Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced
now, right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current
on the outside of the shield according to its characteristic impedance
(skin effect separates current flow on the inside of the shield from the
current on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it?
What if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished.
One way is to run several turns of the coax through a toroid core so
that the impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion
and learn.

John