Manu Abraham wrote:
>> That's a balun, otherwise known as common mode choke. The are
>> explicitly exploiting the fact that coiling adds inductance to the
>> common mode path but not to the differential path.
>
> After writing that only, I thought that attenuation would be in the
> common mode.

It is.  That's exactly the point of a balun.  It adds impedence to the
unwanted common mode signal while adding as little as possible to the real
differential mode signal.

Baluns are often used in RF applications as single ended to differential
converters.  For example, the final transmitter output could be single ende=
d
with respect to ground.  That works fine for the transmitter and for a
single ended antenna such as one that relies on a ground plane.  However, i=
f
you have a symmetric antenna you want both sides with equal and opposite
signals with respect to ground.  A balun between the transmitter and such a
antenna achieves that for various levels of good enough.


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