interference as long as the binary signal is present in order to cause a particularly quick switching reaction in the controllable switching circuit 11 5 8 it is possible to make signal 26 a pulse signal from the response side of detector 18 by differentiation of indicating signal 10 by forming a suitable pulse whose spectral energy either does not form any contribution in the frequency range of the received signals or whose duration in time is so short that it is physiologically not noticed it is possible to transmit such pulse via antenna line 12 to antenna installation 21 with controllable switching circuit 11 5 8 without causing interference with the reception fig3 shows antenna installation 21 having intermediate frequency signal 19 which in the case of interference is keyed up as a signal burst that is selectively transmitted via antenna line 12 and by way of intermediate frequency filters if the indicating signal is to be transmitted to the antenna installation 21 of the invention for example via a line carrying pre applied dc voltage or via another signal line 24 it is possible according to a further embodiment of the invention to transmit such signal 26 as the envelope curve of a signal burst of high frequency and to recover said envelope curve in the antenna installation 21 in order to trigger the change over action the.