Gary, N4AST wrote:
"So 1000 watts is 31.6% of full scale. I thought all Ham wattmeters did
that. Of course it is highly dependent on the Z the wattmeter sees."

The Bird has more than one scale on the same meter face. It`s possible
to scale them any way the designer wants. With the complications of
special circuits, the scale could be made linear. I for one wouldn`t
want it.

31.6 is the sq. rt. of 1000, approximately. Power is proportional to the
sq.of volts or amps. So, if you want to represent watts as a function of
transmission line volts and, or transmission line amps, you can print
the scale of a linear meter so that it advances as the sq. rt. of the
current. That is, current increases to 141.4% of its previous value, and
the watts indication doubles. That`s because 1.414X the volts multiplied
by 1.414X the amps equals 2X the watts.

In the Bird, the impedance is always supposed to be 50 ohms. That Zo
enforces a lock on the volts to amps ratio the Bird must work wiith. So,
all you need to know is the volts or amps in the incident wave and you
can calculate its power. Same for the reflected wave. The Bird very
effectively separates the incident wave from the reflected wave by
setting their samples exactly equal. In the forward wave the incident
volts and amps are exactly in-phase. In the reflected wave, they are
exactly out-of-phase. So, when set to measure power in the forward
direction, the two equal samples, voltage and current, add. The equal
reflected wave samples exactly cancel. Reverse the arrow on the Bird.
That reverses the polarity of the current sample. Now the forward
(incident) wave samples cancel, and the reverse (reflected) wave samples
add.

Zo only permits wave propagation in which volts and amps are exactly
in-phase or 180-degrees out-of-phase. The arithmetic is simple and Zo is
the friend of the Bird wattmeter.

Best regards, Richard Harrison, KB5WZI