In article ,
dg wrote:

I don't want to say any more than I have to about this, I don't want to
contaminate the answer. Lets just say 2 people disagree about something. I
am trying to keep it simple.

Lets take for example a simple piece of coax. 1 center conductor, 1 outer
braid. The cable is carrying AC in the RF range to an antenna. Why coax?

Coax has a nice property. When used as directed, it carries RF on its
center conductor and on the inside of its outer braid/shield/conductor
(equal currents, opposite direction at any given point in the coax).
There is little or no RF current on the outside of the braid/shield,
and the EM fields created the current flow on the conductors are
confined to the inside of the coax.

This has a number of advantages. Very little of the RF escapes the
coax (there's a small amount of leakage for single-braid coax, less
for double-braid, and very little indeed for hardline). This reduces
the potential for RF interference to nearby devices. Very little
external RF can leak into the cable. Because the RF fields are
confined to the inside of the cable, you can route coax right next to
or through metal objects (pipes, other cables, chickenwire in a stucco
wall, etc.) without the presence of this metal affecting the RF. Coax
cables with a solid dielectric hold the center and outer conductors at
a consistent distance, which doesn't change very much when you bend
the coax (within reason), and this means that the cable doesn't tend
to suffer from severe impedance "bumps" when bent or coiled.

The convenience of coax does come at a price. The geometry and
physics of a concentric-conductors cable tends to require that the
cable have a relatively low characteristic impedance. This can result
in higher losses per foot, in some cases, than could be achieved using
various configurations of two- or four-conductor balanced line (given
appropriate impedance matching at both ends).

Do you think the outer braid could be correctly referred to as a "shield"?

It's often referred to by that term. It's common to see coaxial
cables referred to as "double-shielded" or "quad-shielded". Sometimes
two layers of braid are used, sometimes two or more layers of braid
are interspersed with metal-foil or metallized-plastic-film layers.

If you really want fun, get involved in the discussions about the pros
and cons of various shield configurations for cables used in
multiple-frequency duplex operations (e.g. amateur-radio and other
repeater systems). Receiver desensitization, intermodulation and
noise from within the cable become... well, let's say "interesting"...
in those applications.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
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