"Antonio Vernucci" wrote in
:

The Bird actually measures a combination of capacitive coupled
voltage and inductively coupled current. There is a app note on the
Bird website. Find: "Straight Talk About Directivity".


Thanks for pinpointing that nice document.

However the document dwells on the directivity of the meter, and I
could not find there any mention of the impedance tolerance issue I
had raised.

I wrote some notes on the operation of the Bruene type VSWR meter at
http://www.vk1od.net/transmissionlin.../VSWRMeter.htm . The Bird 43 is
not a Bruene type meter, but a similar derivation could be done, and for
all practical purposes, the explanation applies.

Keep in mind that the usual practice of calibrating a VSWR meter is to
adjust it for nil reflected indication with a load of 50+j0. Without
arguing the tolerances implications, its indicated VSWR can only be
applied exactly to an adjacent low loss line with Zo=50+j0... which you
do not have, so you must expect some error in the measurements.

If you read the article I gave earlier, you will see the plots of VSWR
along a specification RG58C/U line with a 50+j0 +/-0% load. Those are the
indications you would expect of a *perfectly* calibrated Bird 43 on
RG58C/U exactly meeting the specification from which the RLGC parameters
were derived.

These are very small effects, but they exist.

Layer on top of that cable tolerance and you have more variation.

But, if you are making measurements using an antenna as a load, I
respectfully submit you probably are not in a sound position to assert
that there is zero common mode current effect.

As a brain teaser, think of the situation in which rho, the magnitude of
the voltage reflection coefficient Gamma could be greater than 1. Of
course many think they have proven that cannot happen my citing
measurements made with a Bird 43... but can it capture what is happening?

Owen