Electromagnetic Shielding, 2nd Edition

Appendix BMagnetic Shielding

For almost a century the subject of shielding extremely low-frequency (ELF) and very low-frequency (VLF) magnetic fields has been of interest [1]. The interest originated from the design necessity to protect part of the circuit in radio-receiving apparatus from the disturbing effect of the radiated field in its neighborhood [2–4]. Over the decades the awareness of possible interferences from radiated fields rapidly increased. Apart from special applications, interference on electronic systems became more and more evident with the widespread use of computers. Such interference occurred mainly as frame disturbances on CRT displays when the external magnetic flux density exceeded fixed values [5]. In the past decade a more serious issue about the possible health hazards for persons being exposed to magnetic fields at low frequencies has led to a renewed interest in the subject [6]. Nowadays, shielding of low-frequency magnetic fields is a topic of interest for a number of applications, ranging from the mitigation of power-line sources to protection of sensitive equipment, as schematically depicted in Figure B.1.

At low frequencies the magnetic field is due either to the electric current flowing in conductors of various geometries or to the magnetization of surrounding ferromagnetic materials. The classical strategy for reducing quasi-static magnetic fields in a specific region consists of inserting a shield of appropriate material, whose properties ...