Mr. Leung, Here is an excerpt from the FCC Rules pertaining to measurements below 30 MHz using a loop antenna.
"At frequencies below 30 MHz, measurements may be performed at a distance closer than that specified in the regulations; however, an attempt should be made to avoid making measurements in the near field. Pending the development of an appropriate measurement procedure for measurements performed below 30 MHz, when performing measurements at a closer distance than specified, the results shall be extrapolated to the specified distance by either making measurements at a minimum of two distances on at least one radial to determine the proper extrapolation factor or by using the square of an inverse linear distance extrapolation factor (40 dB/decade)." Bill Stumpf D.L.S. Electronics 166 South Carter Street Genoa City WI 53128 bstu...@dlsemc.com phone: 262.279.0210 _____ From: owner-emc-p...@ieee.org [mailto:owner-emc-p...@ieee.org] On Behalf Of Ken Javor Sent: Tuesday, June 21, 2005 9:19 AM To: emc-p...@ieee.org Subject: Re: Near field H-field measurement. A limit specified at 300 meters is outside my experience. I have seen limits at 1 m (military ), and 3, 10, and 30 meters (commercial). I'm not sure how to extrapolate below 30 MHz where you are using the EMCO loop. But above 30 MHz where you are only measuring electric field, the extrapolation is 1/r, that is, electric field rolls off linearly with increasing distance. It does not matter that the sources are magnetic loops - at 30 MHz and up you are so far above the intentional emissions that the radiating source doesn't look like a magnetic loop anymore, it is a fortuitous conductor, and you are in its far field so the impedance of the field is that of free space. From: Y W Leung <leungderek2...@yahoo.com.hk> List-Post: emc-pstc@listserv.ieee.org Date: Tue, 21 Jun 2005 13:56:25 +0800 (CST) To: emc-p...@ieee.org Subject: Near field H-field measurement. 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
