I was misinterpreted. If a radiating element is electrically long, then the radiation from it can cause currents to flow in an isolated piece of metal according to J = sigma E.
But if the radiating element and the surrounding shield are electrically short and we are specifically shielding against the magnetic component of the radiation, then in order for eddy currents to flow and the shield to work, the shield must be oriented so that it provides a low impedance loop which is cut or penetrated by the magnetic field lines, in accordance with Faraday's Law. > From: "Robert A. Macy" <[email protected]> > Date: Thu, 15 Sep 2005 20:27:32 -0700 > To: Ken Javor <[email protected]> > Cc: [email protected] > Subject: Re: When is a cable shield not a cable shield? > > Ken, > > May be I don't understand what you are saying. My > apologies. I thought you were saying that a floating > shield is only effective against a magnetic field source > *if* that floating shield is larger than the wavelenth of > the frequency it is trying to shield, which of course is > not true. > > I thought you were saying that if, say, I have a loop of > wire that is 1/2 inch in diameter carrying 200mA at 100MHz > [which of course will cause you to fail radiated Class B] > that when I put a 2 inch plate of floating conductive > material over this loop that it will not effectively do any > shielding, because I *must* shield with much larger than > the wavelength of 100 MHz, which is around 118 inches, or > 10 feet? > > - Robert - > > On Thu, 15 Sep 2005 19:33:09 -0500 > Ken Javor <[email protected]> wrote: >> 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 >> > - 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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