Richard Clark wrote:
and ponder the implication of a negative power

Power to the load equals forward power minus reflected power.

The negative sign for power in that case is associated with
direction of the Poynting vectors, positive for forward and
negative for reflected.

Constructive interference power minus destructive interference
power equals zero.

The negative sign for power is associated with destructive
interference.

Both of those cases are simply mathematical conventions.
--
73, Cecil http://www.w5dxp.com