Guided-wave Characteristics of Periodically Nonuniform Coupled Microstrip Lines for Harmonic Suppression in Filter Design

1. Introduction The parallel-coupled microstrip line was extensively characterized in the past and has been gaining a wide application in the bandpass filter design since 1958 [1]. However, one common and critical problem of this type filter design is to seriously suffer from spurious passbands at the harmonics of the operating frequency. So far, great effort has been made to suppression of these harmonic passbands [2]-[6]. In this way, the two effective methods have been successfully explored, i.e., equalization of modal phase velocities and differentiation of traveling route lengths for coupled even-and odd-mode. In [2], an overcoupled resonator was constituted to extend the odd-mode phase length, thus compensating the phase velocity difference between two modes. Recently, the strip-width modulation technique is developed to make up the " wiggle-line " filter with good harmonic suppression [3] over a wide frequency range. In parallel, the corrugated coupled microstrip line [4] is presented to extend the actual traveling path for the odd-mode such that the 1 st-harmonic be suppressed by canceling the different velocity in the propagation behavior of two modes. Moreover, the square grooves [5] are periodically etched on the parallel-coupled lines and the coupled meander lines are constituted [6] to equalize the phase velocities of the odd-and even-mode field distributions for the same purpose. Based on the above discussion, extensive investigation on the dispersion characteristics of these periodically nonuniform coupled microstrip lines (PNCML) become the most important issue in allowing one not only understanding the operating principle of these filters and also optimizing their filtering behaviors via simple and efficient synthesis procedure [1]. Consequently, guided-wave characteristics of the even and odd dominant modes in the PNCWL need to be promptly studied. In [7], a spectral-domain approach is utilized to analyze the effective dielectric constants of the even-and odd-mode, i.e. β e and β o , in the periodically nonuniform coupled microstrip lines (PNCML). But, it does not take into account the frequency dispersion and also costs intensive numerical expenses, so that it may not be suitable for CAD-based design of microwave circuits. Very recently, FDTD algorithm is executed to analyze the propagation characteristics of PNCML [8]. Unfortunately, only the effective dielectric constants [7, 8] are calculated and no reported work to date has been carried out to derive characteristic impedances of these two modes regardless of any shaped PNCML. In this work, our developed hybrid method of moments and short-open calibration …