Compact Second-Order Bandstop Filter Based on Dual-Mode Complementary Split-Ring Resonator

A compact dual-mode bandstop filter is presented in this letter. From an equivalent circuit viewpoint, a second-order response is obtained by designing a two-pole notch filter with inductive coupling. This equivalent circuit is then implemented by using a dual-mode complementary-split-ring resonator. The proposed bandstop filter prototype has dimensions of 0.1λg× 0.18λg, where λgdefines the guided-wave wavelength. An experimental validation at the center frequency f0of 2 GHz shows that the proposed filter can extend the harmonic-free upper passband up to 4.2f0.