A Parallel-Coupled Microstrip Bandpass Filter with Hook Feed-Line for Wide Harmonics Rejection

In this decade, compact bandpass filters are essential components in microwave systems for passing desired signals and suppressing unwanted signals. Microstrip bandpass filter is rapidly going up in the design and fabrication in RF front-ends of wireless communication systems. The microstrip structure is well-known because of its light weight and simple design method. As knowing, the bandpass filter characteristics need strict requirements for bandpass filters including low insertion loss, high selectivity and wide upper stopband [1-2]. The configurations of the microstrip resonators such as the stepped impedance resonator (SIR) might be the most popular candidate for the filter design for reduced size and the controllable second harmonic [3]. Parallel-coupled line resonator is a basic structure for designing to be a bandpass filter, but it generates harmonic signals. A parallel-coupled line bandpass filter with over coupled end stages has been proposed to compensate the difference in phase velocities by extending the electrical length of the odd mode for 1 and 2 harmonic rejection [4]. Besides that, the stacked SIRs in parallel coupled-line have been proposed for suppressing undesired bands [5]. In order to enhance the circuit performance, a bandpass filter must also have wide stopband response. Many useful methods have shown promising results dealing with the harmonic problems. The electromagnetic-bandgap (EBG) based filter has shown over 25 dB rejection at the second harmonic [6]. The asymmetrical structure can be used to suppress the harmonic responses [7].