Balanced Low-Loss Ka-Band μ-Coaxial Hybrids

Miniature hybrid couplers are designed in an air-filled microfabricated rectangular coaxial line technology for operation near 36 GHz. The size of the couplers is less than 2.7 mm by 2.7 mm in area and 420 μm in height. Air-filled copper μ-coaxial cable provides less than 0.3 dB/cm loss at Ka-band. Coaxial-to-CPW probe transitions on the ports provide means to perform 4-port VNA measurements with an external SOLT calibration. After implementing a de-embedding procedure, measurements show return loss and isolation better than 20 dB, with output amplitudes of 3.2 dB±0.1 dB, and output phase differences of 90° ± 2.0° from 34.5 to 36.5 GHz. The return loss and isolation minima are shifted in frequency as compared to the optimal output amplitude and phase frequency. This frequency difference is 1.5 GHz near 36 GHz with this technology, however branch line couplers using branch-length and reactive tee-junction compensation methods are presented. The improvement is experimentally verified; the frequency differences are 0.5 and 0.2 GHz for the two compensated designs.