A compact highly selective bandpass filter with wide upper-stopband

In this paper, a compact highly selective bandpass filter with wide upper-stopband using short-circuited microstrip coupled-line hairpin resonator is presented. The equivalent transmission-line circuit model is developed to analyze the performance of the proposed filter. There are two transmission poles existing at each resonance of a single short-circuited microstrip coupled-line hairpin unit. The feeding lines are capacitively coupled to the resonator at the location where the finger of the hairpin is divided into two segments at a ratio of 1∶2. As a result, the first harmonic appears at the frequency which is 5 times of the fundamental frequency and three transmission zeros are also generated to improve the skirt effect. Therefore, a compact bandpass filter with a wide upper-stopband is achieved. Finally, the proposed filter is fabricated to verify the design principles, and the measured results agree well with the predicted ones.