"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.)

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