Abstract: A flexible line distributed antenna, defining a perimeter of interest, radiates energy supplied from a transceiver. Motion around the flexible line causes the reflected energy which is intercepted by the flexible distributed antenna to be coupled back to the transceiver as a doppler frequency for processing in an alarm system. Reflected energy from the antenna to the transceiver is amplified and rectified into a DC voltage. This DC voltage is integrated to produce a time varying signal to a level detector that responds to a predetermined value of the integrated DC voltage. A response by the level detector provides an actuating signal to an alarm indicator through alarm logic. Ambient conditions around the extended flexible line distributed antenna are monitored by external sensors having outputs for controlling amplifier gain and triggering the alarm logic. For improved motion detection reliability, reflected energy from the antenna is coupled to a transceiver providing two output signals.