Determination of modal amplitudes by resolution of radiation patterns

A simple and accurate method for the determination of the relative level of excitation of higher-order modes in over-moded waveguides is presented. The method relies on the change of amplitude and phase of the radiation fields of the open ended waveguide when the length of the guide is varied. Good agreement with the theoretical results is observed for a circumferentially corrugated waveguide operated near 10 GHz. Only the change in length and not the length of the guide itself need be precisely known.