On 7/29/2015 6:31 PM, rickman wrote:
On 7/29/2015 6:40 PM, Jeff Liebermann wrote:
On Wed, 29 Jul 2015 15:19:29 -0400, rickman wrote:

I am having trouble forming an image of this. What exactly is the
source of the "returning RF"?

This might help:
http://www.antennex.com/w4rnl/col0606/amod100.html
The first few paragraphs are the applicable parts. Quoting a few
tibits:
"Fig. 1 presents one traditional way to portray the situation
at the dipole feedpoint. Its general purpose is to show why
the insertion of a balun is important as a precautionary measure
in dipole construction."

"However, the current from the braid has 2 paths: the right
leg of the dipole in the figure and the outer side of the
coaxial cable braid."

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.


The rest of the article deals with modeling issues and problems.

When modeling a balun, I use three conductors for the coax. The usual
inner and outer conductors, which are assumed to handle only
differential current and therefore do not radiate, and a mysterious
3rd conductor on the outside, which carries all the common mode
current that does the radiating.

And how is this third wire connected? Why do you see current in it? Is
this just due to the voltage drop across the rest of the coax? If so, I
would expect the current flow to be the same phase as the inner shield
current.

Consider a half-wave dipole fed by coax.

Pretend that the feed point of the antenna has the RF applied (which it
does when you consider that the coax is just a medium to bring the RF up
to that point). What does the the feed point see? It sees one antenna
element on the coax center wire and two antenna elements on the shield
(element + shield). The outside of the shield is now an antenna element
going down toward the earth while the wire attached to the shield is the
other actual element. So the 'balanced' antenna is now unbalanced due to
the coax becoming one of the elements.

This is easy to model in EZNEC. And, you can see the results of changing
the length of the coax as well as the effects of installing impedances
in the coax.