Cecil Moore, W5DXP is alleged to have written:
"Nor will we find a negative scalar quantity accompanied by the claim
that the negative sign indicates a change in direction as you have
done."

Have not read Hscht, but I`ve read Terman and realize that a
transmission line can guide a wave only forward and backward. Sometimes
backward is considered the negative direction.

Terman gives an example on page 90 of his 1955 edition:
"When the load end of the line is shorted, that is Eload =0 , reference
to Eq. (4-14) shows the reflection coefficient has the value -1.0 on an
angle of 0-deg.= 1.0 on an angle of 180-deg. As in the open-circuited
case, the reflected wave has an amplitude equal to the amplitude of the
incident wave. However, the refleection now takes place with a reversal
in phase of the voltage and without change in phase of the current."

Best regards, Richard Harrison, KB5WZI