A local component (antenna array) with fine structure is mounted on an electrically large body (antenna base), which cannot be handled by using the regular commercial EM software based on MoM or FEM, even FDTD method due the huge memory requirement and long simulation time. GEMS software, based on parallel processing technique, can be used to easily simulate this type of problems in a very reasonable time through a small cluster. In the experiments, the Vivaldi antenna array is mounted on a large PEC base, if we simulate the Vidaldi antenna only in the GEMS software, the relative error in the far field pattern will be as high as 20 percent, which is not acceptable in the practical applications. Therefore, we include the large PEC base inside the GEMS simulation, the relative error in the far field pattern will drop to 3 percent.