Necessity for calculating boundary temporary field components in reduced finite-difference time-domain method(R-FDTD) is discussed.A periodic symmetric R-FDTD method is proposed.Temporary components in supplement calculation are given based on symmetry and periodic boundary conditions(PBC).Computational domain is reduced to 1/4 and lateral field components of a symmetry plane are obtained with symmetry relations.Compared with original FDTD,memory storage is reduced to 1/6 while ensuring accuracy. It is shown that the improved algorithm agrees with FDTD method by numerical simulation of EM impulse responses tO infinity wire mesh reinforcement and reinforced concrete layer.The improved algorithm has advantages in saving memory storage and calculation time.

The basic functions in the method of moment (MoM) for 2D electromagnetic (EM) scattering problems that is symmetric about X-axis and Y-axis are obtained with the represention theory of finite group.We construct the group and get its regular representations.The tentative functions according to the four center symmetric positions are shown.While the regular representation is transformed into an irreducible one,orthonormal basic functions composed of the tentative functions are obtained.

The Original Generalized Image Method can only deal with four aspect symmetric scattering body.On the basis of the original GIM,a developed method termed Extended GIM is proposed.The Extended GIM can save a great deal of calculation time and storage of computer when useing it for dealing with the scattering sturcture of higher symmetry than the four aspect symmetric one.A six-aspect symmetric example is also given.