Derek and Robert,
The fields from an electric dipole drop off as: Magnetic: 1/r^2 and 1/r Electric: 1/E^3, 1/E^2, and 1/E For a magnetic dipole (loop): Magnetic: 1/r^3, 1/r^2 and 1/r Electric: 1/E^2, and 1/E This link provides the formulas that will allow to to calculate the field strength versus distance. http://www.conformity.com/0102reflections.html I have correlated these formulas to NEC-2 to confirm that NEC-2 can be used to find the H or E field strength at various distances. Dave Cuthbert Micron Technology From: owner-emc-p...@ieee.org [mailto:owner-emc-p...@ieee.org] On Behalf Of Robert A. Macy Sent: Tuesday, June 21, 2005 8:58 AM To: Y W Leung Cc: emc-p...@ieee.org Subject: Re: Near field H-field measurement. Near field?! You're way inside near field below 200KHz. At those fundamental frequencies, you are correct. Magnetic fields drop at the rate of inverse cube from a dipole. You didn't say the size of the EUT. However, if it is smaller than 1/5 the distance to the antenna, you can correctly assume the "nonradiating" magnetic field is dropping at inverse cube. I'm reluctant to rely on any accuracy using a loop to measure E-Field at that low a frequency. Should use a field probe. The E Field should drop off as the inverse square function. In the range of 50-200KHz, measure E-Field and H-Field at two distances and you should see those ratios. - Robert - On Tue, 21 Jun 2005 13:56:25 +0800 (CST) Y W Leung <leungderek2...@yahoo.com.hk> wrote: > Dear experts, > > I am measuring the fundamental and spurious emissions > from an EUT ( with interanl loop antenna) of fundamental > frequencies of about 50kHz to 200kHz. The measuring range > of spurious emission is up to 1GHz. > > The limits are according to a local standard from a local > regulatory body. > The emission limits for both fundamental and spurious > emissions are quoted in electric field ( dBuV/m) and > magnetic field (dBuA/m), means the emissions from the EUT > must not exceed the limits of both E field AND H-field. > The spectrum analyzer display (dBuV/m) because it already > included the transducer factor ( magnetic field antenna > factor:-55dB/m and the cable loss). > > I am using EMCO 6502 loop antenna for the radiated > emission measurement at 10 measuring distance. Since the > limits are quoted by the regulatory body are at 30 and > 300 meters distance, so I use extrapolation factor (value > of 3) i.e. 20 log (300/10)^3 to determine the > corresponding limit at 10 meters from a 300 meters limit > which equals to 88.3dB mark up for the H-field. > > My questions are : > > 1. From theory, if both Tx and Rx antennas are loops > antenna, under near field conditions, H-fields is > predominant, so the H-field is inversely proportional to > D^3, so I use the extrapolation factor 3. For H-field > measurement , I just use the reading from the spectrum > analyzer plus the cable loss and magnetic antenna factor > (-5dB), and use this calculated value to compare with the > extrapolation value. please correct me if I am wrong. ( I > know there is another approach of extrapolate the > measured value and compare with the limits). Please > correct me the method and extrapolation factor if I am > wrong. > > > 2. Since the requirement of the limits are in both > H-field and E-field. Which extrapolation factor (2 or 3) > should be used to determine the limit from i.e. 300 > meters to 10 meters? The reason of supporting if use 2 is > because under near field coditions, the E-field is > inversely proportional to D^2. But now both Tx and Rx > antennas are loop antennas, so finally I use 3 to > extrapolate the limit of E-field ( i.e. from 300 to 10 > meters) and compare with the spectrum analyzer reading, > please comment and/or correct me if I am wrong. > > Thanks advance for your any comment or opinion. > > Regards, > > Derek Leung. > > > > > > > > > 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 > emc-p...@ieee.org > > 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 emcp...@ptcnh.net > Mike Cantwell mcantw...@ieee.org > > 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: > > 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 emc-p...@ieee.org 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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org 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: 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 emc-p...@ieee.org 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 emcp...@ptcnh.net Mike Cantwell mcantw...@ieee.org 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: http://www.ieeecommunities.org/emc-pstc
