The present invention relates to a multiband transmit-receive coupler-separator with a very wide band of the OMT (“OrthoMode Transducer”) type for microwave-frequency telecommunications antennae. This coupler comprises a port for propagating all of the frequencies, a body and a port for propagating high-frequency bands, these three portions being coaxial, and wide-band coupling slots for propagating the low-frequency bands made in the body and each associated with a waveguide, and it is characterized in that its body joining the two ports has a shape of revolution the profile of which changes according to a multipolynomial law, constantly decreasing from the port with the largest cross section to the port with the smallest cross section. This coupler can operate in order to couple and separate very wide bandwidths (the overall use of this coupler-separator being more than one octave), and two or four wide-band coupling slots are necessary for the propagation of linear and circular polarizations after recombination.