A compact co-planar waveguide (CPW) center-fed substrate-integrated-waveguide (SIW) slot antenna array is proposed to achieve narrow -plane beamwidth and low sidelobe levels for automotive radars such as across-the-road (ATR) radar. The antenna consists of an array of SIW slot elements, a CPW-SIW transition, and a CPW power divider. A new type of CPW-SIW transition is proposed to minimize the blockage aperture of the slot antenna array for suppressing the sidelobe levels of the array. A parallel center feeding configuration is applied to avoid beam squinting. The antenna prototype printed onto a single-layer Rogers 5880 is with 32 4 slot elements and an overall size of 195 mm 40 mm 0.79 mm. Measured results show that the proposed antenna exhibits a gain of 22.8 dBi, an efficiency of 67 , a return loss of 10 dB, a sidelobe level of 21 dB, and a fixed boresight beam of 4.6 in -planes over 24.05–24.25 GHz, suitable for ATR radar.