Design of Tri-Band Bandpass Filter Using Modified X-Shaped Structure for IoT-Based Wireless Applications

This letter introduces a straightforward method for designing a tri-band bandpass filter using a loaded symmetrical dual-step impedance resonator (D-SIR) structure, augmented by multiple open and short circuit stubs. The resulting filter operates at 2.2, 7.9, and 14.1 GHz, showcasing high-band selectivity. Experimental measurements confirm six transmission zeros and six transmission poles across the passbands, yielding fractional bandwidth (FBW) of 42.6%, 63.8%, and 27.9%. Particularly noteworthy is the broader upper stopband rejection, enhancing the filter’s overall performance. The design achieves impressive performance metrics, with minimal insertion loss (IL) of 0.71, 0.87, and 0.76 dB, accompanied by substantial return loss (RL) of 23, 18, and 13.8 dB across the three passbands. Furthermore, the fabricated design have a compact size of ( $0.20\times 0.15$ ) $\lambda _{g}$ on a Rogers Duroid RT/5880 substrate. Remarkable agreement between measured and simulated results underscores its viability for Internet of Things-based communication systems.