Abstract: A microwave coupler with improved efficiency receives microwave energy in a principal mode TEm,n (m and n being whole numbers and n being not zero) and gives a quasi-optical energy beam. It comprises a mode converter that receives energy in the principal mode and converts a part of it into an auxiliary mode TEp,q (with p and q being whole numbers, q close to one and not zero, p greater than q). The energy in the principal mode and in the auxiliary mode get propagated in a radiator and emerge in the form of the quasi-optical beam by an aperture that coincides with a minimum electrical field resulting from the electrical field of the principal mode and the electrical field of the auxiliary mode. The disclosure has applications notably to the field of gyrotubes.