A simple model of 1x4 linear array of microstrip antennas was used to analyse the influence of phase difference between antenna elements on the performance of beamforming at 8.9 GHz. Simulations with full wave CST Studio were performed over a number of phase sets. Among them we identified the most constructive and the most destructive cases based on the maximum gain, the total efficiency and the polarization. It resulted that [0;0;0;0] phase case provided 10.6 dB higher gain, 46.5% higher efficiency and much more spatial uniformity of polarization along the propagation path than the [0;180;0;180] case. Field strength computations and vector representations proved the complexity of beamforming when one also takes into account the mutual coupling of the elements.