Termination conditions for the application of FD-TD techniques in close homogeneous waveguide and planar structures

The successful application of FD-TD methods to homogeneous waveguide and millimetric planar transmission lines presupposes a satisfactory answer to the problem of infinite line termination (Absorbing Boundary Condition ABC) in presence of high frequency dispersion. In the present contribution, a rigorous termination condition specific to homogeneous waveguide is analyzed, that is based on the convolution proprieties of the modal characteristic impedance of the 'accessible modes'. We evaluate, also, the efficiency of some commonly employed ABC's and to present a technique that reduces reflection error to less than 1\% for microstrip and coplanar line circuits up to and beyond 50 GHz, that is lower than the discretization and round-off errors introduced by the very FD-TD technique.