The deviation of an electron beam is measured in a drift tube of a linear accelerator, the wave to be decoupled having a frequency range that corresponds to a multiple of the basic frequency of the acceleration field. Coupling probes, a mixer-based receiving concept with high dynamics and sensitivity, a method for evaluating the measuring signals and a calibration method for calibrating out non-linearities are specified. Disruptive influences through the acceleration field are minimized by the measurement method according to the invention and the frequency range to be evaluated. The high evaluated frequencies also offer geometrically small coupling probes which one can introduce into a drift tube in which only the field of the electron beam to be evaluated exists.