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A novel bandwidth enhancement method using loading multiple parasitic structures to realize broadband miniaturized antenna is proposed. Based on the proposed strategies, one miniaturized wideband dual-polarized dipole antenna covering LTE/CDMA/GSM bands (690–960 MHz) for base-station application is designed. The antenna consists of two folded bowtie-shaped dipoles fed with a pair of crossed baluns, and is further loaded on a ceramic block with high permittivity ($\varepsilon$) for miniaturization. A parasitical square loop is loaded for bandwidth enhancement at a lower frequency. In addition, four split rings with a small square loop are firstly adopted for antenna resonance tuning, the impedance bandwidth is thus improved effectively. The antenna size is only $0.18\,\times \,0.18\,\times \, 0.22\,\,\lambda _{0}$ at the center frequency. The measured result of the fabricated prototype exhibits a relatively large operation bandwidth of 32.73% (VSWR < 1.6) with stable radiation patterns and relatively flat broadside gains. Moreover, to verify the feasibility of the proposed miniaturized antenna element in the base-station application, an array of $2\,\times \, 4$ is further designed. The measured results of the array indicate isolation over 20 dB with an impedance bandwidth of 32%. Due to the good performance and compact feature, the proposed miniaturized broadband antenna element can find large application potential in multiband wireless communication base stations. [ABSTRACT FROM AUTHOR]

High-performance miniaturized antennas are one of the goals that have been pursued in the design of wireless communication systems for implantable medical devices. In this article, a 915 MHz novel-shape loop antenna conforming to the inner wall of the capsule is developed by comprehensively adopting meander and symmetrical structure patch, folding technique, and vector current reversal technique. Through electromagnetic simulation, testing, and experimentation, the antenna shows good characteristics such as small size, ultra-wideband, circular polarization, strong robustness, and safety. Specific performances include the following: 1) the size of the developed miniaturized loop conformal antenna is only 30.5 mm $\times $ 4.6 mm $\times $ 0.05 mm and the maximum gain is −21.5 dBi at 915 MHz, which solves the problem of low gain due to the small antenna radiation area; 2) the antenna has a 4.5 GHz ultra-wideband impedance matching bandwidth ($\vert S_{11}\vert < -10$ dB) from 0.5 to 5 GHz. Even if the parameters such as the implantation depth and the thickness of the capsule shell change, or there exists a wireless power transfer system, the antenna can still cover the required frequency band with strong robustness; and 3) the specific absorption rate (SAR) of human tissues radiated by the antenna is far lower than the safety limit in IEEE standards and ICNIRP guidelines. The experimental platform using pork to simulate human tissues was built for antenna performance testing and communication testing. The measurement results verified the good resonance performance, radiation characteristics, and omnidirectionality of the developed antenna. The antenna link loss model is also given, which has been used in wireless communication and localization. [ABSTRACT FROM AUTHOR]

The derivation of the probability density of irradiance for the received electromagnetic wave propagating through random inhomogeneities has applications in many diverse fields, such as optical wireless communications, radar, and imaging systems, to name few. The conventional approach in this regard is to combine distribution models, with each being found valid under different conditions, in a modulation process, e.g., the extended Rytov theory. However, in these techniques, the nature of the physical processes responsible for the randomness is not considered directly. Moreover, it is not clear how the model parameters are related to the physical environment. Recent research studies found that in random media, such as underwater, the exact physical processes in effect can significantly alter the distribution of the received signal intensity. In this article, a novel intensity distribution model is derived based on Rytov theory of propagation through weak turbulence. The physical parameters of the medium are directly included into the model parameters. Therefore, the resultant model is general enough to describe different physical environments. The utility of the model is validated by studying the effect of the medium parameters on the distribution through simulations. [ABSTRACT FROM AUTHOR]

In this article, a novel design of broadband circularly polarized (CP) folded reflectarray (FRA) antenna is proposed. The CP FRA consists of two kinds of metasurfaces (MSs) and a linearly polarized (LP) open waveguide antenna as the feeding source. The bottom MS functions as the main reflector, which is capable of realizing cross-polarization conversion and beam focusing simultaneously. The upper MS is utilized as a subreflector, which could reflect LP wave coming from the feeding source and transmit its orthogonal counterpart into CP wave. For demonstration, the MS-assisted CP FRA antenna is fabricated and measured. Numerical results are consistent with experimental ones, illustrating a peak gain of 24.1 dBi and a peak aperture efficiency of 51% at 9.9 GHz. The 3 dB gain bandwidth and 3 dB axial ratio (AR) bandwidth are achieved as 27.5% (8.8–11.6 GHz) and 43.9% (8–12.5 GHz), respectively. The proposed CP FRA provides a new avenue to realize broadband high-efficiency CP antenna, which may find great potentials in modern wireless communication systems. [ABSTRACT FROM AUTHOR]

The “time” factor in time-modulated arrays (TMAs) offers a new degree of freedom for securing wireless communication. However, the widely used periodic time modulation introduces limited randomness for eavesdropping receivers, and thus gives weak security. To meet this challenge, this work proposes a TMA with chaotic-enabled phase modulation (CPM) for secure transmission. The proposed CPM technique offers the chaos features of high randomness, unpredictability, and initial condition sensitive characteristic, to increase the information uncertainty for eavesdropping receivers. An optimization-based time sequence design approach is introduced to design optimal CPM time sequences for secure transmission in a given direction. Thus, a variety of security strategies including beam scanning, directional modulation (DM), and chaotic encryption are simultaneously enabled by the optimized CPM time sequence. Moreover, the CPM is a radio frequency (RF)-domain security technique without requiring any baseband modulation modules, which exhibits a simplified hardware architecture. Numerical analysis and experimental results are presented to illustrate the effectiveness of the proposed CPM technique. The results indicate that the security is improved by lowering the bit error rate (BER) between the legitimate receiver and the transmitter, and reducing the mutual information between eavesdropping receivers and the transmitter. [ABSTRACT FROM AUTHOR]

This communication presents a novel compact tri-band filtering antenna for 5G Sub-6 GHz n41 (2.515–2.675 GHz), n78 (3.4–3.6 GHz), and n79 (4.8–5 GHz) applications. The antenna consists of a central dual-band element and four single-band elements placed around. Simulated results show that the tri-band filtering antenna has an impedance bandwidth ($\vert S_{11}\vert < -10$ dB) of 6.6% (2.51–2.68 GHz), 6.3% (3.39–3.61 GHz), and 5.5% (4.73–5 GHz), covering Chinese 5G Sub-6 GHz band. The distance between single-band elements along both the x-axis and y-axis is $0.31\lambda _{0}$ ($\lambda _{0}$ is the free-space wavelength of the center frequency at the lowest band), realizing a compact structure of $0.95\lambda _{0} \times 0.95\lambda _{0} \times 0.2\lambda _{0}$. Since this tightly spaced structure will easily cause strong coupling, additional filtering structure is added, achieving high isolation over 47 dB between the central and the peripheral elements. The final antenna realizes stable gain in all three frequency bands, showing satisfactory in-band average radiation efficiency and high out-of-band gain-suppression level, making it suitable for 5G Sub-6 GHz wireless communication applications. [ABSTRACT FROM AUTHOR]

We present a C-band amplifying reconfigurable intelligent surface (ARIS) integrated with power amplifier (PA) to enhance the reflected electromagnetic (EM) signals. An aperture-coupled patch element with two orthogonal hourglass-shaped slots is elaborately constructed to ensure a broad operation bandwidth. A power combining and dividing network is also introduced to combine the 2 $\times2$ elements into a subarray to reduce the number of PAs and the system cost. The simulation results indicate that the achieved gain of ARIS is 7.7–12.2 dB from 5.0 to 6.0 GHz. A prototype of the proposed ARIS is designed, fabricated, and experimentally verified. The measured results agree well with the simulation. The proposed ARIS is promising for applications in future wireless communication systems for signal enhancement and coverage extension. [ABSTRACT FROM AUTHOR]

A metasurface (MTS)-based direction of arrival (DoA) estimation method is presented. The method exploits the properties of space–time modulated reflective metasurfaces to estimate in real-time the impinging angle of an illuminating monochromatic plane wave. The approach makes use of the amplitude unbalance of the received fields at broadside at the frequencies of the two first-order harmonics generated by the interaction between the incident plane wave and the modulated metasurface. Here, we first describe analytically how to generate the desired higher order harmonics in the reflected spectrum and how to realize the breaking of the spatial symmetry of each order harmonic scattering pattern. Then, the 1-D omnidirectional incident angle can be analytically computed using +1st- and −1st-order harmonics. The approach is also extended to 2-D DoA estimation by using two orthogonally arranged 1-D DoA modulation arrays. The accuracy of 1-D DoA estimation is verified through full-wave numerical simulations. Compared to conventional DoA estimation methods, the proposed approach simplifies the computation and hardware complexity, ensuring at the same time estimation accuracy. The proposed method may have potential applications in wireless communications, target recognition, and identification. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) have been often proposed as a means for restoring nonline-of-sight links in wireless communications. Because the path loss changes with the relative distance of the RIS from the transmitter and the receiver and their relative orientations, the quality of the communication is expected to depend strongly on the RIS placement in an RIS-aided link. In this work, the optimal placement of the RIS is studied with respect to both position and orientation and it is shown that even for an ideal RIS, i.e., lossless, polarization preserving, and with a uniform response with respect to the incident angle, the quality of the communication is greatly affected by the RIS orientation. To provide insight into the optimal choice, the analysis is based on an analytical model that treats the RIS as a continuous surface of finite size that steers the incident beam toward a prescribed direction. By imposing a desired minimum threshold on the received power, it is studied how the efficiency of the RIS depends on both its position and orientation. The decisions that have to be made concerning the placement of the RIS are illustrated with examples of D-band indoor scenarios. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surfaces (RISs) are of potential use in wireless communication systems due to their ability to perform beam steering, harmonic wave manipulation, and polarization conversion. However, the main challenge in designing RISs is to obtain their electromagnetic (EM) responses, which is time-consuming and computationally prohibitive. In this work, an efficient modeling method is proposed for fast evaluation of the reflection coefficient and far-field patterns of RISs. The proposed method utilizes the microwave network theory to compute the accurate reflection coefficient of RISs elements under the illumination of incident waves from arbitrary directions. Compared to the classical equivalent circuit model, it enables the separate design of meta-atoms and tunable devices at the network level. The proposed method is verified by the full-wave simulation results of two RIS elements and experimental results of a 3-bit RIS. This work forms a fast and feasible route to obtain the responses of RISs, which allows the RISs to find extensive applications in many scenarios. [ABSTRACT FROM AUTHOR]

We present the design and experimental validation of wideband dual-circularly-polarized (dual-CP) reflect-arrays (RAs) based on dual-functional-layered unit cells with independent control of circular polarizations. The proposed dual-CP RA cells comprise a circular polarization selective surface (CPSS) on the top based on a bandwidth-enhanced dual-meandered Pierrot structure that implements polarization selection of CP waves and a reflective half-wave plate (HWP) on the bottom. The dual-CP phase delay of the RA cells relies on the Berry-phase (BP) dictated by the rotation of the CPSS and HWP elements for achieving reflective beamforming of CP waves of opposite handedness. The diffractive properties of an infinite periodic gradient array of the dual-CP RA cells in a triangular lattice are investigated, which numerically verifies the dual-CP anomalous reflection capability. Two proof-of-concept RAs operating at X-band are designed, fabricated, and characterized. By replacing only the bottom functional layer of the RA, symmetrical and asymmetrically distributed dual-CP beams can be obtained. A measured peak gain of higher than 23.5 dBic is achieved, with a joint bandwidth of 11% for an axial ratio < 2 dB and a gain variation < 1 dB. The improved bandwidth performance, high-resolution BP compensation, and printed manufacturing process without using any metallic cavities make the demonstrated CPSS-based dual-CP RAs promising candidates for wireless and satellite communications. [ABSTRACT FROM AUTHOR]

We propose a polarization-twisting metasurface antenna (PTMA) to generate the dual-band dual-circularly polarized (CP) radiation by integrating a dual-linear-to-circular polarization converter (DLCPC) with a linear polarization conversion metasurface (LPCM) in the Ku-band. The DLCPC possesses the functionality of polarization selection and is capable of transforming the linearly polarized (LP) waves into right hand CP (RHCP) waves in low-frequency (LF) and left hand CP (LHCP) waves in high frequency (HF). Based on the DLCPC, the dual-band dual-CP PTMA is designed and fabricated, and then, the measured results indicate that the PTMA is capable of realizing the RHCP radiation in LF and the LHCP radiation in HF. In the meanwhile, the common bandwidths of −1.5 dB gains and 3 dB axial ratios are 12.25–12.95 and 14.00–14.55 GHz, respectively. Our design, by integrating the DLCPC and LPCM to construct the dual-band dual-CP PTMA, should, thus, pave the way for building up more efficient wireless communication systems with the requirement of different CPs as the receiver and the transmitter at downlink and uplink in the Ku-band. [ABSTRACT FROM AUTHOR]

A novel metamaterial-loaded, miniaturized, broadband, wide-beam dielectric resonator (DR) antenna is presented for low-cost wide-angle beam-scanning applications. The mushroom structure is loaded on the DR structure, so the proposed DR radiator reduces the size reduction (low profile, $0.09\lambda _{0}$) and achieves a broadened bandwidth with a dual-mode response. The realized 3 dB beamwidth of the DR radiator is widened from 90° to 194° by introducing a pair of miniaturized equal-amplitude and out-of-phase Huygens sources. Owing to its compact size and wide beamwidth, a five-element low-cost wide-angle beam-scanning phased array is presented by using the proposed wide-beam DR elements. The measured −10 dB impedance bandwidth covers the 5G-78 band well (3.30–3.80 GHz, 14.1%). The realized scanning angle of the fabricated linear array can be extended from ±36° to ±65°, and its 3 dB beamwidth scanning coverage can up to ±90°. In virtue of miniaturized size, wideband, low cost, and good beam-scanning ability, the presented element, and the resulting wide-angle beam-scanning array are promising candidates for wireless communication and low-cost phased array applications. [ABSTRACT FROM AUTHOR]

A flat-focal-plane dual-metasurface lens (dual-metalens) is presented to lower the scan loss and sidelobe level (SLL) of a metalens antenna. The dual-metalens consists of two metasurfaces with different phase shift distributions and focal ratios. This proposed metalens ensures that the focal points of 0° and 30° incident plane waves are on a flat plane, which lowers the beam scan loss and SLLs by reducing the comatic aberration of the metalens. The measurement shows that the proposed antenna achieves a maximum gain of 26.6 dBi at 27.5 GHz with a 10 $\lambda _{0}$ -diameter aperture, where $\lambda _{0}$ is the wavelength at 28 GHz in free space. The antenna only has a 1.9 dB scan loss with an SLL lower than −14.1 dB at 28 GHz for a scan angle of ±29°. The 3 dB gain frequency range is from 26.8 to 29.3 GHz, which is suitable for the 5G millimeter wave wireless communication. [ABSTRACT FROM AUTHOR]

The active and dynamic controls of the terahertz (THz) waves are highly demanded due to the rapid development of wireless communication systems. Recently, reconfigurable intelligent surface (RIS) has gained significant attention due to their tremendous potential in controlling electromagnetic (EM) waves. Particularly, RIS-based wireless communications are promising for improving the system’s performance by properly designing the reflection coefficient of the unit cell. In this work, we investigate a graphene-based RIS for active and dynamic controls of THz waves. The RIS design consists of a rectangular graphene meta-atoms periodic array placed over a metallic grounded silicon substrate. An equivalent circuit modeling of the RIS design and its solution is provided. The RIS performance is numerically analyzed. The graphene-based RIS achieves nearly 100% reflection in the operational frequency ranging from 0.1 to 4 THz. The perfect reflection is insensitive to the polarization and the incident angles. Moreover, 100% absorption in the graphene-based RIS is achieved by electrically reconfiguring the meta-atom response via the chemical potential of the graphene. The graphene RIS also achieves the anomalous reflection performance by controlling the number of unit cells and phase gradient of RIS. In view of the effective THz wireless communication environment, this article finally presents a simple RIS-aided communication model to study the impact of the proposed graphene-based RIS on the signal-to-noise ratio (SNR). The results reveal that the proposed THz RIS with graphene meta-atoms is promising for efficient and intelligent THz wireless communications. [ABSTRACT FROM AUTHOR]

So far, the environment has been considered as a source of fading, clutter, and blockage, with detrimental consequences for efficiency and robustness of communication systems. However, the intense research developed toward beyond-5G communications is leading to a paradigm change, in which the environment is exploited as a new degree of freedom and plays an active role in achieving unprecedented system performances. For implementing this challenging paradigm, the use of intelligent surfaces able to control the propagation of electromagnetic (EM) waves has been recently proposed. In this framework, metasurfaces have emerged as a promising solution, thanks to their field manipulation capabilities achieved through low-cost, lightweight, and planar structures. The aim of this article is to review some recent applications of metasurfaces and cast them in the scenario of next-generation wireless systems. In particular, we show their potentialities in overcoming some detrimental effects presented by the environment in wireless communications and discuss their crucial role toward practical implementation of a smart EM environment. [ABSTRACT FROM AUTHOR]

Recently, reconfigurable intelligent surface (RIS) has attracted continuous attention in wireless communications. Because the normal incidence condition of electromagnetic waves cannot always be satisfied in practical applications, the angular insensitivity in RISs has become an important issue. The angular sensitivity in RISs may lead to the failure of RIS-assisted wireless communication networks that rely on the reciprocity of wireless channels. In this work, an angle-insensitive 3-bit RIS meta-atom is proposed. By introducing metallic vias in the substrate of programmable meta-atoms, the interference induced by multiple reflections in the multilayered structure is restrained, and the decoupling between adjacent meta-atoms makes the metasurface insensitive to the incident angle. Hence the angular insensitivity can be achieved. An RIS is fabricated using the proposed angle-insensitive programmable meta-atom with metallic vias, which is capable of maintaining stable phase and amplitude responses at oblique incidences. Both simulated and measured results show stable angle performance. A maximum of 315° phase range and eight digital coding states with 45° stable interval are obtained for wide incidence angles from 0° to 60°. Finally, numerical simulations and experimental results of anomalous reflections are performed to verify the angular reciprocity of the proposed RIS, which is important in wireless communications. [ABSTRACT FROM AUTHOR]

A wideband microstrip antenna with a low profile is reported for green civilian wireless communication. This design employs 12 solar cells for photovoltaic generation and mobile communications. The solar-cell layout is suitable for practical applications. Based on this deployment of solar cells, the radiation structure integrated with solar cells is formulated. With the dual-aperture coupling method, a broadband performance of 54.8% has been achieved with a low profile of $0.068\lambda _{0}$ , where $\lambda _{0}$ is the free space wavelength at the center frequency of 2.3 GHz. More than 60% of the total area is covered by solar cells, so its ability for photovoltaic generation is improved effectively. The proposed dual-function device can offer an enhanced ability of photovoltaic generation and broadband performance with a low-profile structure. [ABSTRACT FROM AUTHOR]

In this study, an ultraminiaturized implantable antenna is developed for biotelemetry applications. It has triple-band characteristics in the industrial, scientific, and medical (ISM) bands of 902–928 MHz for data telemetry and 2400–2483.5 MHz for power-saving applications and midfield band (1824–1980 MHz) for wireless power transfer simultaneously. The size of the proposed antenna is 0.068 $\lambda _{g}\,\,\times0.048\,\,\lambda _{g}\,\,\times0.0036\,\,\lambda _{g}$ and it is the smallest size of any triple-band implantable antenna ever devised without using shortening vias. Such a compact size is achieved by meandering the shape of the radiating patch and adding open-ended ground slots. Unlike most miniaturized implantable antennas reported in the available literature, no shortening pin is used and this makes the fabrication of the antenna very easy. The bandwidths measured in minced pork muscles are 14% and 24% in the lower and upper ISM bands, respectively, and 12% in the midfield band. The maximum values of the realized gain of the proposed antenna obtained in the heterogeneous skin phantom at 915, 1900, and 2450 MHz are −27.2, −22.2, and −19.9 dBi, respectively. In addition, specific absorption rates (SARs) in the heterogeneous skin and muscle tissues at the three resonance frequencies are calculated and show compliance with the IEEE standards for ensuring human safety. Finally, the range of the biotelemetry link is evaluated by calculating the wireless communication link budget at data rates of 7 kb/s, 100 kb/s, and 10 Mb/s at resonance frequencies. [ABSTRACT FROM AUTHOR]

In recent years, there are increasing interests and applications on radar–communication integration for system miniaturization, cost reduction, and so on. Moreover, thanks to the common functions of hardware components including transceiver, antennas, and even basic signal/data processing, the integrated design of radar and communication systems becomes feasible. In this article, through design of proper time sequences, waveforms, and recognition algorithms, radar sensing and wireless communication can be integrated into 4-D antenna arrays simultaneously. In addition, with the high design degrees of freedom of 4-D arrays, the proposed approach can also provide a low probability of intercept capability. Selected numerical and experimental results are provided to validate the effectiveness of the proposed approach. [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 novel low-profile vertically polarized planar endfire smart antenna that can achieve 360° beam-steering and high beam-shaping capability is proposed. The microstrip antenna is printed on a single thin substrate with a vertically polarized omnidirectional driven source and 36 tunable slot-integrated microstrip vertically polarized resonators (SMVR) for shaping the endfire beams. To verify the design approach, a prototype operating at the 5 GHz WLAN band is fabricated and tested. The results show that the antenna operates from 5.55 to 5.75 GHz and is capable of scanning a single endfire beam from 0° to 360° in the full azimuth plane. The scanning resolution and the maximum realized gain are 15° and 8.2 dBi, respectively. Moreover, multiple radiation pattern shapes including omnidirectional beam, dual-beams, and tri-beams can also be generated by the antenna. The overall antenna height is only $0.06~\lambda $ at 5.6 GHz. This design provides a new antenna configuration for low-profile vertically polarized endfire beam-steering and beam-shaping antenna and is promising for future wireless communication systems with lost-cost beamforming property. [ABSTRACT FROM AUTHOR]

The Internet-of-Things (IoT) applications require orientation-insensitive wireless devices to maintain stable and reliable communication. For those reasons, antennas providing a wide quasi-isotropic and circular polarization (CP) coverage are very attractive. However, achieving wide quasi-isotropic and CP coverage simultaneously is challenging. In this work, we show that properly designed sloped dipoles on a 3-D structure can maximize the CP coverage (theoretically up to 100%) even with equal-phased feed to the dipole elements. We derive the conditions and present the design graphs for the optimum slope angle for the dipole elements on a 3-D hexagonal-shaped package to achieve a wide quasi-isotropic and CP coverage simultaneously. Based on the proposed theory, a practical antenna has been designed and fabricated using additive manufacturing. The measured results demonstrate a 7 dB isotropy of 92% and a CP coverage of 85%, which matches well with the predicted results from the theoretical analysis and full-wave simulations. [ABSTRACT FROM AUTHOR]

A novel high-efficiency polarization-selective bifunctional metasurface (MS) is proposed, which can realize cross-polarized focusing transmission for left-handed circular polarization (LHCP) incidence and co-polarized specular reflection for right-handed circular polarization (RHCP) incidence. As an example of its practical application, the designed MS is applied to form an RHCP folded transmitarray antenna (FTA) whose feed source (FS) is integrated with the bottom metallic surface. To prove the concept, a prototype of millimeter-wave (MMW) FTA based on the proposed MS is numerically simulated, physically implemented, and experimentally measured. The measured results coincide well with the simulated ones, showing a peak gain of 23.6 dBi at 27.7 GHz with the peak aperture efficiency of 43.1%, a 3 dB gain bandwidth of 15.8% (25–29.3 GHz), and a 3 dB axial-ratio (AR) bandwidth over 29.6%. The proposed high-performance antenna has the advantages of low profile, high efficiency, integration design, and broad bandwidth, which may have promising applications in the fifth-generation MMW communication system. The proposed MS and its application to high-gain antenna may promote the uses of MS to real-world applications in satellite communication, radar detection, and wireless communication. [ABSTRACT FROM AUTHOR]

The concept of dual-polarized reconfigurable reflectarray (RRA) featuring dual-polarized beam control is proposed and demonstrated using a dual-channel programmable metasurface. The p-i-n diodes are incorporated into each metasurface element as the active components to provide independent phase control in two orthogonal polarization channels. By altering the working states of each p-i-n diode, well-defined beam scanning can be obtained. Element design, focal diameter ratio selection, and discretized phase distributions are carefully optimized to ensure stable radiation performance of the proposed RRA. As verification, a prototype consisting of 20 $\times20$ elements is fabricated and measured, which is controlled by a field programmable gate array-based controller. Measured results show that wide-angle beam scanning up to ±40° can be obtained around 7.45 GHz, with peak gains of 21.13 and 20.89 dBi in dual channels, corresponding to aperture efficiencies of 16.35% and 15.47%. The proposed dual-polarized RRA brings a new freedom in polarization manipulations for versatile beam scanning, which provides promising applications in wireless communication and tracking/detecting systems. [ABSTRACT FROM AUTHOR]

The magnetic induction (MI) technique enables wireless communications in extreme environments. However, the existing MI systems rely on large antennas and high power, which is not suitable for Internet of Things (IoT) and other mobile devices. Although the new metamaterial-enhanced MI ($\text{M}^{2}\text{I}$) technique can theoretically enhance the MI signal by 30 dB, none of the existing real-world systems achieve the performance due to: 1) the inherent metamaterial loss is significant; 2) the complicated metamaterial structure is not optimized; and 3) an $\text{M}^{2}\text{I}$ system with fixed parameters cannot keep the optimal performance in dynamic environments. In this article, an active and reconfigurable meta-sphere is designed, optimized, and implemented to approach the theoretical $\text{M}^{2}\text{I}$ enhancement bound. Specifically, the new $\text{M}^{2}\text{I}$ meta-sphere is realized by a spherical array of active and reconfigurable coil units. Given fixed meta-sphere size and power constraints, there exist complicated trade-offs among coil number, coil size, and the power consumption. Based on the theoretical design, the prototype of the active and reconfigurable $\text{M}^{2}\text{I}$ meta-sphere system is implemented and tested. Significant power gain and range extension are proved through comprehensive analytical deduction, COMSOL Multiphysics-based simulations, and real-world experiments. [ABSTRACT FROM AUTHOR]

Low-cost beam coverage is one of the main goals of indoor wireless communication. We propose a low-cost beam steering (BS) antenna by using a hemispherical active frequency selective surface (AFSS) and its embedded varactors. At the same time, in the 3-D spherical coordinate system, we propose a universal design method for this type of antenna based on the principle of phase compensation. The designed BS antenna can realize energy gathering and beam switching in the quasi-full airspace on the ground. BS without phase shifter and reasonable allocation of energy brings the advantages of low cost. The proposed approach provides a feasible scheme for improving the robustness and energy saving of wireless communication. [ABSTRACT FROM AUTHOR]

Frequency-multiplexing metasurfaces are typically attributed to the integration of multiple resonators, where mode coupling may significantly affect their performances. In this communication, we develop a dispersion-assisted dual-phase hybrid strategy for dual-frequency independent control of amplitude and phase with a single resonance. Therein, the frequency and phase decoupling can be achieved by precise dispersion control between two frequency states, while the amplitude distribution of each meta-atom can be controlled by properly allocating the energy of copolarized and cross-polarized waves. As a proof of concept, a bi-functional meta-mirror shaping multifocal points at two different frequencies have been experimentally demonstrated. The measured results are in good agreement with simulated results, showing the diffraction efficiencies (DEs) of 38% and 45%, and a signal-to-noise ratio (SNR) of 8.36 and 10.35 at $f_{1}$ and $f_{2}$ , respectively. In addition, another three meta-mirrors with tailorable amplitudes at dual frequencies are designed to further verify our strategy. This method offers an alternative platform for designing high-performance devices with dual-frequency wavefront manipulations, which may find potential applications in microwave integrated systems and wireless communication systems. [ABSTRACT FROM AUTHOR]

In this article, a single-sheet metasurface (MS) for independently manipulating the transmissive phase and amplitude of linearly polarized electromagnetic (EM) wave is proposed. The amplitude modulation is achieved via imposing a rotation angle to flexibly control the ratio of polarization conversion, while the phase modulation is realized by tuning the physical parameters of the unit cell to change its resonance. By integrating the proposed single-sheet MS, a low-profile metalens antenna operating around 12.2 GHz is designed, fabricated, and experimentally validated. In particular, spatially varying amplitude on the MS aperture is designed to meet the Taylor distribution, thus reducing the sidelobe level (SLL) of the metalens antenna. In addition, special MS elements rotation strategy is performed to maintain good polarization isolation-level (PIL) performance. The measured results show that the proposed metalens antenna achieves the peak gain of 25.3 dBi at 12.6 GHz with 3 dB gain bandwidth of 16.4% (11.2–13.2 GHz). Within the 3 dB gain bandwidth, this metalens antenna achieves SLL lower than −20.5 dB with an average value of −23 dB and PIL lower than −22.2 dB with an average value of −26.5 dB. Compared with metalens antenna with phase-only modulations, the proposed metalens antenna realizes a 5.7 dB average SLL suppression. The proposed metalens antenna is a promising candidate in point-to-point communications for wireless and satellite system applications. [ABSTRACT FROM AUTHOR]

The rapid development of artificial electromagnetic (EM) materials provides unprecedented freedom to manipulate EM waves. In recent years, reflective spectrum conversion technology based on EM materials has been widely studied due to its multiharmonic generation capability. The possibility of its application in wireless communication is explored. However, the research on spectrum conversion is mainly based on periodic modulation waveforms. The modulation effect of nonperiodic for EM materials has not been studied so far. In this article, a nonperiodic time-varying active frequency selective surface (AFSS) is proposed to manipulate the spectral distribution of harmonics. By dynamically controlling the amplitude of the reflected echo, a continuous Doppler spectrum is generated. On this basis, the imaging characteristics of nonperiodic time-varying AFSS are analyzed in detail, and the defocusing phenomenon of the target image is discovered. Finally, the radar imaging experiments for nonperiodic time-varying AFSS are conducted to verify the validity of the proposed method. [ABSTRACT FROM AUTHOR]

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space-/frequency-domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first-order harmonic can be obtained. While, in other directions, the opposite first-order harmonic can be easily controlled by changing the dimension of I/Q paths, and the objective first-order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first-order harmonic can be used for beam scanning by predesigning the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2 bit unit cells loaded with dynamically switchable pin diodes are used as I/Q modulation. Both the analytical and numerical results demonstrate that space- and frequency-domain manipulations of the reflected fields by the first-order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking. [ABSTRACT FROM AUTHOR]

The era of wireless communications began at the turn of the 20th century, when Guglielmo Marconi used electromagnetic waves to transmit telegraph signals from ships to stations onshore. To understand how radio signals propagate is critical for wireless communication system design. With the development of wireless communications, much effort has been made to characterize radio propagation in different frequency bands and physical environments. Radio propagation and wireless channel modeling are essential for communication system simulation, channel emulator design, wireless system planning and optimization, and the development of regulations and standards in wireless communications ,. [ABSTRACT FROM AUTHOR]

A hemispherical Luneburg lens (LL) with the focal plane near lens surface is proposed using a transformation optics (TO)-based metamaterial (MTM) lens (metalens) concept for wide-angle beamsteering applications. The TO technique has long been implemented to reduce the longitudinal dimension of an LL. However, the transformed lenses, usually having a long focal length, suffer from limited volume reduction, severe reflection, narrow beamscanning range (BSR), and low capture efficiency. The proposed TO mapping in this article, engendered by transforming the upper and lower parts of the original LL separately, preserves the original beamsteering property of an LL in half a space with the focus retained at its flat surface before exerting any index approximations. The effects of the approximations and the discretization on the performance of the transformed LL are then investigated and analyzed comprehensively. After that, a hemispherical lens with a radius of 60 mm is exemplified at X-band and fabricated using a ten-layered nonresonant gradient index MTM. The designed TO-based LL is fed by a stacked patch antenna linearly shifted at a distance of 4 mm (a focal-to-diameter ratio of 0.03) beneath the lens to achieve a wide beamsteering range of ±41° with a maximum gain of 15.7 dBi and a gain tolerance of 1.7 dB. The proposed TO-based metalens antenna features compactness with a near-surface focal plane, wide BSR, and easy fabrication, which shows potential applications in far-field imaging, radar, and 5G wireless communication. [ABSTRACT FROM AUTHOR]

This communication presents a novel single-layer, low-profile broadband metasurface (MTS)-inspired antenna with monopole-like radiation characteristics. To excite symmetric surface waves (SWs) propagating along an MTS consisting of an array of $4\times4$ metal square patches, a horizontal center-fed monopole structure is used at the center of the MTS. The proposed antenna is further embedded within a square ring to improve the gain and H-plane patterns roundness at the upper-frequency band. The presented antenna achieved a −10 dB bandwidth of 32.62% (5.08–7.06 GHz) and a low profile of $0.06\lambda _{0}$ ($\lambda _{0}$ is the free space wavelength at the center of the operating band) distinctive features due, respectively, to the effective excitation of different TM SW resonant modes that are close to each other and the in-phase reflection property of the MTS. The planar structure, single-layer, and low-profile merits make the proposed antenna an ideal candidate for various wireless communications systems, such as wireless local area network (WLAN) at 5.2/5.8 GHz, WiMAX at 5.5 GHz, and car to car (C2C) at 5.85–5.925 GHz. [ABSTRACT FROM AUTHOR]

Since wireless communication technology has been widely applied in our daily lives, wireless devices exhibit explosive growth. A reliable, efficient, and repeatable over-the-air (OTA) testing method is vital for the research and development of wireless devices. Due to the particular advantages of the reverberation chamber (RC), it has become a popular facility to perform OTA tests nowadays. In the RC-based OTA tests, loading the RC with radio frequency (RF) absorbers is a standardized procedure to control the frequency selectivity of the RC channel according to the cellular telecommunications industry association (CTIA). However, this procedure involves iterative manual calibrations and, therefore, is tedious and time-consuming. Moreover, it may be difficult to tune the coherence bandwidth (CBW) to a desired value accurately given fixed RF absorbers. In this work, a locally switchable metamaterial absorber (MA) is proposed for fine-tuning of the CBW of the channel in the RC. It is experimentally verified that the CBW can be easily controlled by switching on and off of the absorbing/reflection units of the MA. The proposed MA can provide fine-tuning of the CBW in an efficient and convenient manner. [ABSTRACT FROM AUTHOR]

System design and deployment of millimeter-wave (mmWave) wireless communication systems for outdoor coverage need sophisticated channel models to describe the wave interaction with illuminated physical objects. However, the conventional physical–statistical model cannot accurately predict the site-specific mmWave channel characteristics when involving complex system configurations and geometric information of transceiver relative locations. A framework of machine learning (ML) assisted channel modeling approach is proposed, in which the statistical models are leveraged for inter-cluster-level channel characterization and the propagation properties within each kind of clusters are predicted using a hybrid physics-based and data-driven approach. In particular, with a focus on the mmWave through-vegetation-scattering effect, a set of dedicated directional channel measurements and ray-tracing simulations is performed in an identical vegetated street canyon environment at 28 GHz for the performance evaluation of the proposed approach. Moreover, the training results and model validation in different environments show that comparing with the physical–statistical model, the proposed hybrid model, which adds the environment features to the artificial neural network (ANN) as inputs, has higher prediction accuracy and greater generalization ability in terms of the site-specific through-vegetation cluster parameters, such as vegetation attenuation, delay spread (DS), and angular spread. [ABSTRACT FROM AUTHOR]

Due to the unique advantages of long-distance, non-relay, and flexible deployment, high frequency (HF) communication plays a vital role in military communication, disaster relief, and global broadcasting, etc. To satisfying the spectrum planning requirement for the next generation intelligent HF communication system, the machine learning method is introduced to develop the long-term prediction method of the usable working frequency for HF wireless communication. Especially, the refined mapping model of maximum usable frequency (MUF) propagation factor for one hop on the F2 layer is first reconstructed by using the statistical machine learning method. Then, the new mapping models of conversion factors of optimum working frequency (OWF) and the highest probable frequency (HPF) are proposed by using the fine-grained solar activity parameters and coupling with two geomagnetic activity parameters. This proposed model has higher prediction accuracy for the MUF, OWF, and HPF over Asia. Compared with the International Telecommunication Union (ITU) recommended model, the root-mean-square errors of MUF, OWF, and HPF are reduced by 1.18, 1.64, and 1.06 MHz, and the accuracies are improved by 10.89%, 15.47%, and 9.10%, respectively. The proposed model can achieve intelligent frequency planning for HF communication and has great potential in terms of improving HF communication quality, reliability, and efficiency. [ABSTRACT FROM AUTHOR]

We give an overview of recent developments in the modeling of radiowave propagation, based on machine learning (ML) algorithms. We identify the input and output specification and the architecture of the model as the main challenges associated with ML-driven propagation models. Relevant papers are discussed and categorized based on their approach to each of these challenges. Emphasis is given on presenting the prospects and open problems in this promising and rapidly evolving area. [ABSTRACT FROM AUTHOR]

We demonstrate an efficient back-fire to forward wide-angle beam steering leaky-wave antenna (LWA) based on a large dispersion gradient planar spoof surface plasmon polariton (SSPP) transmission line (TL) with split-ring units. To convert the highly confined SSPP waveguiding mode to a radiation mode, two arrays of metallic patches are periodically arranged on both sides of the SSPP TL. The patches are designed as two combined half-elliptical shapes to mitigate the open stopband effect and enhance the radiation performance. We propose a theoretical method to interpret the mechanism and predict the directional pattern of the LWA. To verify the proposed method, the SSPP TL and LWA prototypes are fabricated and measured. The calculation, simulation, and experimental results match well and show that the main beam of the LWA can steer a wide range of 112° from back-fire (−90°) to the forward quadrant of 22° passing through the broadside in 6.3–11 GHz, demonstrating a large scanning rate of 2.07°/%. Furthermore, the LWA also possesses the advantages of low profile, high average gain (12 dBi), high average efficiency (95%), and low sidelobe level (<−13 dB). This work provides an alternative route to achieving wide-angle LWAs for promising microwave radar and wireless communication applications. [ABSTRACT FROM AUTHOR]

Electromagnetic signals in the ultralow frequency (ULF) range below 3 kHz are well suited for underwater and underground wireless communication thanks to low signal attenuation and high penetration depth. However, it is challenging to design ULF transmitters that are simultaneously compact and energy efficient using traditional approaches, e.g., using coils or dipole antennas. Recent works have considered magneto-mechanical alternatives, in which ULF magnetic fields are generated using the motion of permanent magnets, since they enable extremely compact ULF transmitters that can operate with low energy consumption and are suitable for human-portable applications. Here we explore the design and operating principles of resonant magneto-mechanical transmitters (MMT) that operate over frequencies spanning a few 10 s of Hz up to 1 kHz. We experimentally demonstrate two types of MMT designs using both single-rotor and multirotor architectures. We study the nonlinear electro-mechanical dynamics of MMTs using point dipole approximation and magneto-static simulations. We further experimentally explore techniques to control the operation frequency and demonstrate amplitude modulation up to 10 bits-per-second. We additionally demonstrate how using oppositely polarized MMT modules can permit systems that have low dc-field but do not sacrifice the ac magnetic field produced. [ABSTRACT FROM AUTHOR]

An axial mode solid dielectric helical antenna (DHA) has been proposed, which is fabricated by using the 3-D printing technique. Fed manners of this antenna have been studied, including conformal coaxial probe and conformal metal strip line. The operating mode of dielectric helix has been analyzed in detail. Circular polarization is achieved by choosing appropriate sizes of the dielectric helix. Furthermore, a miniaturized axial mode DHA with deployable property is implemented. Measured results indicate that the proposed antenna has good radiation performance, such as circular polarization, high gain, and high radiation efficiency. Specially, the radiation patterns of the DHA have good symmetry in the 2 dB axial ratio range. The significant advantage of miniaturized DHA is light in weight, which is only 2.7% of the weight of a metal helical antenna with similar radiation characteristics. The antenna is a good candidate in wireless communication system. [ABSTRACT FROM AUTHOR]

We demonstrate a new approach to implementing directional modulation that leverages distributed array dynamics to change the antenna array pattern over time. With appreciable spacing between the elements, the spatial phase pattern varies rapidly, such that small physical motions result in large phase variations. We define an average radiation pattern that characterizes the space–time modulation of the array pattern and calculate a spatial information filter based on the additional modulation imparted on transmitted data. The concept is demonstrated experimentally using a two-element 1.5 GHz transmit array with relative motion between the elements. The array consists of two nodes, each with one dipole antenna transmitting BPSK data, and each with two log-periodic antennas used to estimate the relative separation of the nodes and correct the beamsteering phase of the transmitters. The node separation is varied dynamically over a range of $0.98\lambda $ – $4.7\lambda $. We investigate the use of linearly and sinusoidally varying internode dynamics, and demonstrate a low bit-error-ratio of approximately 10−5 at the desired transmission direction, indicating high throughput, and a high error ratio of approximately 10−0.3 at angles outside of the mainbeam, indicating corrupted data, at a signal-to-noise ratio of 12 dB. [ABSTRACT FROM AUTHOR]

Polarization reconfigurable antennas at millimeter-wave (mm-wave) frequencies are important technologies for 5G and beyond wireless communications systems. This article presents an mm-wave multi-linear polarization (LP) reconfigurable antenna. It employs a resonant TM510-mode air-filled cavity as the base, and a circular array of slots is uniformly etched on its top surface to provide a rotationally symmetrical “excitation” for the multi-LP antenna. An upper substrate printed with engineered patch layers is introduced above the slots for an in-phase high-gain composition, and it is mechanically rotated to facilitate the multi-LP reconfiguration. Each slot radiation is separately monitored and analyzed by using single-slot cavity models. A final multi-LP antenna that can switch among five LP states at mm-wave band was constructed. To overcome the classic high sidelobe problem associated with circular arrays, a modified bottom layer is introduced to the multi-LP antenna. The measured overlapped bandwidth of the optimized multi-LP antenna ranges from 28.58 to 28.99 GHz. For all of the five LP states, the simulated and measured sidelobe levels are −13 and −10 dB, respectively. The measured realized gains are varied from 15.5 to 16.8 dBi in the overlapped operating bandwidth. This is the first time to report an mm-wave multi-LP reconfigurable antenna. [ABSTRACT FROM AUTHOR]

Simple and accurate expressions for path gain are derived from electromagnetic fundamentals in a wide variety of common environments, including line-of-sight (LOS) and non-line-of-sight (NLOS) indoor, urban canyons, urban/rural macro, outdoor–indoor, and suburban streets with vegetation. Penetration into a scattering region, sometimes aided by guiding, is the “universal” phenomenon shared by the diverse morphologies. Root mean square (rms) errors against extensive measurements are under 5 dB, better than 3GPP models by 1–12 dB rms, depending on the environment. In urban canyons the models have 4.7 dB rms error, as compared to 7.9 dB from linear fit to data and 13.9/17.2 dB from LOS/NLOS 3GPP models. The theoretical path gains depend on distance as a power law with exponents from a small set {1.5, 2, 2.5, and 4}, specific to each morphology. This provides a theoretical justification for widely used power-law empirical models. Only coarse environmental data are needed as parameters: street width, building height, vegetation depth, wall material, and antenna heights. [ABSTRACT FROM AUTHOR]

In this work, a wideband low-sidelobe slot array antenna for wireless communication system at E-band is presented. The slots are excited by a parallel feeding network where the Taylor distribution is applied. Two types of wideband unequal power divider with compact structures are designed for various output ratios to fit the limited feeding network space. One is integrated vertically inside a 2 $\times $ 1 elements subarray using a set of pins with asymmetrical offset. The other type is designed inside an E-plane T-junction with unbalance impedance at two output ports. To avoid main lobe tilt and asymmetrical radiation patterns, the feeding network is designed to be biaxially symmetrical, and the feed port is placed in the center of the back. A prototype is fabricated with brass and measured in an anechoic chamber. It achieves a peak gain of 25.8 dBi and a fractional impedance bandwidth of 19.2% with the reflection coefficient below −10 dB. The measured radiation patterns agree with the simulated patterns very well, and the sidelobe level (SLL) is lower than −21 dB. [ABSTRACT FROM AUTHOR]

This article proposes a generalized three-dimensional (3-D) geometry-based stochastic model (GBSM) channel model with an arbitrary center point of the scatterer distribution. In the proposed channel model, scatterers are assumed to be Gaussian distributed about an arbitrary point within a spheroidal shell having the transmitter and the receiver located at its focal points. Exact closed-form expressions are obtained for the joint probability density function (PDF) of the angles of arrival (AoAs) and time of arrival (ToA), and the marginal PDF of ToA. Moreover, approximate closed-form expressions are derived for the marginal PDFs of AoAs in the elevation and azimuth planes. The accuracy of the analytically derived AoA and ToA PDFs are verified by comparing with the PDFs obtained using computer and ray-tracing simulation. Furthermore, we study the characteristics of the analytically-derived AoA and ToA PDFs for different propagation scenarios. By choosing the center point and standard deviation of the scatterer distribution appropriately, our proposed 3-D channel model can be used to evaluate the performance of wireless communication technologies and systems in indoor environments. [ABSTRACT FROM AUTHOR]

This article presents a wideband dual-circular-polarization (dual-CP) antenna with high port isolation based on a grooved-wall CP horn combined with an orthomode transducer (OMT) in the W-band. A pair of grooves on the inner wall of the horn serves as an inbuilt polarizer within the horn antenna, which is capable of generating both left-hand circular polarization (LHCP) and right-hand circular polarization (RHCP) without any septum. The continuous linearly tapered groove design also allows this multisection grooved-wall CP horn to achieve low reflection coefficients over a wide bandwidth, thereby realizing wideband dual-CP with high port isolation when it is used together with a wideband OMT. A prototype of the proposed dual-CP antenna is designed in simulation and measured in an experiment. The simulated and measured results agree very well, showing that port isolation >32.5 dB over 31.6% bandwidth from 80 to 110 GHz is achieved with axial ratio (AR) < 2.8 dB and reflection coefficient $< -20$ dB for both LHCP and RHCP. Rotationally symmetric radiation patterns over the entire operating bandwidth are also observed with a measured gain of 19.6 ± 2 dBic. [ABSTRACT FROM AUTHOR]

This article presents a design of a compact wideband filtering dipole antenna with an omnidirectional radiation pattern. We analyze the radiation of three kinds of half-wavelength dipoles (strip, U-shaped strip, and folded strip). It is found that these three diploes produce omnidirectional radiation patterns when they work on half-wavelength mode. In the proposed design, we make these three dipoles couple with each other to achieve wide bandwidth. In addition, both electric coupling and magnetic coupling are observed at the lower band and the upper band, respectively. Therefore, two radiation nulls can be achieved without any extra circuit, which makes the proposed antenna filtering property and compact. A prototype is fabricated and measured to verify our idea. The results show that the proposed antenna has a 10 dB bandwidth of 42.51%, an in-band gain of ~2.4 dBi, and an out-of-band radiation suppression of more than 10 dB with a total size of ${0.4}{ {\lambda }}_{{0}}\,\,{\times }\,\, 0.06{ {\lambda }}_{{0}}$. The above advantages of wide bandwidth, filtering property, small size, and easy fabrication make the proposed antenna have potential applications on various wireless communication systems. [ABSTRACT FROM AUTHOR]

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]

Neuroscience research in nonhuman primates (NHPs) often requires multiday neural recordings from freely moving animals inside their home cages, making ultralow-power uplinks using wireless backscatter communication highly desirable. Previous work reveals that the channel transfer function (CTF) of a standard NHP home cage in the 915 MHz and 2.4 GHz industrial, scientific, and medical (ISM) bands resembles a resonant cavity exhibiting deep nulls throughout the cage volume, which are particularly acute for round-trip backscatter paths. In this work, we investigate a novel application of passive antenna mode stirring via switched parasitic antennas (SPAs) to reduce the magnitude and prevalence of deep nulls in the cage CTF. We present a system leveraging four cage-ceiling-mounted SPAs with two dynamically controlled impedance states each, yielding 16 total mode stirring configurations. We compare the frequency-domain power ratio measurements at 126 positions throughout the cage taken with and without passive antenna mode stirring. In the 915 MHz ISM band, the optimized SPA configuration improved the maximum two-way insertion loss in 68% of testing locations, reducing the worst case two-way insertion loss by 60.2 dB. In the 2.4 GHz ISM band, the maximum two-way insertion loss was improved in 53% of testing locations, reducing the worst case two-way insertion loss by 35.6 dB. This approach eliminates the deepest nulls in the cage volume and leads to significantly improved link margin for a backscatter-based wireless brain–computer interface (BCI). [ABSTRACT FROM AUTHOR]