An excitation assembly comprises a symmetrical OMT and two splitters respectively connected to two pathways of the OMT. The OMT comprises a cross junction comprising a central waveguide parallel to an axis Z and four lateral ports oriented in two directions X, Y, the first splitter consisting of an input waveguide and of two output ports coupled to two lateral ports, oriented in the direction X, by respective connection waveguides. The first splitter is located on a lateral side of the OMT, orthogonally to the direction X, and its two output ports are formed one above the other in a lateral wall of the input waveguide, the upper output port being placed facing a first lateral port of the OMT to which it is connected by the first connection waveguide. The difference in electrical length between the two connection waveguides is equal to λ/2.