Owen Duffy wrote in
:

....
This is a property of any directional wattmeter of this type when the
sensor is calibrated for an impedance that is real (ie non reactive),
irrespective of the load or the type of coax that might be attached to
either side of the instrument.

I should have posted a link to an explantion of why this is so. The article
at http://www.vk1od.net/VSWR/VSWRMeter.htm contains an explanation.

Owen