Method in low voltage net data transmission system for keeping the signal level of transmission constant on the net voltage rail or on wall outlet. In the method the feedback signals are taken wired or wirelessly from one or some locations of the actual apparatus or supply cable to the measuring and handling unit (60) of transmitting signals and further to the process unit (70) of sample and holding circuits (S & H) or of corresponding means and control means (CONTROL), by which unit the control signal (U.sub.RC) and/or U.sub.LC) is taken to steer the output signal or output voltage of blocks (10, 20, 40 and/or 50) in a depending way from load impedance (Z.sub.LOAD) and from the series impedance (Z.sub.W) of supply cable (L.sub.W) so that the amplitude (U.sub.LOAD) of transmission signal level (U.sub.LOAD) on voltage rail or some location of the supply cable or on wall outlet is constant or almost constant.