We have also performed some experimentation with our compact semi-anechoic chamber (prescan) and OATS, both at 3 meters. Our experience correlates with Doug's. For devices as small as say a shoebox, our correlation is typically within 2 dB. For systems that are 1.5 meters and floor mounted with a few interconnecting cables, our correlation is typically within 4 dB. We have seen an occasional emissions in the chamber that did not show up on the site. As we use a bilog in the chamber and use bicon and log periodic on the site, we did not pursue the "why". I would assume that it is related to the imperfect absorption of the chamber compared to the open space of the OATS.
Another part that might make a difference in your measurements is the conditions at each site. Our chamber is a fairly constant 72 degrees, 50% humidity producing a constant NSA. The OATS, however, varies in its moisture content which we have learned contributes to the variation between chamber and OATS. When the site is dry, we get a "down the middle" NSA; when it is soaked, we get closer to the NSA limit and also a larger variation between the chamber and OATS as would be expected. We have not yet discovered why the moisture affects the NSA. On the other hand, the OATS is compliant and we have learned to deal with the "degrees" of variation, so at this point we live with it. Don Umbdenstock > ---------- > From: Doug McKean[SMTP:[email protected]] > Reply To: Doug McKean > Sent: Monday, January 07, 2002 1:44 PM > To: EMC-PSTC Discussion Group > Subject: Re: Comparing EMI test results > > > Not to rain on the parade, but be careful when > comparing semi-anechoic chambers versus OATS > that are of different distances, i.e. 3m vs. 10m. You > may get not only widely different measurements > even when compensating for the distances, but you > may also obtain different frequencies. On two > occasions, I've had frequencies show up rather > prominently in the chamber prescan not to be > seen to any significance at an OATS. > > In other words, to get a fairly reliable comparison > between a chamber and a site, in my opinion, you'll > first have to compare a 10m chamber with a 10m > OATS. Then, work backwards to a 3m chamber. > > You will also have to develop a fudge factor for the > size of the EUT. The size can effect results as well. > You may get good correlation with a small tabletop > EUT and terrible correlation to a much larger device. > Possible reasons for this I'll leave to discussion. But > suffice it to say, the closer the product is to a point > source the better the correlation. The farther the > product is from a point source, the worse the > correlation. At least in my experience. > > Name the site with which you will be getting an > official measurement as your standard, then work > all the others in reference to it. The standard site > will be the one which will have the final say. > > Sorta been there ... > Done that ... etc ... > Sorta gave up. > > - Doug McKean > > > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > [email protected] > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: [email protected] > Dave Heald [email protected] > > For policy questions, send mail to: > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > No longer online until our new server is brought online and the old > messages are imported into the new server. > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: [email protected] Dave Heald [email protected] For policy questions, send mail to: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
