Implementation of transparent sources in FDTD simulations

Sources can be embedded in a finite-difference time-domain (FDTD) grid in any one of several ways. Depending on the particular implementation, the embedded source corresponds physically to a hard field source (applied field), a transparent current source (impressed current), a finite-impedance voltage source, or some other physical excitation. While the implementation of any of these sources is a straightforward procedure in FDTD simulations, ensuring an accurate correspondence between the physical source and its numerical implementation is challenging. In this work, we describe the implementation of a new field source, referred to as a transparent field source, that couples the same fields into the FDTD grid as a hard field source. Unlike the hard field source, however, the transparent source does not scatter energy, i.e., the usual FDTD update equation applies to the source node. The implementation is described both in terms of a single node and in terms of an array of nodes. The latter is discussed in the context of parallel-plate waveguide excitation.