A novel microstrip slot antenna backed by a ground plane with a U-shaped feed is presented. Based on the simulation, its impedance bandwidth for VSWRs ≤ 2 covers the range of 5.3 to 18.5 GHz, whose relative bandwidth reaches 110.9%. In addition, the good gain performance is achieved. These features make it an attractive canditate for a compact portable terminal in the personal, mobile satellite and wireless communication systems with large bandwidths.