Mr.. Leung, At low frequencies, you may have undesirable effects from underground pipes / etc... Or your EUT may be significantly large compared to the antenna to influence the measurement. This makes the assumption of 3rd order rolloff uncertain. The FCC guidance is to measure at 2 distances to establish the extrapolation factor that you do have [due to external effects / size of EUT vs measuring distance / etc.] or to use a 40 dB / decade [or inverse square] extrapolation factor. If you are measuring at 10 meters, take another measurement of the fundamental frequencies at 20 meters. If you truly have a 3rd order rolloff of the field then the difference of the H-field readings at 10 meters and 20 meters will be 18 dB different. This corresponds to 20 * [ rolloff factor] dB/ decade. For a factor of 3, you have a 60 dB / decade rolloff of the field. If you do not have a difference of 18 dB, you can calculate what the correct extrapolation factor is. Hope this helps. Best regards, Mac Elliott
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. 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