Focused on the methods of near-zone to far-zone field transformation in solvingscattering problem by FDTD method

two new approaches are presented in this paper. One is directly obtaining scattered far field by adopting Stratton-Chu integral formula. Another method isapplying Laplace transformation and convolution technique which leads to a unified formula forscattered far field in time domain for both 2-D and 3-D scattering problems

and makes 2-D problem a special case of 3-D problem. Numerical results of simple scatterer are given to show the validation of the both algorithms.