1. Introduction The millimeter-wave front-end module for an automotive radar sensor is generally composed of an antenna placed on the front surface of the sensor head and a millimeter-wave circuit inside it. As the antenna for the radar sensor, several types of antennas, a slotted waveguide antenna 1) , a triplate antenna 2) , a dielectric lens antenna 3) , a dielectric leaky wave antenna 4) and a microstrip antenna 5) , have been studied. The microstrip antenna is still a candidate in order to realize low cost and low profile millimeter-wave front-end module for the radar sensor. The microstrip antenna is usually connected to the millimeter-wave circuit through a waveguide. Thus a microstrip line to waveguide transition is required to connect them. Since high SNR (Signal to Noise Ratio) for automotive radar is required to detect a target at a long distance of 150 m, not only the millimeter- wave components but also the transitions used to connect them are desired to be low loss. In addition, 13 R&D Review of Toyota CRDL Vol. 37 No. 2 Abstract A new type of microstrip line to waveguide transition fabricated on a single layer dielectric substrate is proposed. As a result of experiments, low transmission loss of 0.4 dB was realized at the design frequency of 76.5 GHz. Bandwidth of the transition was numerically investigated by the finite element method. It was clarified that the bandwidth of the transition became wider as the cross section of the waveguide became smaller and twice as wide as that of a conventional microstrip patch antenna element fabricated on a dielec- tric substrate with the same parameters. In addition, the effect of error in the relative position between the dielectric substrate and the waveguide was also investigated. It became clear that degradation of transmis- sion characteristics was less than 0.1 dB for a manufacturing accuracy within ±0.1 mm. Millimeter-Wave Microstrip Line to Waveguide Transition Fabricated on a Single Layer Dielectric Substrate Hideo Iizuka, Toshiaki Watanabe, Kazuo Sato, Kunitoshi Nishikawa Research Report Keywords Millimeter-wave, Transition, Microstrip line, Waveguide, Finite element method Special Issue Millimeter-Wave Radar for Automotive Applications