Abstract: In this paper, a novel highly compact ultra-wideband (UWB) antenna with two asymmetric patch grounds is proposed for UWB applications. Using the Semi-elliptical radiator and two patch grounds which are mounted to the back side of the antenna, the broad impedance matching and small form factor are achieved. The key parameters that affect the performance of the antenna are studied and optimized. The proposed antenna has been fabricated and measured. The experimental results show that the designed antenna can provide ultra-wide bandwidth with S₁₁＜-10 dB, covering standard UWB bandwidth of 3.1~10.6 GHz, and the average gain of the antenna is 4.5 dBi. The antenna features a very small physical size of 14 mm×18 mm=252 mm².