The sensor has a transmission-and receiving circuit (18) exhibiting a mixing function for mixing transmitted and receiving signals. The antenna elements (10) comprise two receiving antennae (12, 14) that are arranged symmetrical to a transmitting antenna (16). A transmission-and receiving circuit (18) produces two intermediate frequency signals (ZF1, ZF2) by mixing of signals delivered by the transmitting antenna with signals that are received by the receiving antennae. A subtraction unit (28) is provided to deliver difference (D) of the intermediate frequency signals as an output signal.
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