When the radiating or receiving source (wire or antenna) is a significant fraction of the separation between wire and antenna or antenna and antenna, then not only are the (1/r to the n) calculations imprecise, they are WRONG. All those 1/r to the n equations are based on a simplification that ignores the length of the radiating element relative to the separation between radiator and receptor. Therefore, the equations break down when this situation is not obtained in reality.
---------- >From: CARTER <[email protected]> >To: "'Ken Javor'" <[email protected]>, CARTER <[email protected]>, CARTER <[email protected]>, "'[email protected]'" <[email protected]>, franz gisin <[email protected]>, [email protected] >Subject: RE: Rack populating??-Rationalize it!! >Date: Fri, Jul 28, 2000, 10:38 AM > > I agree that the extrapolations are not, shall we say, extremely precise. > Nor are most of the measurements we do - regardless of our confidence in our > equipment and our expertise. All the more reason to try to make test > environments repeatable, and EUT's configured for maximized (worst case?) > emissions. > > -----Original Message----- > From: Ken Javor [mailto:[email protected]] > Sent: Friday, July 28, 2000 3:40 PM > To: CARTER; carter; '[email protected]'; franz gisin; > [email protected] > Subject: Re: Rack populating??-Rationalize it!! > > > > > ---------- >>From: CARTER <[email protected]> >>To: "'Ken Javor'" <[email protected]>, CARTER > <[email protected]>, > "'[email protected]'" <[email protected]>, Franz Gisin > <[email protected]>, [email protected] >>Subject: RE: Rack populating??-Rationalize it!! >>Date: Fri, Jul 28, 2000, 10:14 AM >> > >> I have long forgotten the equation for computing free space transmission >> loss, but it seems to me that when radiated power, frequency, and distance >> are known, one can certainly predict field intensity at any other > distance. >> If that is not true, we have made a large number of terrestrial microwave >> transmission paths work purely by accident. > > MICROWAVE LINKS WORK IN THE FAR FIELD. >> >> Also, CISPR 22 11.2.1: >> "NOTE - If the field-strength measurement at 10 m cannot be made because > of >> high ambient noise levels or for other reasons, measurements of Class B >> EUT's may be made at a closer distance, for example 3 m. An inverse >> proportionality factor of 20 dB per decade should be used to normalize the >> measured data to the specified distance for determining compliance. Card >> should be taken in the measurement of large EUT's at 3 m at frequencies > near >> 30 MHz due to near field effects." > > 10 METER TO 3 METER SCALING WORKS ONLY WHEN THE EUT IS SMALL WITH RESPECT TO > 3 METERS, AND WHEN THE ANTENNA DIMESNIOS ARE SMALL WITH RESPECT TO 3 M. IF > YOU TRIED TO DO A 3 METER SITE ATTENUATION WITH TWO 30 MHz TUNED DIPOLES, > YOU WOULD FIND YOU NEEDED A CORRECTION FACTOR TO ACCOUNT FOR THE NEAR FIELD > EFECTS. >> >> Someone at IEC thinks its possible. And many labs do, in fact, test at 3 >> meters. > > THERE HAVE BEEN A LOT OF PAPERS WRITTEN SAYING THE EXTRAPOLATION IS FAR FROM > ACCURATE. >> >> I was not suggesting that it would be wise or meaningful to do these > things. >> My point is simply that it is wise to maximize the emissions from an EUT > to >> insure that we are not missing emissions which may be above the limits, > and >> that the end user of the equipment is not likely to inadvertently create >> such a situation. > > UNDERSTAND. >> >> _\\|//_ >> (' O-O ') >> ooO-(_)-Ooo >> >> Mark Carter >> AM Communications, Inc. >> [email protected] <mailto:[email protected]> >> Voice: 215-538-8710 >> Fax: 215-538-8779 >> >> >> -----Original Message----- >> From: Ken Javor [mailto:[email protected]] >> Sent: Friday, July 28, 2000 2:51 PM >> To: CARTER; '[email protected]'; Franz Gisin; >> [email protected] >> Subject: Re: Rack populating??-Rationalize it!! >> >> >> There is a very big, very important error in Mr. Carter's point number 2. >> You absolutely CANNOT extrapolate from 10 m to any other distance, unless >> that other distance also happens to be both in the far field of the EUT > AND >> the far field of the measurement antenna. That is the reason for a 10 m >> measurement in the first place. Everyone would be doing three meter >> measurements if it weren't for issues related to three meters not being > far >> enough away. >> >> Extrapolating a 10 m measurement to one airline seat away is totally >> impossible. And completely unnecessary: the victim protected by the 10 m >> measurement is not some arbitrary "gizmoid," it is a radio receiver. You >> aren't supposed to be operating a radio receiver on the airplane. More to >> the point, the problem with laptops and other personal electronics on a >> commercial transport is not EMI to other personal electronics, but >> interference with aircraft antenna-connected receivers, whose antennas are >> mounted external to the aircraft. >> >> ---------- >>>From: CARTER <[email protected]> >>>To: "'[email protected]'" <[email protected]>, Franz >> Gisin >> <[email protected]>, [email protected] >>>Subject: RE: RE: Rack populating??-Rationalize it!! >>>Date: Fri, Jul 28, 2000, 7:35 AM >>> >> >>> >>> Jim, >>> >>> I differ in opinion on at least two counts: >>> >>> 1. CISPR 22 9.1 states "An attempt shall be made to maximize the >> disturbance >>> consistent with typical applications . . . etc." This can and should be >>> construed as creating a worst-case scenario. Worst case is always assumed >> to >>> be within the range of operating conditions reasonably expected. If a >> laptop >>> will have higher emissions when it is in flames, no one (at least no one > I >>> know) is suggesting that it be tested that way. Setting a laptop ablaze > is >>> not within the range of reason. >>> >>> 2. The intent of making measurements at 10 meters, or 3 meters, or with > an >>> absorbing clamp, or any of the other requirements is not to insure that > no >>> interference will occur at 10 meters, 3 meters, or in a clamp, but that >> test >>> results are repeatable. If the limit is 40 dBuV/m at 10 meters, and I > test >>> at 20 meters, will the EUT pass? Of course not. If I have test results >> from >>> a 10 meter set-up, I can compute the effect of an emission at any other >>> distance from the EUT to the next airline seat, or whatever. If I wanted >> to >>> know that. The point is that we're all on the same page that way. >>> >>> If you do the math, you can compute the interfering field strength of an >>> emission from a device in the next airline seat, and if you know the >>> immunity of the gizmoid in that seat, you can pretty much be certain that >>> will not be any disruptive interference. >>> >>> My 2 cents >>> >>> >>> -----Original Message----- >>> From: [email protected] [mailto:[email protected]] >>> Sent: Friday, July 28, 2000 10:36 AM >>> To: Franz Gisin; [email protected] >>> Subject: Re:RE: Rack populating??-Rationalize it!! >>> >>> >>> >>> forwarding for Franz >>> >>> ____________________Reply Separator____________________ >>> Subject: RE: Rack populating??-Rationalize it!! >>> Author: Franz Gisin <[email protected]> >>> Date: 7/27/00 6:48 PM >>> >>> Worst case is not relevant when it comes to defining >>> EMC test configurations for ITE equipment. I do not >>> know of any ITE EMC test standard that specifically >>> states "worst case" must be used. Eveywhere I look I >>> see words like "typical" or "representative" or >>> "minimum". >>> >>> I am willing to bet that 99.9% of you, when you bought >>> your last car, did not insist the car be at least 6 dB >>> under the smog limits under worst case conditions >>> before you bought it (e.g. ask the car manufacturer to >>> load the car down with bricks, take it to a very steep >>> hill, facing the front of the car up the hill instead >>> of down, and then pushed the accelerator all the way >>> to the floor - with the engine running but still cold >>> - before they measured the peak emission levels rather >>> than quasi-peak). Whenever I ask EMC engineers if >>> they do this when they buy a car, they think I am >>> crazy, and yet they see nothing wrong with doing >>> exactly the same thing themselves when it comes to >>> defining EMC test configurations and test methods. >>> >>> Claiming that a configuration more than what the >>> regulations ask for is "failing", is as ridiculous as >>> claiming that a configuration less than what the >>> regulations ask for is "passing". Not a day goes by >>> that I don't hear one or the other being carelessly >>> brandished about. >>> >>> If you want to bring reality into the picture (usually >>> a bad idea when it comes to law and regulations so >>> forgive me for going there anyway), we should make >>> sure all products which are tested at a 10 meter >>> distance are spaced at least 10 meters apart when >>> installed at our customer's sites. Laptops on >>> airplanes should be spaced at least 20 rows apart >>> (this assumes the flight attendants verify each laptop >>> comes from a reputable manufacturer they can >>> personally vouch for as having an EMC department >>> populated by people of integrity). >>> >>> It is worthwhile to note that the ITE EMC emission >>> standards in the US and most of the rest of the world >>> are designed to reduce the probability of >>> interference, not completely eliminate it - just as >>> smog standards are not designed to prevent death >>> should some frustrated EMC engineer decide to take his >>> life by running his smog-compliant car inside a closed >>> garage. >>> >>> How many EMC engineers do you know who are >>> uncomfortable with the "probability" aspects of the >>> regulations and decide to take it upon themselves to >>> get as close as possible to eliminating all potential >>> for interference by distorting the test configuration >>> sections of the standards until they fit under their >>> personal definition of quality? There are a lot of >>> places where quality (mean time between failures, for >>> example) means more to a customer (including car >>> owners) than whether the unit was overdesigned to meet >>> EMC and/or smog standards. I don't feel I am any less >>> of an EMC engineer or manager for making that >>> statement. >>> >>> I would like to close by saying all the comments above >>> are meant to act as a catalyst, food for thought so to >>> speak, rather than be an accurate rendition of how I >>> (or any of the companies I have worked for, am working >>> for, or will work for) perceive what this crazy EMC >>> discipline is all about. Feel free to discuss it as >>> you wish. If any of you want to dialog eyeball to >>> eyeball, look me up at the International EMC symposium >>> in Washington DC next month. I prefer cold beers >>> (domestic or imported) over wine. >>> >>> Franz Gisin >>> Sometimes an EMC Engineer >>> Sometimes an EMC Manager >>> Always Opinionated as .... >>> >>> >>> ------------------------------------------- >>> This message is from the IEEE EMC Society Product Safety >>> Technical Committee emc-pstc discussion list. >>> >>> To cancel your subscription, send mail to: >>> [email protected] >>> with the single line: >>> unsubscribe emc-pstc >>> >>> For help, send mail to the list administrators: >>> Jim Bacher: [email protected] >>> Michael Garretson: [email protected] >>> >>> For policy questions, send mail to: >>> Richard Nute: [email protected] >>> >>> >>> ------------------------------------------- >>> This message is from the IEEE EMC Society Product Safety >>> Technical Committee emc-pstc discussion list. >>> >>> To cancel your subscription, send mail to: >>> [email protected] >>> with the single line: >>> unsubscribe emc-pstc >>> >>> For help, send mail to the list administrators: >>> Jim Bacher: [email protected] >>> Michael Garretson: [email protected] >>> >>> For policy questions, send mail to: >>> Richard Nute: [email protected] >>> >>> >> > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. To cancel your subscription, send mail to: [email protected] with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Jim Bacher: [email protected] Michael Garretson: [email protected] For policy questions, send mail to: Richard Nute: [email protected]
