any reference point you like and the phase
difference is simply (beta)(l), where "beta" is the phase shift per unit
length, and "l" is the number of units (length) between the selected
points.

Whether "magnitude" changes depends on definition. Magnitude sometimes
means maximum amplitude of a wave and does not ordinarily change.
However, magnitude used as a synonym for instantaneous amplitude varies
sinusoidally with time.

At complete nulls, volts or amps, depending on the null site we choose,
forward and reverse volts or amps are equal and opposite in both phase
and amplitude at every point in the electrical cycle. Their combination
is always zero.

At node maxima, volts and amps when we have a complete reflection on a
line with negligible loss produce doubles of volts and amps. Maxima of
volts are displaced from maxima of amps by 90-degrees.

Best regards, Richard Harrison, KB5WZI