Comparison of efficient power combining techniques

Abstract

Summary form only given. In microwave systems, the system scenario may require high output power. In this case, high-output power modules must be built to operate in microwave frequency range. In recent high power microwave systems, obtaining high power is commonly achieved by combining output powers of a bunch of transistor amplifier. In this approach, possible maximum efficiency is desired in order to reduce cost and thermal extraction.Regarding the power combining operation, apart from the operation frequency bandwidth, insertion loss of the combiner is a critical parameter. This loss is due to material losses, and generally increases with path length. In this study, by keeping the path lengths approximately the same, three power combining methods are investigated and implemented. The methods are compared by both efficiency properties and applicability to different microwave applications. Efficiency comparison is made at a single frequency, 17 GHz. The first method is combining by microstrip lines. An 8-way Wilkinson combiner is designed and implemented at 6-18 GHz band (Fig. 1 (a)). In order to have the possible shortest path lengths, inputs are placed at the edges of an octagon, where the output branch is also on the same plane. Loss per branch turned out to be 1.5 dB at 17 GHz. The second method is combining by suspended substrate striplines (SSS). A 4-way combiner consisting of branchline couplers is designed and implemented at 16.5-17.5 GHz band (Fig. 1 (b)). The average introduced loss per branch is observed to be 0.9 dB at 17 GHz. The last method is combining by microstrip probes in an air filled coaxial waveguide. An 8-way combiner is designed and implemented in 15-18 GHz band (Fig. 1 (c)). Insertion loss per branch is measured as 0.5 dB at 17 GHz. It is seen that the more the dielectric material is eliminated by one of the methods and the best efficiency is obtained by that method. In terms of efficiency, the best method turns out to be combining by microstrip probes in air medium, a relatively new technique. Moreover, the structure enables enough bandwidth for recent radar applications. Furthermore, the most efficient approach is to integrate low loss SSS technique (second method) to Wilkinson structure in an application having much wider frequency band of operation.