Transmission circuit for a transponder system for transmitting a digital signal via a transmission antenna for radio waves at a prescribed carrier frequency, wherein a) a PWM signal generator for obtaining a pulse-width-modulated signal at a prescribed clock frequency is provided and, in order to transmit the digital signal, the digital signal is overlaid on the PWM signal, b) the signal produced in this manner actuates two semiconductor switches (Tl, T2) in push-pull mode via a level converter, wherein one semiconductor switch is connected to the supply voltage and the other semiconductor switch is connected to the earth potential and both semiconductor switches (Tl, T2) have the respective other connection connected to the input of a bandpass input filter, c) the output of the bandpass input filter has the transmission antenna connected to it, d) a control loop for the current (I_Ant) through the transmission antenna is provided, e) wherein the current through the transmission antenna is recorded (R_I_Ant) and is compared with a setpoint value in a comparator, and in the event of a discrepancy f) the control loop produces a control signal for matching the pulse/pause ratio of the PWM signal to the PWM signal generator, characterized in that g) prior to the transmission of the digital signal a test sequence for the PMW signal is transmitted for a prescribed test period and in the course of this the current through the transmission antenna is recorded and the pulse/pause ratio of the PWM signal is regulated such that the current approximately corresponds to the setpoint value, and h) subsequently the digital signal is sent for a prescribed transmission period and in the course of this the pulse/pause ratio is kept constant. a) a PWM signal generator for obtaining a pulse-width-modulated signal at a prescribed clock frequency is provided and, in order to transmit the digital signal, the digital signal is overlaid on the PWM signal, b) the signal produced in this manner actuates two semiconductor switches (Tl, T2) in push-pull mode via a level converter, wherein one semiconductor switch is connected to the supply voltage and the other semiconductor switch is connected to the earth potential and both semiconductor switches (Tl, T2) have the respective other connection connected to the input of a bandpass input filter, c) the output of the bandpass input filter has the transmission antenna connected to it, d) a control loop for the current (I_Ant) through the transmission antenna is provided, e) wherein the current through the transmission antenna is recorded (R_I_Ant) and is compared with a setpoint value in a comparator, and in the event of a discrepancy f) the control loop produces a control signal for matching the pulse/pause ratio of the PWM signal to the PWM signal generator, characterized in that g) prior to the transmission of the digital signal a test sequence for the PMW signal is transmitted for a prescribed test period and in the course of this the current through the transmission antenna is recorded and the pulse/pause ratio of the PWM signal is regulated such that the current approximately corresponds to the setpoint value, and h) subsequently the digital signal is sent for a prescribed transmission period and in the course of this the pulse/pause ratio is kept constant.