A really big field test of all the emergency communications happened on 9/11. We know the results. Subsequently there have been efforts to fix the problem. However, the individuals of the service providers, me and others, use our cell phones as backup. They seem to inflict and suffer much less EMI.
Marko Radojicic <[email protected]> wrote: Extrapolating from the near field (3m) to the really-really near field (10cm) is not possible, unfortunately. The only way, IMO, to know that two pieces of equipment will work harmoniously together is to actually test the configuration. This conversation has been interesting, and shocking, to me that emergency services that use intentional transmitters do not test all the pieces together before putting them in a life-critical situation. Remember the British warship during the Falklands war that could not use their radio and radar at the same time and thus was hit by a missile? That was a situation that should never have been allowed to go to combat. I was surprised that no criminal negligence lawsuits were brought up. [If it had been an American (land of the lawyers) ship, it probably would have been.] ...Marko From: [email protected] [mailto:[email protected]] On Behalf Of Kunde, Brian Sent: Thursday, June 28, 2007 10:23 AM To: Ken Javor; [email protected] Subject: RE: Internal Margins for Emissions Testing Comparing the emissions limits at 3 meter (lets say 40dbuV/m) verses the field strength of 3V/m (immunity test), if my math is correct, isn't 3V/m about 130dbuV/m (e-field)? At 10cm (4 inches), isn't the maximun field strength of a Class B product about 70dbuV/m? Am I doing this right? Most products should be able to handle that (not counting transmitters). The Other Brian From: [email protected] [mailto:[email protected]] On Behalf Of Ken Javor Sent: Thursday, June 28, 2007 11:28 AM To: [email protected] Subject: Re: Internal Margins for Emissions Testing I don't understand part of the following paragraph: "Again, I'm not sure what you mean. A test distance of 3 m should not be assumed to mean that below 3 m interference is certain. The test level is set higher than that. On the other hand, it isn't set so high that interference at 10 cm cannot occur." I get the first sentence. I don't understand the second sentence. We are talking about emission limits and testing here, correct? And if we are discussing emission limits, then it would make more sense (to me) if the last sentence read, "On the other hand, it isn't set so low that interference at 10 cm cannot occur." > From: John Woodgate > Date: Thu, 28 Jun 2007 16:06:09 +0100 > To: [email protected] > Subject: Re: Internal Margins for Emissions Testing > > In message <[email protected]>, dated Thu, > 28 Jun 2007, Gary Stuyvenberg writes: > >> In reference to your first comment, can you explain this further? >> That sounds like circular reasoning to me. Does not the EMC >> Directive take precedence and set the tone for the standards? > > I don't think I understand the question. The Directive just has two > Essential Requirements: > > - not to cause interference, and > > - not to be unduly sensitive to it. > > The standards do not say 'no emissions' and 'infinite immunity'; they > obviously set what a majority, at least, of the relevant national > standards committees consider reasonable, practicable and economically > supportable. >> >> Responding to your second comment, it was assumed any other products >> experiencing interference were compliant. Also, by performing >> immunity testing at specific distance and field strength, are you not >> defining encroachment limits for your product? > > Again, I'm not sure what you mean. A test distance of 3 m should not > be assumed to mean that below 3 m interference is certain. The test > level is set higher than that. On the other hand, it isn't set so high > that interference at 10 cm cannot occur. > > What you can't do is to calculate from the emission limit and test > distance, and the immunity limit, the separation below which > interference is inevitable. This is because many external factors > influence the field strength at any point. A calculation would be > valid in free space, provided the result gave a distance in the far > field of the source. But not otherwise. > -- > 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. 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