FDTD analysis of periodically nonuniform coupled microstrip lines

Periodic wave guiding structures have been widely used in microwave millimeter-wave circuits due to their slow-wave and frequency-selective characteristics. In a periodic structure, the dispersion relation is an important topic in designing the circuits. The FDTD method is used to analyze the propagation characteristics of periodically nonuniform coupled microstrip lines (PNCML). It is found that the propagation constant of the odd-mode is increased more significantly than that of the even-mode, when the dimensions of the stub are increased. The intersection of the dispersive even- and odd-mode propagation constants shows that the structure is useful in the design of high-performance directional couplers and bandpass filters.