Millimeter-wave technologies have been developed for applications to high-speed wireless communication systems and high angular resolution microwave sensors. Figure 1 shows one of the possible configurations of the millimeter-wave RF module when a microstrip antenna is used. The microstrip-to-waveguide transitions are necessary at the connection of the waveguide with the antenna feed and with the planar RF circuit. We have developed planer microstrip-to-waveguide transitions on a single-layer dielectric substrate in the millimeter-wave band [1], [2]. Via holes surround a rectangular waveguide to prevent leakage of parallel plate mode transmitting in the substrate. However, an additional process for via holes increases production cost and manufacturing errors of via-hole positions increases insertion loss of the transitions. Then, we have proposed a choke structure for replacement from via-hole arrangement [3]. The size of the waveguide short pattern composed of the metal plate on the substrate is designed to form choke structures in the substrate around the waveguide. The impedance of the open circuit with a quarter wavelengths transmission line is equivalent to the short circuit. Therefore, the impedance at the point along the waveguide profile is equivalent to the short circuit, when the open circuit at the edge of the metal plate is spaced by a quarter wavelengths from the waveguide profile in the substrate. First, the planar microstrip-to-waveguide transition is designed. Then, we optimized the choke structure for the transition in this work. The perturbed characteristics of the transition are discussed and the simulated performance is presented in this paper.
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