Abstract: An antenna system for producing a millimeter wave beam having a Gaussian distribution uses a high power source and a partial mode converter operatively coupled to the source and reviving from the source a T.sub.m,n mode wave which might either be TE.sub.m,n or TM.sub.m,n'. The converter delivers a mixture of modes including the mode T.sub.m,n at a relative power rate of 70 to 85% and at a relative phase 0, and at least one of the following additional modes, namely T.sub.m,n-1 at a relative power rate of 10 to 20% and at relative phase .pi., and TE.sub.m,n+1 at a relative power rate of 10 to 20% and at relative phase .pi.. The mixture of modes is applied to a collimation reflector (3, 10), having a concentration point (5) constituting the antenna system outlet. The antenna system can convert any T.sub.m,n mode into a linearly polarized narrow beam of very high power (about 1 MW) and and of high frequencies above 100 GHz.