Abstract: A jammer (20) for jamming the readout of contactless data carriers (2, 2?) by a reader (1) emitting electromagnetic scanning signals (SS) has an air interface (11) for receiving electromagnetic signals, analyzing means (15) for analyzing the electromagnetic signals received by the air interface, and jamming-signal generating means (18) for generating a jamming signal (DS). The analyzing means (15) for analyzing the electromagnetic signals received are arranged to identify scanning signals (SS) from the reader (1) among the electromagnetic signals received and, when scanning signals (SS) are detected, to transmit a control signal (CS) to the jamming-signal generating means (18), which latter in turn generate the jamming signal (DS) and transmit it via the air interface (11) on receipt of the control signal (CS).