Owen Duffy wrote:
"Roger Sparks" wrote in
:


"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.



Many simple reflectometer designs do indeed sample the line over a short
length of line, and that short length may be 100mm or more.

Ideally, they would take the sample at a point. (Since a point has zero
length, I can't quickly think of a sampling technique that truly takes a
point sample.)

The voltage sample is easy... measure the voltage using an infinitely
thin probe.

The current sample is measured in a similar way by measuring the
magnetic field over a infinitely small segment of the conductor. There
are sensitivity issues or bandwidth issues, but there are lots of very,
very small magnetic field probe schemes around.


If one says, "point sample" == "less than 1/1000 wavelength), I think
it's actually pretty straight forward, certainly for 100 MHz or less.
(3mm is 1/1000 lambda).






Although sampling over a non-zero length limits their accuracy somewhat,
if that length is kept sufficiently short, they are still able to provide
sufficiently accurate measurements.

Owen