Eddy currents exist everywhere the conductor looks like a
shorted turn on a transformer.
Imagine a perfectly balanced pair of wires, driven in a
perfetly balanced manner. A wire down, a wire back - a
loop. That pair will radiate right out through its loop.
A simple plate of conductor placed over the loop will get
eddy currents generated in it that "cancels" to lower
radiation.
The plate does not have to be terminated to have this
effect. Simply covering the surface of the loop is
sufficient. Plus with the wires perfectly balanced and the
drive pervectly balanced there will be no residual signal
put upon the plate, so the plate does not radiate, or
reradiate.
- Robert -
On Thu, 15 Sep 2005 14:41:56 -0500
Ken Javor <[email protected]> wrote:
> See response to Mr. Woodgate. Shorted fibers are not an
> issue, my argument
> holds if the shield material is a solid copper pipe. You
> won't get eddy
> currents flowing in that pipe until the ends are
> terminated so currents can
> flow in a loop. The magnetic vector flows around the
> circumference of the
> pipe, but that is not the path the eddy currents nee to
> take. They must
> flow longitudinally down the pipe. The cable becomes the
> center of a
> coaxial transmission line; the shield is the coaxial
> transmission line
> shield and return conductor.
>
> > From: "Robert A. Macy" <[email protected]>
> > Date: Thu, 15 Sep 2005 11:28:26 -0700
> > To: Ken Javor <[email protected]>
> > Cc: [email protected]
> > Subject: Re: When is a cable shield not a cable shield?
> >
> > Yes, it can.
> >
> > The fibres are all shorted out making it a pseudo-metal
> > plane.
> >
> > Plus, the loops of interest are pretty much contained
> in
> > between the two ends of the shield.
> >
> > To see how much effect there is measure the inductance
> of
> > the loop inside shield and ouside the loop.
> >
> > - Robert -
> >
> > On Thu, 15 Sep 2005 11:50:33 -0500
> > Ken Javor <[email protected]> wrote:
> >> I have a problem with this as well. A non-magnetic
> >> shield material protects
> >> against magnetic field radiation by providing a path
> for
> >> eddy currents to
> >> flow in. The eddy currents flow in such a way as to
> >> cause a magnetic field
> >> that opposes the originating field (Lenz' law). In
> order
> >> for the eddy
> >> currents to have sufficient magnitude to cause field
> >> cancellation, the
> >> shield must provide a very low impedance short
> circuit.
> >> An open-circuited
> >> shield braid cannot perform this function.
> >>
> >>> From: "Robert A. Macy" <[email protected]>
> >>> Date: Thu, 15 Sep 2005 09:16:09 -0700
> >>> To: "Hudson, Alan" <[email protected]>
> >>> Cc: [email protected]
> >>> Subject: Re: When is a cable shield not a cable
> shield?
> >>>
> >>> Alan,
> >>>
> >>> Think also in terms of magnetic fields, or "area
> >> loops".
> >>>
> >>> The area defined by the small loop between signal
> lines
> >> and
> >>> ground lines now have rather opaque metal plates over
> >> them.
> >>> Starting around 10KHz the conductive "shield" kills
> all
> >>> the magnetic fields coming out. Which were probably
> >> more
> >>> significant than the E-Field contributions.
> >>>
> >>> - Robert -
> >>>
> >>> On Thu, 15 Sep 2005 16:22:31 +0100
> >>> "Hudson, Alan" <[email protected]> wrote:
> >>>> G'Day!
> >>>>
> >>>> Came across an oddity today which has failed to
> >> stimulate
> >>>> any memories (from
> >>>> about the time the old King died!) of basic
> >>>> electromagnetic theory. A
> >>>> well-known computer Company supplies what it calls
> >>>> "shielded ribbon cables".
> >>>> When we examine one of these we find it's
> essentially
> >> a
> >>>> metallic braid
> >>>> sleeve around a standard ribbon cable. The braid
> isn't
> >>>> connected to anything
> >>>> (not even a drain wire at the connector ends).
> >>>>
> >>>> So what level of shielding is this going to give -
> if
> >>>> any?
> >>>>
> >>>> I could imagine signal lines being capacitively
> >> coupled
> >>>> to the braid and
> >>>> then capacitively coupled to any 0V lines in the
> >> ribbon.
> >>>> But I can also
> >>>> imagine signal lines being capacitively coupled to
> >> other
> >>>> signal lines.
> >>>> Similarly with external signals hitting the braid -
> >> they
> >>>> could go to either
> >>>> the signal or 0V lines - yes/no?
> >>>>
> >>>> I know I should know the answer to this, but
> >> obviously,
> >>>> today at least, if I
> >>>> had a brain I'd be dangerous!
> >>>>
> >>>> :-)
> >>>>
> >>>> Regards,
> >>>>
> >>>> Alan
> >>>> --
> >>>> Principal Engineer,
> >>>> BAE Systems Limited,
> >>>> UK
> >>>
> >>> -
> >>> 2005 IEEE Symposium on Product Safety Engineering
> >>> 3-4 October Schaumburg, IL
> >>> http://www.ieee-pses.org/symposium
> >>>
-
2005 IEEE Symposium on Product Safety Engineering
3-4 October Schaumburg, IL
http://www.ieee-pses.org/symposium
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