Richard Harrison wrote:
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.


We're all agreed on what happens with the Bird's sampling lines and
detector. The only differences arise from each person's attempt to
condense it all into a couple of sentences, and aren't worth arguing
about.

But Richard cuts it too short when he claims that:
A Bird Thruline wattmeter uses a directional coupler to separate
forward direction power from reverse direction power.

That claim confuses two different things: what the line-loop-detector
hardware physically *does*; and what the indicated results *mean*. The
first of these is agreed; the second is not.

The disagreement is entirely about the interpretation - in other words,
it's about the theory about standing and travelling waves. Richard
habitually misses out this step, which makes it look as if the Bird
wattmeter "proves" the physical existence of forward and reverse
travelling waves of power.

It doesn't. Everything that you see printed on the Bird's meter scale,
and in the Bird literature, represents that company's particular
interpretation of theory about waves on transmission lines. The details
of that theory are *not* agreed within this newsgroup, which means that
- to some people - the two halves of Richard's claim do not join up.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek