The voltage reflection coefficent, sometimes designated uppercase gamma,
sometimes lowercase rho, is the ratio of the reflected voltage to
forward voltage. It doesn't represent power at all, and you have to make
some assumptions of questionable validity to try and associate it with a
power.

People who like to speak of "forward power" and "reverse power"
calculate that the ratio of "reverse power" to "forward power" is equal
to the square of the magnitude of the voltage reflection coefficient,
providing that the characteristic impedance of the line is assumed to be
purely real.

The reflection coefficient is a complex number, so it isn't restricted
to "positive" and "negative", but can have any phase angle. Its
magnitude, like the magnitude of any complex number, is a real number,
so is square is always positive. (Complex numbers can be written with a
negative magnitude, but this isn't commonly done. If it were, though,
the square of the magnitude would still be positive.) Incidentally, the
magnitude of the reflection coefficient is also often designated as
lowercase rho, so any question about "rho" is unclear unless you say
whether you're talking about the complex reflection coefficient or just
its magnitude.

The complex reflection coefficient can have a value of exactly -1, or a
magnitude of 1 with a phase angle of 180 degrees. This simply means that
the reflected voltage is equal in magnitude to the forward voltage, and
exactly out of phase with it. The sum of forward and reflected voltage
has to equal the total voltage at any point along the line. If you
terminate a line with a short circuit, the reflection coefficient is -1
and the forward and reverse voltages at the end of the line are equal
and opposite, so they add up to zero. Which is the voltage you have at a
short circuit.

Roy Lewallen, W7EL

Jim Kelley wrote:
Ur right, thanks Dave. I meant to say voltage rather than power. Let
me ask the question properly.

Tom,
Since rho represents the fraction of forward voltage that is reflected,
what does a negative value for rho indicate?

Thanks and 73,

Jim AC6XG