Wayne wrote:


"John S" wrote in message ...

Consider a center-fed dipole. The generator is at the center.

----------GEN-----------

A balanced antenna. Familiar, yes?

Now consider attaching a feed line to it. JUST the shield!


----------GEN-----------
|
|
| - line shield
|


What happens? Well, there is one radiating element on one side and two
radiating elements on the other side (element + shield). Not balanced now,
right?

The current divides on the right side such that the current on the left
side is equal to the sum of the currents on the right side. This is the
message David was trying to get across, I think.

So, okay, what are the currents in the two elements on the right side?
Without further knowledge, dunno. BUT note that there will be a current on
the outside of the shield according to its characteristic impedance (skin
effect separates current flow on the inside of the shield from the current
on the outside of the shield).

Well, the outside current we don't really want, so how do we stop it? What
if we install a large impedance just below the GEN that runs down?


----------GEN-----------
Z
|
| - line shield
|

This is the purpose of providing a device (Z) that impedes the current
flowing down the line. There are several ways this can be accomplished. One
way is to run several turns of the coax through a toroid core so that the
impedance reduces the current.

Again, I am no expert on this. I'm just trying to generate discussion and
learn.

John

This is similar to the way I always viewed the current.

In summary, at the balanced antenna, the center conductor path is
straightforward.
However on the shield half, the current has two possible paths: the antenna
half connected to the shield and the outside of the shield.

This would cause half of the dipole to radiate less, with the remainder
being radiated by the outer shield.

The proportion of the current flowing down the outer side of the shield
seems easier to stop than to calculate

flame shields up!

It is trivial to calculate the current on the shield and the effect it
has on the antenna pattern with any antenna simulation software.

If you do that, you find that, just as you would expect, the effect is
dependant on how long the shield portion is.


--
Jim Pennino