"Keith Dysart" wrote in news:1174507951.363436.150330
@y66g2000hsf.googlegroups.com:

....
The scale on the meter could be labelled "furlongs per fortnight" and
this would still be true.

After exploring alternative explanations for the various phenomena,
the explorer will be ready to understand what a directional wattmeter
really measures (as opposed to what its meter scale claims to
indicate)
and truly understand what inferences can be correctly made from its
indications.

Keith,

I drafted an article exploring the operation of a Breune type directional
wattmeter, it is at http://www.vk1od.net/VSWR/VSWRMeter.htm . The Bird 43
responds in the same way, but from a different sampler construction.

The article gives a simple derivation of the meter response, and deals
with the legitimacy of scaling the instrument in Watts.

If one takes measurements with the instrument, it is true that the power
at a point is "forward power" less "reflected power", and the
manufacturer has scaled the instrument in Watts to facilitate that
calculation, but that does not imply that the value of "forward power" or
"reflected power" has any stand alone value, the ratio of the two is
meaningful, the difference of the two is meaningful, but one alone is
meaningless.

To illustrate the lack of stand-alone value of the "forward power"
reading, one could place two such instruments, one calibrated for 50 ohm
and another calibrated for 100 ohm in tandem at the tx, and then a load.
The two instruments will indicate different "forward power" and different
"reflected power".

Notwithstanding the fact that the "forward power" and "reflected power"
readings are each not of stand alone, the difference between "forward
power" and "reflected power" has meaning and will be the same for each
instrument.

I agree with you that a lack of understanding of the instrument can be
used to prop up bogus explanations and concepts, even leading to people
citing the Bird 43 user manual like it was a respected and authoritative
text.

Owen