At frequencies where skin effect is fully developed, and that is a
reasonable assumption for most practical coaxial cables at HF, the
current on the inside surfaace of the outer conductor is equal to but
opposite in direction to the current on the outside surface of the
inner conductor. This is TEM mode propagation.

At the end of the isolated outer conductor, this current must flow
somewhere, and it flows around the end onto the outside surface of the
outer conductor (effectively changing direction as it does so). So, at
that point, the current flowing on the outside of the outer conductor
is exactly equal to the current flowing on the outside of the inner
conductor.

Leaving aside the effects of changing Zo by substitution of coax for
plain conductors:

If you use two coax lines in parallel with the shields isolated, it
makes very little difference, the current that would have flowed on
the two plain conductors now flows on the outer of the coax lines. The
common mode current is the sum of the currents in both coax shields,
as it would be for plain conductors.

If you join the shields together at each end, the sheilds together now
carry the common mode current. A different equivalent circuit, but
almost the same outcome.


The last paragraph above is where I lose you..... when the shields
are joined together. Yes, I understand inside the shield RF
current flowing around the end and to the outer side.... HOWEVER, the
OTHER center conductor is inducing RF current flowing in the opposite
direction. Since both these inside currents are 'shorted' at the ends
of the two shields, I fail to see how you can have any current flowing
on the outer shield since the two opposite currents should cancel....
????


Ed