A study on an improved absorbing boundary condition for a multimode waveguide analysis using the FDTD method

When the FDTD method is applied to the analysis of a multimode waveguide, a problem is the treatment of the absorbing boundary corresponding to the matching at the termination. In the present study, the electromagnetic field in the cross section of the waveguide one cell inward from the termination is expressed as a linear combination of the eigenmodes of the waveguide. For the propagating mode, Mur's absorbing boundary condition is applied by means of the modal phase velocity, and the electromagnetic field at the termination is derived for each nonpropagating mode analytically by means of the modal attenuation constant. Finally, a linear combination is created at the termination to make a simple absorbing boundary. As a specific application, this procedure is applied to a waveguide with an H-plane discontinuity for which the modal transmitted power is calculated, demonstrating the effectiveness of the procedure. Since the present technique is based oil waveguide eigenmode expansion in the frequency domain, it can be used only for single-frequency analysis. (C) 2004 Wiley Periodicals, Inc.