>“if you move a source towards / away from the antenna, the signal level should >follow the inverse square law – correct?” >At most frequencies, the wavelength is long compared to the dimensions of the >shielded enclosure, so moving relative to the source is not done in a >far-field >condition and thus far-field relations don’t hold up.
IMHO, at most frequencies the wavelength is SMALL compared to the room, but at CRITICAL frequencies (<100 MHz) I agree with you Ed. The boundary between close and far field is ( if I may refer to Henry Ott' works) around Lambda/2pi , so even with a 2-fold margin the close-field/far-field boundary is extremely small compared to the room (even if 3 of 5 meter). >the distance between absorber and a reflective metal backing is important for >ensuring that the returning wave is in anti-phase” It is recommended by several tile (ferrite) manufacturers to add a certain dielectric distance between wall and ferrite tile, to shift the "resonance" frequency (the freq where incident and reflected wave "cancel") down to a (desired) lower frequency at the cost of a slightly higher reflection coefficient. Additional effect is a wider dampening bandwidth of the tile-wall combination . The optimum dielectric spacing is said to be tile thickness dependent. Our FAR was built to this principle, and although I cannot compare (never build it without spacing) it functions satisfactorily. Regards, Ing. Gert Gremmen Approvals manager ------------------------------------------------------------------------------------ + ce marking of electrical/electronic equipment + Independent Consultancy Services + Compliance Testing and Design for CE marking according to EC-directives: - Electro Magnetic Compatibility 2004/108/EC - Electrical Safety 2006/95/EC - Medical Devices 93/42/EC - Radio & Telecommunication Terminal Equipment 99/5/EC + Improvement of Product Quality and Reliability testing + Education Web: www.cetest.nl (English) Phone : +31 10 415 24 26 ------------------------------------------------------------------- This e-mail and any attachments thereto may contain information that is confidential and/or protected by intellectual property rights and are intended for the sole use of the recipient(s) named above. Any use of the information contained herein (including, but not limited to, total or partial reproduction, communication or distribution in any form) by persons other than the designated recipient(s) is prohibited. If you have received this e-mail in error, please notify the sender either by telephone or by e-mail and delete the material from any computer. Thank you for your co-operation. From: Ed Price [mailto:edpr...@cox.net] Sent: Friday 29 April 2016 11:28 To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: Re: [PSES] Anechoic Chamber Questions Jim: “if you move a source towards / away from the antenna, the signal level should follow the inverse square law – correct?” At most frequencies, the wavelength is long compared to the dimensions of the shielded enclosure, so moving relative to the source is not done in a far-field condition and thus far-field relations don’t hold up. “the return loss characteristic gives the amount of absorption” Yes, sort of. You may get some reflection from the front surface of the absorber. That’s a result of the absorber, but not exactly an absorption effect. The signal that does go through the absorber material then has to reflect off the shielded enclosure wall and then travel back through the absorber. So, the return signal has actually experienced two passes through the absorber and likely some scattering at the reflection. Still, it all adds up to a return loss. ☺ “with hybrid + tile absorber is it just a case of adding the return loss of the hybrid to the return loss of the tile” Just moved past my level of experience, but I would expect that to not be true. I think you will see reflections due to mismatch at the absorber/tile interface, and this will affect the overall return loss. We need an expert to say how much. “ The reflected signal will be shifted 180° by the massive impedance discontinuity (the metal wall), and I never heard of spacing the absorber off the reflective surface for any advantage. There will be some small phase shifting in propagation through the absorber, as the velocity of propagation will be different from air. I have heard of critical thickness and internal spacings for absorbers intended for stealth aircraft, but these are very frequency selective. Paywalls are more like absorbers than reflectors, right? Ed Price WB6WSN Chula Vista, CA USA From: Pawson, James [mailto:james.paw...@echostar.com] Sent: Friday, April 29, 2016 12:14 AM To: EMC-PSTC@LISTSERV.IEEE.ORG Subject: [PSES] Anechoic Chamber Questions Hello all, I have some questions about anechoic chambers that I need some help with and I’m sure there is some expertise in this group that can help. 1. My understanding is that an anechoic chamber is meant to simulate a reflection-less, free space environment. Therefore if you move a source towards / away from the antenna, the signal level should follow the inverse square law – correct? 2. When comparing absorber types (hybrid + ferrite tile vs. foam absorber) the return loss characteristic gives the amount of absorption at a particular frequency – correct? 3. If I wanted to compare effectiveness of foam absorber with hybrid + tile absorber is it just a case of adding the return loss of the hybrid to the return loss of the tile to achieve a final figure? My understanding is that the hybrid helps match the wave impedance from free space to that of the tile. Is the return loss of hybrid + tiles _together_ greater than the individual return losses of the separate components? Manufacturers that I’ve looked at list the data separately. 4. I have been told that the distance between absorber and a reflective metal backing is important for ensuring that the returning wave is in anti-phase (or at least as much as possible) with the incoming signal. However information on acceptable limits for this distance seems sporadic or in rarefied scientific papers behind paywalls. Does anyone have any info or experience on this point? Many thanks for your time, I’m trying to get a handle on our chamber’s performance and any answers will help. Regards, James - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <sdoug...@ieee.org> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher <j.bac...@ieee.org> David Heald <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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