Many researchers have studied the numerical dispersion error inherent in the FDTD method, including Taflove [2], Choi [158], and Kim and Hoefer [159]. In 1993 Ray [160] presented a dispersion analysis for a non-orthogonal FDTD mesh, while in 1994 Monk and Süli [51,52] presented a convergence analysis of Yee's algorithm for non-uniform rectangular grids. Shlager et al. [161] have presented a comparison of the dispersion errors of several orthogonal-grid FDTD methods. Cangellaris and Wright [46] analyzed staircasing errors at PEC surfaces for the regular FDTD method. Recently, Shlager [162] presented a new criterion for insuring that staircasing errors are small when modeling horn-like and wedge-shaped objects.