Conditions of Exciting Coils Used in Evaluation of Shielding Factor of Magnetically Shielded Rooms for Uniform Magnetic Field

摘要

To shield the environmental magnetic noises, magnetically shielded rooms (MSRs) are used for biomagnetic measurements, etc. The sources of environmental noises are far away from the MSR, in such a way that almost uniform magnetic field is applied to the MSR. To evaluate the shielding factor ${rm SF}_{o}$ for the uniform magnetic field in a construction site, the magnetic noises are applied by coils. However, SFs depend on the size and position of the coils, so a standardization of the method for accurately evaluating ${rm SF}_{o}$ using coils is being discussed in the Japan Electronics and Information Technology Industries Association (JEITA). In JEITA, we have already proposed and verified the method of estimating ${rm SF}_{o}$'s by smaller coils placed near MSRs in the low frequency region without eddy current effect. In this paper, the suitable size and position of coils, such that the errors of the ${rm SF}_{o}$'s estimated and directly evaluated are within 10%, on various shapes of MSRs are obtained using 3-D linear magnetostatic field analysis. Moreover, to correct SFs when the magnetic noises are applied to MSRs through a reinforced concrete wall, etc., the effect of wall on the applied magnetic field is also obtained from the experiment. Finally, the suitable coil conditions and the correction method are verified by the experiment using an actual double-layered MSR.