I. Introduction
The microstrip bandpass filter is one of most important components in microwave systems. Resonators are key building elements in filter design. Various kinds of resonators have been proposed in the past, e.g., hairpin [1], [2] and open-loop resonators [3]–[7]. Without special measures, most of the resonators exhibit harmonic response, which degrades the system performance. To suppress the harmonics of these open-ended resonators, various methods have been proposed in the past [8]–[15]. A bandstop filter with harmonic suppression is achieved by cascading open loops with a combination of open stubs and spurlines [8]. Modification and perturbation to the width of the coupled-line open-ended resonators are also utilized to suppress the spurious response such as the corrugated structures [9], wiggly lines [10], fractal-shaped lines [11], and periodic grooved lines [12]. All of these structures [8]–[12] suppress the second harmonic through the creation of Bragg reflections. Adjustment of the odd/even-mode phase velocities is also proposed to suppress the second harmonic [13] in which grooved substrate along the outer edge of the coupled line is cut to make the even/odd-mode phase velocities equal. Stepped-impedance resonators are also used to obtain wide stopband [14], [15]. Several stepped-impedance resonators, which have the same fundamental resonance frequency, but different spurious resonances, are combined together to attain multiple spurious suppression. While most research in harmonic suppression of bandpass filters has been focused on open-ended resonators, little study has mentioned the combination of open- and short-ended resonators together. Furthermore, the role of selecting the coupling region between resonators for spurious harmonics suppression has not been fully investigated.