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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.

Regarding the capacitive nature of input/output impedances of millimeterwave amplifiers, it is necessary to utilize wideband inductive elements for the design of input/output impedance matching circuits. In this paper, the design and optimization of an ultra-wideband transmission line inductor with an inductance of less than 100 pH are introduced for using at frequencies above 50 GHz. The reason of using this inductor is its superior performance compared to that of a spiral inductor and single-coil inductor. The inductor consists of three rectangular microstrip line segments with nonequal and variable characteristic impedances. The design is done by calculating the impedance matrix using planar circuit analysis (PCA) based on a planar waveguide model, segmentation/desegmentation methods, and an intelligent algorithm in MATLAB. With this design method, the effect of discontinuities, fringing fields at the edges of the microstrip, and the conductor as well as dielectric losses can be considered while minimizing the dispersion effect. In this study, to reduce the calculation time of the impedance matrix, the doubly infinite series are reduced to a singly infinite series by summing the inner sum for a rectangular segment whose one side is shorted and three other sides are open.

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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.