This paper presents a new kind of highly flexible frequency-agile bandpass filters (FA-BPFs) based on the novel synchronously tuned dual-mode resonator (STDR). The bandwidth (BW), BW variation tendency, passband selectivity (stopband rejection level), and frequency tuning range of the filter can be predefined individually. Benefiting from the unique characteristics of the STDR, the FA-BPF with very simple and highly flexible design/control procedures is achieved. Due to the proposed geometry, two self-adaptive transmission zeros are introduced and move with the passband. The predefined mechanism is investigated in detail, and the simple design and predefined procedures are summarized. Then, three 0.75-1.7 GHz single-band examples with elliptic response are developed to achieve three predefined absolute BWs (ABW). The design techniques and filter superiority are confirmed by the experiments. Moreover, aiming at China 2G/3G/4G cellular wireless/mobile communication system (up to band 40:0.825-2.65 GHz), a novel intrinsically switchable single-/dual-band FA-BPF is presented based on the proposed STDR. An example with a constant ABW 1 dB and a fractional bandwidth 1 dB (FBW 1 dB) is designed to validate the theory and analysis. The FA-BPF is able to operate as a highly selective dual-band FA-BPF with 0.76-1.78 GHz/1.61-2.63 GHz tuning ranges, and also can be switched to single-band operation with the continuous tuning range of 0.76-2.63 GHz.

In this paper, a novel approach for high-order modes excitation using substrate integrated waveguide (SIW) is proposed for the implementation of dual polarization cavity backed slot antenna. The antenna consists of a resonant SIW cavity with eight radiating slots and two separated SIW feeding networks. The vertically linear polarization (VLP) and horizontally linear polarization (HLP) are realized by adopting the TE430 and TE340 modes with different signal schemes. The field distributions and the surface currents of the TE430 and TE340 modes are used to analyze and illuminate the radiation mechanism. To further validate the proposed design, a 2×2 polarization-diverse SIW cavity backed antenna array is fabricated and tested. The measured results show that the impedance bandwidth (S11 < -10 dB) for the two LP states are 10.73-10.9 GHz and 10.75–10.83 GHz, respectively, while the 3-dB axial ratio bandwidth for the right-hand circular polarization (RHCP) is 10.75-10.83 GHz. The measured peak gains of the two LP modes and CP mode are 13.4, 12.92, and 12.2 dBi, respectively. The proposed approach demonstrates an effective way of high-order modes (TE430 and TE340 modes) generation in a single SIW cavity, which has significant meaning in polarization-diversity applications.

This paper proposes a unified synthesis methodology and the related architecture for fully passband-tunable dual-mode filters with wide tuning range. The methodology and architecture relate the well-developed frequency-fixed synthesis technologies to tunable filters, and can be applied to any tunable filter design and analysis by an element-variable coupling matrix. This general methodology accounts for bandwidth (BW), center frequency (CF), and return loss (RL) (passband ripple or stopband rejection) tuning, and helps to implement the passband-tunable filter. For validation, a novel tunable dual-mode bandpass filter (BPF) based on the architecture is presented. The proposed filter has the ability to arbitrarily construct the two-pole passband within the frequency tuning range as predicted. In addition, three transmission zeros (TZs) are generated by the proposed structure, which results in a highly selective passband and a reconfigurable stopband. A tunable filter with a tuning range from 0.8 to 1.2 GHz is designed and fabricated. The measured filter presents an elliptic response during the frequency tuning. The 50% CF tuning range (0.8-1.42 GHz) with a constant absolute BW₁ dB and a BW₁ dB tuning range from 50 to 500 MHz at the fixed CF are measured. Good agreement among matrix calculation, circuit model simulation and measurement demonstrates the validity of the proposed method and structure. In addition, different states of the stopband with the same passband are obtained to further broaden its application.

A high-gain 4 × 4 substrate integrated waveguide (SIW) circularly polarized (CP) antenna array at a Ka-band with a quadruple-ridge waveguide polarizer is proposed in this letter. The antenna array consists of 16 cavity-backed slot antenna elements, quadruple ridge waveguide polarizers, and SIW T-type power dividers. Dominant resonant mode TE ₁₁₀ is excited in the cavity-backed slot antenna element. The measured impedance bandwidth ( S ₁₁ < –10 dB) is from 35.3 to 35.55 GHz, and 3 dB axial-ratio bandwidth is from 35.24 to 35.57 GHz. In addition, the maximum measured gain of 18.14 dBi at the boresight is experimentally obtained at 35.42 GHz. The antenna prototype was fabricated by a multilayer printed circuit board technology. To the best of our knowledge, the quadruple-ridge waveguide is used as polarizer to produce CP signals for the first time. Compared with an aperture antenna with a conventional rectangle or a circular waveguide polarizer, this work has high aperture radiation efficiency as well as compact size. This design idea may open a new way for development of millimeter-wave high-efficiency arrays.

Miniaturized hexagonal stepped-impedance resonators and their applications to bandpass filters are presented. Based on the odd- and even-mode equivalent circuits, the resonance characteristics of these resonators are investigated. A series of circuit configurations, including the third-order Chebyshev, trisection, and fifth-order cascaded trisection filters, are constructed by the proposed resonators. These filters are very compact and have good stopband responses. Measured results validate the analysis and theoretical prediction with good agreement.