The effect of phase changes along the antenna radiation pattern beamwidth and bandwidth on synthetic aperture radar (SAR) imaging is evaluated by means of electromagnetic simulations. Electromagnetic images have been obtained in order to characterize the effect of the non-uniform phase of the antenna radiation pattern on this type of measurements. Three different situations have been analyzed: i) phase variations across the angular range, ii) nonlinear phase shift across the working frequency band and iii) antenna phase center shift across the bandwidth.