Theory and Operation of Tuned Radio-Frequency Coupling Systems

The subject is the tuned r-f coupling systems commonly used in broadcast receivers, to couple the antenna to the grid of the first tube, and to couple the plate of each r-f amplifier tube to the grid of the following tube. The simple tuned r-f transformer used in the 1923 neutrodyne receiver has been improved by the cooperation of different kinds of impedances in the primary circuit. The "equivalent mutual inductance" is thereby caused to vary with frequency in a predetermined manner, without the use of any moving elements except the tuning condenser; this is also referred to as a varying "effective turns ratio." The gain of an amplifier can be held uniform or made to vary with frequency in any desired manner consistent with the amplifying ability of the tube and the tuned secondary circuit, and without appreciable loss of selectivity. A large variety of these improved coupling circuits is shown and classified in terms of the fixed and varying components of the equivalent mutual inductance. A large number of these coupling systems from commercial broadcast receivers are described in terms of coil structure, electrical constants, and performance. These include antenna and amplifier circuits dating from 1924 to date, and used in unneutralized, neutralized, and screen-grid receivers. Special attention is paid to antenna circuits for unicontrol receivers, whose tuning is substantially independent of antenna capacitance and of the adjustment of a shunt rheostat sometimes used as a volume control.