In this paper, an approach for the synthesis of Ultra-Wideband (UWB) antennas with a notched frequency band is presented. It exploits the effectiveness of a representation based on spline curves to obtain the wideband behavior and the simplicity of using a rectangular slot for the stopband characteristic. A Particle Swarm Optimizer (PSO) is used to guide the synthesis process to a final design that fully satisfies the project guidelines and constraints defined by the user. A representative design example is reported to assess the effectiveness of the proposed approach