Tom, W8JI wrote:
"The directional coupler in a Bird meter samples the across vector
(voltage) from a capacitive divider and adds it to a sample voltage of
the through vector (current) from a current transformer in a
predetermined ratio.

That`s close. In the cartridges is a loop terminated in a diode.
Capacitive coupling of the loop to the center conductor of the precision
ccoax supplies the voltage sample. Inductive coupling of the loop
supplies the current sample.

I`ve described operation several times here and once in this thread, so
I won`t repeat it.

SWR is easy to get from the forward and reflected indications of the
wattmeter.

VSWR = 1 + sq.rt. (ref. PWR / for. PWR)
Divided by 1 - sq.rt. (ref. PWR / for. PWR)

Bird supplies a family of VSWR lines on a graph of forward power vs.
reflected power for those who would avoid the calculation. They can also
supply a slide-rule to do rhe same.

Best regards, Richard Harrison, KB5WZI