You have to be very careful about calculating attenuation through perforations. First, the basic KL/G is correct. As you might expect, G in the equation is either the hole diameter, or the width of a rectangular hole. The equation, however, goes beyond that, when dealing with a thin sheet of perforated metal. There are several correction factors (many of which don't contribute much) which apply. These include attenuation or absorption loss (A), reflection losses (R), correction factor for the number of openings/square (k), correction for coupling between closely space holes (K), and correction for conductor penetration at low frequencies (a) (e.g.-skin depth).
The Air Force Design Handbook DH1-4 is a good source of material for perforated metals. They got some of their data from IEEE EMC Symposium Record from July 1968. Feel free to contact me if you would like to discuss this further. Otherwise, Happy Reading! Bob Martin, P.E., N.C.E. Sr. Technical Manager Intertek Testing Services (978)263-2662 fax (978)263-7086 [email protected] The views expressed are my own and not necessarily those of my employer. ---------- From: WOODS, RICHARD To: 'emc-pstc' Subject: Desiging Openings for EMC Compliance List-Post: [email protected] Date: Monday, July 20, 1998 8:56AM I am about to give a short seminar to our Mechanical Engineers and Designers on enclosure design for EMC compliance. There is only one problem - I have no faith in the theory I have for the attenuation through openings. The following formula is from the "EMC Handbook", Vol 3, by Don White. Assuming the frequencies of interest are below the waveguide cutoff frequency, the formula is A(dB) = KL/G - 20 log N where, K = 32 for round holes or 27 for square holes L = thickness of panel G = hole diameter N = number of holes. According to this formula, one 1/4 inch hole in a 0.090 inch panel would have an attenuation of 11 dB, and ten holes would have no attenuation whatsoever. This does not match my experience in typical ITE. Does anyone have any usable "rules of thumb" for Mechicanical types? Richard Woods Sensormatic Electronics [email protected] Views expressed by the author do not necessarily represent those of Sensormatic.
