Asymmetric Coupled-Resonator Bandpass Filter for Ultra-Wide Stopband

The research presents a novel combination of an asymmetric coupling with a stepped impedance resonator (ACSIR) to realize high performance, especially an ultra-wide stopband rejection. A basic second-order bandpass filter was developed and subsequently extended to a fourth-order configuration to improve key performance metrics, including wide stopband characteristics and compact size, without compromising insertion loss. The use of asymmetric coupling enables optimization of the filter response while preserving a simple configuration. Furthermore, the resonance conditions associated with the asymmetric coupling were examined through mathematical analysis. The proposed ACSIR filter achieved an enhanced fractional bandwidth (FBW) of 15.4%, a low insertion loss of 0.64 dB, and wide stopband rejection extending up to 7.25 f₀ tailored for WLAN applications operating at 2.45 GHz.