This letter presents a novel tri-band filter design using an ultraminiaturized substrate-integrated waveguide (SIW) cavity operated at evanescent mode. Two miniaturized SIW cavities in a face-to-face form are vertically stacked to build the proposed tri-band filter. The top and bottom metal layers form the cavity's broadside walls, and the middle layer possesses three identical patches which are formed by evenly trisecting a circular patch. These patches are shorted to the cavity's bottom wall through one, two, or four via-holes. The patches together with the host cavity exhibit three frequency-distinct resonances that create the filter's triple passbands. The resonance frequencies are independently controlled, and six passband-edge transmission zeros are created for good selectivity. The filter size is only 0.22 λd x 0.22 λd, and the measured in-band minimum insertion losses are 2.31, 2.25, and 3.01 dB for the first, second, and third bands, respectively. It is believed that the proposed tri-band filter has obtained the best circuit area efficiency in the SIW tri-band filter category.

The contribution of this letter is to propose an evanescent-mode ultraminiaturized substrate integrated waveguide (SIW) cavity resonator and its application in bandpass filter (BPF) design. The proposed compact SIW cavity is composed of two dielectric substrates with three metal layers. The top and bottom metal layers form the cavity's broadside walls, and the middle one is a circular patch that shorts to the bottom wall by a ring fence of vias (or a centered single via). This circular patch together with the ring-fence vias (or single via) provides cavity resonator a large loading capacitance to allow a dramatic frequency downshift. Two experimental four-pole BPFs are built to validate the circuit design. The achieved frequency downshift of the proposed SIW cavity, compared with that of its conventional SIW cavity counterpart, is 72.5% (82.3%) for the via-ring fence (single via) case. Not only is the associated area miniaturization factor (MF) comparable with the highest record in the literature, but also the BPFs implemented here are structurally simpler and exhibit a much wider relative upper stopband bandwidth than that of the SIW BPF with a record of high MF.

The contribution of this study is to develop a miniaturised substrate integrated waveguide (SIW) cavity resonator with two evanescent‐mode resonances. The circuit area of the developed dual‐frequency SIW cavity is only 9.2% that of its conventional SIW cavity counterpart. A dual‐band filter is implemented from using two miniaturised SIW cavity resonators. The two‐passband frequencies can be independently controlled. Two transmission zeros are created by the two sides of each passband for enhancing the signal selectivity. A diplexer is also designed from using the proposed SIW cavities. A dual‐band filter and a diplexer are built for experimental verification. Agreements between measured and simulated data are observed. To the best of the authors’ knowledge, the designed dual‐band filter and diplexer have achieved the best circuit area efficiency in the categories of dual‐band SIW cavity filters and diplexers.

The contribution of this work is to propose a half-mode substrate integrated coaxial resonator (HMSICR) and its application in bandpass filter (BPF) design. The proposed HMSICR is formed by evenly bisecting a square substrate integrated coaxial resonator (SICR), which is a cavity composed of two dielectric substrates and three metal layers. The SICR’s sidewalls are mimicked by periodically spaced thru-via arrays, and a circular patch is embedded in the middle metal layer of the SICR with the patch shorted to the cavity’s bottom wall by a circular array of blind vias. This HMSICR can drastically lower the cavity’s resonance frequency. The achieved frequency reduction rate of the proposed HMSICR, as compared with that of its conventional substrate integrated waveguide (SIW) cavity counterpart, reaches 70%. A sample four-HMSICR BPF is built for the circuit verification measurement. To further reduce the sample filter’s area, the composing HMSICRs are vertically stacked in a back-to-back configuration. We believe that its obtained size-reduction rate reaches the highest record.

A dual‐mode balanced bandpass filter (BPF) design with an excellent common‐mode (CM) rejection performance is presented. The balanced filter is composed of coupled substrate‐integrated waveguide (SIW) cavities and a hybrid microstrip/slot feed. The cavities are of a square shape to produce two degenerated resonance modes , TE ₁₀₂ and TE ₂₀₁ , for dual‐mode operation. The hybrid microstrip/slot feed is developed to suppress the unwanted CM signals in both the in‐band and out‐band regions. The measured CM signal rejection levels are over 50 dB. A experiment was conducted to verify this novel balanced BPF design.

The contribution of this work is to propose a compact balanced bandpass filter design using the miniaturised full/half‐mode substrate integrated waveguide cavities (SIWCs) together with the microstrip coupling slot feed. The circuit area of the proposed full‐mode SIWC is only 4.64% of its conventional SIWC counterpart, and the half‐mode SIWC further reduces the circuit area by half. Besides the circuit compactness, the non‐coupling of the microstrip and the slot in the common‐mode (CM) operation provides an excellent CM signal rejection. Prototypes of two‐pole full/half‐mode balanced SIWC BPFs are fabricated and measured to validate the circuit design. Agreements between measured and simulated data are observed. The CM signal rejection levels are larger than 45 dB over a wide‐frequency range with the passband's differential‐mode insertion losses around 1.6 and 0.8 dB for the full‐ and half‐mode designs, respectively.