On Tue, 28 Jun 2005 15:39:57 -0700, Frank Gilliland
wrote:

One of the most misunderstood terms in radio is "common-mode current".
It simply means that current is moving in the same direction, and in
phase, on two or more conductors. It occurs in a coax when current on
the -inside- of the shield is in phase with the current on the center
conductor. Any RF current on the -outside- of a coax has -nothing- to
do with common-mode currents -- it's simply the result of RF spilling
out of the coax or being induced onto it from an external field.
*******

Yes misunderstood.

I have yet to really see any coax of decent quality that has
suffiecient gaps in the shield to allow a 27 Mhz wave to have
appreciable leakage. Even with 80% coverage the holes in the shield
are so little of a wavelength that I would dare say less than 1/10,000
of the energy of the TEM wave propogating down the coax can "leak"
out.

As for common mode currents the coax itself can have induced currents
in the chield from fields radiated from the antenna. Depending on
where the coax is located to a conducting surface, you can develope
various intensity of currents. Yes you need two conductors minimum to
have comnmon mode. Earth can be one of those conductors.