A novel broadband circularly polarized planar monopole antenna fed by coplanar waveguide (CPW) is proposed and fabricated. The proposed antenna consists of a rectangular monopole, an inverted-L strip and an asymmetric ground plane with cutting a horizontal slit on the right ground plane. Firstly, a narrow circularly polarized (CP) radiation at the upper band can be achieved by utilizing the asymmetric ground plane. Then, an inverted-L strip is introduced to obtain broadband CP characteristic matched with wide impedance bandwidth. The measured results demonstrate that a 10-dB bandwidth of 58.8% from 4.8 to 8.8 GHz and a 3-dB axial-ratio bandwidth (ARBW) of 47.8% from 5.375 to 8.75 GHz can be achieved which can completely cover the WLAN (5.725-5.85 GHz) band. Additionally, a 10-dB impedance bandwidth of 24% (3.3-4.2 GHz) with linear polarization is also obtained which can completely cover the WiMAX (3.3-3.7 GHz) bands. In additional, to explain the mechanism of dual-band CP operation, the analysis of magnetic fields distributions and a parametric study of the design are given. Compared to other recent works, a simpler structure, wider axial ratio and impedance bandwidths and a more compact size are the key features of the proposed antenna.
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