In order to effectively improve filter selectivity and out-of-band rejection level, a multi-cavity two-mode dual-passband filter operating in X-band is proposed. By designing a suitable circuit topology, the bandpasss of the filter are formed using TE201 mode in the substrate integrated waveguide (SIW) cavity and the TE101 mode in the half mode substrate integrated waveguide (HMSIW) cavity. In addition, incorporating a T-slot structure in the dual-mode SIW cavity can add additional transmission zeros (TZs) and improve the filter selectivity while achieving miniaturization. The center frequencies of the two passbands are 8.67 GHz and 11.52 GHz, respectively. The inter-band isolation is better than 65 dB with three transmission zeros and maximum insertion loss of 0.48 dB and 0.31 dB, respectively. The proposed filter has a compact structure, low insertion loss, high-frequency selectivity, and the measured results agree with the simulated ones.
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