Apparatus for checking the interference radiation behaviour of a modem for transferring data via a power supply network, comprising: - a network connection to supply an electric alternating current; - a connection connected to a phase conductor, a neutral conductor and a protective conductor of the network connection and which is configured to connect a modem to the phase conductor, the neutral conductor and the protective conductor, wherein the modem is configured to couple a first signal and a second signal in the connection to form a MIMO transfer of the first and second signal by means of the phase conductor, the neutral conductor and the protective conductor; - coupling means connected to the connection comprising an asymmetric impedance stabilisation network (260, 260') and an output, wherein the impedance stabilisation network (260, 260') is connected to the phase conductor, the neutral conductor and the protective conductor on the input side and configured such that the first and second signal can be fed in on the input side if the modem is connected to the phase conductor, the neutral conductor and the protective conductor and the first and the second signal are coupled in the connection, and wherein the asymmetric impedance stabilisation network (260, 260') is connected to the output of the coupling means on the output side, wherein the impedance stabilisation network (260, 260') comprises a first resistor, a second resistor and a third resistor which are arranged in a star structure relative to a common nodal point, wherein the first resistor is connected to the phase conductor, the second resistor is connected to the neutral conductor, the third resistor is connected to the protective conductor and the common nodal point is connected to the output of the coupling means, and wherein the impedance stabilisation network (260, 260') comprises a fourth resistor which connects the common nodal point with the output of the coupling means. - a network connection to supply an electric alternating current; - a connection connected to a phase conductor, a neutral conductor and a protective conductor of the network connection and which is configured to connect a modem to the phase conductor, the neutral conductor and the protective conductor, wherein the modem is configured to couple a first signal and a second signal in the connection to form a MIMO transfer of the first and second signal by means of the phase conductor, the neutral conductor and the protective conductor; - coupling means connected to the connection comprising an asymmetric impedance stabilisation network (260, 260') and an output, wherein the impedance stabilisation network (260, 260') is connected to the phase conductor, the neutral conductor and the protective conductor on the input side and configured such that the first and second signal can be fed in on the input side if the modem is connected to the phase conductor, the neutral conductor and the protective conductor and the first and the second signal are coupled in the connection, and wherein the asymmetric impedance stabilisation network (260, 260') is connected to the output of the coupling means on the output side,