Broadband Millimeter-Wave Hybrid Circulators for NASA Missions

Circulators are used to direct signal flow in millimeter-wave (MMW) transmit and receive systems. For more than 50 years, the Y-junction circulator has been the state-of-the-art MMW technology. They are commercially available with full waveguide band operation up to 40 GHz, although the isolation is generally less than 16 dB. Above 50nbsp;GHz the bandwidth is typically less than 4 GHz due to limitations in the ferrite material properties. The narrow bandwidth makes them unsuitable for many systems.Micro Harmonics invented and patented a new hybrid circulator technology. The prototype developed in the Phase I exhibited unsurpassed performance, covering the entire band from 150-190 GHz with 20nbsp;dB level isolation. For comparison, a state-of-the-art Y-junction isolator has about 3 GHz bandwidth at 160nbsp;GHz. The hybrid theory suggests that even higher levels of performance are possible. During the course of the Phase I work we identified several specific areas in the design that constrain the bandwidth and isolation. In the Phase II effort, we will address these issues and seek to improve the hybrid circulator performance.The hybrid comprises a modified Faraday rotation isolator and an orthomode transducer (OMT). We observed cross-coupling of the vertical and horizontal polarizations in the OMT common section and also in the OMT to isolator transition. This cross-coupling degrades the isolation between the Tx, Rx and antenna ports in the hybrid circulator. The transition is also the prime limiting factor in the bandwidth. Much of the proposed phase II effort is focused on reducing the cross-coupling and improving the bandwidth of the transition.Hybrid prototypes will be developed in seven waveguide bands from 50 GHz to 250 GHz. These prototypes will be delivered to NASA. A hybrid latching circulator or duplex switch will also be designed in the WR-10 band. A detailed analysis of multipaction in the hybrid and thermal analysis are also part of the proposed phase II work.