A hollow waveguide slot array antenna with a total of 338x106.5x28.9 mm3 is proposed as a sensor for radar applications at K-band. The proposed antenna consists of a radiating slot plate with 8x32 slots, a middle plate with 64 cavities and apertures, and a full-corporate-feeding network. The antenna is fabricated with these three parts in which the sub-array feeding technique is employed. The proposed antenna is designed for millimeter-wave short-range radar applications. For this purpose, the antenna radiates and achieves reasonable gain values between 23.72–25.30 GHz frequencies with 6.45% bandwidth. The measurement directivity is 32.1 dBi, and the realized gain of the antenna is measured as 30.83 dBi at 24.5 GHz center frequency with 74.8% total efficiency. The antenna achieves high gain and efficiency with straightforward fabrication. Although there are some fabrication errors and connector losses, antenna measurements are obtained sufficiently consistent with simulations.