Ferro-Inductors and Permeability Tuning

Brief analysis indicates that tuning by variation of inductance in such a manner that L/R of the circuit is kept constant results in constant selectance and amplification throughout the tuning range. Very finely divided and compressed magnetic-core material has been developed for this purpose. Between the prescribed limits of frequency range, iron cores can be so designed as to produce simultaneous inductance and resistance variations of the order of an air-core inductance. This material exhibits extremely low losses at broadcast frequencies and possesses exceptional magnetic stability, from which other than radio applications follow. Constructional details of variable ferro-inductors are given, together with their behavior and application in radio circuits. Because of inherent uniformity of the circuits, they are most applicable in tuned radio-frequency receivers, several forms of which are shown in their performance and construction.