This letter presents the design of a planar antenna for ultrawideband (UWB) applications with a bandwidth of 5.5 GHz over 3.7 to 9.2 GHz and return loss values lower than -10 dB. The antenna geometry is described in terms of a spline-based representation whose control parameters, together with other geometrical descriptive quantities, are determined through a suitable particle swarm optimizer (PSO) in order to fit the UWB requirements. Representative results of both numerical and experimental validations are reported in order to assess the performance of the prototype as well as to give some preliminary indications on the reliability and effectiveness of the whole synthesis approach