Theoretical analysis of log-periodic circuits in microwave amplifiers

The feasibility of applying the log-periodic concept to microwave amplifiers has been investigated. This approach, successfully utilized in antenna design, offers the potential of greater bandwidth at a given power level than attainable from existing uniform devices. Log-periodic devices analogous to klystron, traveling-wave, and hybrid amplifiers have been studied. The dimensions of each section, including the beam diameter, in a log-periodic amplifier are a constant factor times those of the preceding section. Thus the general outline of such devices is conical. These devices have been simulated on a digital computer using a model in which the coupling between periodic sections is provided by one space-charge mode and one circuit mode. The accuracy of this model has been verified by simulation of existing traveling-wave tubes. Computer results on a 100kW log-periodic "klystorn," using a backward-wave circuit with a total taper ratio of 4.6 to 1, indicate that a useful bandwidth of approximately 3 to 1 can be obtained. Increasing the mechanical taper increases the bandwidth in direct proportion. Computer results on log-periodic versions of helix traveling-wave tubes are also given.