Abstract: The theory and numerical techniques of near-field far-field transformations using spherical-wave expansions are studied. The efficient data processing methods to achieve a high degree of accuracy in transformations and at the same time to reduce the memory capacity are developed. The analytical modei is used to validate the feasibility of the theory and computer software. For an X-Band horn antenna, the far-field pattern computed from near-field data, the measured far-field pattern and the far-field patter from theoretical integral equations are compared, demonstrating the excellent numerical accuracy of the transformation techniques.