Finally, an explanation. Thank you, Ian.
On 4/21/2012 5:17 AM, Ian White GM3SEK wrote:
>  The concept of "common mode current" was originally developed for
> twin-wire lines, on which signals are carried by equal-and-opposite
> differential currents. Any inequality or phase error between the
> currents on the two wires can be represented as a separate current
> flowing equally and in phase on BOTH wires - which is what the word
> "common" was originally meant to imply. In other words, common-mode
> current makes a twin-wire line behave like a single wire. Contrary to
> popular belief , both the differential (transmission-line signaling)
> currents and the common-mode currents will coexist freely on the same
> twin-wire line.
> Engineers have borrowed this label "common mode" from twin-wire
> transmission lines, and have also applied it to any current flowing on
> the outside surface of a coax shield, for any reason. This isn't
> accurate usage because the geometry of coax is completely different from
> twin-wire, effectively giving us three conductors instead of two... but
> RF engineers seem to revel in twisting the English language until it
> points newcomers in entirely the wrong direction.
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