Reg, G4FGQ wrote:
"Whatever is inside the box, a little toriod or tiny fraction of
wavelength of line, they all work in exactly the same way."

In-line power meters separate reflected power from incident power for
determinations.

My second edition of the RSGB "Test Equipment For The Radio Amateur" has
an in-line wattmeter diagram on page 4.7 which uses a toriod but
operates much like a Bird wattmeter.

The RSGB meter uses a 4-inch longth of coax which is grounded only on
one end. It carries the power to be measured through the center of the
toroid, and this coax forms a 1-turn primary for the toroidal
transformer. A center-tapped secondary is wound on the toroid. A
capaciitive voltage divider is connected between the coaz center
conductor and ground. This voltage divider and the transformer center
tap share a common load resistor.

The capacitive ratio of the divider is adjusted so that its voltage out
is equal to that of the magnetically induced secondary voltage on either
side of the center tap. The capacitively supplied voltage is
proportional to the coax voltage. The magnetically supplied voltage is
proportional to the coax current.

Reflected current is flowing in the opposite direction in the coax from
forward current in the short piece of coax, so in the direction of
current flow where induced volts in the transformer secondary add to
those capacitively coupled to the common load the total is douple either
contribution. Output of the combo is proportional to the power
represented by the current and voltage samples. Reflected power doesn`t
affect forward power determination in this instance because its voltage
and current samples are out-of-phase and exactly cancel.

Since the start and finish of the toroid`s secondary winding are
180-degrees out-of-phase, when the start of the coil is phased with the
capacitive sample to produce an output proportional to reverse
(reflected) power, the finish end of the coil is phased with the
capacitive sample contribution to produce an output proportional to the
forward (incident) power.

Calibration procedure is given in the book.

Best regards, Richard Harrison, KB5WZI