Building on Hans' comments, I would like to add that all launchers of EM radiation start out as primarily either high impedance emitters (primarily an E-field; a voltage on a dipole) or low impedance emitters (primarily an H-field; a current in a loop). As you move away from the launcher structure, the field begins to "normalize", with energy distributed in both the E & H fields. At some distance, you will find that the ratio of the E-field strength to the H-field strength is 377 ohms. (Volts per meter divided by amps per meter simplifies to volts/amps, which is ohms. It's the law.) This is the far field transition point, and is usually about 3 wavelengths out from the emitter structure.
At 1 GHz, a wavelength is about 30 centimeters. If your shield (with an aperture) is located only 1 cm away from the radiating structure, then you are very much in the near field. You might want to look at Volume 3 of the White EMC Handbook series, Electromagnetic Shielding, by Don White & Mike Mardiguian. (My copy is dated 1988, so there is probably a newer addition available. I think I heard that it was available on CD ROM.) Their Equation 1.15 develops a wave impedance for a pure electric field, at 1 GHz, at a separation distance of 1 cm, as 1800 ohms. This is about 477% higher than the plane wave 377 ohm impedance. (If your source was magnetic, then the wave impedance would be about 80 ohms, which is 21% of the plane wave Z.) Section 2.1 of the book then goes on to explore performance of shielding materials, with Section 2.1.5 dealing with screens and holes and slots. If I'm reading their Equations 2.14 & 2.15 correctly, then a barrier shielding effectiveness will be plane wave SE times the wave impedance ratio. So, for an electric field, the E-field SE would be 4.77 x the plane wave SE. The H-field SE would be 0.21 x the plane wave SE. Using this logic, you could then extend the various formulas and guides for different shapes of apertures by just multiplying the wave impedance ratio times the plane wave SE. That sounds intuitively right to me (although this tough math always gives me a headache). Ed Ed Price ed.pr...@cubic.com Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Services Is Our Specialty Shake-Bake-Shock - Metrology - Reliability Analysis >-----Original Message----- >From: Hans Mellberg [mailto:emcconsult...@yahoo.com] >Sent: Tuesday, June 05, 2001 11:35 AM >To: Neven Pischl; emc-p...@majordomo.ieee.org; >si-l...@silab.eng.sun.com >Subject: Re: Shielding Effectiveness Question > > > >Neven; > >In the near field one must know whether the offending emmiter >is mostly generated >from a magnetic loop or electric field antenna structure. Once >that is known, the >eventual transmitted portion can be computed. If your >shielding is non Fe, Ni or Co >and the source is of a magnetic loop nature then there will be >minimal attenuation. >If the source resembles an electric field antenna (less likely >at lower frequencies) >then the usual shielding formulae may apply since they are >primarily based on >E-fields to some extent and you will have attenuation. > > > > >===== >Best Regards >Hans Mellberg >Regulatory Compliance & EMC Design Services Consultant >By the Pacific Coast next to Silicon Valley, >Santa Cruz, CA, USA >408-507-9694 > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://www.rcic.com/ click on "Virtual Conference Hall,"
