Ka-band surface-mount directional coupler fabricated using micro-rectangular coaxial transmission lines

2.5-dimensional, 10-dB and 20-dB directional couplers, are designed and fabricated using air-core rectangular coaxial transmission lines. The couplers are both smaller than 1.8 by 4.2mm² with probing points. The measured performance of the coupler is in agreement with full-wave simulations. The components are designed such that probing or flip-chip attachment can be done with the same structures. A flip-chip transition from rectangular coax to traditional co-planar waveguide is also presented.