The mixer comprises two parallel conductors placed on each side of a substrate, two diodes mounted in series between the first ends of the two conductors, an incident-wave input connected between the second conductor-end located on the second face of the substrate and a ground line located on the first face, a mixing-wave input connected to means for applying between the ends of each diode an electric field produced by the mixing wave, and a mixed-wave output connected between at least one end of a diode and the ground line. The end of the conductor of the two-wire line is left free and electrically isolated from the ground line by means of a gap.