Perimeter protection in a security installation is achieved by detecting disturbances in a microwave beam sent from a transmitter to a receiver. The transmitter and receiver have associated beam antennas of extended vertical aperture of not less than 0.75 meters to mitigate the effects of ground reflection....http://www.google.fr/patents/US4191953?utm_source=gb-gplus-shareBrevet US4191953 - Intrusion sensor and aerial therefor
