Patent ID: 6946950

Claim:
A mobile body discrimination apparatus comprising a plurality of transponders and an interrogator apparatus, said interrogator apparatus comprising: at least one interrogator antenna; control means; transmitting means controlled by said control means for driving said interrogator antenna to periodically transmit a transmission start command code conveyed by modulated radio waves, directed at each of said transponders which are currently within a communication region of said interrogator antenna, and for transmitting CW (continuous-wave) radio waves in intervals between transmissions of said transmission start command code; interrogator receiving means coupled to said interrogator antenna, for obtaining requested signals from said transponders, conveyed by modulated reflected radio waves from said transponders which are within said communication region; wherein said at lest one interrogator antenna comprises a first antenna and a second antenna, and said transmitting means comprises first transmitting means controlled by said control means for driving said first antenna to periodically transmit said transmission start command code conveyed by modulated radio waves and to transmit CW radio waves in intervals between said transmission of said transmission start command code, and second transmitting means for driving said second antenna to transmit said CW radio waves; wherein said control means comprises means for periodically interrupting said transmission fo said CW radio waves by said second transmitting means and said second antenna, for the duration of each of respective time intervals in which said transmission start command code is transmitted by said fist transmitting means and first antenna; and each of said transponders comprising: a transponder antenna; rectifier circuit means coupled to said transponder antenna, for converting a signal received by said transponder antenna to electrical power for operating circuits of said transponder; transponder receiving means coupled to said transponder antenna, for obtaining said transmission start command code from said modulated radio waves transmitted by said interrogator apparatus; memory means for storing data therein; modulator means controllable for varying a reflection condition of said transponder antenna with respect to incident radio waves; and response timing control means responsive to reception of said transmission start command code by said transponder receiving means for controlling said modulator means to vary said reflection condition of said transponder antenna according to response data comprising at least a part of said data of said memory means, to generate modulated reflected radio waves conveying said response data after a specific delay time has elapsed following reception of said transmission start command code; wherein said response timing control means comprises means for setting a randomly determined value of said delay time, at each reception of said transmission start command code.