that portion with an rf input signal 122 carried on a conductor 118 that has been received by a receive antenna 120 typically in the presence of relative motion between the device and the body the rf input signal 122 is a reflected and doppler shifted version of the transmitted signal 124 the output of the mixer that is the if signal 126 is input into the detection system 104 which if appropriate can instruct an alarm fault generator 130 to generate an appropriate alarm signal to be output via the audio visual output device 110 the detector device also comprises a field distortor 132 for influencing the electro magnetic field of the rf input signal being carried by the conductor 118 preferably the field distortor 132 takes the form of a diode that in response to an applied signal 136 causes a change in the electro magnetic field carried by the conductor in particular the field distortor influences the phase of the rf input signal at the rf input port of the mixer 116 in the absence of motion the mixer 116 acts as a phase detector and produces a dc output signal in response to receiving an input signal on the port that would ordinarily receive the received signal 122 that is on the rf input port of the mixer this input signal is a combination of the received signal 122 reflected from static objects within the protected volume 108 and signals generated within the detector device 102 due to internal reflections.