A compact bandpass filter based on concavo-convex cavity resonators is proposed. The reported resonator is developed from a conventional rectangular cavity resonator by introducing a concave surface. Compared to a conventional rectangular cavity, this new resonator has a low resonant frequency, compact size and a wider relative mono-mode bandwidth. Using the substrate integrated waveguide technique to integrate the concavo-convex cavity on a eight-layer low temperature co-fired ceramics substrate, a demonstration filter is designed, fabricated and tested to show the validity of the proposed structure.Introduction: Recently, substrate integrated waveguide (SIW) circuits have attracted much attention owing to their merits of higher quality factor, low cost, high power handling capability, and easy integration [1,2]. However, their physical sizes are not small enough when compared with the microstrip circuitry, especially in filter design, since the conventional SIW rectangular cavity resonators are too large [3].To reduce the size of SIW filters, this Letter proposes a new SIW cavity resonator, shown in Fig. 1. As seen from the Figure, this new resonator has a concave upper surface to compose a concavo-convex cavity structure, so it is named as the concavo-convex cavity resonator. This new resonator exhibits a lower resonant frequency as well as a wider relative mono-mode bandwidth in comparison to a conventional reactangular cavity resonator. Employing this resonator, a four-pole multilayered SIW cross-coupled filter is designed, fabricated and measured as an exemplary case. Simulated and measured results show that this SIW filter occupies smaller area, and has better stopband performance.

Dual‐mode defected ground structure (DGS) is presented to construct differential wideband bandpass filter (WBPF) with very compact size. This proposed DGS is composed of an etched rectangular loop with a central zigzag tapered structure on the bottom surface. Based on this DGS, the filter exhibits adjustable dual‐mode resonant characteristics under differential mode (DM) operation and widebandstop filtering characteristics under common‐mode (CM) operation. To verify the design concept, one filter to cover the UWB lower‐band has been designed, fabricated, and measured. The fabricated differential WBPF demonstrates a DM 3 dB fractional bandwidth of 59% with lower passband insertion loss of 0.52 dB, wider CM suppression better than 10 dB from DC to 11.5 GHz, and very compact size of 0.08 λg2, where λg is the guided wavelength at the center frequency of the DM passband. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:1210–1213, 2015

Abstract-A new compact interdigital capacitor loaded open slot antenna and its lumped model are presented in this letter. Equivalent model analysis shows that the introduction of the interdigital structure increases the capacitive element of the slot and thus reduces the operating frequency of the slot antenna. And the antenna operating frequency as well as its size can be easily reduced by simply increasing the capacitance of the interdigital capacitor and the characteristic impedance of the slot. Experimental results of the exemplary antenna agree well with those of the full-wave simulation, proving that the proposed open slot antenna structure is viable in antenna design.

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