I was only referring to the two common standard distances, 3 and 10 meters, others being available also but not part of this discussion. I was not thinking of the likelihood of interference and what might have driven the choice of those distances, but since you did... The stories I've been told from guys well retired by now, have been that the primary choice of 3 or 10 meters had to do with definition of far/near field. The US used lambda/2pi, the EU used something with antenna dimensions and ripples in the fresnel zone tapering off (you've seen the graph?). There was also as you mentioned, the chance of interference with 3 meters being a household chance of source/victim, and 10 meters being an office or any place other than a residence. A far as the chance of interference, we can imagine, or know of situations everyday that don't fit choosing 3 or 10 meters as representative of common conditions. I'll go with physics. How I got to the fall, or rise, in emissions was that long ago the US required 3 meter testing and EU required 10 meter testing. So I did one after the other, the direction being immaterial to me. 20 log(3/10) or 20 log(10/3) is about 10 with a change of sign. Or as a standard says 20 dB per decade like 3m to 30m is 20 dB, which assumes 1/r falloff. And fortunately I got to use a chamber that had multiple masts at 3 and 10 meters with multiple antennas and SA's so I could make all these measurements while sitting down and never opening a chamber door. Setup stayed the same. Another fine point, is that some of the standards mentions correcting the reading to mandated distance and another standard/s mentions adjusting the limit to the measurement distance.
John Woodgate <[email protected]> wrote: In message <[email protected]>, dated Tue, 19 Jun 2007, Bill Owsley writes: >I've had a number of products pass at 3 meters to fail at 10 and with >nearly the same profile on the spectrum plot, besides the amplitude >that did not drop the 10 dB as expected. But interference is more likely with less separation, and 3 m separation between source and victim is much more likely these days than 10 m. I think you should look at your results as showing that 'the field strength did not RISE as much as expected between 10 m and 3 m', not the other way round. I suspect that 10 m was standardized 60 years ago as a reasonable distance on an OATS when the radio spectrum was quite quiet and products were much larger. -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - 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 _____ Building a website is a piece of cake. Yahoo! Small Business gives you all the tools to get online. <http://us.rd.yahoo.com/evt=48251/*http //smallbusiness.yahoo.com/webhosting/?p=PASSPORTPLUS> - ---------------------------------------------------------------- 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 ______________________________________________________________________
