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Two bandpass filters based on hybrid spoof surface plasmon polariton (SSPP) and substrate integrated waveguide (SIW) are presented for X-band applications, herein. The dispersion and transmission characteristics of the proposed hybrid SSPP-SIW structures are analysed, and the influence on the variation of their structural parameters is investigated. The proposed dumbbell-shaped SSPP shows more slow-wave effects than the rectangle-shaped SSPP for the same height of grooves, and hence could be a good choice in the design of compact, low loss and highly integrated microwave and terahertz devices. The lower and upper cutoff frequencies of hybrid SSPP-SIW bandpass filters can be adjusted independently by varying the structural parameters of SIW and SSPP units, respectively. The proposed two filters exhibit excellent passband (8-13.15 GHz for filter I and 8-13.5 GHz for filter II) and wide upper-band rejection (>42.5 dB from 14.1 to 20 GHz for filter I and >41 dB from 14.2 to 19.5 GHz for filter II) simultaneously. Their size is very compact, amounts to only about 1λ g � 0.7λ g , where λ g is the guided wavelength in the SIW at the centre frequency. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

This paper presents a novel ultrawideband (UWB) bandpass filter using stepped impedance stub loaded microstrip resonator (SISLMR). The proposed resonator is so formed to allow its four resonant frequencies in the UWB passband, which extends from 3.1 GHz to 10.6 GHz. Moreover, two spurline sections are employed to create a sharp notched-band filter for suppressing the signals of 5 GHz WLAN devices. Experimental results of the fabricated filters are in good agreement with the HFSS simulations and validate the design.

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