Miniaturized Bandpass Filter Using the Metamaterial Half-Mode Technique for Substrate Integrated Waveguide

In this paper, a half-mode evanescent substrate integrated waveguide (SIW) bandpass filter is proposed. The proposed filter operates below the cutoff frequency of the conventional SIW by adopting the metamaterial technique. The operating frequency of the full-mode substrate integrated waveguide (FM-SIW) is 2.2 GHz, while the cutoff frequency of the conventional SIW structure is 5.2 GHz. Fractal slots are cut from the broad wall of the SIW. First-, second-, and third-order bandpass filters are designed, simulated, fabricated, and experimentally validated using a vector network analyzer. The metamaterial technique achieves a miniaturization of about 60% relative to the conventional SIW structure, and the responses are compared with those of its FM-SIW counterpart, with no obvious difference observed. The half-mode SIW (HM-SIW) technique is then used to achieve further miniaturization of about 50% relative to the original FM-SIW structure. The measured one-stage filter covers the 2.4 GHz ISM band with an insertion loss of about 1.2 dB and a return loss of about 40 dB, and cascading additional unit cells is shown to sharpen the filter's selectivity while preserving close agreement between simulation and measurement. Finally, the simulation and measurement results agree well with other work in the literature.