System implications of high power multi-amplifier UHF television transmitters

In recent years power levels for UHF television transmitter systems have been increasing. In the USA 120 and 240 kW stations are commonplace and the general trend is towards even higher powers. The current limit on power output for external cavity klystrons is 60-70 kW peak sync. It is therefore necessary to parallel klystron amplifiers to provide higher powers. Traditional coaxial combiners are not practical at these powers, instead waveguide combiners are used. American systems however use a single drive system, split to feed all the amplifiers. These systems therefore require that all the klystron amplifiers have identical characteristics, there being only one set of corrections for all amplifiers. The stability and matching of each amplifiers is therefore much more important in this type of system. All these aspects must be considered in the design of stable high efficiency klystron systems