Design of the band-pass filter with wide stopband performance using λ/4 forked SIRs

In this paper, a band-pass filter (BPF) using forked stepped-impedance resonators (SIRs) was proposed with having a very wide stopband performance. The circuit composed of two forked SIRs connected with a narrow microstrip with the latter´s middle point shorting to the ground through a via hole. The FSIRs filter operated under a quarter-wavelength resonance condition. The filter was built for WLAN IEEE 802.11b/g (2.4/2.45 GHz) applications. A long narrow coupled microstrip which increasing the stopband range was used to couple I/O signal to/from the resonators. The upper stopband, its frequency range was measured from 2.76 to 23.08 GHz with the criterion of the signal rejection levels greater than 20 dB. The stopband´s upper-end is over 9.5 f₀ with the central frequency of 2.44 GHz. Experiment was conducted and a good agreement was observed between the measurement and simulation.