W5DXP wrote:
Ian White, G3SEK wrote:
As you yourself say, the Bird senses current and voltage samples; and
these are then added (forward) or subtracted (reverse). There is no
physical mechanism inside the Bird by which E x I multiplication takes
place, so power is not directly sensed or measured.

I'm sorry, but that is incorrect. Refresh your memory on how a slide
rule multiplies. Two linear values of length are physically added but
a product is read from the results. The secret to multiplying using
linear scales is to calibrate that linear scales logarithmically.
It's easy to understand for a slide rule, is it not? The same applies
to the Bird wattmeter scale.

You are mistaken. You may want it to be so, but it ain't.

The Bird "power" scale is almost a pure square law, indicating power as
being proportional to V-squared, where V is the voltage detected by the
diode in the slug. No log function is involved.

This isn't just theory. I and another person have independently verified
the Bird 43's scale law by experiment, measuring the meter current at
various power levels and then determining the mathematical law of the
meter scale. We even took the current measurements to the level of
precision where we could see that a few of the Bird's individual scale
marks weren't drawn in quite the right place relative to all the rest.
We confirmed that the calibration fits the square law almost perfectly
near the top end of the scale, but deviates by a few percent near the
bottom end where diode's threshold voltage becomes significant.


The Bird takes two linear voltages and adds them.

Agreed. One voltage is a sample of the voltage on the centre conductor
of the Bird's internal transmission line, and the other is derived by
transformer action from the current on the line. When sensing in the
forward direction, these voltages are made to add in phase; then the sum
is detected by the diode.

When the slug is turned around, the current sample reverses its phase
but the phase of the voltage sample stays the same, so now the two
subtract. With a matched load, the two voltages are arranged to be equal
and opposite so you get a null in the reverse direction. That's what
gives the Bird its directional properties.

But the power scale on
the Bird meter face is scaled logarithmically just like it is on a slide
rule.

Sorry, that is factually incorrect, so everything you concluded from it
is incorrect too.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek