Cecil, W5DXP wrote:
"Nope, they don`t have to be in-phase."

Cecil is right. The third side of a triangle can be longer or shorter
than one of the sides.

I should have said, it is entirely possible that the superposed forward
and reflected voltages can damage an unprotected transmitter, The
reflected voltage can add to the forward voltage applied to the
transmitter.

The phase of the reflected voltage should be out of phase with the
transmitter source voltage to maximize the volts across the internal
impedance of the transmitter if we want to damage an unprotected
transmitter.

Best regards, Richard Harrisin, KB5WZI