A polarizer for two different frequency bands is described for exciting an antenna with a parabolic reflector. The polarizer has a waveguide section capable of carrying in each frequency band two mutually perpendicularly linearly polarized waves. For each frequency band, two waveguides having a rectangular cross-section are connected to the waveguide section separate from each other and mutually offset in the axial direction of the waveguide section. A respective waveguide is connected directly to the waveguide section for each polarization direction of the lower frequency band, whereas for the upper frequency band—from a connecting point (11, 12) on—each of the two waveguides is subdivided into two branches (18, 19, 20, 21) with each having identical rectangular cross-section. The respective branches terminate on the waveguide section at mutually opposing locations which are circumferentially offset relative to each other by 90° for the two polarization directions. To simplify the portion designated for the upper frequency band, the wider flat sides of the two branches (18, 19, 20, 21) abut each other at each of the connecting points (11, 12) in such a way that the front faces of the branches are aligned with each other for connection to the respective waveguide (7, 8). In addition, one of the branches (19, 21) of the two different polarization directions is twisted along its path about an angle of 180°.