This paper describes the ultra-wideband horn antenna design operating in quasi TEM mode. The antenna consists of a triangular plate with an inclination angle above a ground plane and directly fed by a coaxial cable. Broadband characteristics, radiation pattern, high gain, and a small reflection coefficient were achieved. Its performance analysis and main parameters effects were obtained by using ANSYS HFSS R software. In the numerical analysis, an optimized model was obtained in 1.57 to 12.85 GHz bandwidth an 11 to 14 dBi gain. The proposed antenna was manufactured and the measured reflection coefficient results show an operation frequency range between 1.48 and 10.12 GHz, agreeing very well with simulations.