AbstractThe characterization of the electromagnetic field at the cyclotron electron frequency is one important point for the ECRH installation, which is in development at the CEA Cadarache. This characterization is based on a mathematical decomposition of the electromagnetic field over the various radiating modes of an oversized corrugated wave-guide. In order to do this calculation, the principle of which will be exposed in the paper, we have to know the amplitude and the phase of the complete electric field on a plane transverse to the propagation axis. We will see that it is possible to obtain amplitude diagrams of the two crossed polarizations at high and low power. The high frequency low level measurement has been done with classical method at the laboratory. The high level measurement at the output of a gyrotron is made with thermal pictures after short pulses (typically 5 ms). Nevertheless, measuring the phase is not easy at this frequency, thus a numerical method is used. The phase is numerically obtained from several amplitude diagrams according to an algorithm created by Katsenelenbaum and Semenov in 1967. The paper gives the results of this method and we will see that it is possible to measure the phase at 118 GHz at low level. So the results on amplitude and phase allow us to give the portion of the HE11 mode in the complete electromagnetic field.