Yes Ian, I agree with you that "common mode current" on a coax outer shield
is a complete misnomer. It doesn't describe what it is "common to".
If it were really common mode current it would have the same current on the
inner conductors as well as on the outer conductor and be in phase.
A better name for what is commonly called "common mode current" on the coax
outer shield would probably be "outer shield current".
>  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.
> What engineers don't appreciate is that twisted language turns around
> and bites you! What comes out of your mouth goes right back into your
> mind and muddles your own thinking too (see "ERP", "gain", "ground" etc
> :-) Worst of all is that students and hams have to dig down several
> technical levels before they can begin to understand that their mentors
> are often saying one thing but meaning quite another... and often
> without even realising it.
> 73 from Ian GM3SEK
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