Design of a wideband bandpass filter with mixed coupling and source-load coupling

A short stub multi-mode resonator (SSMMR) with mixed electromagnetic coupling, which primarily comprises five impedance sections, is proposed in this paper. Two symmetric stub-loaded stepped-impedance resonators (SLSIRs) are interconnected by an additional impedance line, with two short-circuited stubs. One stub acts as an impedance suppressor to control harmonic resonances, while the other establishes magnetic coupling in the hybrid design. Notably, the stub-loaded stepped-impedance sections are symmetrical, with each SLSIR consisting of four impedance segments. In the SLSIR structure, the two smaller impedance sections generate electric coupling through parallel-coupled lines, and the short-circuited stub, serving as a common grounding via, produces magnetic coupling. By leveraging mixed electromagnetic coupling, source-load coupling, and harmonic effects, the proposed design creates five transmission zeros outside the passband, significantly improving selectivity and out-of-band rejection. Experimental verification demonstrates a fourth-order wideband BPF with a fractional bandwidth of 41.7% and 20dB suppression up to 2.7<i>f</i>₀. The measured results show excellent agreement with simulated responses.