In Chapter 4 we presented the Yee algorithm for the discretization of Maxwell's equations on a regular, orthogonal grid. In Chapter 6 we discussed the numerical accuracy of the discretization, which depends on the choices of Δx, Δy, Δz, and Δt, as well as the medium of interest (∈, µ, and σ) and the frequencies to be simulated. However, the accuracy of the method is further restricted by how we choose to drive the simulation. The choice of a source is thus integral to accurate simulation in the FDTD method.

Sources can be either internal or external to the simulation. Internal sources are those that are driven at one or more grid cells within the simulation space. An example of an internal source might be a hard antenna, whose parameters we wish to investigate in a particular medium. External sources are those which are assumed to be outside of the simulation space. For example, we may wish to investigate the scattering pattern of an object when excited by a plane wave, but the source of that plane wave may be far from the scattering object. The problem, of course, is that in a numerical simulation, all sources must be applied to real grid cells, which means they are by definition “internal.” However, as we will see beginning in Section 7.2, methods have been derived with great success to make sources appear external.