Abstract: An electronic radio frequency identification system comprises a reader 12 and a plurality of transponders to be read by the reader. The reader comprises a recovery circuit 32 for recovering and separating an upper sideband 26.12 and a lower sideband 26.11 of a modulated response signal 26.1 from one of the transponders. The reader further comprises an evaluating circuit 38 for evaluating and selecting one of the upper sideband the lower sideband for output to a next stage, based on the evaluation.

Abstract: A dipole antenna 34 comprises a loop 36 as a first pole and an appendage 42 comprising a strip 44 terminating in a pad 46 as a second pole. The circumference of the loop is in the order of one half-wavelength of an operating frequency and an effective length of the appendage is at least 0.15 times the wavelength. The dipole antenna 34 is shorter than conventional half-wavelength dipole antennas, has a better frequency response and the antenna impedance ZA is dependent on the position of a feedpoint 38 in the loop 36. The antenna may be used with transponders for electronic radio frequency identification systems.

Abstract: A transponder 14.1 for an electronic radio frequency identification system comprises an antenna 18 having an antenna impedance at a feedpoint thereof. The transponder further comprises an electronic circuit 19 having an input connected to the antenna. The antenna impedance having impedance characteristics for developing first and second voltage maxima (36, 38) across the input at first and second frequencies (fL and fH) respectively, thereby to provide a continuous operative frequency band for the transponder including the first and second frequencies.

Abstract: There is disclosed and claimed an electronic identification system including at east one reader and a plurality of transponders to be read by the reader. Each transponder has a normal operational mode 32 wherein it, in response to an interrogation signal from the reader transmits a code 36 to the reader, and a monitor mode 40 wherein it is still sensitive and responsive to signals from the reader, but wherein it does not transmit the code. The reader is adapted to cause a transponder of which the code has been read, to switch from the normal operational mode to the monitor mode, by transmitting a first signal S1 to the transponder. The transponder, while being in the monitor mode, is sensitive to a second signal S2 and responsive upon receipt of the second signal, to reset to the normal operational mode.

Abstract: A transponder 20 for an electronic identification system transponder is characterized in that it presents a high input impedance (>400.OMEGA.) at an input thereof which is directly connected to an antenna 22 with a matched high input impedance. The transponder is aimed at improving the voltage recovered on capacitor C.sub.2 from an interrogation signal and thus the operational range of the system. The modulator 30 of the transponder is arranged to backscatter modulate the interrogation signal at a modulation depth of less than 80%, preferably in the order of 30%. This also results in an improvement of the operational range of the system.