Dave Oldridge wrote:

If the antenna is TRULY balanced and the feedline dressed well away from it
at right angles you should have no common-mode currents on the feedline.
. . .

That only prevents one of the two ways common mode current can be
created, by coupling. It can also be created by conduction. A common
example is a coax-fed dipole, where the current in the outer feedline
conductor splits between the antenna conductor and the outside of the
coax. An equivalent problem can occur when a dipole is fed with
symmetrical line such as ladder line, and one conductor of the line is
connected to the rig's chassis at the rig end. The current on the inside
of the chassis is equal to the current from the "hot" conductor, and
this splits between the transmission line conductor and the outside of
the chassis. A detailed explanation of conducted common mode current can
be found at http://eznec.com/Amateur/Articles/Baluns.pdf.

Roy Lewallen, W7EL