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Transmissive and reflective metasurfaces have been extensively studied for generating an orbital angular momentum (OAM)-carrying beam. However, the existing OAM generators suffer from feed blockage, large profile, OAM mode nonreconfigurable, and narrowband. An OAM generator that integrates multiple functions into a single module is in high demand in OAM-based applications. In this study, we proposed an OAM generator based on a folded reflectarray with high-gain low-profile broadband and programmable characteristics. The proposed multifunctional OAM generator, comprising half the volume of the normal OAM metasurfaces, can generate a high-gain $l=0$ , $l=+1$ , and $l=+2$ OAM-mode-reconfigurable beams with high mode purity over the frequency range of 9.5–10.5 GHz. The proposed OAM generator-integrated multiple functions have potential applications in OAM-based wireless communications and radar detection. [ABSTRACT FROM AUTHOR]

A novel method for the development of a reconfigurable cavity bandpass filter using fluid dielectric is proposed. Dielectric material can produce an effective permittivity ϵeff of the resonant mode when it is loaded into the cavity. Thus, a tube filled with fluid dielectric, e.g., distilled water, can achieve controlled and reversible ϵeff by adjusting the amount of water in the tube. The same manner of resonant frequency can be achieved as the resonant frequency is related to ϵeff, and then frequency tuning is realized. The fluid property can realize easier and faster tuning mechanism than conventional solid dielectric. As ϵeff is affected by the loaded dielectric parallel to the electric field, a triple-mode resonator with resonant modes TE101, TE011, and TM110, which have orthogonal electric fields, is investigated to realize tri-band reconfiguration. The ϵeff, as well as the resonant frequencies, corresponding to each mode can be individually controlled by adjusting their related water posts. Then, reconfigurable single-band and tri-band bandpass filters are designed. A reconfigurable tri-band cavity filter using a triple-mode cavity resonator and fluid dielectric with individual and continuous frequency tuning is reported for the first time. Finally, the reconfigurable tri-band filter is fabricated and measured to validate the concept. [ABSTRACT FROM AUTHOR]

A high roll-off, third-order frequency selective surface (FSS) with quasi-elliptic bandpass response is presented. It is based on substrate integrated waveguide technology. Its unit cell includes three cavities that support at the TE101 mode. Interactions of resonators are dominated by magnetic couplings. The coupling matrix is analyzed and used to design FSS. Guidelines for a coupling-matrix-based FSS design are given. Transmission zeros realized close to the lower and upper narrow passband lead to high roll-off characteristics. In addition, a cross-polarization feature is easily achieved. Finally, a prototype is designed and fabricated to validate the proposed structure. A stable frequency response in passband has been obtained with an oblique incidence of 40° for TE polarization. Good agreement between the simulated and measured results verifies the design concept of bandpass FSS. [ABSTRACT FROM AUTHOR]

A new method for frequency selective surface (FSS) with high selectivity, namely, filtenna–filtenna (FA-FA)-based technique, is proposed, which consists of two filtering antennas and impedance matching network. Based on the reciprocity theorem, the radiation null of filtenna can provide a stopband of the proposed FSS. In this work, it is validated by using a periodic array of back-to-back magnetoelectronic (ME)-dipole antennas with split-ring resonators (SRRs), and a slotted GND plane is used to connect the two ME-dipole antennas for impedance matching. Bandpass response of the proposed FSS is realized by the intrinsic high-pass characteristic of ME-dipole antennas and the bandstop characteristic of SRRs. Four transmission zeros (TZs) and three transmission poles (TPs) are generated, exhibiting a high-order filtering response. TZs and TPs are analyzed by equivalent circuit model and current distributions. The 3 dB fractional bandwidth (${\mathrm {FBW}}_{\mathrm {3\,dB}}$) of 38.5% in the passband and the 20 dB fractional bandwidths $({\mathrm {FBW}}_{\mathrm {20\,dB}})$ of 8.7% in the lower rejection band and 13.8% in upper rejection band are obtained in this design. Finally, a prototype is fabricated and measured to further validate the proposed FA-FA-based technique. Simulated and measured results show that the proposed FA-FA-based FSS is with the distinct advantages, including wide passband, low profile, tunable TZs, and dual-polarized application, which verifies the proposed method of FSSs. [ABSTRACT FROM AUTHOR]

In this article, a class of wideband bandpass filters (BPFs) using triple-mode coaxial resonators has been introduced. Each triple-mode resonator is utilized with TM010 mode, TE− mode, and TE+ mode orthogonal to each other. Two L-shaped feeding probes and a loading disk are adopted to implement the miniaturization of these wideband BPFs. To observe the degree of miniaturization, we set a design specification of the same fractional bandwidth (FBW) of 40% at the same central frequency of 2.5 GHz. After employing the L-shaped feeding probes and loading disk, the efficient circuit size without cavity thickness is reduced by 80% compared with the triple-mode wideband filter without L-shaped feeding probes and loading disk. In contrast, the unloaded quality factors are slightly changed. In addition, another advantage of this work is that the upper stopband is extended to $2.5f_{0}$. Three experimental wideband BPFs in the large, medium, and small sizes are designed, manufactured, and tested for demonstration. Good agreements are obtained between simulation and experiment. [ABSTRACT FROM AUTHOR]

This paper presents a novel polarization-insensitive dual-band frequency-selective surface (FSS)-based electromagnetic shield. The miniaturized FSS unit cell consists of a modified Jerusalem crossed loop and a corner-modified square loop. These FSS elements are arranged in a co-planner configuration over a single-layer Rogers 5880 substrate and simultaneously offer effective shielding in the X- and Ku-bands. Moreover, the FSS manifests polarization-independent and angularly stable band-reject filter characteristics over various oblique angles of incidence for both the TE and TM polarizations with virtuous attenuation at both resonances. In addition, the FSS structure is modelled into an equivalent lumped circuit to better analyze the phenomenon of EM wave suppression. A finite prototype of FSS is fabricated and tested. The simulated and measured results are in good agreement, thus making it a potential candidate for RF shielding/isolation applications. [ABSTRACT FROM AUTHOR]

In this article, a low‐profile ultra‐wideband (UWB) circularly polarized (CP) antenna array with metasurface is presented. The antenna array consists of a 2 × 2 polarization conversion metasurfaces (PCMs) and a sequential feeding network. The PCM is composed of 4 × 4 polarization conversion unit‐cells. The unit‐cell is obtained by putting an ellipse patch into a rectangular ring, connecting the ellipse and the ring with two stubs, and then rotating it by 45°. A linear polarization (LP) slot antenna with a PCM can achieve a CP antenna, which has an impedance bandwidth of about 33.8% (5.4‐7.6 GHz) of |S11| < −10 dB and a 3‐dB axial ratio (AR) bandwidth of about 24.8% (5.8‐7.3 GHz), respectively. The UWB CP characteristics can be achieved when a UWB sequential feeding network is added to the 2 × 2 PCMs. Simulated and measured results show that the proposed antenna array has an impedance bandwidth of about 117% (1.75‐6.75 GHz) of |S11| < −10 dB and a 3‐dB axial ratio (AR) bandwidth of about 105.7% (2.24‐7.35 GHz), respectively. The usable bandwidth can reach 100.3% (2.24‐6.75 GHz). Besides, the proposed antenna array achieves a higher gain compared to the traditional slot array and the peak gain of 12.5 dBic at 6 GHz. The overall size of the antenna is 0.6λ × 0.6λ × 0.03λ (λ refers to the wavelength of the lowest operating frequency in free space). The proposed antenna array can be widely applied to various wireless communication systems. [ABSTRACT FROM AUTHOR]

In this article, a continuously microfluidically tunable filter using two microstrip resonators is proposed. By injecting different amount of water into the tubes, the resonant frequency as well as coupling intensity between two resonators can be controlled. As a result, the fractional bandwidth (FBW) of the proposed band can keep constant when tuning the frequency. As far as the authors know, most of the state‐of‐the‐art microfluidics tunable filters are discontinuous, and the tuning range is limited. In this article, the wide tuning capacity of the frequency is realized by placing some tubes between the microstrip resonators and the ground. By increasing or decreasing the amount of water in the pipes, the local dielectric constant and the equivalent electrical length of the transmission line are changed simultaneously. Therefore, it plays a key role in resonant frequency tuning. In the other hand, the coupling coefficient can be controlled by injecting the amount of water into the tube between the two resonators. Finally, through the analysis of computer simulation technology simulation and measurement, it can be seen that the tuning range of resonant frequency is up to 41.3%, with the central frequency varying from 1.33 to 1.88 GHz. The insertion loss is less than 3.09 dB and the return loss is higher than 15 dB, while the FBW is 3.4% ± 0.2%. The measurement matches well with the simulation results, verifying the proposed design methodology. [ABSTRACT FROM AUTHOR]

A dielectric‐loaded slot antenna with the characteristic of broadband, low profile, and low cross‐polarization is presented. The antenna incorporating a low permittivity (εl=4.1 ${\varepsilon }_{l}=4.1$) and multiple high permittivity (εh=22 ${\varepsilon }_{h}=22$) rectangle dielectric is fed by two rectangle slot, which generates three slot modes and achieves measured 10 dB impedance bandwidth of 88% $ \% $ from 1.57 to 4.04 GHz covering the long term evolution (1.71–2.69 GHz) and sub‐6G (3.3–3.6 GHz). Furthermore, the presented slot antenna with a footprint of 0.81 × 0.56λ02 ${\lambda }_{0}^{2}$ and a low profile of 0.09λ0 ${\lambda }_{0}$ (λ0 ${\lambda }_{0}$ is the wavelength in the air region at the center frequency) appears the performance of measured peak gain of 9 dB without using any dielectric resonator mode. On the basis of the single‐polarized design, the dual‐polarized design with a profile of 0.09λ0 ${\lambda }_{0}$ is investigated and fabricated. According to the measured results, the dual‐polarized prototype achieves a 10‐dB impedance of 60% $ \% $ from 1.6 to 3 GHz. [ABSTRACT FROM AUTHOR]

Novel design concept of reconfigurable full-metal cavity-backed slot antennas (CBSAs) using movable metal posts is proposed in this article. It is found that the cavity mode with purely directional electric field is only affected by the metal post to be placed in parallel to its electric field. Thus, two orthogonal cavity modes TE101 and TE011 can be perturbed by two respective metal posts so as to independently tune the resonant frequencies of these two modes. Electromagnetic energy of the cavity modes can be radiated out through the slots on the top wall of the cavity. In this context, reconfigurable slot antennas with different functionalities, such as single-band, dual-band, dual-mode wideband, the tunable frequency with constant bandwidth, and tunable bandwidth with constant frequency, are thus constituted to demonstrate its attractive design feasibility. The frequency-reconfigurable cavity-back slot antenna with movable metal posts is proposed for the first time. The utilization of the metal posts allows us to achieve a continuous frequency tuning property, while the full-metal structure of the proposed antenna holds a high power-handling capacity and a high radiation efficiency. In final, a prototype of the dual-mode wideband slot antenna is designed, fabricated, and tested to reveal its total efficiency higher than 88% and stable radiation pattern over the tuning band. Good agreement between the measured and the simulated results well validates the presented design concept. [ABSTRACT FROM AUTHOR]

A class of circularly‐polarized cavity‐backed slot antenna arrays with simplified feeding structure are proposed. Power dividing network is an essential feeding structure for the design of conventional antenna arrays, which results in complicated antenna structure. A novel feeding technique based on the electric field of cavity modes is introduced to feed the array elements. The slots on the top wall are directly fed by the electric field without additional feeding structure. Crossed slots with different lengths are introduced to achieve the self‐phased CP radiation as they produce different perturbation on the two degenerate cavity modes. Therefore, high efficiency and simple antenna structure of the CP slot antenna array are achieved. Then, two CP slot arrays with 3 × 3 and 8 × 8 elements are presented to show the designed feasibility. Finally, the 3 × 3 CP antenna array is fabricated and measured to validate the design concept, which can achieve 12.6 dBic gain, 94% total efficiency, 80% aperture efficiency, and −25 dB cross polarization. [ABSTRACT FROM AUTHOR]

In this communication, an $S$ -band wide-bandwidth and wide-beamwidth full-metal circularly polarized (CP) cavity-backed slot antenna is proposed. A pair of triangular prisms placed at the corners of the cavity is introduced to produce TE101 and TE011 modes with orthogonal electric fields. These two modes are used to achieve the CP radiation and radiate the energy through a single-circular slot. By simultaneously varying the prisms’ size and slot radius, the axial ratio (AR) bandwidth (ARBW) improves. The ARBW is further improved by replacing the circular slot with an elliptical slot. As a result, the ARBW increases from 1.0% to 10.3% without complicating the antenna structure and without enlarging the antenna size. Besides, the antenna still operates at two degenerate modes to introduce stable radiation gain and pattern. In addition, the proposed CP slot antenna features a wide ARBW. The final measurement shows that the proposed CP antenna can achieve a 9.5% bandwidth, a stable gain of 7.65 dBic ± 0.15 dBic, and higher than 140° ARBWs in a wide frequency range. The peak total radiation efficiency and aperture efficiency are about 97% and 91%, respectively. [ABSTRACT FROM AUTHOR]

This article tries to address the unstable radiation pattern issue of the planar‐coupled patch antennas with two different methods. First, characteristic mode analysis (CMA) reveals that the coupled patch antennas generate three characteristic modes with distinctive radiation patterns. A slot‐loading scheme is proposed to reshape the current distribution of the higher mode and reduce its sidelobe level significantly. Then a 13.4% bandwidth with consistent radiation patterns and high gain is achieved by exciting modes 1 and 3 while suppressing unwanted modes with an aperture‐coupled feeding. Second, the radiation pattern stability of the coupled patch antennas is further improved by employing shorted parasitic patches. The presented antenna can be equivalent as the same magnetic current array model under the in‐phase and out‐of‐phase modes, and thus the radiation patterns hardly get changed. Moreover, the non‐radioative nature and high radiation quality factor of the higher mode improve the radiation performance in a new principle. It not only enhances the bandwidth up to 18.5% but also results in a sharp filtering edge of gain response. [ABSTRACT FROM AUTHOR]

Programmable metasurface, or reconfigurable intelligent surface is one of the most promising strategies to enhance communication and sensing capacities via arrays of reconfigurable meta‐atoms. The functionalities of conventional programmable metasurfaces are usually limited, and active meta‐atoms are connected with steering‐logic board through cables, which makes the whole system inconvenient for installation and stable use. Here an all‐in‐one integrated multifunctional metasurface for various application purpose is proposed. The integration of metasurface and steering‐logic board produces an all‐in‐one system, which makes it compact, low profile, and stable. The novel meta‐atom structure provides all‐in‐one functionalities, including 1) dispersion‐reconfigurable spatial phaser for analogue signal processing, 2) polarization conversion, 3) beam steering and orbital angular momentum generation for high‐speed communication and, 4) near‐field scanning for sensing. The proposed metasurface also has a broadband property, which is operating from 9.5 to 10.5 GHz with 10% relative bandwidth. Such a design integrates functionalities of signal processing, communication, and sensing in a single programmable metasurface, which makes it especially useful for integrated sensing and communication toward next‐generation wireless networks. [ABSTRACT FROM AUTHOR]

This communication presents the design of 1 bit coding engineered reflector for wideband monostatic/bistatic radar cross section (RCS) reduction under a wide angle of incidence by redirecting the backscattered electromagnetic (EM) energies into wide and countless angles. The proposed surface can achieve an excellent monostatic/bistatic RCS reduction performance of more than 10 dB over a wide frequency range from 60 to 120 GHz under wide angles of incidence up to 75°. This is achieved by designing a wideband anisotropic polarization rotator unit cell with a relative polarization conversion bandwidth of 66.7% and polarization conversion efficiency of more than 99%. The unit cell and its mirrored version are used to represent the “0” and “1” coding states of an optimized 1 bit coding sequence. The distribution of the “0” and “1” coding states and the dimensions of the anisotropic unit cell are optimized carefully, such that the unit cell will have 180° ± 30° reflection phase difference between the $x$ - and $y$ -axes over the whole three frequency bands (V-, E-, and W-bands) and obtains ultrawideband and wide-angle diffusion scattering patterns under oblique incidence up to 75°. The simulation and experimental results show that the proposed coding engineered reflector has nearly uniform diffusive scattering characteristics under normal and wide-angle off-normal incidences of EM waves up to 75° over the three frequency bands. [ABSTRACT FROM AUTHOR]

The independent control of two orthogonal polarizations that possess dual functionalities is highly desirable in antenna and wireless communication technologies. Spin-decoupled metasurfaces with integrated dynamic and geometric phases have been proposed to generate two independent right-handed and left-handed circularly polarized (RCP and LCP, respectively) waves. However, the RCP and LCP phases are correlated by a linear equation of the dynamic and geometric phases. In this article, we propose a meta-atom in which the 1 b phase of two orthogonal polarizations can be controlled independently. Moreover, a high-gain broadband millimeter-wave multidimensional metasurface to generate an $x$ -polarized $l=0$ beam with a 10° beam deflection and a $y$ -polarized $l=+1$ beam independently is simulated, fabricated, and tested. The proposed metasurface operating over the frequency range of 16–28 GHz has the maximum gains of 28.7 and 23.8 dBi for the $l=0\,\,x$ -polarized waves and $l=+1\,\,y$ -polarized waves, respectively. Compared with spin-decoupled metasurfaces, the proposed metasurface provides a simpler design process to generate two independent vortex beams for the two orthogonal polarizations. The proposed vortex metasurface also has the advantages of dual polarization, high gain, wide relative bandwidth, and is broadband. The proposed metasurface has potential application in high-capacity communication systems. [ABSTRACT FROM AUTHOR]

A substrate integrated waveguide (SIW) H-plane horn antenna loaded with tapered multistrip transition and dielectric slab is proposed for broadband high-gain operation. The broadband characteristic of the proposed antenna is obtained by loading a tapered multistrip transition. The theory of coupled resonator is introduced to reveal the mechanism of bandwidth enhancement. On the other hand, a dielectric slab is used to enhance the gain of the proposed antenna. The optimal length of this dielectric slab is determined based on the design principles of a dielectric rod with maximum gain. Besides, the tapered multistrip transition also contributes to the gain improvement by suppressing the feed radiation and prompting the transverse expansion of the surface wave. To verify the design theory and concept, a prototype operating at the K- and Ka-bands is fabricated and measured. The measurement results show that the impedance bandwidth of the proposed antenna is from 20.6 to 38.4 GHz (60%). Within the operating band, a peak realized gain of 19.1 dBi and excellent endfire radiation patterns with low cross polarization (<−30 dB) and high front-to-back ratio (FBR >20 dB) are achieved simultaneously. With its broad bandwidth, high gain, low cross polarization, and high FBR, the proposed antenna is promising for various millimeter-wave applications. [ABSTRACT FROM AUTHOR]

A novel ultra-wideband (UWB) reflectarray antenna for the Internet of Vehicles (IoV) is introduced in this paper. By simply connecting the neighboring reflectarray elements, the proposed reflectarray antenna achieves a remarkable radiation pattern bandwidth of 20 GHz, ranging from 10 GHz to 30 GHz. To explain the operating principles of the proposed reflectarray antenna, the equivalent circuit (EQC) model of the unit cell is built, which also provides an efficient and rapid way to analyze the performance of the proposed reflectarray element. It is found from the EQC analysis that the connected elements can achieve better reflection phase responses than conventional separated elements, thereby improving the array bandwidth. As a proof of concept, a 503-element reflectarray antenna simultaneously covering the vehicle-to-satellite bands (12.25–12.75 GHz/14.0–14.5 GHz/19.6–21.2 GHz/29.4–31.0 GHz), the 24-GHz short-range vehicle radar band (24.25–26.65 GHz) and the 5G millimeter-wave band (27.5–28.35 GHz) is designed, fabricated, and characterized. The experimental results demonstrate that the presented reflectarray antenna can maintain undistorted beams, high antenna gain, low cross-pol level, and moderate aperture efficiency over a bandwidth of 100%, i.e., from 10 to 30 GHz. With its simple and planar aperture as well as excellent performance, the proposed reflectarray antenna can be a promising candidate for vehicles that require reliable high data-rate satellite links and 5G millimeter-wave connections simultaneously. [ABSTRACT FROM AUTHOR]

High-isolation in-band full-duplex (IBFD) cavity-backed slot antennas (CBSAs) in a single resonant cavity are presented in this article. The TE101 and TE011 cavity modes combined with a modified orthogonal feeding structure are first used to design the duplex antenna. The proposed feeding structure can reduce the port-to-port coupling strength and produce a very high isolation. By replacing the crossed radiation slots with two pairs of parallel radiation slots, enhanced radiation gain is achieved. To tackle the narrow bandwidth for the single-mode operation, two orthogonal resonant-iris modes produced by the two feeding slots are introduced to design a dual-mode wideband IBFD antenna. The simulated bandwidth is increased from 0.8% to 5.7%. All the presented antennas with improved performances are designed and realized in a single full-metal resonant cavity. Compared with the conventional design method, high isolation, enhanced gain, and enhanced bandwidth are simultaneously achieved without enlarging the antenna size. Finally, the prototype of the wideband IBFD slot antenna is measured and presented to validate the design concept, which has a 5.8% bandwidth, higher than 95% total efficiency, especially an isolation higher than 72 dB. [ABSTRACT FROM AUTHOR]

With the arrival of the 5G era, the working frequencies of wireless devices are being transferred to a higher spectrum with a wider available bandwidth. However, conventional metal wire antennas used in 5G communication systems are often limited by their resonant structure, which inevitably leads to narrow operating bands and fixed sizes at integral multiples of λ/2 or λ/4. In this Letter, a ceramic inverted-L antenna with a non-resonant architecture is proposed to achieve a wide bandwidth and flexible size. Its leaky-wave radiation is based on the cutoff region of dielectric waveguides, which verifies that the working frequencies of the proposed antenna are related to the cross section rather than the length. Ceramic 3D printing technology is adopted for precise manufacturing of the proposed antenna, which covers the frequency range of 3.28–5.28 GHz, including the bands n77/n78/n79 of the sub-6 GHz spectrum. [ABSTRACT FROM AUTHOR]

This article presents a fully integrated design of probe‐fed open‐ended waveguide filtering antenna by adopting 3‐D printing technology. As for the conventional processing technologies such as computer numerical control machining, they usually suffer from mutually separated parts and inquire post‐processing assembly in the fabrication of complex waveguide structure. Compared with them, the 3‐D printing technology can provide possibility to directly form an all‐in‐one architecture after reasonable design to avoid unexpected concave, thus eliminating the error caused by assembly as can as possible. In this article, the aforementioned manufacture strategy has been successfully applied to waveguide filtering antenna. By adopting a tilted setting in selective laser melting process that is a kind of 3‐D printing technologies, the proposed waveguide antenna with internal structure can be fabricated without flaw and extra supporter. The probe‐fed waveguide antenna is designed herein by reforming the first‐ and last‐order cavities of a waveguide‐fed metal‐insert filter to achieve similar filtering characteristics. This design offers a feasible example of all‐in‐one waveguide filtering antenna and can be extended to other waveguide components. [ABSTRACT FROM AUTHOR]