Dual band bandpass filters with full band reflectionless response and enhanced upper stopband suppression

This paper presents a novel dual-band bandpass filter (BPF) with two-port all-frequency reflectionless characteristics and enhanced upper-band suppression. The design integrates a square ring resonator (SRR) and advanced coupled lines (CLs), incorporating complementary absorptive branches to achieve broadband reflectionless performance. To mitigate upper-band harmonics, radial-stub-loaded resonators (RSLSs) with high-impedance transmission lines (HITLs) are introduced, extending suppression up to 10f₀ without significantly increasing the circuit footprint. A comprehensive theoretical analysis, including parametric studies and transmission zero (TZ) derivation, is conducted to optimize performance. Two prototypes are designed, fabricated, and tested: Prototype I achieves dual-band operation with all-frequency absorption, while Prototype II further enhances harmonic suppression. Experimental results closely align with simulations, with Prototype I exhibiting 3-dB fractional bandwidths (FBWs) of 19.5% and 9.7% at 1.38 GHz and 2.63 GHz, respectively, with insertion losses (ILs) of 0.95 dB and 1.47 dB. Prototype II achieves superior harmonic suppression, with attenuation levels of 22.8 dB over 2.9-20 GHz and 28.5 dB within 3.0-12 GHz. These designs offer high selectivity, inter-band isolation exceeding 45 dB, and full-spectrum absorption, providing an efficient solution for modern RF and wireless communication systems.