Abstract: A sensor system having a sharply defined zone of active sensing comprising a compound antenna system comprising an antenna structure disposed in relation to a shield structure and spaced from the shield structure, the shield structure having an open aperture in front of the antenna structure in the direction of a lobe of sensitivity of the antenna structure. In various embodiments, the shield structure may be layered on the inside between the antenna and the shield structure with an RF absorbing material. The aperture may be formed in part by adjustable panels and the antenna spacing from the aperture may be adjustable by adjusting an antenna mounting position within the shield. The compound antenna system may be coupled to a receiver having a threshold response based on the compound antenna system response characteristic.