Richard Harrison wrote:
Ian White, GM3SEK wrote:
"But here you pick up the pace. Instead of the detailed argument above,
suddenly whole chapters rush by in a single sentence."

Fair criticism. It reflects tiring of posting before its conclusion.

Sure. If you'd kept to the original very steady pace, you'd still be
writing... which is not what we do in newsgroups.

The Bird wattmeter`s firectional coupler distinguishes between incident
and reflected waves by their singular difference. Upon reflection of a
wave, either the voltage or the current it generates is reversed in
phase, but not both.

Yup.

If I can fill this out a little...

Bird takes equal samples of voltage and current from the wave..

This is done by the pickup loop, which is both inductively and
capacitively coupled to the center line. The capacitive coupling gives
the voltage sample, while the inductive coupling gives the current
sample.

The current sample runs through a resistor, which develops a voltage
that is made exactly equal to the direct voltage sample. So now we have
two RF voltages appearing in series. In the forward direction, the thing
is built so that these voltages add in phase.

When you rotate the slug by 180deg, the phase of the current sample
reverses but the phase of the voltage sample does not, so now the two
voltages subtract. If the instrument is terminated in its design
impedance of 50 ohms, the voltages (should) cancel exactly, so the meter
reading falls back to zero. There's a small capacitive tab on the pickup
loop that allows the meter reading to be nulled exactly.

When
there has been a reflection, the samples have opposite polarity and
cancel. When there has been no reflection the samples from that
direction of travel are in-phase and the sample total is double the
contribution of either sample.

Er, yes, pretty much...

To determine reverse power flow, the polarity of one of the samples is
reversed.

And here you've made that big leap again. Where did "power" come from?
Nothing in what you or I have said above explains how come the meter can
read "Watts".

That's because it doesn't actually measure watts. It has been calibrated
in watts under certain specific test conditions, using a different kind
of wattmeter that actually does measure watts.

You don`t need to know how it works to use it

No, you don't. But if you choose to use it as "evidence" in a discussion
about waves and reflections, then you do need to know how it works.

and Bird never advertised
how simple it is as far as I know.

Possibly because it isn't actually as simple as it looks.


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