In this paper, a method of designing a SIW (Substrate Integrated Waveguide) bandpass filter with high selectivity is proposed. Four resonant cavities of the proposed filter are arranged in straight line. The microstrip gradient line is directly fed into the cavities. Two U-shaped slots are etched on the top face of each cavity which will result in the resonant modes reduced and the high modes of SIW cavity pushed far away from the dominant resonant mode. Thus the filter will have both the features of compact size and wide stopband. The center frequency of the filter is designed at 5.2 GHz. The measured results are highly matched with the simulated ones.
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