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 dont 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. Thats 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. J 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 distance between absorber and a reflective metal backing is important for ensuring that the returning wave is in anti-phase 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:[email protected]] Sent: Friday, April 29, 2016 12:14 AM To: [email protected] Subject: [PSES] Anechoic Chamber Questions Hello all, I have some questions about anechoic chambers that I need some help with and Im 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 Ive 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, Im trying to get a handle on our chambers 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 <[email protected]> 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 <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
