Compact bandpass filter with improved upper stopband

A compact bandpass filter (BPF) with a wide upper stopband using a short-circuited microstrip coupled-line hairpin resonator is presented. The equivalent transmission-line circuit model is developed to analyse the performances of the proposed filter. The feeding lines are capacitively coupled to the resonator at the point which divides the fingers of the hairpin into two segments at a ratio of 1:2. As a result, the first harmonic appears at the frequency which is five times the fundamental frequency. In the final design with a grounding via hole, three transmission zeros are generated to improve the skirt effect. Therefore, a compact BPF with a wide upper stopband could be achieved. Finally, the proposed filter is fabricated to verify the design principles, and the measured results agree well with the predicted ones.