"Roy Lewallen" wrote in message
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Coaxial lines do, however, have an interesting characteristic not shared
by other kinds -- the differential and common mode components aren't
simply a mathematical convenience, but are actually physically separate.
If we do the analysis described above, we find that the common mode
current equals the current on the outside of the shield and the
differential current equals the current on the inside. As Owen pointed
out, the differential current is solely on the inside of the shield and
the common mode current solely on the outside. While this makes the
effects of each mode current easier to visualize and sometimes to measure,
the net effects of common mode and differential currents are exactly the
same for coaxial and non-coaxial lines.

Owen Duffy wrote:
In the case of coax, so-called common mode current flows only on the
outside of the outer conductor, and differential mode current flows only
on the inside of the outer conductor and outside of the inner conductor.

I assume that you are talking about a length of coax that is attached to a
free-space antenna. What about the case where the coax shield is grounded
at both ends? (make it a non-ideal ground if you like.) Wouldn't this
create a ground-loop that will cause some of the signal current to flow
through the ground-connection, thus unbalancing the center-conductor/shield
current? In this case, the common-mode current isn't necessarily flowing on
the outside of the shield. (I am asking a question here).

Also, consider the case at frequencies low enough that skin-effect doesn't
apply. Here there is no inside or outside of the coax shield. Still, the
magnetic fields caused by imbalance between center-conductor and shield
currents are the same, with or without skin effect.

At least these are the thoughts I had while I was discussing the
installation of antennas and tuners on boats. There is no end to the
controversy surrounding the grounding of radio equipment on a boat. There
the antennas are typically end-fed wires (usually part of the rigging), and
some combination of radials and seawater connection for the RF counterpoise.

-Paul