On Thu, 10 Jul 2003 12:58:49 -0600, [email protected] wrote: >I have a question on apertures. You may recall the formula that is frequently given for signal attenuation through a small aperture in a large conductive sheet. It is 20LOG(I/2L), where I is the wavelength and L is the slot length. For example, if x is 1/2-wavelength then the attenuation is 0 dB. But I'm not 100% sure what the attenuation is referenced to. If they are referencing it to the E-field that would be present at the aperture location if the sheet were not there to the E-field across the length of the aperture then that makes sense. It seems that we now have a 1/2 wavelength aperture radiating only the signal energy that it has intercepted. > >Let's say it is referenced to the E-field that would be present with no sheet. Now to say that the E-field a large distance away from the 1/2 wavelength aperture has not been attenuated by the aperture is wrong, although this is implied by the formula. Only a fraction of the energy contained in the total incident wave has made it through the aperture. The aperture now acts as a dipole radiating this fraction of the total incident wave. > >So is the attenuation given by this formula to be referenced to the power that would be intercepted by a dipole?
I'm not experienced in radiation issues, so I might be off-base. I remember this subject being discussed in Henry Ott's book, "Noise Reduction Techniques in Electronic Systems". On page 182, he starts talking about shielding with magnetic metals, and about apertures specifically on page 187. It looks like the subject concerns magnetic currents induced in the metal radiating at the point of discontinuity (the slot). Is this the same situation as measuring shielding effectiveness with two antennas? 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: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: [email protected] Dave Heald: [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] Archive is being moved, we will announce when it is back on-line. All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
