Boundary conditions on a planar R tau grid for the absorption and excitation of plane waves and eigenmodes in a rectangular waveguide

A technique is proposed for imposing a boundary condition that provides the excitation and absorption of plane waves and eigenmodes in a rectangular w aveguide. The waveguide is filled with an inhomogeneous dielectric whose pe rmittivity may be greater or less than the permittivity of vacuum. This bou ndary condition is suitable for application in the R tau -grid method used for analyzing the electrodynamic properties of two-dimensional (M) structur es in the time domain.