Wideband In-Line H-Plane Transition From Microstrip Line to Air-Filled Rectangular Waveguide

An in-line H-plane transition from microstrip line (MSL) to air-filled rectangular waveguide operating in millimeter- waveband is proposed. In the transition, a radiator, i.e., a rectangular patch with a semicircular slot structure, is placed above a wedge-shaped cavity. The metal posts A and B are connected to the broad wall of the rectangular waveguide, where those two posts function as a $\pi $ -shaped matching-network to expand the bandwidth of the transition. A back-to-back transition prototype in Ka-band is designed, fabricated, and measured to verify the method. Measurement results show that the maximum insertion loss of 1 dB and return loss of better than 15 dB are obtained over the entire Ka-band (26.5–40 GHz). The proposed transition exhibits wideband performance while maintaining a compact structure. In addition, the proposed transition structure is also verified in W-band (75–110 GHz) through simulation, with a bandwidth covering the entire W-band (S $_{11} \le -15$ dB) and an insertion loss of less than 0.78 dB.