[TowerTalk] To get a truly balanced antenna feed

[jimlux]

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