Low-Loss Hexaferrite Self-Biased Microstrip and CPW Circulators

Nonreciprocal microwave devices, such as circulator and isolators, traditionally employ magnetically soft ferrites biased by hard magnets. The need of external bias remains the major obstacle for the ferrite device integration in RF front-end and active antennas. The known magnet-free nonreciprocal devices use hexaferrites or voltage-biased diodes. They usually suffer from the high RF losses. The causes and mechanisms of losses in self-biased circulators with hexaferrite junction resonators are examined and the ways of major loss reduction are presented. The newly developed La-Co substituted hexaferrite compounds have been explored and are used for the design of self-biased microstrip and CPW circulators. They have enabled reduction of the insertion loss in the Ku band circulators for more than a half, to the level below 0.6 dB, while maintaining the high isolation and return loss across the frequency band about 5%.