The Method of Auxiliary Sources is employed to model the impact of finite ground planes on the radiation from antennas. In many cases the computational cost of available commercial tools restricts the simulations to include only a small ground plane or, by use of the image principle, the infinitely large ground plane. The method proposed here makes use of results from such simulations to model large and moderate-sized finite ground planes. The method is applied to 3 different antenna test cases and a total of 5 different ground planes. Firstly it is validated through comparison with reference simulation results for the case of crossed Hertzian dipoles antennas. Later the method is applied to the cases of real antennas, a slot antenna and a printed dipole phased array, for which corresponding measurements have been conducted. The results obtained with the method agree well with the simulated and measured reference solutions and the method is thus found to be a useful tool in determining the impact of finite ground planes.