Abstract: It is difficult for conventional 77 GHz automotive radar antennas to cover the field of view for both medium-range radar and long-range radar simultaneously. The existing solution is to switch between two sets of antennas for medium-range and long-range radar, at the expense of a larger antenna footprint. Extra insertion loss will be introduced by the switch, which may influence the system signal-to-noise ratio. In this paper, a compact antenna array with a flat-shoulder shaped radiation pattern is presented, which enables simultaneous support for both medium-range and long-range radar application. In order to improve the design efficiency, a mixed optimization method, including non-linear fitting and aggressive space mapping, is utilized to synthesis this special flat-shoulder-shaped radiation pattern. In addition, a compact substrate-integrated waveguide feeding network is designed for realizing a compact array footprint while reducing parasitic radiation. The proposed antenna is fabricated using a low-cost single-layer printed circuit board process. Measurement results demonstrate a peak gain of 19.8 dBi and a desirable flat-shoulder-shaped radiation pattern at 77 GHz, making it a promising candidate for 77 GHz automotive radar applications.