This article introduces a novel transition between coplanar waveguide (CPW) and air-filled rectangular waveguide (RWG). A rectangular radiator etched with a semi-elliptic slot is connected to the center conductor of CPW to realize the transition, which broadens the bandwidth. This direct transition does not require intermediate transition or air-bridges. Moreover, the planar circuit of the transition can be designed with high- and low-permittivity materials (εr = 10.2 and 2.22), which offer

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... benefits in both PCB and MMIC design. Two back-to-back transition prototypes at X-band are designed, fabricated and measured. The 15 dB fractional bandwidths are expanded to 44.7% and 47.6% respectively, which have been demonstrated in both of the transitions (εr = 10.2 and 2.22). The measurement results agree well with simulation results, which validate the feasibility of this design.