Design criteria for active shielding of inhomogeneous magnetic fields for biomagnetic applications

General analytical expressions for the field attenuation and the reaction factor for a spherical active compensated cabin are theoretically derived. The shielding effect of various materials and their thickness on external magnetic disturbances as well as the retroactive effect on the locally generated compensating fields of the compensating coils are then analyzed. A numerical evaluation of the analytical expressions developed is made and directives for practical measures are derived. Comparison with the experimental results obtained from the measurement of shielding properties of an equivalent cubic cabin is made. The so-determined design criteria are then discussed in detail.