Coax has three currents in general if there is something unbalancing the
coax installation. That something could be a coax connected directly to
a dipole center, and lead away closer to one leg than the other. It
could also be a resonant length of coax feeder. In the case of
unbalance between the current flowing in the center conductor and that
on the inside of the shield, you then have a third current that exits
the coax end at the dipole center point, and flows back on the outside
of the shield. This currrent could radiate or cause RF in the shack
problems.
Normal coax action has the currents confined to inside the shield. It
flows on the surface of the inside shield wires, yes, to oppose the
current in the center conductor; but there is a distinct chance of some
shield current dividing at the feed point and not returning via the
inside the shield surface, but flowing on the surface of the outside the
shield wires. Check a Radio Handbook, or any of F. E. Terman's text
books on Radio and Electronics. Another great author of the modern day
is Bernard Grob with his series "Basic Electronics". Don't confuse skin
effect in conductors with the unbalance current flow in an antenna
coupling to a coax feeder.
A contemporary write up of this was in the W5GI article, "The Mystery
Antenna" in CQ a few years ago.
-Stuart Rohre
K5KVH
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