Many passive and wireless devices, such as RFID tags or batteryless wireless sensors, are designed to control electromagnetic field backscattered by the devices, in order to maximize the interrogation range (or the device-to-reader separation distance) and/or to mitigate the electromagnetic clutter. In this Letter, we show that a linearly-polarized electromagnetic field can be converted into a circularly-polarized field by using a passive and chipless device. The device, also called tag, consists of a one-port antenna loaded by a passive impedance, whose value is derived from a rigorous electromagnetic model including structural and antenna scattering modes. The experimental validation of predicted Linear-to-Circular polarization conversion is reported in the case of a patch antenna illuminated by a linearly-polarized field. At the operating frequency of 2.6 GHz, the measured axial ratio of the electric field backscattered by the tag is lower than 1 dB.