Abstract: A diagonal dual-polarized broadband horn antenna. The unique use of diagonal line ridges placed in the corners of the aperture of the diagonal dual-polarized broadband horn antenna. The sides of the diagonal dual-polarized broadband horn antenna are made of any number of materials including dielectric material or metallic material. The operating frequency range of one embodiment of the present invention is approximately 100 MHz to 18 GHz in one embodiment. The present invention is scaleable to allow operation at even higher and lower frequencies. This ability of the present invention to adapt to a number of frequency ranges allows application for a wide variety of electromagnetic testing applications. The use of the diagonal line ridges allows for a broadband horn antenna that is significantly more manageable that conventional broadband horn antennas, while offering operation at common frequency ranges.