On 6/24/16 1:28 AM, Ian White wrote:
Any suggestions how to apply this concept to ladder line/open
feed?
Ed McCann AG6CX
Measurement of common-mode and differential-mode currents on
open-wire line are also very difficult because they require a
complicated arrangement of current transformers and baluns. It can be
done [1] but I have never seen it described in an amateur radio
publication.
I would think that with a pair of current transformers, a pair of High Z
voltage probe, and a suitable four channel coherent receiver, you could
do it by measuring I and V, just like they do for three phase AC power
meters.
The current is easy, but I'm not sure what you would use as a "ground"
reference for the voltage probe that wouldn't itself be subject to all
sorts of issues.
So you'd have to do something with 4 current probes and spacing them
some distance apart.
Would it be worth it? I doubt it.
If you were interested only in measuring unbalanced current, a suitable
big (!) core around the entire feed line with a measurement winding
would do it, just like they use in a Groung Fault Interrupter.
That is why, out of the many thousands of amateurs who claim to be
using "balanced feedline", there are few or none who can actually
*prove* it. All the rest are really just expressing a hope or a wish
which will almost never be granted.
[1] A Google search for < measure "common mode" and "differential
mode" impedances > will produce lots of references, but all of the
measurement techniques rely on idealized baluns within the test
fixture itself. The measurement errors in a high-Z situation remain
completely unknown.
Or they use multiport VNAs and get to the result by algebra. If you
look at the measured test data for a minicircuits RF transformer, you'll
see full 4 or 5 port data.
http://www.minicircuits.com/MCLStore/Agreement.jsp?file=ADT1-6T_S5P.zip
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