Self-Packaged SISL Based Band Pass Filter Design with Wide Stopband Suppression for 5G Applications

This paper presents a compact bandpass filter (BPF) based on substrate-integrated suspended line (SISL) technology for 5G applications. The filter employs stepped-impedance resonators (SIRs) and edge-coupled open stubs to achieve a narrow passband from 4.04 GHz to 4.27 GHz, centered at 4.15 GHz, with a fractional bandwidth (FBW) of 5.54%. Enhanced out-of-band rejection is obtained through symmetrically placed open stubs near the input/output ports, extending the rejection band beyond 12 GHz with attenuation levels below −30 dB. The fabricated filter exhibited a low insertion loss (IL) of less than 0.35 dB and a return loss (RL) greater than 18 dB in the passband. The lumped-element equivalent circuit, even – odd mode analysis, and parametric studies validated the design, while measured results showed good agreement with simulations. The SISL platform offers self-packaging, low loss, and integration advantages, making the filter well suited for RF/microwave systems.