In addition to the previously mentioned applications, the FDTD method has also been applied to problems involving periodic structures, scattering from random surfaces, and to non-destructive evaluation problems.
In 1991, Chan et al. [290] was the first to use the FDTD method in conjunction with periodic boundary conditions to investigate scattering from 2-D randomly rough surfaces. Later, Fung et al. [291] used the FDTD method to model the scattering from 3-D randomly rough surfaces. Navarro et al. [292] used the FDTD method in combination with the Floquet theorem to model scattering from a 2-D metallic strip grating. At the same time, Tsay and Pozar [293] investigated a similar 2-D metallic strip grating problem using a periodic boundary condition. Both these studies presented results only for the normal incidence case. Extensions and improvements, including results for the oblique incidence periodic case were presented shortly after by Prescott and Shuley [294] and by Veysoglu et al. [295]. Ren et al. [198] presented a 2-D FDTD analysis for phased-array antennas, incorporating the Floquet boundary conditions. Harms et al. [296] soon extended the FDTD analysis to full 3-D periodic structures. Recently, Celuch-Marcysiak and Gwarek [297] have investigated waveguide structures which are periodic in the direction of propagation. Also, Cangellaris et al. [298] used a hybrid spectral-FDTD method to investigate the propagation of waves in a guided wave periodic structure.
Other applications of the FDTD method include non-destructive evaluation. Moghaddam et al. [156,157] applied the FDTD method to model a subsurface radar. Their results were restricted to excitation by canonical point or line sources. Maeshima et al. [203] used the FDTD method to model a 2-D subsurface radar, using a more complex cavity backed antenna. He et al. [299] have also analyzed 2-D scattering from the air-earth interface using the FDTD method. Recently, Bourgeois and Smith [300] have applied the 3-D FDTD method to a subsurface radar for the detection of buried pipes, in which they incorporate both a transmitting and receiving bow-tie antenna, fed by parallel-wire transmission lines. In addition, they also model the frequency dispersive characteristics of the earth.