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?

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

Thanks,
--Dan

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
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

The run to my ant tower is a bit over 100 ft, this is coax, underground
in a 1 in plastic pvc conduit--try that with another feedline...

Warmest regards,
John
"dg" wrote in message
...
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?

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

Thanks,
--Dan

Thanks for the writeup.

So, does the braid actually SHIELD or is it a consistent way to ensure
interference will cross both conductors? And along the same line of
thought, from the signal point of view on the inside of the cable everything
is kind of even because of the equal current-opposite direction? I am going
to contaminate my simple question by saying that I feel the braid is almost
incorrectly referred to as a shield-it is a functioning conductor necessary
for the signal to pass.

Also, how about this, reverse the polarity of the AC signal on the input of
the coax. Any difference on the performance of the cable? I mean, if its
AC the "shield" is carrying the same AC that the center conductor is right?
I know the world of RF is like black magic, I'm trying to keep this basic.

Any thoughts or comments are welcome.

THANKS!
--Dan


"Dave Platt" wrote in message
...
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.

In article ,
dg wrote:

Thanks for the writeup.

So, does the braid actually SHIELD or is it a consistent way to ensure
interference will cross both conductors?

Yes.

It performs both functions. It actually does provide a shielding, due
to the "skin effect" - RF resists flowing _through_ the body of a
conductor, and prefers to flow along its surface. It's very difficult
for RF to flow from the inner surface to the outer surface, as it
would have to travel through the thickness of the outer conductor.

That's probably the stronger factor in the interference resistance of
a coax. There's also some which comes from the symmetrical shape, I'd
guess, but I don't think it's anywhere near as much of an issue.

And along the same line of
thought, from the signal point of view on the inside of the cable everything
is kind of even because of the equal current-opposite direction?

Yup.

I am going
to contaminate my simple question by saying that I feel the braid is almost
incorrectly referred to as a shield-it is a functioning conductor necessary
for the signal to pass.

This is not an either-or sort of situation. In a single-shielded
coax, the braid (or hardline) performs both functions. It's a
fully-functioning conductor, and it also acts as a shield which
confines the signal to the inside of the coax. It's a floor wax *and*
a dessert topping!

Single-shielded cables can be somewhat "leaky" - the shielding
efficiency is not 100% if the outer conductor is not a solid, unbroken
surface. That's the reason why many cables are multiply-shielded...
the fields/current that leak out through the inner shields are
confined by the outer ones. In cables such as this, the inner
shields/braids/foils will be carrying almost all of the RF current,
and providing a lot of the shielding efficiency. The outer shields
will be carrying very little RF current - almost the only thing that
these outer shields do, is provide additional shielding.

At the other end of the spectrum are the so-called "leaky" coaxes,
which are deliberately designed with incomplete shields and which can
be counted on to "leak" a certain percentage of their RF energy out
into free space over a specific distance. These are often used to
couple RF into underground spaces (e.g. tunnels), along railroad
tracks, and so forth.

Also, how about this, reverse the polarity of the AC signal on the input of
the coax. Any difference on the performance of the cable? I mean, if its
AC the "shield" is carrying the same AC that the center conductor is right?

As far as the behavior of a given length of coax (away from the ends)
is concerned, there's no difference at all. The RF is AC and has no
polarity.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"dg" wrote in message
...
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?

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

Thanks,
--Dan

Dan,
Basically, the differrence between coax and shielded cable is that coax will
have a controlled characteritic impedance. Shielded wire will hava a
characteristc impedance, but it is not specified. Shielded wire is normally
terminated in an impedance that is orders of magnitude higher than the wire
impedance, and people who use it are more concerned about its capacitance in
PF/foot.

Tam/WB2TT

I second Tam's answer, having suffered through undergrad EM fields.