John/Bob, We actually had some degree of floor reflection and continued to perform antenna height adjustments (1 to 2.7m, due to room limitations). We didn't nearly get the amount of change in measured value versus antenna height as we did with a reflective floor, though. For RI testing, we also had to add a couple blocks of 3' absorber in the middle of the floor. We also used 3' absorber in the side walls and 4' in the end walls. Yes, we interpolated the test distance using the factor 9.5 dB. The region from 30 to 150 MHz, you could "write off", but everything higher was amazingly predictable based on chamber measurements. Fortunately, our equipment rarely had emissions below 150 MHz.
By the way, if anyone would like to build a "real" comb generator, let me know privately and I can send you the DIY details. It produces harmonics every 5 MHz and has useful energy out to 1500 MHz. This was published in the early 90's by RF Design Magazine. Regards, Ken Wyatt Technical Services, LLC 56 Aspen Dr. Woodland Park, CO 80863 Email: [email protected] Web: www.emc-seminars.com (719) 310-5418 (888) 212-4602 toll-free On Apr 13, 2009, at 8:54 AM, John McAuley wrote: Hi Ken Does this mean that the ground reflected component is not necessary to achieve the NSA at 3m? Did varying the receive antenna height make any difference to the NSA. Were you doing a simple linear interpolation between 10m and 3m? What area of waffle tiles did you use? Complete floor coverage? Lots of questions, I know. Thanks John McAuley From: [email protected] [mailto:[email protected]] On Behalf Of Ken Wyatt Sent: 13 April 2009 15:26 To: Elliott Mac-FME001 Cc: [email protected] Subject: Re: [PSES] any recommendations on floor absorber? Hi Mac, et al, At the Agilent - Colorado Springs test lab, we replaced our reflective floor in the 3m chamber with the waffle tiles, covering it over with thin plywood and carpeting. With the antenna / turntable angled from the center line, we were able to achieve NSA without any trouble (30-1000 MHz). We never measured above that, though, as we didn't have equipment standards that required it. However, the comparison between 10m OATS and 3m FAC was almost too good to be true. We were often within 2 dB and hardly ever more than 4-5 dB different. This was data from real products with cables attached and spread out, as well as comb generator data. ...and yes, I know all the affects of near field versus far field and 3m versus 10m distance. We were maybe just plain lucky, who knows? Bottom line is that the waffle tiles did us GOOD! Regards, Ken ---------------------------------------------- Wyatt Technical Services, LLC 56 Aspen Dr. Woodland Park, CO 80863 Email: [email protected] Web: www.emc-seminars.com (719) 310-5418 (888) 212-4602 toll-free On Apr 10, 2009, at 10:51 PM, Elliott Mac-FME001 wrote: Hi Charles - interesting question. I haven't really had any experience with this. Ferrite tiles become more reflective at higher frequencies so may be unlikely to meet the criteria I would bet. However, I have recently seen some "waffle" ferrite walls at a lab recently that actually worked pretty well at not attenuating the incident wave at higher freqs but instead "scattered" the reflections and the dispersement seemed to work fairly well. Correlation to OATS pretty good up to at least 8 GHz. Whether or not it meets the CISPR site requirements would be a good study.... Anybody out there have any luck using ferrite material Best regards, Mac Elliott [ ] General Public ________________________________ From: Grasso, Charles [mailto:[email protected]] Sent: Friday, April 10, 2009 6:45 PM To: Elliott Mac-FME001; Knighten, Jim L; [email protected] Subject: RE: any recommendations on floor absorber? Typically folks with chambers use tile – is that out of the question? ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Elliott Mac-FME001 Sent: Friday, April 10, 2009 2:59 PM To: Knighten, Jim L; [email protected] Subject: RE: any recommendations on floor absorber? Jim Last year I started looking into this for my OATS as well but was primarily looking at the upcoming changes to ANSI C63.4 [absorber with > 20 dB attenuation over 1 GHz over freq range]. New version of ANSI C63.4 will require 2.4m*2.4m square for 3m OATS and proportionally higher for larger distances [about 8m*8m for 10m site..] I found two that meet the requirement, although I am sure there are more out there TDK Model IS-30A ETS Lindgren Model EHP-12PCL Both have distinct pros and cons. The TDK seems like it would be better for anchoring down and leaving on OATS due to closed cell absorber blended with polyethylene which should be fairly impervious to elements [at least that is the theory] The ETS comes in larger pieces and easier for transport but may not be as resistant to elements. Of course price comes into play as well.... I have done some experiments and found that NSA below 1 GHz seems to be OK on 3m site except for 1-2 sites that are out. Could have been operator error as we were doing yearly NSA tests and just stuck the absorber out there to get general idea what would happen. No time to investigate anomaly yet... Am cautiously optimistic that NSA may be OK with absorber on ground plane, which seems counter intuitive to me because I thought that the ground bounce was included in the equation for ideal site attenuation. This doesn't seem to be the case for my 10m sites which are off axis from the 3m site and we have dual masts for doing primarily Part 90 tests [TX], which won't require absorber and TIA 603 will actually point to old ANSI standard which doesn't have it. There seems to be an edge effect in VPOL if I leave the 3m foam down when I do my site attenuation for my 10m sites. Need more investigation. Using the ANSI C63.5:2006 AFs for NSA may help as well - need to do comparisons. My hope is that my OATS with the absorber will meet the CISPR 16 [site requirements as well. May need some turntable treatment but hopefully not as much of an issue as trying to meet it in a chamber from what I have been hearing. Anyway - more info than you probably needed but though I would share with you and the rest of the group [especially since you have an OATS]. Hope this helps [at least point towards some potential absorber!] Best regards, Mac Elliott [ ] Motorola Confidential Restricted (MCR), [ X ] Motorola Internal Use Only [ ] General Public ________________________________ From: [email protected] [mailto:[email protected]] On Behalf Of Knighten, Jim L Sent: Friday, April 10, 2009 1:57 PM To: [email protected] Subject: any recommendations on floor absorber? I am looking for advice as I prepare for radiated emissions testing above 1 GHz according to the CISPR test method, which becomes mandatory in 2010. I have an OATS with an all weather fiberglass dome (with a 4m diameter turntable) in which I testensembles of floor-standing racks of equipment (each rack ~6 ft high, 2000 lbs.) Does anyone have recommendations to share with me as to candidate absorber materials for placement on the floor to meet the CISPR 16-2-3 measurement method for measuring radiated emissions above 1 GHz? I am guessing that important characteristics for absorbers may be: fragility or durability, absorptionfrequency range, cost, and size. I cannot easily raise my EUT to a height off the floor to clear the absorbers, not can I configure my EUT to place certain chassis higher or lower in the rack. I appreciate your help in advance. Jim __________________________ James L. Knighten, Ph.D. EMC Engineer Teradata Corporation 17095 Via Del Campo San Diego, CA 92127 858-485-2537 – phone 858-485-3788 – fax (unattended) - ---------------------------------------------------------------- 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.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. Website: http://www.ieee-pses.org/ 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]> - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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.ieeecommunities.org/emc-pstc Graphics (in well-used formats), large files, etc. can be posted to that URL. 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