Ian, G3SEK wrote:
"What are those V`s and I`s doing in your reply?"

Cecil is right on target. Power is VxI. The Bird takes carefully
balanced samples of V&I. In the incident wave the samples are in-phase.
In the reflected wave the samples are 180-degrees out-of-phase. By
exactly balancing the two samples, the reflected wave cancels. Only the
incident wave is sensed. To sense the reflected wave only, the polarity
of one of the two samples is switched.

I am now more dangerous. Until today, I had never seen one of Kraus`
books. I now own the third edition of "Antennas (for all applications)".
Kraus says there are "fundamental" units and "secondary" units. The
"Ampere" is a fundamental unit. All secondary units can be derived from
the fundamental units.

In the Bird, the given is a 50-ohm line. Power is 50(I)(I), since I=E/R
= E/50, and E = 50(I) = V.

Power = EI

The Bird is accurate enough using a sample of E and a sample of I and
scaling to determine power. Multiplication is just scaling or the number
of times you repeat a number in an addition column. In the Bird, scaling
is an electrical adjustment.

Best regards, Richard Harrison, KB5WZI