Abstract: This invention relates in general to a passive responder of a position finding system including at least one microwave antenna and a resonator body. The resonator body is mounted adjacent the rail of a railroad track and is connected to a microwave antenna which points upwardly such that it receives radiation from a transmitter carried by the train. The radiation opening of the receiving antenna is adjacent the rail head and the antenna is covered with a cover member comprising dielectric material which comes into contact with the wheel as each wheel rolls past. The contact of the wheel with the dielectric cover assures that ice, snow, and other foreign material will be continuously cleared from the receiving antenna so as to assure optimum operation of the transponder.