A new design methodology is presented for multi‐band evanescent‐mode band‐pass filters (BPFs) realisation by integrating substrate‐integrated waveguide structure with multiple short‐circuited irises. The electro magnetic (EM)‐behaviour of the distributed‐element structure is modelled with the lumped‐element equivalent circuit in a wide frequency range. Arbitrary transmission poles (TPs) and transmission zeros (TZs) corresponding to the different irises are characterised. Moreover, a rigorous prescribed filtering function is developed to design dual‐band evanescent‐mode BPFs in terms of independently tunable TPs and TZs. The proposed structure is reported to not only demonstrate miniaturised high quality‐factor dual‐band BPFs but also obviate the design challenges of multi‐band BPFs below the cut‐off frequency. Furthermore, two triple‐ and quadruple‐band evanescent‐mode BPFs are experimentally examined for the proposed concept verification.

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A single-layer substrate integrated waveguide (SIW) is developed to design a dual-band band-pass filter (BPF) operating below the cutoff frequency of the SIW, known as evanescent-mode excitation. Gap-coupled excitation is used to demonstrate the multiple transmission poles (TPs) and transmission zeros (TZs) below the cutoff frequency of the SIW. The structure is reported to realize two independent evanescent-mode poles on a single-layer SIW which reduces the size and complexity of the structure compared to those of the recent multi-layer evanescent-mode structures. Lumped-element equivalent circuit is employed to describe the EM behavior of the structure for TZs and TPs realization. A compact single-layer dual-band SIW filter is fabricated based on the proposed structure. A good agreement is reported between the measured and simulated performances.

In this paper, an optimization-driven methodology is proposed for the design of substrate integrated waveguide (SIW) bandpass filters (BPFs) with predefined passbands. The width between the metallic walls of via-holes is governed by a truncated Fourier series to achieve the desired filtering performance. The theory of rectangular waveguide is used to establish the optimization framework and obtain the series coefficients under predefined physical constraints. Two types of end-terminations are studied; specifically, with and without SIW-to-microstrip transitions. To validate the proposed methodology, two Ku-band BPF prototypes with 2.5% and 5.8% 15-dB fractional bandwidth (FBW) are designed, simulated, and measured. Furthermore, the half-mode SIW (HMSIW) concept is incorporated in one prototype to facilitate a miniaturized physical structure. Simulations and measurements are in close proximity with passband matching and transmission losses better than-15 dB and-2.5 dB, respectively. The proposed methodology allows for designing BPFs with predefined wideband or narrowband FBW by modifying the underlying physical constraints and optimization parameters. The resulting filters are planar, compact, and have wide stopband rejection. In addition, a derivation for the characteristic impedance of the SIW line is provided, which can be used to find the optimum SIW-to-microstrip transition without performing a parametric study. INDEX TERMS Bandpass filter (BPF), half-mode substrate integrated waveguide (HMSIW), SIW-tomicrostrip transition, narrowband, rectangular waveguide, substrate integrated waveguide (SIW), wideband.

A new class of back-to-back integrated aperture-and gap-coupled discontinuities is proposed for substrate-integrated waveguide bandpass filter design. The developed structure is shown to take advantage of both discontinuities in the design of cavity and/or planar resonators with an optimum performance including higher quality factor accompanied by transmission zero realisation, wider upper stop-band with second harmonic suppression, and a considerable size reduction. The measured unloaded quality factor has been increased by a ratio of 60% in comparison to the conventional gap-coupled structures.