A 50-Kilowatt Broadcast Station Utilizing the Doherty Amplifier and Designed for Expansion to 500 Kilowatts

Abstract

Radio station WHAS purchased the first commercial transmitter employing the Doherty high-efficiency amplifier and has installed it in a completely new transmitting plant which is designed for expansion to 500 kilowatts output. The transmitter extends the use of negative feedback for noise suppression and all vacuumtube filaments are heated with alternating current, completely eliminating rotating equipment except for fans and pumps. Heat for the building is obtained from the water-cooling system, in which porcelain tubes entirely replace rubber hose for the insulating sections. Six-inch concentric transmission line of refined design is used to convey energyfrom the transmitter to the antenna and for the suppression of harmonics. The novel construction of the feed line to the shunt-excited half-wave antenna makes unnecessary the coupling apparatus ordinarily used between the transmission line and the inclined lead to provide the capacitive reactance required at that point in the circuit. The theory and practice followed in the design of the high-efficiency amplifier and other special features are touched upon as the entire plant is described. Performance data of the transmitter and the antenna system are also included.