Spatial subgridding in FDTD

In the traditional finite difference time domain (FDTD) method: the studied structure is modeled as elementary cells, which sizes have to be small eno ugh to get close to the reality. Then, this discretization is applied to th e whole calculation volume. even though some zones do not need such fine di scretization. Consequently, this numerical tool requires an important calcu lation capacity to increase the precision of the grid mesh. For very large dimension structures, computer limitations no longer allow a sufficiently a ccurate discretization. The aim of this article is to discuss some methods which allow the EM study of large structures containing elements of very un equal size as compared to the wavelengths used, while preserving an accepta ble calculation cost. This approach combines finite differences of variable precision and different-sized cells in the same calculation domain. It all ows a fine discretization of small structures and a rough discretization of the other elements of the Ehl problem. This spatial subgridding gives a ve ry original possibility of undertaking EM zooms on some specific parts of t he calculation domain thus allowing a better modelling of the studied struc ture while preserving good accuracy and an acceptable cost.