Early techniques used to truncate the FDTD computational domain have included differential-based ABC's, such as those proposed by Merewether [11], Engquist and Madja [12], Lindman [13], and Mur [14]. These early techniques were vastly improved in the mid-1980's by formulations proposed by Higdon [15,16], Liao et al. [17], and Keys [18]. Many other extensions of these differential-based ABC's have since been proposed. The interested reader is referred to the on-line database described in Sec. xv to survey such extensions.