A method and apparatus for transmitting a directional reply signals in response to Air Traffic Control Radar Beacon System and Mode Select signals interrogation signals. Two directional antennas independently acquire an interrogation signal and are connected by means of a configurable switch to a transponder....http://www.google.de/patents/US6313783?utm_source=gb-gplus-sharePatent US6313783 - Transponder having directional antennas
