On Dec 16, 7:42*am, Bob wrote:
Guess I am wondering how can emi noise get thru the outer shield and to
the signal carrying conductor.

Common mode RF signals cannot exist on the inner conductor of an
*ideal* coax feedline and is forced to the outside of the braid by the
laws of physics. The coax has to be less than ideal for common mode to
exist on the inner conductor. But common mode signals don't have to
make it to the coax inner conductor to cause noise problems in a
single-ended unbalanced receiver circuit. A differential amplifier is
often used in control circuits to minimize common-mode problems. A
link-coupled transformer will accomplish much the same function for
amateur circuits.
--
73, Cecil, w5dxp.com