Abstract: Disclosed is a coaxial waveguide antenna. In a basic form the coaxial waveguide horn for the antenna comprises an inner section operating in a .SIGMA. (sum) mode only and an outer section operating in a .DELTA. (difference) mode only. Where required for broadband (i.e., for more than 1 octave), intermediate sections operating in .SIGMA. and .DELTA. modes are provided. The basic structure operates in the linear mode; for polarization diversity and circular polarization, a polarizer is placed in front of the horn, or a dielectric sheet is positioned within a coaxial waveguide section at an angle of 45.degree. as to the E field, or by feeding the coaxial waveguide with six or more number of probes.