A calibration device for adjusting the forward power and reflected power measured via a single measuring directional coupler which is connected to a high-frequency transmitter, the forward power that is attenuated with ak+x and decoupled being used as a reference value. The calibration device comprises a changeover switch that is embodied as a double switch and is connected to the control and display unit via a control wire. A control signal that actuates the changeover switch is transmitted by the control and display unit, whereby switching occurs from a measurement of the reflected power to a measurement of the forward power. The forward power and the reflected power are measured via the same measuring directional coupler and by means of the same measuring sensor which is composed of a low pass and a rectifier.