rickman wrote:
On 7/29/2015 3:14 PM, Ian Jackson wrote:
In message , John S
writes
On 7/29/2015 1:16 PM, rickman wrote:

Perhaps someone can explain the issue of current in the coax shield.
Current gives rise to a magnetic field. But the current in the inner
conductor is opposite and would create a magnetic field that would
cancel the field of the outer conductor, no?

What am I missing?


Skin effect. The currents on the inside of the shield and on the
outside of the shield see different things. They each have no idea
what the other is doing.

As for magnetic field, I must step aside. I can only report what the
gurus say (nothing that I've found).

Even though the coax shield is grounded at the shack end, both halves of
the antenna get fed push-pull (in anti-phase) with the RF signal flowing
on the outer skin of the inner conductor and the inner skin of the shield.

However, at the antenna end, the returning RF on the shield side of the
antenna doesn't know that it should stay on the inside of the shield.
Because of the skin effect, it happily makes for the outside, whence it
flows back to shack, and through the shack grounding connections.

I am having trouble forming an image of this. What exactly is the
source of the "returning RF"? Is this reflected RF at the impedance
mismatch at the feedpoint? If so, the situation being discussed has no
impedance mismatch, so no returning RF. Is the returning RF from the
signal being radiated from the antenna inducing current in the shield?
If so, doesn't the inner conductor also pick up the radiated signal?

The energy IN the coax is not carried by either conductor, but in the
field between the conductors.

https://en.wikipedia.org/wiki/Coaxia...al_propagation

Once you connect to coax to something, the outside of the shield looks
like another current path with some impedance of it's own.

Likely easiest to visualize on a vertical antenna as being another radial.

See http://www.eznec.com/miscpage.htm and in particular the article
"Baluns: What They Do and How They Do It".


--
Jim Pennino