An integrated circularly polarized (CP) microstrip antenna array for generating orbital angular momentum (OAM) beams at S band is presented， which adopts coaxial feeding. To obtain excellent polarization performance, the parameters of the antenna structure are simulated and optimized by the CST software, then the overall size of proposed antenna is 0.416λ0×0.416λ0×0.026λ0, the 3 dB axial ratio (AR) bandwidth is 3.79~3.85 GHz and S11＜-10 dB bandwidth is 3.74~3.96 GHz. The proposed antenna has simple and compact structure, which is easy to implement. The antenna array consists of six identical CP elements that rotate 60 degrees clockwise sequentially. According to the simulated and measured results, the proposed antenna array can generate OAM beams with mode l = -1 under the condition of same amplitude and phase feeders, which can effectively avoid the design of complex network structure.
