Bandwidth Extension of Planar Microstrip-to-Waveguide Transition by Controlling Transmission Modes Through Via-Hole Positioning in Millimeter-Wave Band

This paper presents a design technique to achieve a broadband planar microstrip-to-waveguide transition in a millimeter-wave (mmWave) band. In the conventional planar microstrip-to-waveguide transition, via holes are located around the rectangular waveguide and microstrip line to prevent power leakage due to the generation of a multi-transmission mode. Therefore, a single-transmission mode is dominant at the input port of the transition, with a narrow bandwidth of the single resonance. In the broadband planar microstrip-to-waveguide transition, via-hole positioning is utilized to add inductance to constrain the predominance of the single-transmission mode at the input port of the transition. The double-resonant frequency yielded by excitation of the grounded coplanar waveguide transmission mode and parallel plate transmission mode is obtained by controlling the positions of holes adjacent to the microstrip line. Moreover, to simplify the structure and meet the requirement of high assembly accuracy in fabrication, two holes adjacent to the microstrip line are maintained, but the remaining holes are replaced by a choke structure that performs the equivalent function to the via-hole arrangement. The influences of the multi-transmission mode and choke structure on the characteristics are investigated by electromagnetic analysis, and the feasibility is confirmed by experiments in this work. A double-resonant frequency and a broad bandwidth of 10.6 GHz (13.8%) are obtained. The measured results of the broadband planar microstrip-to-waveguide transition using via-hole positioning show an insertion loss of 0.41 dB at the center frequency of 76.5 GHz.