The invention relates to a source for a reflector antenna, comprising: a pseudo-cavity, a first sigma excitation device for exciting the pseudo-cavity in such a way as to generate a sum channel signal via a coaxial waveguide, a second excitation device for exciting the pseudo-cavity in such a way as to generate a difference channel signal, the second device comprising eight probes angularly distributed around a principal emission axis of the source, and a difference supply circuit for supplying the eight excitation probes according to the two modes TE21.