This paper proposes a novel substrate integrated waveguide (SIW) higher order mode cavity fed 2 × 2 -element slot array. The higher order mode cavity feeds all of the slot elements simultaneously, and the resonant mode is excited only by a slot aperture, which helps simplify the feeding network. To enhance the bandwidth, the array is designed as an asymmetric structure with different slot sizes and locations. In this way, new resonances are introduced and thus broaden the bandwidth. Furthermore, the proposed 2 × 2 -element slot array is used as subarray to implement a differentially fed 4 × 4 -element array. The measured results demonstrate that this simple structure antenna array features wideband, high radiation efficiency, good aperture efficiency, and broadside radiation.