A Miniaturized Dual-Band Bandpass Filter Using Embedded Resonators

A compact microstrip dual-band bandpass filter (BPF) using the embedded open-loop ring resonator is proposed. The filter can achieve two passbands without using the external dual-band impedance transformer in the input/output feeding line. Furthermore, by using the developed design method, the ratio of the first passband to the second passband can be controlled. A compact microstrip dual-band BPF operated at 2.45/5.7 GHz is demonstrated. The designed filter has low insertion loss, high rejection between the two passbands, and a very small size of 11.5 × 4.65 mm² (0.094λ₀ × 0.039 λ₀), λ₀ being the free-space wavelength of the center operating frequency of the 2.45 GHz. The good agreement between simulation and measurement is obtained.