Loop-Induced Magnetic Fields with Negligible Side-Lobes

In schools for the hard-of-hearing where adjacent classrooms are equipped with non-carrier induction loop systems, it is necessary to realize a geometric discrimination between the classroomfields. One approach to reaching the required 24 dB per meter rolloff at the boundaries is the orthogonal field principle reported first in 1965 by de Boer, Bosman and Joosten (ref. 2, 3, 4). In this paper a method is described and results are given for an heuristic development of this principle in the case of classrooms about 8 m in length with permissible roll-off ranges of 2 m. It is shown that it is also possible to discriminate in a vertical direction if a secondary field at the ceiling is used. The heuristic approach achieves very good results in this case and overcomes the mathematical difficulties arising when the field is calculated analytically.