This article presents a wideband and low-profile omnidirectional circularly polarized antenna. It consists of a circular slotted top patch connected with a circular ground plane by four shorting vias and eight extended curved branches. These curved branches are introduced around both the patch and the ground plane with proper angle separation, achieving a 90-degree phase difference between the vertical and horizontal radiating parts. In this way, the wideband circularly polarized characteristic is realized. The final optimized structure is very compact, with a dimension of only 32~2×π×6mm~3 (0.37~2×π×0.07 = 0.03 λ_0~3). Besides, the measured results agree well with the simulated ones, with a simulated/measured impedance bandwidth of 28.4/32.2 and an axial ratio bandwidth of 46.7/42.3 being obtained. And the maximum realized gain is 1.1 dBic, with a gain variation of less than 1.0 dB.
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