"Jeff" wrote in
.com:

How can that be? If the meter is basically a directional coupler then
the forward power is just that. Subtracting any reflected power will
just give a stupid answer. The only errors will be due to the
directivity of the coupler, which will give a band of uncertainty
which varies with VSWR, and the error due to the accuracy of the
detectors.

My Bird does not subtract any reflected power to give a forward power
reading!! It can't I need to rotate the slug to read reverse power.

Jeff, without commenting on whether Cecil's assertions are right or
wrong, you seem to have some misconceptions about what is measured with
your Bird (presumably 43).

The so called "forward power" and "reflected power" are notional values,
but not actual power "components". The only power is the average rate at
which energy passes a point, and it is in one direction or the other.

In fact the power can be calculated taking "forward power" minus
"reflected power", but only in the case where the sampler is calibrated
for Zo being real (as it is in a Bird 43).

My article at http://www.vk1od.net/VSWR/VSWRMeter.htm describes the
operation of a Bruene type of VSWR meter and discusses the power
measurement issue. Though the sampler in the Bird is different to the
Bruene sampler, the Bird samples V and I in a very small region (
regarded a point ) and sums them in the same way as the Bruene circuit.

Owen