Re: [SI-LIST] : Shielding Effectivness Question
Neven, To analyze the noise picked up by a wire that crosses a hole/slot in a shield, look at: [831] Lin, Guoxian, "Electromagnetic Excitation of a Wire Crossing a Long Slot in an Infinite Plane," Electromagnetic Compatibility 1989, Zurich, Switzerland, Mar. 7-9, 1989, pp. 89-92. [938] Nakano, Hisamatsu, Yamauchi, Junji, Eda, Masahiro, and Iwasaki, Takeshi, "Numerical Analysis of Electromagnetic Couplings Between Wires and Slots Using Integral Equations," 4th International Conference on Antennas and Propagation (ICAP 85), Coventry, UK, Apr. 16-19, 1985, pp. 438-442. [966] Parmantier, J. P., and Aparicio, J. P., "Electromagnetic Topology: Coupling of Two Wires Through an Aperture," Electromagnetic Compatibility 1991, Zurich, Switzerland, Mar. 12-14, 1991, pp. 595-600. [1211] Taylor, Clayborne D., Marcum, Frank, Prather, William D., and Herrmann, Carl C., "On Using a Sense Wire to Quantitate the Magnetic Flux Leakage Through an Aperture in an Electromagnetic Shield," IEEE Transactions on Electromagnetic Compatibility, vol. 31 no. 4, pp. 337-341, Nov. 1989. My article on "Designing Electronic Equipment for ESD-Immunity" is being edited by Printed Circuit Design magazine, and should appear on their webpage http://www.pcdmag.com/ pretty soon now. Just last week I received several E-mails from Andy Shaughnessy asking for clarification of points in my article. My list of references, which is supposed to be posted with the article, covers 70-some books and booklets, and about 1300 pertinent standards/reports/papers/articles gleaned from engineering and physics publications going back to the mid-1970's. John Barnes Advisory Engineer Lexmark International To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
Re: [SI-LIST] : Shielding Effectivness Question
RE: Product Safety: A Matter of Law or Litigation?Neven, I did some rather intensive analysis of shielding effectiveness over the course of several weeks a while back. Results were extremely informative. I've read a couple of fields and waves books for slot analysis. Rau is the one I prefer. I'm at work, but I'll get the name of it and the ISBN as well. I've done some experiments with "small" stubs and loops with reference to wavelength. And I was surprised to find a rather predictable approx -10dB rolloff per several decades from lo to high frequencies. Results were debatable. Converting the wire stubs over to slots by way of Babinet makes me think that the roll-off for slots is similar. And with this information, I started my little analysis in the chamber on products. I haven't seen nor am I really worried about problems with holes. Unless of course they're big. Slots are your worst violator. This is because of the mechanism by which apertures work. It's, in my opinion, more difficult to impose a voltage differential across a round hole than it is across a slot. Slots are more directional than wires by a factor of roughly 10. That makes your product particularly susceptible during emissions. Anywho, I'll get you the name of the book and specifics for ya. And if you want, maybe you can pop over here to do some testing. - Doug McKean To unsubscribe from si-list or si-list-digest: send e-mail to majord...@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu
Re: [SI-LIST] : Shielding Effectivness Question
Neven, The phenomenon you describe is pretty well understood and covered in the literature. What follows is from memory but is essentially the idea, so check the literature! Basically what you have, with a small opening, is a waveguide operating below its cutoff frequency. While it is true that power incident on the input port of such a waveguide does not propagate through it, in the sense of a propagating mode, electromagnetic fields will exist within the guide and will decay exponentially along the length of the guide. These are referred to as cutoff or evanescent modes. Unfortunately, when the evanescent mode reaches the other end of the waveguide, it will propagate in the space beyond, albeit attenuated significantly. Propagation through a waveguide is proportional to the complex exponential e^(-jBz). In a waveguide operating below it's cutoff frequency, beta (B), the propagation constant becomes negative-imaginary, B=-jA. The result is an exponential decay of the field strength as e^(-Az). The amount of attenuation is then dictated by the position within the guide, z, as measured from the input, and the value of A. Beta, B is the propagation constant and is computed for rectangular wave guide as: B = sqrt(k^2 -kc^2) k = 2*pi*freq*sqrt(ue) kc = sqrt( (m*pi/a)^2 + (n*pi/b)^2) a = long dimension of waveguide cross section b = short dimension of waveguide cross section m and n are the mode indices When kc > k the guide is in cutoff and B = -j(kc^2 - k^2) = -jA. In the case of a rectangular waveguide operating in cutoff we are only interested in the lowest frequency mode, TE10, The others operate at much higher attenuations and we want worst case. In this case m=1 and n=0. So my best guess is your attenuation should be something like 20log(e^(-Az)) dB, for a single rectangular waveguide, of length z, operating below cutoff. This could be easily extended to circular waveguide as well, if you have round holes. Well, that's my WAG at your question. As I said, this is covered in the literature. Any EM text or microwave engineering text will have the governing equations. One of my favorites is Microwave Engineering by David Pozar. Perhaps others can suggest more. Simulation could be done quite effectively using a field solver. We use Ansoft's HFSS. There are others as well. I would not dismiss simulation completely, but like you, I appreciate an analytical understanding. It keeps you out of trouble! Well most of the time anyway... I hope this helps. Cheers, -Rob Hinz Consulting Engineer SiQual, Signal Quality Engineering 18735 SW Boones Ferry Road Tualatin, OR 97062-3090 (503) 885-1231 http://www.siqual.com/ At 09:16 AM 6/5/2001 -0700, Neven Pischl wrote: I would appreciate if anyone could let me know if there are any references (books, application notes, anythig ..) that deal with shielding efectivness in cases when a source is close to an (electrically small) opening in a shield (enclosure). In such a situation, the field will penetrate through the hole and leak even if the size is much smaller than the wavelength. I am particularly interested in situation when high-frequency source, such as a PCB edge or a component operating at (say) 1 GHz and above is in proximity of the venting holes, "small" gaps in the chassis etc. All references that I have deal with uniform plane wave propagating incident to a metal plane with a slot or hole, in which case it is enought o have electrically small size of the opening (e.g. lambda/10) to efficiently block any field propagation through the barrier. I can't find any useful reference that deals in any analytical way with the situation I am intersted in. I believe I might get some answers using some of the simulation programs, but at the moment I am more intersted in the analysis of the problem than in simulating it. Thank you, Neven Pischl Rob Hinz SiQual Corporation r...@siqual.com phone (503)885-1231 x30 fax (503)885-0550
