Transition radiation as a source of circularly polarized radiation

Transition radiation generated by a moving charge after crossing two gratings, one after another, is considered. The gratings are made of parallel metal wires. Gratings planes are parallel to each other and are perpendicular to the charge velocity. Conductors of the first gratings are perpendicular to conductors of the other. It is shown that the resulting transition radiation has elliptic polarization. The degree of ellipticity and direction of rotation depend on distance between gratings and on charge velocity