Dave wrote:
the voltage on the braid is not zero on the inside, it varies along with the
wave traveling along the inside of the coax. and the currents are exactly
balanced inside the coax also, they have to be or it wouldn't work. this
notion of balanced vs un-balanced transmission lines is totally confusing to
most people, in a proper system, say just with a dummy load on a coax the
currents on the shield exactly balance the current on center conductor. so
why do we go through all this stuff with bal-uns?? and coax chokes?? the
currents are already balanced, so WHY?? come on you gurus out there,
explain this one!

The currents on the inside are always balanced -- they're purely
differential mode. The purpose of baluns or common mode chokes is to
reduce the common mode current which, on coax, flows entirely on the
outside.

If you're driving a dummy load from your transmitter, the common mode
current will be zero in an ideal system and negligible in a real system.
A balun or common mode choke will do nothing in that situation.

See http://eznec.com/Amateur/Articles/Baluns.pdf for a more complete
explanation.

Roy Lewallen, W7EL