Jim Kelley wrote:
. . .
Voltages on a transmission line do not determine reflection
coefficients.
Reflection coefficients are determined by characteristic impedances, not
virtual ones.

73, Jim AC6XG

I disagree with this. When applied to transmission lines, the (voltage)
reflection coefficient is, as far as I can tell, universally defined as
the ratio of reflected to forward voltage to reverse voltage at a point.
So a reflection coefficient can be, and often is, calculated for every
point along a line, not just at discontinuities or points of actual
reflection. This can be done with nothing more than the knowledge of the
values of forward and reflected voltages at the point of calculation.

As it turns out, the value of the reflection coefficient at any point
will be equal to (Z - Z0) / (Z + Z0), where Z is the impedance seen
looking down the line toward the load at the point of calculation. I'm
very leery of the use of "virtual" anything, since it often adds an
unnecessary level of confusion. But if I were to calculate a reflection
coefficient at some point along a continuous line, I could replace the
remainder of the line and the load with a lumped load of impedance Z,
and maintain exactly the same reflection coefficient and forward and
reverse traveling waves in the remaining line. I wouldn't object, then,
if someone would say that there was a "virtual impedance" of Z at that
point when the line was intact, since all properties prior to that point
are unchanged if a lumped Z of that value is substituted for the
remainder. (I personally wouldn't call it "virtual" -- I'd just call it
the Z at that point, since it's the ratio of V to I there.) The point is
that the reflection coefficient was the same before and after the
substitution of the remaining line with a real lumped Z. Before the
substitution, reflection was occurring at the load. After, the
reflection is occurring at the new, substituted Z load. Yet the
reflection coefficient and traveling waves remain the same on the
remaining line.

A reflection coefficient isn't the cause of anything. It's simply a
calculated quantity used for computational and conceptual convenience.
Only an impedance discontinuity causes reflections, but we can calculate
a reflection coefficient at any point we choose, with its value being
well defined and unambiguous.

Roy Lewallen, W7EL