Ferrite Core Inductors

The extent to which the theoretical capabilities of wave filters and networks can be realized in practice usually depends on how high a Q, ratio of reactance to resistance, can be obtained in the inductors. In the voice and carrier telephone frequency ranges the dissipation in mica capacitors is so small compared to that in inductors that it can generally be neglected. Even paper capacitors, in the lower frequency ranges, compare favorably in Q with the best available coils. Consequently, there has been considerable incentive to develop improved magnetic materials that will permit the realization of higher Q inductors for filter and network use. Work along this line has resulted in the development of the permalloys, and later the molybdenum permalloys which, powdered, insulated and pressed into shapes, have become the standard materials for wave filter coils in the voice and carrier frequency ranges. 1,2 Although the permalloys represent a vast improvement over the soft iron that preceded them they share the fundamental disadvantage of all metals, that they are good conductors. A n y conductor in the vicinity of an alternating magnetic field has eddy currents induced in it, and, if the conductor is the core of a coil, the power loss associated with these currents appears as added resistance in the coil windings. To restrict the eddy current paths it is customary to powder the material and insulate the particles with some kind of inorganic filler. However, there 265