[Antennas] Current Flow in Coax?

Fri Sep 28 21:58:40 EDT 2012

Andy,
Thanks for your reply. I follow your explanation once current starts to flow on 
the inside of the shield, but I am still puzzled by why the current is only on 
the shield inside.
The source is connected to the center conductor and to the shield. The moment 
current begins, why would the current in the shield not choose the outside or 
divide, with some flowing on the inside and some flowing on the outside of the 
shield? 
It seems the current on the skin of the center conductor must be causing 
current to flow on the inside of the shield, but I still do not know why. 
73,
Paul
> > I understand this must be the case to cancel radiation from the coax, but can
> > someone point me to a technical explanation of how the forces operate to
> > "pull" the current to the inside of the shield?
>> One way to look at it is this:
>> If the electric and magnetic fields are properly contained within the
> coax and none are on the outside, then those fields ... which are what
> cause the current to flow ... can only move electrons on those
> surfaces they touch ... namely, the inner wire and the inside wall of
> the shield. The RF fields themselves do not penetrate through good
> conductors, so the fields that are inside, can't move electrons on the
> outside wall of the coax shield.
>> The only way to get current on the outside of the shield, is for there
> to be (electro-)magnetic fields on the outside of the coax. Likewise,
> the reverse is true; the two go hand-in-hand. Wherever there is RF
> current, there are electro-magnetic fields adjacent to it, and
> vice-versa.
>> The way I see it, the forces do not precisely "pull" the current to
> the inside of the shield. Rather, there is nothing there (no fields)
> on the outside of the shield, or many skin-depths into the metal
> thickness, that could make current flow there. So the result is that
> the current ends up being confined mostly to the inner wall, not
> precisely that it was "pulled" there by some sort of force.
>> Andy
>