I disagree with this part: "Near field or far field only matters when we want to know what one will be >from the other, and this can be calculated."
Theoretically you could map the entire wave-front surface at a given separation from the EUT and connected cables and using an electrical analog of the optical Huygens wave-front principle you could extrapolate the field at larger separations. But practically speaking you would have to separate out the radial and tangential electric fields, and you would have to accurately determine the vector nature (direction) of each portion of the wave-front as well as its magnitude, and this would have to be done in small enough "patches" that the magnitude and direction of the field vectors would be constant over the patch and it isn't going to happen. The only practical solution is to use a probe that looks like the one the victim of rfi is going to be using, connected to a receiver of equal to or greater sensitivity than the victim, and similar bandwidth, and verify that the level of rfi at a particular separation is sufficiently low as to not interfere with a predetermined level of broadcast reception. And the measurement is made in such a way as to maximize emissions, in the traditional manner. And you cannot of course extrapolate the near field from the far field; not that anyone is trying. > From: Cortland Richmond <[email protected]> > Date: Thu, 23 Jun 2005 15:53:34 -0400 > To: ieee pstc list <[email protected]> > Subject: RE: ??? RE: ??? RE: Near field H-field measurement. > > Bob Heller asked: > >>> How would one go about accurately measuring low frequency fields say at > 10 > kHz where the far-field even at lambda/6 is 5000 meters? << > > Accuracy and what regulatory authorities require are not always the same > thing. It isn't hard to measure the magnetic field accurately, given a loop > whose response is known (and which is either well balanced or shielded > against E-field coupling) and an accurate receiver, analyzer, etc.. Near > field or far field only matters when we want to know what one will be from > the other, and this can be calculated. > > But do our regulatory agencies want to know the (accurate) result in > dBpA/m? Usually not. They want dBuV/m -- and to simplify things, probably > tell us to assume far-field conditions. Our measurements would then be > reasonably precise -- and inaccurate. This is not necessarily a bad thing. > It makes measurements faster and easier and *so long as limits are properly > crafted* still protects victim devices. If everyone uses the same > (inaccurate) method with reasonable precision, we'll get results that > aren't wildly different from each others', so the tests we do will have a > usable relationship to the real world even though the numbers are > "predistorted." > > (It still bugs me. If an accountant predistorted our corporate bosses' > books this way someone would go to jail. But in this case, it is probably > good enough. C'est la vie!) > > > > > Cortland Richmond > KA5S > > ---------------------------------------------------------------- > 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/listserv/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: > > Richard Nute: [email protected] > Jim Bacher: [email protected] > > All emc-pstc postings are archived and searchable on the web at: > > http://www.ieeecommunities.org/emc-pstc > 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/listserv/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: Richard Nute: [email protected] Jim Bacher: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
