Some observations about shielding extremely low-frequency magnetic fields by finite width shields

Shielding of extremely low-frequency magnetic fields is limited by fields penetrating through imperfect material and leakage around the edges of shields which do not completely enclose the shielded region. The latter effect, more specifically the effect of source geometry and shield type on leakage, is the subject of this work. Particular attention is paid to elementary cylindrical dipole sources since the dominant fields of complex two-dimensional (2-D) sources can be represented as those of dipole sources. When possible, insight is gained from a simple analytic model for shielding by semi-infinite and finite width perfect electric and perfect magnetic shields. In cases for which the simple model is not valid, insight is gained from numerical results. It has been found that source orientation and shield type are critical issues and can cause differences in shielding effectiveness in excess of 20 dB