I am not familiar with GR 1089, but assuming there are electric field emission limits from 60 Hz to 30 MHz, the magnetic field limit would then be the electric field limit minus the free space conversion factor. The magnetic limit would be independent of the electric field emissions, it depends only on the electric field limit. And it is not a correct assumption to say that the magnetic field emissions will be 377 Ohms down from the electric field emissions. Especially in this frequency range, where you are in the very near field, the field impedance will track closely with the impedance of the source circuit, which in general will vary widely from 377 Ohms. Your electric field measurement device (not an antenna in the strict sense of the word but rather an electric field probe) senses only the electric field. Likewise a magnetic field probe does not respond well to the electric field. Incidentally if your electric field limits are low (on the order of 1 mV/m or less), you will need a device with wide dynamic range to make the measurement, since the electric field at the power line frequency can be on the order of 1 Volt/meter. This means both a good amplifier on the electrically short dipole or monopole, and a receiver with a narrowly tunable bandwidth near the power frequency. Electrometrics makes an electric field dipole. There used to be wave analyzers with 1 Hz measurement bandwidths in the audio frequency range. There still may be, but I haven't purchased anything like this new in awhile.
---------- From: "Sam Wismer" <[email protected]> To: "EMC Forum" <[email protected]> Subject: GR 1089 List-Post: [email protected] Date: Mon, Jul 29, 2002, 5:11 PM To all; Section 3.2.2(Radiated Emissions Magnetic Fields) says: Radiated emissions from the EUT shall not exceed the levels of field strength obtained from the following equation in the frequency range of 60 Hz through 30 MHz: H = E-51.5dB In this equation, E is the electric-field intensity in dBmV/m as obtained from Table 3-1; H is the magnetic-field intensity limit in dBmA/m; and the constant, 51.5, is the decibel equivalent of the free-space wave impedance of a plane wave. Extrapolation of magneticfield intensity limits is based on the electric-field intensity obtained from using the equations of Table 3-1 for the particular distance greater than 3 meters. What is the procedure here? 1) Calculate your E-field limits depending on your measurement distance. 2) Measure the E-Field and compare to the E-field limits. 3) Calculate your H-field limits depending on your measurement distance. 4) Measure the H-field and compare to your limits. Im sure this isnt right since the H-field limits are the E-Field limits minus 51.5 dB, so there would be no reason to re-measure the field strength. What am I missing here? Questions: Kind Regards, Sam Wismer Engineering Manager ACS, Inc. Phone: (770) 831-8048 Fax: (770) 831-8598 Web: www.acstestlab.com
