Broadband Millimeter-Wave Hybrid Circulators

Abstract

A new broadband hybrid circulator has been developed for use at millimeter-wave (MMW) frequencies. The hybrid circulator comprises a Faraday rotator and an orthomode transducer (OMT) which work together to produce the circulator function. The initial hybrid circulator prototypes employ an asymmetric OMT and are designed to have 20 dB isolation over 24% fractional bandwidths. However, the architecture can be modified to cover full rectangular waveguide bands by using a symmetric OMT. The hybrid circulator bandwidth does not degrade at the higher MMW frequencies like the Y-junction. For example, a well-tuned Y-junction circulator designed to operate at 60 GHz has a 20 dB fractional bandwidth near 10%. But the bandwidth drops to 2% for a Y-junction circulator operating at 160 GHz. Test data are presented for two hybrid circulator prototypes covering the bands 53–68 GHz in WR-15 and 150–190 GHz in WR-5. The measured insertion loss of the hybrid circulator is typically less than 0.8 dB over the 53–68 GHz band and near 2 dB over the 150–190 GHz band. The measured VSWR is less than 1.4:1 in both bands. The hybrid circulator can be scaled to at least 400 GHz.