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.

Introduction

MILLIMETER-WAVE (mmWave) technologies have been applied in various applications of broadband high-speed wireless communication systems, such as fixed wireless access [1], wireless LAN [2], 5G antenna systems [3] or high angular resolution automotive radars [4]–[۷]. The next generation of ultra-wideband (UWB) automotive radar requires a wider frequency bandwidth [8]. Antennas in mmWave systems with a high gain and a narrow beam even when the physical size of the antenna aperture is very small could be developed. Such antennas are appropriate for meeting the demand for high traffic capacities and beam steering capabilities in mmWave applications. Some mmWave applications have been studied and commercialized on the market. Antennas for these products are designed depending on the specifications required for the systems such as performance, physical size or production cost. Several types of antennas have been developed for mmWave systems, including dielectric lens antennas [9], folded reflector antennas [10], slot antennas [11], [12], etc. The microstrip array antenna is one of the most attractive options for realizing a low cost and a low profile, which can be easily integrated into the RF circuit of mmWave devices. Techniques for integrating a microstrip array antenna into an RF circuit were developed by using a microstrip-towaveguide transition.