Joel Koltner wrote:
"Powers don't add, field strengths do"

"Add" is a rather loosely defined term. A more technically
precise statement would be: "Powers don't superpose, field
strengths do." When fields superpose, they still must obey
the conservation of energy principle, i.e. the total energy
before the superposition must equal the total energy after
the superposition.

Given two RF waves in a transmission line and the phase angle,
A, between the two electric fields, the following Power equation,
published in QEX, gives us a valid method of "adding" two powers.

Ptotal = P1 + P2 + 2*SQRT(P1*P2)*cos(A)

Reference: "Wave Mechanics of Transmission Lines, Part 3",
by Steven R. Best, VE9SRB, "QEX", Nov/Dec 2001, (Eq 13),
page 4.

The last term is known in optics as the "interference"
term, positive for constructive interference and negative
for destructive interference. Angle A, the phase angle
between the two electric fields, determines the sign
of the last term and thus whether interference is
destructive or constructive.

Reference: "Optics", by Hecht, 4th Edition:
Chapter 7: The Superposition of Waves
Chapter 9: Interference
--
73, Cecil http://www.w5dxp.com