High selectivity tri-band microstrip bandpass filter with controllable passband frequencies

Abstract

A new short-ended resonator with open and shorted stubs loaded is proposed. Resonant property and current distribution are analysed. The analysis reveals that the three passband frequencies of the proposed filter can be conveniently controlled. Furthermore, to improve the filter selectivity, the source-load coupling is adopted to create six transmission zeros outside the three passbands. Thus, a tri-band bandpass filter with high selectivity can be achieved. One filter with three passband frequencies at 1.57, 2.4 and 3.5 GHz is designed and fabricated to provide an experimental verification on the design strategies.