TEM mode excitation and detection in FDTD analysis

A large class of electromagnetic problems, suitable for numerical analysis, is the class consisting of N-port networks where each port has a defined terminal and source plane on some transmission line or waveguide. A typical method for exciting the electromagnetic field at the source plane is to place a soft or hard source in the vicinity of the conductors that form the waveguide. These sources are designed and placed to excite the dominant mode. For open structures like microstrips, the line sources also excite surface and radiation waves. We show, in this paper, that impressed electric and magnetic surface currents on the source plane can be designed to couple exclusively to the mode of interest.