Improvements in the simulation of absorbing boundaries for the finite-difference time-domain method

For about twenty years, numerical techniques such as the TLM or FDTD methods have been extensively used for the modeling microwave components. Nevertheless, the memory size of the computers imposes a limitation to their use and it becomes necessary to introduce absorbing boundaries in order to simulate open space propagation. During the last few years, many authors have worked on this problem without obtaining a perfectly satisfying solution. We compare the solutions proposed by authors in terms of efficiency, dispersion, and spurious modes. These solutions are tested in the two formulations of the FDTD method: expanded nodes (Yee´s scheme) and condensed nodes. These comparisons enable the authors to suggest some improvements in the simulation of absorbing boundaries such as the use of non-centered finite-differences formulas for Mur´s (1981) absorbing boundaries. The comparison of the expanded nodes and condensed nodes formulations is extensively developed in the case of the Berenger´s (see J. Comp. Phys, vol.114, no.2, 1994) absorbing boundaries.