There is not much point to further discussion, because I just keep repeating myself. The case you are discussing is partially valid where the shield material is large relative to a wavelength, and it is reflecting an electromagnetic field. The currents induced in this case are due to the electric field vector, according to J = sigma E. But even here, without an adequate shield termination the currents induced to flow on the inside of the shield will escape to the outside and re-radiate.
If the shielding is specifically for the low frequency magnetic component of the field then the eddy currents flow due to Faraday Law induction, which is a vector phenomenon, and then the shield has to form a loop penetrated by the magnetic field vector, or it won't work. > From: "Robert A. Macy" <[email protected]> > Date: Thu, 15 Sep 2005 15:27:54 -0700 > To: Ken Javor <[email protected]> > Cc: [email protected] > Subject: Re: When is a cable shield not a cable shield? > > 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 This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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