There will be NO compatibility issues between the two equipments you posit over the frequency range of the immunity/emission tests. The fields do not scale as you suggest. What you suggest is a worst case and you will encounter no problem with such a separation as you calculate, but it is TOTALLY unnecessary.
The error is a common one and it causes a lot of mischief. The equations that predict field intensity as a function of distance always assume that the radiation source dimensions are small relative to the source to point-of-interest separation. When the source is a cable or slot in a chassis and the separation from that source is commensurate with the source size, this analysis is inaccurate. Notice I have not mentioned wavelength at all in this discussion. It is not necessary to discuss far-field / near-field issues here, the problem is more basic and underlies the derivation of equations of both near and far field performance. Consider a single example for the sake of illustration. This is not your problem, but it is easier to visualize. Consider a cable 5 cm above ground with an rf potential relative to ground. Imagine that potential to be 10 mV, common mode. My experience says this is very high relative to what would allow a specification level emission at 3 or 10 meters at 30 MHz and up, but this is a worst case analysis. Some of the designers on this forum might be able to weigh in with a more authoritative number. What is the maximum field intensity you would measure if you were to bring your antenna in from 3 meters in close? Clearly the electric field is bounded by the quotient of the rf potential and its separation from ground, or 0.01 V/0.05 m = 0.2 V/m. And you could never measure that with an antenna because that field intensity only exists between cable and ground. In your world ground as I described it really doesn't exist and the issue is more cable-driven currents radiating electromagnetic fields. The potentials here are not cable-to-ground, but along the cables themselves, which makes it harder to visualize the electric field. But the issue is the same: the field gradient does not increase without bound but is limited to a low value by the relatively low cable drive and the finite separations that generate the gradients, which separations are assumed to be identically equal to ZERO in the equations usually applied to perform these calculations. Ken Javor on 1/9/02 11:18 PM, peter.pou...@invensys.com at peter.pou...@invensys.com wrote: > > Hi Folks. > > At the moment I'm examining as a generic case, the potential for > interference with Item A (tested to comply with 3V/m radiated immunity) > caused by Item B (tested to comply with FCC or EN Class A [industrial] > emissions). > > Using simple inverse distance ( E2 = E1 x d1/d2 ) extrapolation (assuming > dominant interfering frequencies will be in the far field), I come up with > a required separation distance of approximately 75cm to ensure the 3V/m > immunity limit of Item A isn't exceeded by the 47dBuV/m emissions from Item > B. > > Based on this, I'd expect then the risk for EMC problems should be > relatively low provided: > 1. A minimum separation of 1m was used between Items A & B; > 2. No direct interconnection of A to B via cables; > 3. Use of a mains filter and/or separate power supply sources for A & B; > 4. The nature of Item B is such that no significant low (eg.power) > frequency magnetic fields are emitted; > > Does anyone have any experience to suggest that the minimum separation of > 1m under theses conditions would not be adequate? > > Thanks, > > Peter Poulos > Design Engineer > Foxboro Transportation > (Invensys Rail Systems Australia) > > > > ------------------------------------------- > 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: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: pstc_ad...@garretson.org > Dave Heald davehe...@mediaone.net > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > 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: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org 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.
