Dave, We have run the measurements at 300 m, 30 m, and 10 m and found the fall off of 1/R^3 apply because the magnetic field is being measured using a loop antenna. The far field E field rule does not apply for a magnetic field.
Regards, Frank de Vall Sr. Engineering Manager - Compliance Assa Abloy ITG and HID Corporation drcuthbert@micron .com Sent by: To owner-emc-pstc@ie <leungderek2...@yahoo.com.hk> ee.org cc <emc-p...@ieee.org> Subject 06/22/2005 08:29 RE: 標題: RE: Near field H-field AM measurement. Derek, in regards to the magnetic field measurement from 50 kHz to 1 GHz: You can figure that your measurement is in the far-field when the distance is >1/6 wavelength. At a distance of 10 meters this occurs above 5 MHz. So above 5 MHz the field will drop as 1/d and the difference between 300 meters and 10 meters is 30 dB. Below 5 MHz what is the difference between 300 meters and 10 meters? Let's run a NEC-2 simulation and see what we get. The antennas are 1 meter square loops in free space. I am comparing the current induced in the two receive loops placed 10 and 30 meters from the transmit loop. FREQ 0.25 MHz 82 dB 0.50 69 1.00 57 2.00 44 4.00 33 8.00 30 I next placed the antennas 1 meter above perfect ground and obtained the same results. These results can be mathematically confirmed (I will leave that up to you) using the formulas I sent. It would be great if someone could run actual measurements at 10 and 300 meters. Once again we find that the FCC rules-of-thumb do not reflect reality. Dave Cuthbert Micron Technology 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