A compact planar ultra wideband (UWB) antenna optimized for on-body communications is presented. The antenna is characterized in free space, on a homogeneous phantom modeling a human arm, and on a realistic high-resolution whole-body voxel model. The results are presented in terms of reflection coefficient, radiation pattern, efficiency, and E-field distribution. The simulation results for the return loss and radiation patterns are in good agreement with measurements, and good on-body propagation features are demonstrated. Time domain behavior of the proposed antenna is investigated for some relevant on-body paths, and the effect of modulations schemes on the on-body system performance is discussed.