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Summary: This paper proposes a wideband quasi‐reflectionless bandpass filter with high‐selectivity based on signal‐interference method. The bandpass filter is composed of a bandpass section and two absorptive sections, which consist of a quarter‐wavelength coupled line and signal‐interfering/resistively loaded path, respectively. The bandpass section mainly performs bandpass filtering characteristics while the absorptive sections are applied to consume the reflected signals. By virtue of rationally setting electric lengths and characteristic impedances of the absorptive section to form signal interference, signal dissipation within a wider frequency range and higher selectivity can be realized. Finally, the quasi‐reflectionless bandpass filter was designed and demonstrated with well‐matched measurement and simulation results. Compared with conventional works, the proposed bandpass filter can simultaneously attain wideband, high‐selectivity, and two‐port quasi‐reflectionless performance within a single‐layer topology and well‐integrated structure owing to the signal‐interference technology. The proposed technique may be extensively applied in the RF front‐end and decreases the effect on the stability of communication systems caused by reflected signal. [ABSTRACT FROM AUTHOR]

Summary: This paper proposes a high‐selective crossover with a third‐order filtering characteristic, high passband isolation, and multiple controllable transmission zeros based on a coupled topology using single‐ and dual‐mode resonators. By virtue of the odd‐ and even‐mode feature of the stub‐loaded dual‐mode ring resonator, not only can the two resonant modes for different passbands be relatively independently controlled, but also better isolation can be achieved. With a single‐mode resonator and stub‐loaded dual‐mode resonant coupling topology, the resultant crossover can perform third‐order filtering performance in two distinct passbands/channels and generate several transmission zeros outside the desired bands to improve the isolation between the passbands. Experimental results demonstrate that, compared with conventional works, the filtering order of the proposed crossover is promoted and its passband isolation can be significantly improved by about 8 dB. The proposed work may facilitate its widespread applications in high‐performance multichannel integrated microwave systems for avoiding interference between multiple intersecting signal paths. [ABSTRACT FROM AUTHOR]

The previous work on reflective linear polarization converters has mostly focused on wideband characteristics, while the out-of-band selectivity has received rare attention. To achieve wideband and high selectivity simultaneously, a wideband reflective linear polarization converter with quasi-elliptic filtering characteristic is proposed based on multi-mode resonant metamaterial. Through the loading of the cross-shaped slotlines and inter-unit connecting lines, the inherent degenerate modes of a square patch can be excited and separated, and two additional resonant modes and two independently controllable conversion zeros are introduced to achieve excellent wideband and quasi-elliptic filtering characteristics. After the principles of generating in-band resonant modes and out-of-band conversion zeros are revealed, the mechanism of polarization conversion is analyzed in detail from the perspective of the amplitude-phase imbalance between its polarization components. Finally, a polarization converter is designed, fabricated, and measured. The main-polarization reflection coefficient is less than − 10 dB in fractional bandwidth of 50%, wherein the polarization conversion rate is 90%, and the polarization conversion rate squareness is about 0.9. The proposed converter with excellent frequency selectivity provides great potential for applications in multi-band and multi-functionality systems for reduced isolation band. [ABSTRACT FROM AUTHOR]

This article proposes the synthesis design method of the dual-band differential phase shifters with independent filtering passbands and predetermined phase shifts. The filtering unit of the proposed phase shifter adopts a simple branch-line structure, which possesses its characteristics of controllable dual-band frequency ratios and bandwidths. Meanwhile, the Schiffman phase alignment element holds the capability of achieving different predetermined phase shifts in dual band. In our design, dual-mode resonator and dual-mode J-inverter models for the filtering units are equivalently established, and the amplitude and phase closed-form formulas for the proposed dual-band phase shifter are further numerically obtained for the first time. This proposed work features new synthesis method, integrated higher order filtering response with two independently prescribed bandwidths, and concurrent predetermined dual-band phase shifts. Design procedures are provided in detail for a quick design. Finally, two dual-band phase shifters with different frequency ratios and phase shifts are designed and demonstrated. The experimental and measured results match well. [ABSTRACT FROM AUTHOR]

In this paper, a kind of coplanar waveguide (CPW)‐fed series stub structure is proposed and its application in wideband bandpass filters (BPFs) is developed based on the coplanar strip (CPS) dual stubs. Compared with its conventional CPS dual‐series‐stub counterparts, the proposed one excited by even‐mode CPW lines possesses enhanced immunity against unexpected resonance of the CPS‐based series stubs. By virtue of this attractive performance, desired resonance can be well realized with predicted in‐band characteristics. Subsequently, the application in its constituted wideband BPFs is investigated with synthesis design method. In final, one third‐order and one fifth‐order BPFs are designed, fabricated, and measured to verify the proposed structure. Measured results are found in reasonably good agreement with the synthesized and the simulated ones. [ABSTRACT FROM AUTHOR]

In this study, a class of wideband bandstop filters is proposed based on wideband 180° phase shifters. Different from traditional signal–interference-based bandstop filters, they make use of a 180° phase shifting in a wide operating band in principle. After numerical analysis of the phase responses of the two coupling-line paths, the optimal solutions for the largest 180° phase-shifting bandwidth or minimum impedance ratio can be obtained. Then, the wideband bandstop filter based on these two unique coupling lines is intensively analysed to reveal that three transmission zeros are generated in the stopband as the two paths have a 180° phase difference, thus exhibiting its wideband property. Moreover, by enlarging the impedance of the two paths, good out-of-band characteristics of the bandstop filter can be attained, while in-band performances are almost unchanged. Next, two types of bandstop filters with extended bandwidth are proposed by cascading an extra delay line for both paths, and they hold similar filtering characteristics as the aforementioned one. Finally, the design process and demonstration are provided. The measured results of the three fabricated filters show that Type-I, Type-II, and Type-III filters can satisfactorily obtain a 3 dB bandwidth of 108, 122, and 130% with 25, 14.7, and 11.6 dB rejection level, respectively. [ABSTRACT FROM AUTHOR]

In this study, a class of wideband bandstop filters is proposed based on wideband 180° phase shifters. Different from traditional signal–interference‐based bandstop filters, they make use of a 180° phase shifting in a wide operating band in principle. After numerical analysis of the phase responses of the two coupling‐line paths, the optimal solutions for the largest 180° phase‐shifting bandwidth or minimum impedance ratio can be obtained. Then, the wideband bandstop filter based on these two unique coupling lines is intensively analysed to reveal that three transmission zeros are generated in the stopband as the two paths have a 180° phase difference, thus exhibiting its wideband property. Moreover, by enlarging the impedance of the two paths, good out‐of‐band characteristics of the bandstop filter can be attained, while in‐band performances are almost unchanged. Next, two types of bandstop filters with extended bandwidth are proposed by cascading an extra delay line for both paths, and they hold similar filtering characteristics as the aforementioned one. Finally, the design process and demonstration are provided. The measured results of the three fabricated filters show that Type‐I, Type‐II, and Type‐III filters can satisfactorily obtain a 3 dB bandwidth of 108, 122, and 130% with 25, 14.7, and 11.6 dB rejection level, respectively. [ABSTRACT FROM AUTHOR]

This communication proposes a wideband low-profile circularly polarized patch antenna based on a broadband 90° modified Schiffman phase shifter and a meandering L-shaped microstrip feed. The whole antenna consists of a broadband hybrid feeding network and a circular patch radiator. The hybrid network has two branches that operate in the first and second Schiffman periods, and thus can achieve 113% 10 dB return loss bandwidth and 75% phase shift bandwidth within 90°± 4.6° under a relaxed coupling strength of 1.58. To reduce the profile, the planar microstrip L-shaped feeding probe is properly meandered, which provides not only broadband impedance matching, but also an extra degree of freedom in the reduction of cross polarization, thus achieving a wide axial ratio (AR) bandwidth. Simulated and measured results show that the antenna has indeed achieved a low profile ($0.05~\lambda _{0}$) with 50.5% impedance bandwidth, 47.8%AR bandwidth, and 34% gain bandwidth (>3 dBi). [ABSTRACT FROM AUTHOR]

In this article, a class of improved Schiffman phase shifters with two C-section coupled lines separately operating in the first and second phase periods is presented and analyzed. Theoretical model and closed-form synthesis formulae are utilized to analyze the phase performances of the proposed phase shifters, which reveals significant differences compared to the phase shifters solely operating in the first or second phase period. Compared to the traditional counterparts, the proposed phase shifters can achieve a larger phase shift value and a wider phase shift range at the same impedance ratio. Moreover, under the same impedance ratio, the phase shifter can obtain wider phase shift bandwidth. These features are believed to be very useful in the limitedly realizable coupling strength for a wide phase shift range case. Finally, two sets of comparative experiments, including the operations in the first phase period, second phase period, and the proposed type, are designed and fabricated to validate the proposed approach. The simulated and measured results agree well with the theory and achieve a 33.6% (36.7%) increase in phase shift bandwidth when the phase shift value is 90° (150°) under the impedance ratio of 1.5822 (2.0312). [ABSTRACT FROM AUTHOR]

This study proposes a class of differential phase shifters based on the Schiffman type‐C and type‐F networks with wide phase shift range. After the phase and bandwidth properties of these two distinct networks are numerically analyzed, two unique advantages of wide phase shift range and wide phase shift bandwidth are revealed. Next, the optimal design parameters for different phase shift values up to 405° are presented to allow for a quick design process. To verify the proposed approach, a 180° differential phase shifter is designed, fabricated and measured. Good performance is achieved with an impedance ratio of 1.959 and 3.7° phase deviation over a phase shift bandwidth of 61%. [ABSTRACT FROM AUTHOR]

In this letter, a pin‐loaded triple‐mode square patch resonator is proposed for bandpass filter (BPF) design. By loading short‐circuited pins underneath the patch, the patch resonator not only enables DC‐blocking, but also brings out one additional resonant mode, thus a triple‐mode patch resonator can be formed up under operation with a pair of degenerate modes. According to the field distributions, these three resonant modes can be tuned by virtue of loaded slots and pins, while these two degenerate modes can be properly split by offset‐positioning the two sets of pins. In final, a pin‐loaded triple‐mode patch BPF is designed, fabricated and measured to validate the predicted results. [ABSTRACT FROM AUTHOR]

This paper proposes a class of balanced wideband phase shifters with wide phase shift range and good common-mode (CM) suppression. Both the balanced filtering units in the reference and main branches consist of horizontal half-wavelength branches and vertical quarter-wavelength loaded stubs but with dissimilar sizes. The differential-mode (DM) section of these two units can achieve ($2N + 1$)th-order Chebyshev amplitude response while with discrepant phase characteristics, which can be utilized in the design of balanced phase shifters with large phase shift. As another interesting feature, the proposed balanced filtering unit functions as the conventional branch-line bandstop response in CM, and its frequency selectivity can be further enhanced by using the so-called signal-interference method. The proposed balanced wideband phase shifters have the advantages of wide phase shift range, low phase deviation, high level and wide range for CM rejection in both two branches. The 30°, 60°, 90°, 120°, 150°, and 180° balanced phase shifters are then studied and designed. As the typical examples, 30°, 90°, and 180° phase shifters are designed, fabricated, and measured. The 30° and 90° phase shifters have a fifth-order Chebyshev response, while the 180° phase shifter is of fifth-order and third-order Chebyshev responses in the reference and main branches, respectively. The measured results show that the phase shift values are 30.14° ± 2.2°, 90.75° ± 4.05°, and 180.1° ± 5.5°, the phase shift bandwidths are 73%, 68.5%, and 66.8%, and the CM rejections are −23.3, −22, and −14.6 dB, respectively. [ABSTRACT FROM AUTHOR]

In this paper, a dual-band filtering antenna with independent predetermined frequencies and low cross-polarization levels is proposed. In order to realize the independent control of frequencies of two radiation passbands, multiple additional structures such as rectangular slots, rectangular parasitic patches, and U-shaped slots are integrated on the traditional rectangular patch antenna to introduce new resonant modes and radiation nulls to the antenna, simultaneously. Since the generation structures of each frequency point are different, they do not affect each other. Besides, to obtain low cross-polarization levels, each additional structure is carefully adjusted such that the current vectors of each resonant frequency along the cross-polarized direction are quite weak. The proposed antenna operates in the 5G sub-6G (4050–4294 MHz) and the Wi-Fi 5.8 GHz band (5700–5920 MHz), and achieves cross-polarization levels of −37.8 and −36.8 dB in the lower and higher frequency bands, respectively. Compared to other multi-band filtering antennas, the proposed one has the advantages of independent predetermined frequencies and lower cross-polarization levels. [ABSTRACT FROM AUTHOR]

In this paper, a class of balanced wideband phase shifters with good filtering property and common-mode (CM) suppression is proposed, synthetically designed, and implemented. Unlike the conventional coupled-line reference branch one, the proposed balanced phase shifter has its T-shaped structure in both of main and reference branches, so as to achieve superior passband selectivity and CM rejection simultaneously. The synthesis method is then presented to design the proposed balanced wideband phase shifters with the prescribed phase shift value, passband ripple, maximum phase deviation, and fabricated bandwidth. The 30°, 45°, 60°, 75°, and 90° balanced phase shifters are theoretically designed and demonstrated. To validate the design method, two prototypes with different fractional bandwidths (FBWs) and phase shift values (Case 1: FBW = 66.7%, typical phase shift value is 30°; Case 2: FBW = 55.6%, typical phase shift value is 90°) are designed, fabricated, and measured. The synthesized, simulated, and measured results are found in good agreement with each other. [ABSTRACT FROM AUTHOR]

In this paper, a wideband metasurface-loaded (MTS-L) rectenna system is proposed to capture electromagnetic (EM) energy at arbitrary azimuth angles. The radiation patterns of different modes in the original MTS configuration are analyzed using the characteristic mode theory, and potential modes with omnidirectional radiation are screened out. By the arrangement of patches, the roundness performance of the radiation pattern can be ameliorated, and the omnidirectional characteristic is obtained over a wide frequency band. Subsequently, the surface current density of the selected mode is carefully and artificially designed to facilitate probe excitation as well as refrain from introducing complex power-combining networks. A wideband rectifier circuit is designed as the load of the proposed antenna. Eventually, measured results show that it operates from 4.6 to 9.6 GHz with a fractional bandwidth of 70.4%, and the peak system efficiency is 52.2%. The proposed system demonstrates excellent potential for wireless power transmission and EM energy harvesting in indoor environments. [ABSTRACT FROM AUTHOR]

Modified coupled feed-lines (MCFLs) and coupled-line stubs (CLSs)/coupled-line stub-loaded resonators are proposed and applied to design wideband balanced filters to obtain high selectivity and common-mode suppression. MCFLs are implemented by loading short- and open-circuited stubs at the end of the traditional parallel-coupled feed-lines to perform bandpass and bandstop response, respectively. Impedance matching and additional transmission poles guarantee that the filters have wideband differential-mode (DM) passband. Transmission zeros can be allocated to suppress the common mode through different electrical length of stubs. Moreover, the CLSs and its stub-loaded resonators have the property of dual transmission zeros outside the passband, which improves the selectivity of the DM passband. Specifically, two prototypes based on CLSs with second harmonic suppression and CLS-loaded resonators with sharp rejection (\Delta f{{3dB}} / \Delta f{{20dB}}=0.97) are designed and fabricated for demonstration. The simulation and measurement results are in good agreement. [ABSTRACT FROM PUBLISHER]

ABSTRACT A sharp-rejection dual-band bandstop filter using transversal signal-interaction with three paths, consisting of a transmission line path and two coupled paths, is proposed. By tuning the coupling coefficient and odd-mode impedance of the coupled lines, the center frequency ratio and bandwidth can be controlled, respectively, and the transmission line impedance is used to allocate the transmission poles to sharp the attenuation rate. A compact dual-band bandstop filter prototype is designed and fabricated. The simulation and measurement results agree well. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:657-660, 2015 [ABSTRACT FROM AUTHOR]

A flexible triple-band and polarization-insensitive rectifying metasurface (RMS) is presented in this article. The surface consists of three parts: a periodic metal unit with integrated diodes, a DC filter composed of series inductor and parallel capacitor, and a load resistor. Due to the direct integration of rectifying diodes and interconnected branches within and between the units, complex matching networks can be eliminated and the overall structure is simplified. A 6 × 6 array is fabricated and measured. The results show that at a low input power of 0 dBm, the RMS has efficiency of 51.81%, 44.25%, and 42.01% at 2.9 GHz, 5.2 GHz, and 5.77 GHz, respectively. The efficiency remains consistent under different polarization angles, demonstrating the polarization insensitivity property of RMS. Additionally, the flexible substrate enables high efficiency in the S-band and Wi-Fi bands even under a certain range of bending, revealing its superior flexibility and adaptability. Therefore, it is well suited for ambient energy harvesting and wireless self-powered miniaturized internet of things devices. [ABSTRACT FROM AUTHOR]

In this article, a class of microstrip-to-coplanar strip (MS-to-CPS) transitions with wideband balanced-type topologies, efficient synthesis design approach, and well-verified experimental de-embedding are proposed and designed. By virtue of the electromagnetic field conversion along the transitions, the unwanted common-mode (CM) signal could be intrinsically rejected while the differential-mode (DM) counterpart is capable of being converted through a very wideband. In this context, equivalent circuits of such transitions under DM operation are first developed and synthesis design is derived so as to determine each characteristic impedance in these transmission-line (TL) models. Propagation parameters for various CPS lines with varied physical dimensions are then investigated. Experimental de-embedding procedure is described and executed to extract the fundamental responses of a single transition inherently. As a result, a third-order and two different fifth-order MS-to-CPS transitions with respective fractional bandwidth (FBW) of 100%/111%/111% are finally synthesized, designed, fabricated, and measured. Measured results agree well with the theoretical, simulated, and extracted ones, strongly demonstrating and verifying that these proposed transitions feature indeed not only the intrinsic CM suppression but also the wideband DM filtering performances. [ABSTRACT FROM AUTHOR]

In this article, two coplanar-strip (CPS) wideband balanced bandpass filters with intrinsic common-mode (CM) suppression are proposed based on shunt-stub structures. As previously reported with higher integration and greater flexibility, the double-sided shunt-stub structures could make sufficient use of various transmission lines (TLs), such as double-sided parallel-strip line (DSPSL), microstrip line (MSL), and slot line (SL). Compared with its conventional dual circuit, the proposed structure herein is capable of realizing ideal CPS-based resonant characteristics without degraded performance. As such, a DSPSL-loaded CPS triple-mode resonator and a microstrip-to-CPS (MS-to-CPS) transition are developed as two distinctive double-sided shunt-stub element units. Meanwhile, their equivalent circuit models are generalized for analysis of these proposed shunt-stub units and the corresponding synthesis design is further theoretically derived. For experimental verification, one fifth-order and one seventh-order balanced filters are designed, fabricated, and measured. It has been well demonstrated that both the wideband differential-mode (DM) filtering property and intrinsic CM suppression are satisfactorily achieved at the same time, with a certain range of available bandwidths. [ABSTRACT FROM AUTHOR]

In this article, a simple kind of coplanar stripline (CPS) open-circuited series stub structure and its application in wideband bandpass filters (BPFs) are proposed and developed. Compared with its conventional dual-shunted CPS series stub counterparts, the proposed one is capable of achieving desired characteristics as the specified transmission-line (TL) prototypes behave. Based on such equivalent prototypes, three types of CPS open-circuited series stubs are then investigated to reveal the immunity against unexpected resonance caused by the underlying surface-wave-like mode. For demonstration, two examples, a second-order and a third-order wideband BPFs, are synthetically designed for equal-ripple Chebyshev response with prescribed fractional bandwidth (FBW) and return loss ($RL$). Finally, these two proposed filters with a respective FBW of 49%/70%, an $RL$ of 16.426 dB, and a central frequency of 2 GHz are synthesized, simulated, fabricated, and measured. Measured and simulated results are reasonably in good agreement with each other, thus firmly validating the flexibility of the presented CPS series stub structure in the application of wideband BPFs. [ABSTRACT FROM AUTHOR]

A broadband antenna with simultaneous flat gain and filtering characteristic is proposed in this paper. To achieve a flat gain response, rectangle slots and a bandpass filter are loaded on the antipodal Vivaldi antenna. The loaded rectangle slots adjust the phase of the surface currents to widen the operating bandwidth, as well as reduce the far-field radiation cancellation of the reverse currents in the end-fire direction, thus improving the gain. In addition, a bandpass filter is added to the antenna to further equalize the currents amplitude and suppress out-of-band signals. The measured results show that the proposed antenna achieves an impedance bandwidth of 101.9% (3.99–12.28 GHz) and an out-of-band suppression level of 15.5 dBi in the upper stopband. What is more, an extremely low gain fluctuation of ± 0.464 dBi is realized in the 4–12 GHz. Compared with conventional broadband antennas, the antenna features simple structure and better performance, which achieves broadband with simultaneous flat gain response and filtering characteristic. The proposed antenna may be applied in high-precision radar sensing system. [ABSTRACT FROM AUTHOR]

A dual-band quasi-single-layer absorber based on multimode resonance is proposed. Different from the traditional multi-substrate or multi-resonator composite structure, by virtue of stub- and slot-loaded technologies, only one single-substrate printed resonator is utilized to achieve multi-mode resonant characteristics for two absorption bands. The bandwidth of the two absorption bands can be adjusted by structural parameters. Through the detailed analysis of the surface current distribution in different modes, the generation principle of the multi-mode dual-band characteristic is revealed. Finally, the dual-band absorber is fabricated and measured, achieving dual-band absorptions in the frequency range of 7.64–10.8 GHz and 15.04–17.32 GHz with fractional bandwidth of 34.3% and 14.1%, respectively. The advantages of low-profile simple structure and controlled dual bandwidth make it very suitable for radar detection and stealth fields. [ABSTRACT FROM AUTHOR]

Summary: A tunable circularly polarized square patch antenna with parasitic elements is designed for a wide frequency tuning range and high gain characteristics. The proposed antenna is constructed by one main patch and four semi‐elliptic parasitic units. By loading four varactor diodes and adjusting their capacitance values, the tunable feature is performed to reallocate the corresponding working frequency. Moreover, the diagonal corners of the antenna are cut and loaded with varactor diodes, which provide the appropriate perturbation between the two orthogonality modes of the antenna, so as to ensure the circular polarization characteristic in the entire operating tuning band. The experimental results demonstrate that the reflection coefficient and axial ratio are less than −13 dB and 3 dB, respectively. The proposed antenna features a relatively wide continuously tuning range of 24% within 1.9‐2.3 GHz and a stable gain of over 7 dBi with a radiation efficiency of above 85%. [ABSTRACT FROM AUTHOR]

A dual-band quasi-single-layer absorber based on multimode resonance is proposed. Different from the traditional multi-substrate or multi-resonator composite structure, by virtue of stub- and slot-loaded technologies, only one single-substrate printed resonator is utilized to achieve multi-mode resonant characteristics for two absorption bands. The bandwidth of the two absorption bands can be adjusted by structural parameters. Through the detailed analysis of the surface current distribution in different modes, the generation principle of the multi-mode dual-band characteristic is revealed. Finally, the dual-band absorber is fabricated and measured, achieving dual-band absorptions in the frequency range of 7.64–10.8 GHz and 15.04–17.32 GHz with fractional bandwidth of 34.3% and 14.1%, respectively. The advantages of low-profile simple structure and controlled dual bandwidth make it very suitable for radar detection and stealth fields. [ABSTRACT FROM AUTHOR]

A metamaterial‐based frequency reconfigurable microstrip antenna is proposed with wideband tuning range and enhanced gain. By co‐designing the main reconfigurable microstrip patch unit and the upper‐layer located reconfigurable negative‐permeability‐metamaterial resonant loop, the gain of the resultant proposed antenna is readily improved. The wideband reconfigurability of the microstrip antenna and the metamaterial unit is realized by tuning the capacitance values of the varactor diodes through altering their bias voltages. Furthermore, the effect of different number of reconfigurable metamaterial units on the gain of the proposed antenna is extensively explored, which shows the radiation pattern is largely improved since the electromagnetic energy is more concentrated and directed. Finally, the proposed antenna is fabricated and measured. The experimental results demonstrate that the operating range of the proposed antenna is within 8.6–10.3 GHz, and the reflection coefficient is less than −10 dB, and the gain is above 6 dBi. The radiation efficiency of the antenna is above 85%. The electrical size of the antenna is 0.58λ0 × 0.58λ0. As expected, the proposed antenna has the main advantage of enhanced gain performance, while performing comparable frequency tuning range. [ABSTRACT FROM AUTHOR]