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?

You fail to understand the difference between a choke balun and a
voltage balun.

A voltage balun forces the voltages at the output to be equal.

A choke balun provides a very high impedance to the shield path.

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.

Nope, it is just the opposite.

A high impedance path prevents the flow of current.

I = E/Z

From high school algebra, as Z appoaches zero, I approaches infinity,
and as Z approaces infinity, I approaches zero.

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.

Nope, the current comes from the end of the coax and the outside
of the shield is a separate current path from the inside of the
coax and the antenna.

Maybe this is one of those pointless distinctions and both ways of
looking at it are correct.

Nope, there is a very big point to it.

Read www.eznec.com/Amateur/Articles/Baluns.pdf for a detailed discussion
with pictures showing the current flow.



--
Jim Pennino