A compact modified C-shaped monopole antenna with broadband circular polarization is proposed, fabricated and measured. The antenna structure is simple and only consists of combined modified C-shaped radiation patch and an improved ground plane with the overall size of 25 × 25 × 1 mm 3 . By cutting the corner on the modified C-shaped patch and adding triangular stubs on the ground plane, the wide impedance bandwidth and axial ratio bandwidth are achieved. The design process of the antenna is given, and the circular polarization mechanism of the circularly polarized antenna is analyzed from the surface current distributions. The measured impedance bandwidth is 95.2% (4.4-12.4 GHz) with return loss better than 10 dB, and the measured 3 dB axial ratio bandwidth is 96.8% (4.42-12.72 GHz). The peak gain is above 3.0 dBi within the working band, which indicates that it is suitable for application of ultra-wideband (UWB) wireless communication systems and satellite communication systems. K E Y W O R D S axial ratio, circular polarization, monopole antenna, ultra-wideband (UWB)

A novel smiling-face-shaped band-notched ultra-wideband (UWB) antenna is designed in this paper. First, antenna bandwidth is broadened by adding a pair of small semi-circular stubs (ears) and etching a pair of symmetrical C-shaped slots (eyes), a triangular slot (nose), and a U-shaped slot (mouth) on the patch. Second, by adjusting the length of the ground plane and creating a stepped slot on the ground plane, antenna bandwidth covers 3-12 GHz. Third, a pair of L-shaped slots (hands) is etched on the ground plane to create a notch, which can effectively suppress the interference signal in worldwide interoperability for microwave access (WiMAX) (3.4-3.6 GHz) and satellite C band (3.7-4.2 GHz). Lastly, the proposed antenna is fabricated and measured. The measured results indicate the proposed antenna has good performance and can be widely used in UWB systems.