In this letter, a broadband proximity coupled millimeter-wave microstrip array antenna is presented for automotive radar applications. The antenna array consists of a microstrip line and a series of trapezoidal radiating elements that are periodically arranged on both sides of the microstrip line, at intervals of about half the guided-wavelength. The introduction of the trapezoidal radiating patch enhances the excitation coupling while suppressing out-of-band frequencies, and it has a wider impedance bandwidth than the rectangular patch. In the design of proposed antenna, the normalized resistance of the trapezoidal radiating element is controlled by adjusting the gap with the microstrip line, so that a low-sidelobe level (SLL) can be achieved. Taking the 77-81 GHz frequency band allocated to automotive radar applications as an example, a 1×16 linear array is designed and fabricated. The measured SLL is better than -20 dB. The measured gain of 1×16 array is higher than 15 dBi over the operating frequency range of 77-81 GHz. The 1×16 linear array can achieve an impedance bandwidth of 7.6% (75.6-81.6 GHz).
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