Reactance Networks for Coupling between Unbalanced and Balanced Circuits

The problem of transferring electrical energy from unbalanced sources to balanced loads is usually handled by interposing an isolation transformer between the source and the load. In situations where the isolation transformer proves unsatisfactory, resort must be had to other means. Alford has described certain 2-mesh reactance networks which often serve as satisfactory substitutes for the isolation transformer. In this paper a general theorem is derived for the arbitrary 4-terminal linear passive network, demonstrating necessary and sufficient conditions for satisfactory matching of unbalanced sources and balanced loads. The design of networks satisfying these conditions is independent of the impedances of the source and load; a network satisfying the conditions of this theorem will match an unbalanced source of any arbitrary internal impedance to a balanced load of any arbitrary impedance. In general, any particular reactance network satisfies these conditions at only a few isolated frequencies, at most. In the particular case of the isolation transformer they are satisfied at all frequencies. Certain previously known networks are shown to be special cases of this general class; lattice networks satisfying the conditions of the theorem are described in some detail. The paper concludes with a brief discussion of a well-known network often used to match partly unbalanced sources to balanced loads.