Coupling matrix synthesis of orthomode transducers

Abstract

Orthomode transducers (OMTs) are key components in modern antenna feed systems when dual-polarizations are involved, which are commonly encountered in satellite communication, remote-sensing, etc [1]. As the increasing demanding of communication capacity, the OMTs are also used to transmit and receive two independent signals with orthogonal polarizations, which doubles the capacity of the link. Although many types of OMTs have been proposed and implemented in practical systems [2], the design is mainly based on experience, cut-and-try, or full-wave optimizations [3]. As the requirements of OMT become severe, such as low-loss, compact size, wide band, dual-band, even multi-band, the design difficulty increases substantially. However, unlike the filters and multiplexes, few synthesis theory has been developed [4]. Recently, the concept of coupling matrix, well-developed synthesis theory for two-port network [5,6], is extended to multiport case [7], and used to synthesize the coupling matrix of diplexers. In this paper, the multiport coupling matrix theory is adopted to represent the performance of OMT, such as return loss, isolation, cross-polarization. The initial values of the corresponding coupling matrix are synthesized according to the specified return loss and bandwidth by ignoring the weak cross couplings, and then the whole coupling matrix is optimized to achieve all the specifications including the cross-polarization and isolation. Two examples of single-band and dual-band OMTs are presented to validate our approach.