Correction:

Roy Lewallen wrote:
. . .
Reflection coefficients are complex numbers, so they can't properly be
described as "positive" or "negative" except in the special cases of +1
and -1. In all other cases, the can only be described by their magnitude
and angle, or real and imaginary component. . .

It's also reasonable to talk about positive or negative reflection
coefficients when you're restricting the possibilities to ones having an
angle of zero or 180 degrees. This would be the case, for example, when
dealing with lossless lines (purely real Z0) connected to other lossless
lines or to a purely resistive source or load.

Roy Lewallen, W7EL