[TowerTalk] current balun question

[jimlux]

On 3/20/17 4:34 AM, Máximo EA1DDO_HK1H wrote:
> Guy, my question is NOT about common mode current.
>
> Is NOT about how much impedance a choke provide.
>
>
> My only question is to know if (and how) a "common mode choke" is able to balance different "differential currents".
>
>
> Do you remember the famous I1, I2 and I3? Where I3 is the common mode current the choke suppress.
>
> So ,my question is about I1 and I2.
>
> If I1 and I2 are different values at feed point, Is a choke balancing them? Make them equal values into the coax (not antenna side, only coax side) ?
>
> Or a choke has no effect in I1 and I2 differential mode currents?
>
>

Yes it does force them the be equal..

Think of the two wires as a and b, so the current in one wire is Ia and 
the current in the other wire is Ib.

Ia = -Ib (because there's no where else for the current to go)..

Say that a is connected to the center pin, which is I1, and b is 
connected to the outer shield, the currents are I2 (inside the coax) and 
I3 (outside the coax)

So Ib = I2+I3

if we force I3 to zero (by putting a big impedance in that leg)
then Ib=I2

so I1=-I2, which is what you want.