AbstractThe 140 GHz ASDEX-Upgrade ECRH system uses four gyrotrons to generate a total power of 2 MW in a Gaussian beam for 2 s. External magnetic perturbations, originating from the poloidal stray magnetic field of the tokamak air core transformer, and also from the cryomagnet of the adjacent gyrotron, influence the startup of the oscillation and the electron beam deposition in the collector. The transmissions are partly quasioptical and partly corrugated HE-11 waveguide lines. Their losses have been determined calorimetrically.