On Dec 10, 10:08 am, "Roger Sparks" wrote:
"Keith Dysart" wrote in message

...

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The directional ammeter measures instantaneous
Vt and It, does the above arithmetic and presents
If. A directional ammeter that presents a single
number rather than the time varying If has probably
converted the instantaneous values to RMS.
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...Keith

I don't think that the directional ammeter reads instantaneous Vt and It.
The circuits I am thinking of sample a length of line (NOT A POINT) so the
sample records average voltage (or current) from a period of time.

If I understand the methodology of the directional ammeter correctly, it
extracts energy from the wave from both magnetic (current) and voltage
components. If the components are in phase, they add, and that only occurs
with the wave going in the design direction. Yes, this is a reading of
power, not voltage or current individually. Current and voltage are
related by the Zo of the transmission line, so if we know current, we also
know voltage, and visa versa.

Agreed?

Only partly. If you look at the element on an instrument
like a Bird 43, you will find that it is both capacitively
and inductively coupled to the line. The capactive
coupling is sensitive to the total voltage on the line
at the point of the element, while the inductive coupling
is sensitive to the total current in the line.

The subtraction (or addition) is done in the element
where the voltage sample and the current sample
(scaled by Z0) are subtracted before being applied
to the diode. The output of the diode is the
rectified instantaneous difference of the voltage and
scaled current from the equations originally provided.
This is fed to an average responding meter which has
a scale marked to show (Vf**2)/Z0 (i.e. power).

You are correct that the element does not sample
at a point, but rather over the width of the coupling
element. This is done because of design
limitations and is one of the sources for error in the
instrument, though small if the wavelength is long
compared to the element.

There are many ways to obtain the instaneous
voltage and current for the subtraction (or addition).
Some designs measure the voltage by using
an electrical connection to the line, so these are
essentially measuring at a point. Other designs
measure the current by detecting the voltage
drop across a resistor in series with the line.

Diagrams of the internals of the Bird 43 element
are available in the Operations Manual he
http://www.bird-electronic.com/produ...uct.aspx?id=81

....Keith