Thank you very much to everyone who has replied, online or offline. If I remember correctly, there was an IEEE paper to compare test data taken at 3m and 10m. It concluded as Bob's comment. The paper was forwarded by a chief engineer, then. I remember it was published in 1993 by an Australian author(s). Regarding NSA at 3m chambers, based on chamber proposals I have received, chamber manufacturers are able to build 3m chambers (about 9m x 6m x 6m, sizes varies by manufacturers) within the required +/- 4 dB NSA (volumetric measurement). The best NSA can be achieved is 3 dB for a 3m chamber. BSMI (Taiwan's government authority for EMC and Safety) has a 10m chamber within +/- 2 dB NSA. I wonder a 30m chamber (if there is one) may have a better NSA. Grace
On 6/20/07, Bob Richards <[email protected]> wrote: I don't think anyone has mentioned it yet, so I will. Due to the effects of the required reflective ground plane, the difference in the theoretical NSA between a 3m and 10m site does not equal 10dB through much of the frequency range. The 10dB rolloff (actually, 10.5dB) really only applies to free space. It has also been my experience that 3m measurements in a 10m chamber more closely follow the theoretical NSA than do measurements at the 10m distance. At my previous employer, the proximity of the walls caused a deviation of almost 4 dB at the 10m distance (from 60 to 100 MHz as I recall), while the 3m measurements were very much closer to theoretical NSA, since the walls were farther away relative to the measurement distance. Generally speaking, everything else the same (like absorber performance) the larger chambers will perform better. Bob Richards, NCT. [email protected] wrote: Just as a point of clarity.....................the +/- 4 dB NSA range applies to OATS as well so there could be as much as 8 dB difference between one OATS and another and both could have good NSAs. The +/- 4 dB is not a parameter just for volumetric measurements but applies to all test sites. With volumetric measurements it is more difficult to attain the +/- 4 dB. And I have to agree with an earlier email. A lot 3 meter "chambers" that I have seen are "home made" and would not pass NSA. Bob Heller 3M EMC Laboratory, 76-1-01 St. Paul, MN 55107-1208 Tel: 651- 778-6336 Fax: 651-778-6252 ======================================= ========================================================== - ---------------------------------------------------------------- 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 <mailto:[email protected]> [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: <mailto:[email protected]> [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 ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
