Grace, I believe you already have the answer on the bare minimum distance from the quiet zone to the wall according to the emissions standards from John Woodgate. However, that only answers what is required by the standard as opposed to what spacing is needed to have an effective chamber.
Generally, the chamber with its ferrite tiles will work best if the wall & ferrite treatment is located at the distance where the signals are in the far field. Attached are some comments from Dave Eckhardt (Sun Louisville EMC Engineer). - - - - - - - - - - - - - - - - - - - - - - Generally, the following expression is accepted as the “boundary” between near field and far field conditions (complex wave impedance vs. “mostly” real wave impedance) for an EM wave spreading from a source or antenna: L = 2 DD / (lambda) where: L = distance from source or antenna to far field conditions D = largest physical dimension of EUT or antenna lambda = free space wavelength The lowest frequency which is required to be measured for FCC or EN55022 is 30 MHz. The free space wavelength at 30 MHz is then: lambda (30 MHz) = c / frequency = 10 m If the largest dimension of the product is roughly 5 m as we have, then the minimum distance to any chamber wall treated with ferrite/cones should be: L = 2 (5 m) (5 m) / 10 m = 5 m Note this distance is to the EUT-facing physical boundary of the chamber wall treatment to the nearest physical conducting structural member of the EUT. This distance should assure that the propagating EM waves from the largest product are relatively close to a plane wave and reasonably orthogonal to the chamber walls. Note that the ferrite wall treatment REQUIRES orthogonal incidence for a good match. Non-orthogonal incidence generates standing waves within the chamber and renders the wall treatment less effective resulting in questionable volumetric performance of the chamber. In this example, the EUT end of the chamber (the quiet zone) should be a minimum of 5 meters from the rear and side walls. - - - - - - - - - - - - - - - - - - - - - - For a more detailed treatment and derivation of the near-field versus far-field, Dave recommends reading Balinas' textbook on antennas. Based on this discussion, the 1 meter bare minimum distance described in EN50147-2 would only adequately handle a product whose longest dimension was under 2.2 meters in length. Bottom line with chambers, the bigger the chamber, the better the low frequency performance. Of course, the larger chambers do cost a little more money. Our chamber quote increased by 20% to go from 1 meter to 5 meters spacing for a chamber with a 6 meter turntable, but you get what you pay for. * Monrad L. Monsen * Product Compliance Program Manager Storage Group* Sun Microsystems* [email protected] Grace Lin wrote: > Dear Members, > Is there any minimum distance requirement between the edge of > turntable (quiet zone) and the tip of absorbers in a 3m semi-anechoic > chamber (interior metal-to-metal dimensions of 28'x18'x18')? I get an > impression that one meter is required. I didn't find this distance > requirement from ANSI C63.4. Clause 5 of EN 50147-2: 1996 states: > "The vertical and horizontal polarization measurements in the rear > position may be omitted if the closest point of the construction > and/or absorbing material is at a distance greater than 1 m from the > rear boundary of the tes volume." > I assume keeping a 1 m distance is a good idea. However, a chamber > manufacturer has a difficulty to do so. > Could you please share your knowledge and comments? > Thank you very much. > Best regards, > Grace > - ---------------------------------------------------------------- > This message is from the IEEE Product Safety Engineering Society > emc-pstc discussion list. Website: http://www.ieee-pses.org/ > > To post a message to the list, send your e-mail to [email protected] > > Instructions: http://listserv.ieee.org/request/user-guide.html > > List rules: http://www.ieee-pses.org/listrules.html > > For help, send mail to the list administrators: > > Scott Douglas [email protected] Mike Cantwell [email protected] > > For policy questions, send mail to: > > Jim Bacher: [email protected] David Heald: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
