Abstract: A compact device for launching circularly polarized electromagnetic waves utilizes a hollow waveguide terminated by a horn. Within that structure, a septum fin extends from the waveguide into the horn. The septum fin divides the hollow waveguide into two smaller waveguides, each of which is capable of independently supporting the TE.sub.1,0 mode of wave propagation. The fin is a tapered plate that has its maximum width within the waveguide and its minimum width in the horn. A signal injected into one port of the divided waveguide emerges from the horn as a circularly polarized wave having its polarization vector rotating clockwise whereas a signal injected into the other port emerges from the horn as a circularly polarized wave with its polarization vector rotating in the counter clockwise direction.