Re: Mythbusters: V/I ratio is forced to Z0

Dave wrote:
ah, but if the bird does establish a 50 ohm 'environment' within its 40mm housing then the reading that was actually obtained is indeed correct. so now, how else could you read zero reflected power at that point if it didn't establish that environment? 

Would you like for me to go through the math of a voltage and
current sampled at a point without disturbing anything?

Assume the voltage is sampled such that 100v of net voltage
is sampled as 10 volts.

Assume the current is sampled such that two amps of net current
is sampled as 10 volts.

If these samples are in phase, adding them will result in 20v.
That equals 200 watts of forward power on the meter.

Subtracting them will result in zero volts. That equals zero
watts of reflected power on the meter.

The sampled net voltage and net current determine the 50 ohm
calibration. No 50 ohm environment required.

If the sample point is in 75 ohm coax with no reflections, the
sample voltage will be 12.3 volts and the sample current will
8.165 volts. Subtracting those sample values doesn't yield zero
so 41.35 watts of reflected power will be reported where none
actually exists.

So unless the mere writing of my words on this newsgroup
establishes a 50 ohm environment, the Bird doesn't need to
establish a 50 ohm environment to obtain the same above
results.

If the Bird indeed did control the environment into which
it was inserted, it would be violating a design goal for
measuring instruments and Bird wouldn't be selling many
of those highly intrusive devices. Bird wants the meter to
be as unobtrusive as possible and it is.
--
73, Cecil, http://www.qsl.net/w5dxp
.

Relevant Pages

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