This paper presents a compact F band radar sensor with on-chip antenna for microwave imaging applications. The frontend chips are fabricated in a 0.13 μm SiGe:C technology and feature a wide bandwidth from 120~140 GHz and a compact size of 0.633 mm2 and 0.689 mm2 with on-chip antenna. The on chip antenna is optimized for small size, with a profile of only 570μm by 250μm. The gain of the on-chip antenna is between 0 dBi to -3.3 dBi at 128 GHz. The transmitter and the receiver consume 175mW and 205mW respectively, and exhibit a range resolution of 1.5 cm.