Abstract: A sensitive intrusion detection system has an RF excited antenna located within the area to be protected and a leaky coaxial cable extending around the perimeter. The presence of an intruder alters the coupling between the antenna and the coaxial cable thereby changing the signal received by the cable. The detection system is responsive to incremental changes in the in-phase and quadrature components of the received signal. When these components are plotted against each other a cardioid-like curve is obtained in the .DELTA.I,.DELTA.Q plane. By tracking both magnitude and angle of this curve as it is generated a sensitive detection mechanism is provided. When the variations in magnitude and angle exceed a threshold an alarm is sounded. To avoid the possibility of intruders using a particular path would gives a null angle response, a second cable adjacent to the first may also be employed.