Coupling between two oscillatory circuits, with some applications

(1) Introduction. The different methods of coupling two oscillatory circuits are conveniently classified as follows ¿ (i) Indirect coupling through a field of force common to both oscillatory circuits. (ii) Direct coupling through an impedance common to both circuits. (iii) Direct coupling through an impedance not included in either oscillatory circuit. (2) The Physics of Tuned Coupled Circuits. An easy method of obtaining the values of the two resonant frequencies for each system is described. The method is based upon current considerations and is applicable to the case when the two circuits are tuned to the same frequency. The method is illustrated by applying it to a type of inductive coupling. (3) The Frequency Variations. The properties peculiar to the two resonant frequencies for each type of coupled circuit are discussed, and coupling units are described by which the resonant frequencies of the directly coupled circuits may be made continuously variable. (4) Applications. These coupling units are applied to an aerial circuit in such a manner that each resonant frequency of the system can be varied independently of the other, over any desired range of frequency. By this method of dual reception each wave-length is properly tuned and one aerial only is employed. Finally, the coupling units are applied to the intervalve system of a high-frequency amplifier, thereby enabling the amplifier to be readily tuned to any frequency and to amplify two signals independently and simultaneously.