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A method of loading surface corrugation in a 2×2 slot planar array with element spacing in the E plane beyond a wavelength is proposed for suppressing high grating lobes. This antenna array adopts a corrugated structure as a secondary radiation source to make effective spacing of elements within a wavelength. The authors investigate numerically and experimentally the performance of the antenna, and find good agreement between the measured and simulated results. Compared with conventional flat 2×2 slot arrays, the results show that the gain of the corrugated slot antenna array is significantly improved by about 6×dB, the half-power beamwidth is narrowed by 12× in the E plane and 10° in the H plane and the high grating lobe level occurring in the E plane of the flat slot array is sharply suppressed as well. Moreover, it is found that the introduction of a secondary source in this slot array can also make the element spacing in the H plane larger than a wavelength without an obvious grating lobe. The radiation mechanism of this antenna is well described and the role of surface corrugation in this antenna is demonstrated as the secondary radiation source which is responsible for the improved antenna gain and suppressed grating lobe.