On 7/30/2015 4:28 PM, rickman wrote:
On 7/30/2015 2:01 PM, Dave Platt wrote:
In article , rickman
wrote:

Yes, I read that, but it doesn't really explain this current. Later
they make the statement, "the current on the braid outside side is the
sum of currents other than transmission line currents on the entire
coaxial cable structure". This is pretty clear, but still does not
explain the source, or maybe I should say "why" the current flows on the
braid and not the antenna.

Don't ask "Why does current flow on the braid?". Ask "What would *stop*
current from flowing on the braid?".

Current flows on *all* paths that have less than an infinite
impedance. That's its nature.

Remember, I = E / R (or, for AC/RF, I = E / Z). "I" (current on the
braid) will be nonzero, if the voltage at that point is nonzero (E !=
0) and the impedance down the braid at that point is not infinite.

The effect of a balun is to place a high "choking" impedance in series
with the outside of the feedline braid, thus "choking off" the current
flow.

I can't say I agree with your "choking" impedance idea. The coax
connects to the balun in the same way it connects to the antenna. The
balun can have no effect on the impedance of the coax shield. Just as
you ask, "What would *stop* current from flowing on the braid?" when
connected to the antenna what will stop the current from flowing on the
braid when connected to the balun?

The balun is an impedance that the RF sees as it starts to travel down
the outside of the coax toward the transmitter. But you know about
common mode currents, I think.

The only thing that will stop the current from flowing on the outside of
the shield when connected to the balun is if the balun presents a much
lower impedance path for the current than does the shield.

Think of a common mode choke. However it is mechanically implemented,
its purpose is to provide a block to common mode currents and allow
differential currents only.

The only way your suggestion makes sense is if the current actually
comes *from* the antenna and the balun prevents that current from
returning to the feed line.

Actually, your statement is the key. The feed point of the antenna IS
the current source. One end of the current source is applied to the
'hot' side of the antenna and the other end is applied to the other
element plus coax shield. As far as the feed point is concerned it has
one element on one side and two elements on the other side.