Wideband miniature rat-race using novel phase inverter with fishbone slot-line structure

A new-type of wideband miniature rat-race is presented. The rat-race has been technologically realised by integration of a complementary- conducting-strip transmission line (CCS TL) and a CCS phase inverter (PI) with a fishbone slot-line structure in CMOS 0.18 μm 1P6M process. This unique design of a fishbone slot-line structure operated with a higher slow-wave factor can substantially reduce the size compared with the straight slot-line configuration and can easily be integrated into the two-dimensional perforated ground plane for the CCS TL-based rat-race with the CCS PI. The prototype has an area of 302 × 472 μm which is only 3.3% of the area of the conventional ring hybrid. Measured results are consistent with simulated ones whose operating frequencies cover the Ka band (26-40 GHz). The percentage of bandwidth of the rat-race is up to 90% under these conditions: the amplitude imbalance smaller than 1.5 dB, the phase imbalance smaller than 10°, and the isolation larger than 20 dB, which is much wider than that of the conventional version.