On Wed, 12 Oct 2005 22:09:51 GMT, Owen Duffy wrote:

The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself.


All,

Can I offer the suggestion that the key to understanding why this is
so, it to understand the sampler.

I don't have the detail of the physical and electrical parameters of
the Bird sampler, however I suspect that like so many other
reflectometers, it comes down to a device that samples independently
the net current and the net voltage associated with any travelling
waves, and those RF samples are in proportion and phase relationship
that when algebraicly added and rectified, they produce a DC voltage
that is proportional to the power flow in one direction only (provided
that Zo is real). The proportions calibrate the instrument for a
specific V/I ratio.

Apart from mentioning that Zo must be real, and I will address that in
another thread, Zo in the region where the sample is unimportant to
the "proportion and phase relationship..." bit. Zo of the through line
is important only to the extent that you would generally:
- not want a significant transformation of impedance between the load
terminals and the calibrated sensor at that calibrated V/I ratio.
- not want a significant transformation of impedance between the
generator and load terminals at that calibrated V/I ratio to minimise
disruption to the system being measured.

Everyday we use instruments to measure something, somewhere and apply
that knowledge to infer something else, somewhere else using
appropriate other knowledge. For example, you might measure the
voltage drop across a cathode resistor and make some reasonable
inference about anode current using appropriate other knowledge.
Making measurements with a Bird 43 in one place and inferring the
situation somewhere else using appropriate other knowledge is
reasonable.

For example, I may have an antenna system (say a loop) that uses a
transmission line transformer (TLT) to transform the loop terminal Z
to 50 ohms. I can attach the generator end of the TLT to the Bird 43,
attach the Bird 43 via 50 ohm cable to a transceiver that is rated for
a nominal 50+j0 ohm load, and proceed to adjust the antenna / TLT for
zero reflected power indication on the Bird 43 knowing that I can
reasonably infer that the load presented to the transceiver will be
approximately 50+j0 ohms using the knowledge that Bird readings
indicate Z at that point is 50+j0 and there will be insignificant
transformation on the 50 ohm cable to the transceiver. This is a
proper and sound application of the instrument.

Did I get that wrong?

Did I need to mention environments?

Owen
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