The design and operation of high-power broadcast transmitter units with their outputs combined in parallel

The paper describes a method, developed by the British Broadcasting Corporation, of obtaining very large r.f. output power for broadcasting purposes by combining the outputs of two or more similar transmitter units. The principle is analogous to paralleling the outputs of a number of a.c. generators, with the difference that, in the case of transmitters, the frequency of the separate units is identical, the variables being phase and loading. The method of ensuring equality of the r.f. output voltage and modulation amplitude of the separate transmitter units is described. The circuits for combining and matching a variable number of transmitters to a common load are detailed, and a number of special aspects relating to the design of transmitters for parallel operation, including the principle of ?drive suppression? for protection purposes, are discussed. The design of the circuits and operational procedure developed for the Droitwich high-power medium-wave station, which has a power output to the aerial of 400 kW carrier provided by two 200-kW class B modulated transmitters in parallel, are described together with operating experience. Mention is made of a second station, the Ottringham high-power long-wave station of the B.B.C. European service, in which an aerial power of 800 kW was obtained by means of four similar 200-kW transmitters operated in parallel, and which will be fully described in another paper. Appendices contain some data on the design of the modulated-amplifier output circuit of the transmitters used, and a description of an r.f. impedance monitor for fault protection on the high-power r.f. chain.