Efficient FDTD calculation of multi-port S parameters for microstrip and stripline circuits

The finite difference time domain (FDTD) method has been used for the calculation of scattering (S) parameters for a number of years. A difficult problem is separating the interaction between the source excitation and the time domain reflections. In a previous paper Luebbers and Langdon (see IEEE Transactions on Antennas and Propagation, vol.44, no.7, p.1535-9, 1996) presented the idea of a voltage source with an internal source resistance as a means for exciting a microstrip patch antenna. The resistive source feed also allows one to position the feed very close to the structure, thereby reducing the number of cells and computer memory needed. In addition the number of time steps needed can be drastically reduced, especially for a resonant structure. We extend the resistive source approach to the calculation of multiport S parameters. Each FDTD calculation determines one column of the matrix. For example, if port 1 is excited then that calculation provides S/sub 11/, S/sub 21/, "...", S/sub N1/.