A method monitors a line for changed ambient conditions. The line has a measuring line with a predetermined length and a measuring conductor surrounded by insulation having a known dielectric constant. In the method, an analog signal having a predetermined frequency is generated, the signal is divided into a reference signal and an operating signal, the operating signal is fed into the measuring conductor, a return signal obtained from the operating signal is combined with the reference signal and by a phase shift between the reference signal and the return signal, a measure is determined for the changed condition, particularly for a temperature change.