A novel broadband monopole antenna with circular polarization (CP) is designed and fabricated. It consists of a C-shaped monopole, an open-loop on the backside of the monopole and a modified ground plane. By placing a rectangular open-loop coplanar with the ground plane and connecting a rectangular vertical stub to the ground plane, wide impedance bandwidth is achieved, and axial-ratio (AR) bandwidth of the antenna could be improved simultaneously. The measured results exhibit a 65.2% (4.14 GHz, 4.28-8.42 GHz) CP bandwidth, and the 10-dB impedance bandwidth is 87.7% (6.66 GHz, 4.26-10.92 GHz). Finally, by placing a reflector underneath the antenna with its structural dimensions unchanged, one can obtain unidirectional radiation patterns with greatly improved gains. The advantages of the proposed antenna are the simple structure, a compact size, and a broad 3-dB AR bandwidth. Index Terms-Circularly polarized (CP), axial-ratio (AR), broadband, monopole antenna.

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