Abstract: A device for a contactless transmission of data includes a data transceiver with an antenna. The impedance of the antenna of the data transceiver is purposely mismatched. A reactance is introduced into the antenna in order to tune the antenna to a frequency different from its resonant frequency. In the event that the antenna impedance has to be adapted to the characteristic impedance of a line of unknown length, the loss resistance of the antenna is deliberately increased. The power required to produce a magnetic alternating field with the data transceiver for operating at least one portable data carrier is reduced.