In the first embodiment, the inventive sensor comprises first and second antennas, a pulse generator provided with two outputs, wherein the first output thereof is connected to the input/output of the first antenna and the second output is connected to the input/output of the second antenna. The output circuit of the sensor consists of two detectors, wherein the input of the first detector is connected to the input/output of the first antenna and the input of the second detector is connected to the input/output of the second antenna. The output of the first detector is connected to the first input of a differential amplifier whilst the output of the second detector is connected to the second input thereof. The output of the differential amplifier is connected to the low frequency filter and to a feedback device whose output is connected to one of the inputs of the differential amplifier. The output of the low frequency filter is connected to a comparator input. In the second variant, the input circuit of the sensor consists of two detectors each of which is provided with two inputs. The first output of the pulse generator is connected to the input/output of the first antenna and to the second input of the first detector. The input/output of the first antenna is connected to the first input of the second detector whilst the second output of the pulse generator is connected to the second input of the second detector though a delay circuit.