Ian, G3SEK wrote:
"1. The Bird "wattmeter" does not in fact sense power. At the detailed
level, it senses voltages and currents in its internal transmission
line---."

Since the ratio of voltage to current in either direction is always Zo,
all you need are voltage and current samples of one direction at a time.

The Bird is not alone in determining power from such voltage and current
samples. The dynamometer does too.

A d-c meter is useless on a-c because the needle tries to move first one
way then the other as the a-c current through it alternates. Inertia
keeps the indication on a-c close to zero.

The dynamometer eliminates the flicker and gives an appropriate
deflection on a-c. It does this by replacing the d-c meter`s permanent
magnet with an a-c coil. This is arranged to torque the needle upscale
no matter whch direction the current takes through the coils. The
moveable coil is always deflected the same way by interaction with the
fixed coil.

It is simple to calibrate a dynamometer for power and feed current
through one coil and place volts across the other. It senses volts and
amps but it reads out power, as does the Bird wattmeter.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

It is simple to calibrate a dynamometer for power and feed current
through one coil and place volts across the other. It senses volts and
amps but it reads out power,

Correct...

as does the Bird wattmeter.

Wrong. The Bird has no second meter coil, and no other physical
mechanism to multiply the E sample by the I sample.

As Walt says (thank you, sir) the Bird is *calibrated* in terms of power
using a matched load.

I've made my point. There's an Internet abbreviation called
IAFWAFIAWMWQ... so I'll be wise, and quit.


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

Ian White, G3SEK wrote:
Wrong. The Bird has no second meter coil, and no other physical
mechanism to multiply the E sample by the I sample.

Remember when nomograms were popular? Do you think you could
design a nomogram with variables of voltage and current to yield
power? Where's the physical multiplication mechanism?
--
73, Cecil http://www.qsl.net/w5dxp



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