This article proposes a new class of box-like substrate integrated waveguide (SIW) filters with wide stopband. First, a two-pole doublet bandpass filter with one finite-transmission zero (FTZ) is proposed using a new planar dual-mode rectangular SIW cavity, which is realized by ₂₀₁ and TE₁₀₃ modes. TE₁₀₂, TE₂₀₂, and TE₁₀₄ modes are suppressed intrinsically by arranging feeding lines at the horizontal centerline of the dual-mode SIW cavity. Then, a four-pole box-section filter with one FTZ is proposed using one dual-mode and two single-mode rectangular SIW cavities, and the two single-mode cavities have different width-length ratios to further improve the stopband performance. To further improve the cutoff skirt, a new design method for five-pole quasi-elliptic SIW filter is proposed, which is achieved by the four-pole filter loaded by one single-mode SIW cavity. Some design examples with the center frequency of 10 GHz are presented to verify the performance of the proposed dual-mode SIW filters and the design method.

High-selective bandpass filters (BPFs) based on novel multiple-mode substrate-integrated waveguide (SIW) cavities are proposed in this article. The multiple-mode SIW cavities are realized by a method of folded SIW (FSIW) structure, and its characteristics are analyzed in detail. Dual modes TE $_{102}$ and TE $_{201}$ in the normal SIW as well as modes TE $_{\mathrm{1/2,0,1/2}}$ , TE $_{\mathrm{1/2,0,3/2}}$ , and TE $_{\mathrm{3/2,0,1/2}}$ in the FSIW can be exploited to realize multiple-mode cavities for BPFs’ design because their resonant frequencies can be controlled flexibly. To demonstrate the multiple-mode SIW filters’ design, two single-band triple-mode SIW BPFs with three finite-transmission zeros (FTZs) are proposed first. Then, dual-band and single-band quad-mode SIW BPFs with four FTZs are proposed. FTZs’ locations of the proposed filters can be controlled well, and their controllability is analyzed. Finally, three SIW filters are designed, fabricated, and measured to verify the proposed filters and method. The proposed multiple-mode SIW filters have the advantages of high selectivity, compact size, and flexibly controllable FTZs’ locations and bandwidth (BW).

In this paper, a new polarization insensitive, low profile frequency selective rasorber (FSR) is presented with a switchable transmission band. This FSR consists of a lossy layer, a bandpass layer and a switchable layer where PIN diodes are mounted and parallel fed. The lossy layer is composed of compact fractal elements with lumped resistors, which miniaturizes the unit dimensions. The bandpass layer is composed of a grided square loop and the switchable layer is integrated by coaxial hybrid square loops with embedded bias network, whose configuration reduces the impact of bias lines on high frequency transmission and consequently achieves polarization insensitivity. A prototype was fabricated and measured, which verified that the 3dB transmission window covers 8.9-9.8 GHz, and the -10dB wideband absorption covers 2.3-8.2 GHz. Good agreement is obtained between the numerical simulation and measurement results.

This letter reports a novel realization of frequency-dependent coupling (FDC) structure, which can realize both the positive and negative slopes. The FDC is constructed by two coupled stepped-impedance grounded coplanar waveguides with opened ends. The characteristics of the FDC are studied, which indicates that the proposed FDC has advantages of the flexibly controllable slope, zero of position, and weak/strong strength. For the verification of the characteristics of the proposed FDC, an inline-line fourth-order substrate integrated waveguide bandpass filter (BPF) with one large positive slope and a third-order triplet BPF with one small negative slope for additional finite transmission zero with controllable position were designed, fabricated, and measured. Measured results agree well with simulated ones.

A novel high‐selective triple‐mode substrate integrated waveguide (SIW) bandpass filter using higher‐order resonant modes is proposed in this Letter. The new triple‐mode SIW resonator is realised by the second degenerate dual modes TM ₂₁₀ and one perturbed higher‐order mode TM ₀₂₀ in a circular SIW cavity. The resonance of TM ₀₂₀ mode can be independently moved to that of dual modes TM ₂₁₀ by changing the size of a floating circular patch. The proposed triple‐mode SIW filter can produce three finite‐transmission zeros (FTZs), which can also be controlled well by the angle between two feeding lines. For the demonstration, a triple‐mode SIW filter with the centre frequency of 13.5 GHz was designed, fabricated and measured. The proposed triple‐mode circular SIW filter has the merits of the high‐quality factor, high selectivity and controllable bandwidth as well as FTZs.