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In this work, an ultra-wideband and high-efficiency reflective linear-to-circular polarization converter based on an anisotropic metasurface is proposed, which is an orthotropic structure with a pair of mutually perpendicular symmetric axes u and v along ±45 • directions with respect to the vertical y axis. The simulated and experimental results show that the polarization converter can realize ultrawideband linear-to-circular polarization conversion at both x-and y-polarized incidences, its 3dB-axialratio-band is between 5.8 and 20.4 GHz, which is corresponding to a relative bandwidth of 112%; moreover, the polarization conversion efficiency (PCE) can be kept larger than 99.6% in the frequency range of 6.1-19.8GHz. In addition, to get an insight into the root cause of the LTC polarization conversion, a detailed theoretical analysis is presented, in which the conclusion is reached that in the case of neglecting thelittle dielectric loss, the axial ratio (AR) of the reflected wave can be completely determined by the phase difference between the two reflection coefficients at u-and v-polarized incidences, and any anisotropic metasurface can be used as an effective LTC polarization converter when the phase difference is close to ±90 • .