The FDTD method has been used in conjunction with other techniques. In 1982, Taflove and Umashankar [285] used a hybrid FDTD-Method of Moment approach to investigate coupling problems. More recently, other hybrid FDTD techniques were introduced. In 1993 Aoyagi et al. [301] used the Yee algorithm in conjunction with the scalar wave equation to reduce the computations needed to model a Vivaldi antenna, while Cangellaris et al. [298] used a hybrid spectral-FDTD method to analyze propagation in anisotropic, inhomogeneous periodic structures. Lee and Chia [302] introduced a hybrid ray-FDTD method and used it to investigate scattering from a cavity with a complex termination. In 1994, Mrozowski [303] introduced a hybrid FDTD-PEE (partial eigenfunction expansion) method to speed up the FDTD method when solving shielded structure problems.