Richard Harrison wrote:

Bird Technical Series #1 may be enlightning:
"By proper combination of the two samples (derived from volts and amps)
we obtain an RF voltage proportional to the square root of main line
power---."

"---the scale which has been marked in watts corresponding to the power
being sampled from the main line."


IF Z0 is perfectly resistive, or if the line is
distortionless, then

Pdelivered = Pforward - Preflected
= |V+|^2/Z0 - |V-|^2/Z0

is perfectly correct (W.C. Johnson, pages 150-151).

The Bird voltmeter readings (scale marked in
watts) take into account phase relations between
forward and reflected voltage waves and forward
and reflected current waves.

The formula and the Bird Company assume that low
loss line is being used, so that Z0 is nearly
resistive and the error due to non-resistive Z0 is
small compared to the sum of all other system errors.

If Z0 is sufficiently reactive that I have to
doubt the usefulness of the Bird meter, then I
need to take other action with respect to the
lossiness of the coax. My 600 W PA was not
intended to be a heating element for the coax.

There is an old saying "if you look hard enough
you can find fault with anything".

Or anyone, including Terman and Bird.

Bill W0IYH