Ian White, GM3SEK wrote:
"But here you pick up the pace. Instead of the detailed argument above,
suddenly whole chapters rush by in a single sentence."

Fair criticism. It reflects tiring of posting before its conclusion.

The Bird wattmeter`s firectional coupler distinguishes between incident
and reflected waves by their singular difference. Upon reflection of a
wave, either the voltage or the current it generates is reversed in
phase, but not both.

Bird takes equal samples of voltage and current from the wave.. When
there has been a reflection, the samples have opposite polarity and
cancel. When there has been no reflection the samples from that
direction of travel are in-phase and the sample total is double the
contribution of either sample.

To determine reverse power flow, the polarity of one of the samples is
reversed.

You don`t need to know how it works to use it and Bird never advertised
how simple it is as far as I know.

Best regards, Richard Harrison, KB5WZI