Mode Characterization in Electrically Large Waveguides using Compact FDTD Algorithm

The compact 2-D FDTD algorithm is used to study the mode characteristics in electrically large waveguides. The dominant modes in these waveguides have a longitudinal phase constant beta_z that is very close to the unbounded wavenumber beta₀. The global phase error caused by the deviation of the numerically computed transverse wavenumber from its analytical value is studied. It is shown that this error increases sharply for the dominant modes if the time step is taken according to the maximum value dictated by the stability criterion. It is found that in order to reduce this error, the time step should be reduced by a factor that depends on the ratio beta_z I beta₀. Numerical examples of electrically large waveguides are given to support our claims.