Compact dual-band bandpass filter with ultra-wide upper-stopband

Proposed is a compact dual-band bandpass filter (BPF) using stepped impedance resonators (SIRs). To improve the upper-stopband of the proposed filter, tri-section folded SIRs are used. By adjusting the impedance ratio and electronical length ratio of the tri-section SIR, the out-of-band of the filter can be improved. A 2.09/2.62 GHz dual-band experimental BPF has been designed and fabricated to demonstrate the proposed method. The measured results are in good agreement with the electromagnetic simulation. The spurious frequencies of the upper-stopband are suppressed below 20 dB from 2.79 to 22.13 GHz. The circuit only occupies 0.29 λg × 0.13 λg.