Compact Dual-Band Bandpass Filter Using U-Shaped Stepped-Impedance Resonators with Parallel Coupled Structures

This paper proposes a dual-band bandpass filter using stepped-impedance resonators (SIRs) with parallel coupled structures.The proposed filter adopts U-shaped SIRs with parallel coupled lines (PCLs) that have interdigital and comb-line shorted ends.The central PCLs build an upper passband and a transmission zero, and the two U-shaped SIRs build a lower passband.Four resonators and coupling structures are theoretically analyzed to derive its scattering parameters.A novel dual-band bandpass filter is designed and fabricated using the induced scattering characteristics.The measured results show that the fabricated dual-band bandpass filter has an insertion loss of less than 1.02 dB in the lower band of 2.45 GHz and of 3.01 dB in the upper band of 3.42 GHz, and a band-to-band isolation of more than 40 dB, from 3.14 to 3.2 GHz.