Method for evaluating shielding factor of magnetically shielded rooms taking account of eddy currents using exciting coil

Abstract

To shield magnetic noise, magnetically shielded rooms (MSR) are used for biomagnetic measurements, etc. To evaluate the shielding factor SF (SF=B n / B s , B n and B s are the flux densities at the center of the room without and with MSR, respectively), the magnetic noise is applied by coils. However, SF depends on the size and position of the coils, so the standard of JEITA (Japan Electronics and Information Technology Industries Association) EM-4502, in which the shielding factor (SF o ) of MSR for the uniform magnetic field in the extremely low-frequency region of less than 1 Hz without eddy current effect is estimated using coils within 10% error [1-2], has been already published [3]. Currently, a standard method for the low-frequency region, e.g. 50 Hz, with eddy current effect is being discussed. In a previous work [4], it was shown that the coil should be placed in front of the door of MSR and a much larger estimation error than 10% should be permitted for practical use in the standard to estimate SF o for the low-frequency region.