In this paper, applications of the discrete Green's function (DGF) in the three-dimensional (3-D) finite-difference time-domain (FDTD) method are presented. The FDTD method on disjoint domains was developed employing DGF to couple the subdomains as well as to compute the electromagnetic field outside these subdomains. Hence, source and scatterer are simulated in separate subdomains and updating of vacuum cells, being of little interest from a user point of view, can be avoided. In the developed method, the field radiated by a single subdomain is computed as a convolution of DGF with equivalent current sources measured over two displaced Huygens surfaces. Therefore, the computed electromagnetic field is compatible with the FDTD grid and can be applied as an incident wave in a coupled total-field/scattered-field (TFSF) subdomain. In the developed method, the DGF waveforms are truncated using the Hann's window and windowing parameters assuring accuracy of computations are pointed out. The error of the field computations varies between -90 dB and -40 dB depending on the DGF length and excitation waveform. However, if the DGF length is equal to the number of iterations in a simulation, the presented DGF applications return the same results as the direct FDTD method.
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