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Abstract The authors presents a low‐profile wideband circularly polarised (CP) millimetre‐wave (mmWave) elliptical integrated lens antenna (ILA) fabricated by dielectric 3D printing technology. The proposed lens is integrated with a dielectric polariser and an air cavity for phase compensation, which can transfer incident waves from linearly polarised (LP) to planar CP waves over a wide bandwidth. Compared to the classic bifocal ILA, the proposed lens has a low profile. The phase compensation is performed by introducing a specifically shaped air cavity, which greatly reduces the length and weight of the lens. The polariser causes the two electric field components Ex and Ey to degenerate with an orthogonal phase difference, which results in the wideband characteristics of the antenna. The polariser, air cavity and lens can be conveniently and precisely fabricated by low‐cost 3D printing technology. Measurements show that the proposed ILA has a wide axial ratio (AR) bandwidth of 21.9–32.6 GHz, while the gain reaches 22.54 dBic with a 38% size reduction and a 60.7% weight reduction. Benefiting from the low profile, high gain, wide bandwidth and low cost, the proposed ILA has great potential to be the candidate antenna for 5G applications and satellite communications.

The authors presents a low‐profile wideband circularly polarised (CP) millimetre‐wave (mmWave) elliptical integrated lens antenna (ILA) fabricated by dielectric 3D printing technology. The proposed lens is integrated with a dielectric polariser and an air cavity for phase compensation, which can transfer incident waves from linearly polarised (LP) to planar CP waves over a wide bandwidth. Compared to the classic bifocal ILA, the proposed lens has a low profile. The phase compensation is performed by introducing a specifically shaped air cavity, which greatly reduces the length and weight of the lens. The polariser causes the two electric field components Ex and Ey to degenerate with an orthogonal phase difference, which results in the wideband characteristics of the antenna. The polariser, air cavity and lens can be conveniently and precisely fabricated by low‐cost 3D printing technology. Measurements show that the proposed ILA has a wide axial ratio (AR) bandwidth of 21.9–32.6 GHz, while the gain reaches 22.54 dBic with a 38% size reduction and a 60.7% weight reduction. Benefiting from the low profile, high gain, wide bandwidth and low cost, the proposed ILA has great potential to be the candidate antenna for 5G applications and satellite communications. [ABSTRACT FROM AUTHOR]

Abstract This article uses the ray inserting method to design a low‐cost inhomogeneous lens in the Ku‐band. The focal length to the diametre ratio of the designed lens is 0.45, which is lower than most designed lenses in this area. Also, no approximation is considered to yield the refractive index profile. The refractive index changes from the centre to the sides have been made in two sections to limit the maximum calculated refractive index of the structure to the Plexiglass refractive index. The designed lens proposed a good match to the source and surroundings. The theoretical design is validated by the finite difference time domain scheme and full‐wave simulation. The lens structure is realised and fabricated on a multilayer Plexiglass sheet. The results of simulations and experiments indicate good performances of the designed lens in the wide frequency bandwidth.

This article uses the ray inserting method to design a low-cost inhomogeneous lens in the Ku-band. The focal length to the diametre ratio of the designed lens is 0.45, which is lower than most designed lenses in this area. Also, no approximation is considered to yield the refractive index profile. The refractive index changes from the centre to the sides have been made in two sections to limit the maximum calculated refractive index of the structure to the Plexiglass refractive index. The designed lens proposed a good match to the source and surroundings. The theoretical design is validated by the finite difference time domain scheme and full-wave simulation. The lens structure is realised and fabricated on a multilayer Plexiglass sheet. The results of simulations and experiments indicate good performances of the designed lens in the wide frequency bandwidth. [ABSTRACT FROM AUTHOR]

It is demonstrated that combining phased array antennas with dielectric lenses constitutes a suitable solution to increase the energy efficiency in next‐generation communication systems. A dielectric lens is designed to increase the gain of an array antenna working in the 3GPP n257 band (26.5–29.5 GHz). The simulated results demonstrate that the dielectric lens increases the gain between 0.4 and 1.3 dB in a large scanning range from 0° to 60° at 28 GHz; meaning that the output RF power can be decreased by up to 26% for a same equivalent isotropic radiated power required from the base station. [ABSTRACT FROM AUTHOR]

Abstract In millimetre wave (mmWave) frequency band, radio resource management (RRM) performance is characterised in plane wave conditions using radiated over‐the‐air test method, which is a challenging combination of protocol and radio frequency performance measurement. This article presents a novel design of multiple lens‐based compact antenna test ranges (CATRs) to simulate multiple angles of arrival from different base‐stations in RRM measurement. Only four lens‐based CATRs are proposed to create plane wave conditions at the device under test with six different angles of arrival and capable of accurately constructing the wireless propagation condition and minimising the diffraction and scattering between adjacent CATRs. The lens‐based CATR is the key element in the RRM measurement system, which is composed of a metasurface lens for phase alignment and a 2 × 2 feed array antenna for the emission of electromagnetic signals. The function and capability of the proposed lens‐based CATR is validated through simulation and measurement, that the worst‐case amplitude variation in the quiet zone region is lower than ±0.8 dB at 26 GHz, and phase variation is lower than ±7.5° in the designed quiet zone, which could be used in the fifth‐generation mmWave RRM measurement.

Abstract It is demonstrated that combining phased array antennas with dielectric lenses constitutes a suitable solution to increase the energy efficiency in next‐generation communication systems. A dielectric lens is designed to increase the gain of an array antenna working in the 3GPP n257 band (26.5–29.5 GHz). The simulated results demonstrate that the dielectric lens increases the gain between 0.4 and 1.3 dB in a large scanning range from 0° to 60° at 28 GHz; meaning that the output RF power can be decreased by up to 26% for a same equivalent isotropic radiated power required from the base station.

In millimetre wave (mmWave) frequency band, radio resource management (RRM) performance is characterised in plane wave conditions using radiated over‐the‐air test method, which is a challenging combination of protocol and radio frequency performance measurement. This article presents a novel design of multiple lens‐based compact antenna test ranges (CATRs) to simulate multiple angles of arrival from different base‐stations in RRM measurement. Only four lens‐based CATRs are proposed to create plane wave conditions at the device under test with six different angles of arrival and capable of accurately constructing the wireless propagation condition and minimising the diffraction and scattering between adjacent CATRs. The lens‐based CATR is the key element in the RRM measurement system, which is composed of a metasurface lens for phase alignment and a 2 × 2 feed array antenna for the emission of electromagnetic signals. The function and capability of the proposed lens‐based CATR is validated through simulation and measurement, that the worst‐case amplitude variation in the quiet zone region is lower than ±0.8 dB at 26 GHz, and phase variation is lower than ±7.5° in the designed quiet zone, which could be used in the fifth‐generation mmWave RRM measurement. [ABSTRACT FROM AUTHOR]

Abstract This paper presents a framework for achieving machine learning (ML)‐assisted direction‐of‐arrival (DoA) accuracy enhancement using a millimetre‐wave (mmWave) dynamic aperture. The technique used for the enhanced DoA estimation accuracy leverages an over‐sized lens‐loaded cavity antenna connected to a single RF chain in the physical layer and a computational method in the computational layer of the framework. It is shown for the first time that by introducing a reconfigurable mode‐mixing mechanism inside the over‐sized lens‐loaded cavity hardware, a greater number of spatially orthogonal radiation modes can be achieved giving rise to many cavity states. If the best cavity state is determined and selected by means of design exploration using a contemporary ML‐assisted antenna optimisation method, the computational DoA estimation accuracy can be improved. The mode‐mixing mechanism in this work is a randomly oriented metallic scatterer located inside an over‐sized constant−ϵr lens‐loaded cavity, connected to a stepper motor that is electronically controlled by inputs from the computational layer of the presented framework. Measurement results in terms of near‐field radiation mode scans are included in this study to verify and validate that the proposed ML‐assisted framework enhances the DoA estimation accuracy. Moreover, this investigation simultaneously provides a simplification in the physical layer implementation of mmWave radio hardware, and DoA accuracy enhancement, which in turn lends itself favourably to the adoption of the proposed framework for channel sounding in mmWave communication systems.

We report on the first demonstration of dynamic beam steering using a scanning lens phased array. A scanning lens phased array relies on a combination of mechanical and electrical phase shifting to dynamically steer a high-gain beam beyond the grating-lobe free region using a sparse array. These two concepts have been demonstrated separately in the past, here we present, for the first time, a prototype demonstration where active mechanical and electrical phase shifting are combined. For this purpose, we have developed a sparse 4x1 scanning lens phased array at W-band (75-110 GHz) capable of beam steering a directive beam (>30 dBi) towards ± 20° with low grating lobe levels (around -10 dB). The lens array is fed by a waveguide-based leaky-wave feeding architecture that illuminates the lenses with high aperture efficiency over a wide bandwidth, which is required in the proposed scanning lens phased array architecture. The electrical phase shifting has been implemented using IQ-mixers around 15 GHz in combination with x6 multipliers to reach the W-band. The mechanical phase shifting relies on a piezo-electric motor, which is able to achieve displacements of the lens array of 6 mm with an accuracy of a few nanometers. The entire active array is calibrated over the air with an ad-hoc quasi-optical measurement setup. Resulting measurements show excellent agreement with the anticipated performance.

Recent work has proposed that millimeter-wave beam steering antennas consisting of lens antenna subarrays (LASs) reduce hardware complexity. This manuscript extends the concept to 2D beam steering with extended hemispherical dielectric lenses (EHDLs). To accomplish this, we introduce a design process to maximize scan range and side lobe level (SLL) performance. The design process first employs the solution of the geometric disk covering problem to identify the initial positions of the feed antennas such that the subarray size, $M$ , is minimized. This process is followed by systematic 3D full-wave simulation-based parametric sweeps of lens geometry and feed antenna positions to maximize scan range and minimize SLL. Finally, we demonstrate this process with a 38 GHz antenna consisting of $L$ = 7 LASs and $M$ = 17 feed antennas per LAS. The resulting antenna has a ±36° field of view, −9.5 dB SLL, 5° half-power beamwidth, and ~20 dBi maximum realized gain. Compared to the existing literature on subarray-based beam-steering antennas, this antenna performs with a more extensive scan range while offering a comparable SLL performance.

In this letter, we derive a new closed-form analytical formula for the refractive index of a telescopic lens based on inhomogeneous graded index (GRIN) material. The telescopic lens is composed of two cylindrical confocal GRIN lenses, one divergent and one convergent. The scope of the telescopic lens is to magnify the aperture of a flat antenna placed at the first interface of the divergent-convergent lens doublet. This telescopic GRIN lens is useful for reusing already available medium-gain arrays and bring them to a high-gain, occupying a reduced volume as compared to traditional lenses or multireflector systems. The formulation used here is based on the solution of a regularized ray-congruence integral equation recently introduced by the authors, and applied in a previous work to treat convergent cylindrical GRIN lenses. The formulation is extended here to the case of a telescopic lens leading to analytical formulas for the two inhomogenous refractive indexes that involve five independent parameters; these parameters are eventually optimized to get the maximum gain boost. [ABSTRACT FROM AUTHOR]

This work presents a parallel-plate waveguide (PPWG) fanbeam antenna achieving a $\pm 48^\circ $ field of view with an average maximum scan loss of $2.7\,\textrm {dB}$ over the Ka-band for a flat feed surface. This approach uses a novel compound gradient index (GRIN) lens system, wherein two GRIN lenses are cascaded axially: an “aperture” lens at the antenna output provides the bulk of the phase collimation, while a “focal” lens near the feed surface corrects the spillover losses and provides additional phase collimation. This arrangement represents a wide-scanning alternative that requires 76% of the axial depth and only 37.3% of the actual material compared to a Luneburg lens fanbeam antenna with similar aperture size. A maximum scan loss of $2.58~$ dB at 48° and a scan loss exponent of 1.5 are achieved at $40~$ GHz. [ABSTRACT FROM AUTHOR]

The transformation optics (TO) technique has been widely applied in the volume reduction of Luneburg lens (LL) in both optical and microwave regimes. However, it is found that the focus is usually not located on the surface of the transformed LL anymore after applying TO. The focus location shift results in a limited volume reduction, severe reflection from the new lens surface, and limited scanning range with increased spillover loss. This article theoretically studies and proposes a TO-based LL antenna with zero focal length by considering the phase mismatch caused by space discontinuity. The study first reveals that the transformation of an LL alone without sustaining the original boundary inherently deteriorates the original focusing property even before exerting any approximation. To validate the idea, a flat metamaterial LL antenna using TO is proposed, approximated, and fabricated using an integrated dielectric and conductive 3-D printing manufacturing process. The feeding patch antenna remains on the surface of the transformed lens. The thickness of the proposed antenna is reduced to 1/3 of a traditional LL in its boresight, while a ±20° beam scanning range is achieved with linearly shifted feedings. The study verifies that it is essential to ensure phase matching at the radiation boundary to maintain the original focusing property of an LL. The information derived from the study evidences the conditions of the enforcement of TO in electromagnetic problems such as dielectric lenses. [ABSTRACT FROM AUTHOR]

Current spaceborne wind scatterometers have shown failure due to mechanical wear from moving parts used to conically scan a directive beam. In order to eliminate the risk of mechanical rotation failure, an antenna design with no moving parts that achieve the conical beam sweep is desirable. In this article, a lightweight, 3-D-printed, inhomogeneous lens antenna is presented as an all-electronic alternative. Compared to the previous on-axis fed lenses, the real-life application requiring conical scanning necessitates significant developments regarding the synthesis, optimization, and prototyping of the off-axis fed lens. The lens antenna is designed using curved-ray geometrical optics coupled to particle swarm optimization to determine the optimum lens surface shapes and material inhomogeneity while obtaining a design with minimum volume and therefore mass. An 18 cm lens is designed using this approach. Instructions on how to 3-D-print the inhomogeneous lens are given using a unit cell design, which permits additive manufacturing using fused deposition modeling (FDM) techniques. The lens is measured for far-field pattern performance at various locations along with the ring-shaped focus of the azimuthally symmetric lens. The measurements agree well with predictions obtained by full-wave simulations. These all-electronic alternatives are expected to prolong the lifetime of future spaceborne wind scatterometers. [ABSTRACT FROM AUTHOR]

This paper presents a framework for achieving machine learning (ML)‐assisted direction‐of‐arrival (DoA) accuracy enhancement using a millimetre‐wave (mmWave) dynamic aperture. The technique used for the enhanced DoA estimation accuracy leverages an over‐sized lens‐loaded cavity antenna connected to a single RF chain in the physical layer and a computational method in the computational layer of the framework. It is shown for the first time that by introducing a reconfigurable mode‐mixing mechanism inside the over‐sized lens‐loaded cavity hardware, a greater number of spatially orthogonal radiation modes can be achieved giving rise to many cavity states. If the best cavity state is determined and selected by means of design exploration using a contemporary ML‐assisted antenna optimisation method, the computational DoA estimation accuracy can be improved. The mode‐mixing mechanism in this work is a randomly oriented metallic scatterer located inside an over‐sized constant−ϵr lens‐loaded cavity, connected to a stepper motor that is electronically controlled by inputs from the computational layer of the presented framework. Measurement results in terms of near‐field radiation mode scans are included in this study to verify and validate that the proposed ML‐assisted framework enhances the DoA estimation accuracy. Moreover, this investigation simultaneously provides a simplification in the physical layer implementation of mmWave radio hardware, and DoA accuracy enhancement, which in turn lends itself favourably to the adoption of the proposed framework for channel sounding in mmWave communication systems. [ABSTRACT FROM AUTHOR]

This article presents a design concept of integrative transmitarray antenna (TAA) with gain-filtering and low-scattering characteristics. The integrative TAA element combines a bandpass frequency-selective surface (FSS) element and a square dielectric post with antireflection pyramidal structures (SDPP), achieving the simultaneous control of in-band transmitted waves and out-of-band reflected waves. The FSS element provides the phase-shifting and gain-filtering capabilities around the center operating frequency $f_{0}$. Meanwhile, the SDPP elements with different heights are utilized to reduce the scattering cross section (SCS) based on the phase cancellation principle. To realize wideband and wide-angle SCS reductions, also to mitigate the in-band radiation degradation, the optimal SDPP arrangement is determined with subarray segmentation. In the experiment, a $42\times42$ -element integrative TAA is designed. Against only the FSS TAA, the simulated results show that loading the optimal SDPP array can realize the scattering reduction in a 10:1 bandwidth up to 60°, including $\sim 5/\sim 10$ dB SCS reductions in 0.5– $1.5~f_{0}$ /1.5– $5~f_{0}$ , respectively. Moreover, the integrative TAA achieves gain-filtering responses with out-of-band suppression levels higher than 20 dB. After fabricating the FSS TAA and the SDPP array by the technologies of standard PCB fabrication and 3-D printing, respectively, the proposed design is further demonstrated by experiments. [ABSTRACT FROM AUTHOR]

In this work, we show that the near field of leaky-wave resonant antennas (LWAs) radiating into a dense medium can be locally represented as a spherical wave in a certain solid angle around broadside using an accurate definition of the phase center. The near field in this solid angle can be efficiently evaluated through the integration of the spectral Green’s function along the steepest descent path (SDP). Beyond this solid angle, defined as the shadow boundary angle, a residual contribution due to the leaky-wave pole must also be added to fully describe the near field. It is found that this shadow boundary angle can be used to define the phase center and geometry of a truncated lens that couples well to leaky-wave antennas, even in electrically small-to-medium sized lenses and low-contrast cases. To demonstrate the applicability of the proposed study, we combine the SDP field calculation with a Fourier optics (FO) methodology to evaluate the aperture efficiency and radiation patterns of small-to-medium sized lenses in reception. A truncated silicon lens with a diameter of only four free-space wavelengths is presented with almost 80% aperture efficiency. Excellent agreement with full-wave simulations is achieved, which demonstrates the accuracy of the proposed design and analysis methodology. [ABSTRACT FROM AUTHOR]

This article presents a parallel-plate lens beamformer for continuous wide-angle scanning. The design is based on a compact dual-lens system with extended scanning range and is optimized using a previously developed geometrical optics technique. Beam steering is accomplished with a mechanical feed system based on the noncontact characteristic of groove gap waveguides, offering large bandwidth, low profile, and mechanical ruggedness. The proposed concept is validated by an all-metal prototype of a $20.5\lambda $ lens operating in the uplink Ka-band allocated to satellite communications (27.5–31 GHz). Good agreement is obtained between the simulated and measured performances. The measured return loss is greater than 12 dB over the entire frequency band and beyond. High scanning performances are achieved over an angular range of ±50° (±14 beamwidths), with maximum scan losses in the order of 3 dB and good pattern stability over the entire band. The proposed solution is particularly suited for next-generation satellite terminals requiring compact broadband antennas with continuous beam-steering capability over a large angular range. [ABSTRACT FROM AUTHOR]

A filtering transmitarray antenna (TAA) with high-gain in-band radiations at the $X$ -band and an out-of-band low-scattering characteristic is presented in this communication. The proposed composite TAA element consists of a dual-polarized resistive sheet, a fixed bandstop frequency-selective surface (FSS) element, and an adjustable bandpass FSS element also serving as a phase shifter. The resistive element is constructed by inserting a parallel inductor–capacitor (PLC) resonant structure into the center of a lumped-resistor-loaded metallic dipole. A transparent frequency-domain window at the center operating frequency ($f_{c}$) is created by the PLC resonance and is further shared by both FSS elements, hence allowing for high in-band transmission herein. At frequencies below or above $f_{c}$ , the whole composite TAA element acts as an absorber with the bandpass or bandstop FSS elements as the ground planes for the resistive sheet, respectively. The measured results show that the composite TAA can achieve gain-filtering responses with out-of-band suppression levels ≥25 dB, ~8 dB scattering cross section (SCS) reductions in 4–7 and 14–20 GHz, and the radiation gain of 25.3 dBi simultaneously. Because of these merits, the proposed design can relieve the stress on filtering circuit designs, also offering an attractive solution in stealth technology. [ABSTRACT FROM AUTHOR]

Conformal, steerable lens antennas are of particular interest for millimeter-wave (mm-wave) antenna designers, as they enable low cost solutions for applications such as 5G mobile communications, radio-wave imaging, and satellite communications. Recent advances in additive manufacturing technology have opened up new possibilities for realizing graded-dielectric electromagnetic devices. In this letter, a compressed Luneburg lens fabricated from multimaterial 3-D printing is presented. Such a device has a steep dielectric gradient and cannot be easily realized using an effective medium approach that has become typical of 3-D printed graded-index devices. Instead, five different dielectric filaments were used to print the lens with a 100% filament fill-factor. The lens is excited by a WR-10 open-ended waveguide probe across the 75–110 GHz band, and achieves a bore sight gain of 22 dBi, and $-$ 3 dBi scan angle of 25 $^\circ$ at 84 GHz. [ABSTRACT FROM AUTHOR]

Metamaterials (MTMs) are artificial structures characterized by flexible and designable electromagnetic (EM) properties, including permittivity, permeability, and refractive index. Because of their subwavelength‐scale composing units, MTMs are able to manipulate the propagation of EM waves at extremely fine resolutions and offer solutions for new‐concept devices and components with novel functionalities and challenge‐breaking properties. From among the attractive applications of MTMs, this review article focuses on MTM lenses (also termed metalenses) developed in recent years for the purpose of microwave engineering, including beam forming, beam steering, and imaging. Bulk lenses made of 3D MTMs and planar lenses made of 2D transmission‐type metasurfaces are both involved and investigated. Different categories of metalenses, together with the theory, principles, methods, and advantages of their designs, are introduced and discussed. Potential and future applications of metalenses at microwave frequencies are also presented.

We present a cylindrical, multibeam, modified Luneburg lens antenna. The proposed antenna features low-profile, good impedance match, high gain, and wide angle scanning capability. We tapered the thickness of a dielectric disk along its radius to achieve the required refractive index profile of the lens in a nondispersive fashion. Seventeen dielectric-filled waveguide feeds, arranged circumferentially around the focal arc of the lens, provide almost invariant azimuthal scanning in the range of ±60° from broadside direction. The lens and the interior of the waveguide feeds are made of the same dielectric material, offering a smooth impedance transition from the feed antenna to the lens. A prototype of the antenna was fabricated and characterized. Measured reflection coefficient is better than −10 dB from 14 to 16 GHz. The isolations between feeds remain higher than 24 dB over the entire operational frequency range. We measured a maximum antenna gain of 20.5 dB with only 1.5 dB gain loss near the maximum 60° scanning angle. The proposed high-gain, multibeam antenna design can be used in a wide range of applications including automotive and surveillance radars. [ABSTRACT FROM AUTHOR]

This letter presents the design and characterization of an octave-bandwidth, small-aperture, linearly polarized, high-gain horn antenna operating in the 6–12 GHz band for active arrays. The aperture size is limited by the waveguide feed cutoff at lower frequencies and by grating lobes at higher frequencies. The radiating structure is composed of an E-plane flared horn with an exponential double ridge fed by a low-loss waveguide-to-microstrip transition, eliminating the need for additional connections between the antenna elements and the beamforming and amplification circuits. The horn is loaded with an artificial dielectric with 1 < εr,eff <  2.2, which lowers the cutoff frequency and improves phase distribution over the aperture. The lens and feed loading structure are designed together to improve matching without degrading the radiation pattern. A prototype is fabricated in brass using split-block machining, and the lens is machined from 16 parallel dielectric slabs. The measured performance is in agreement with simulations. [ABSTRACT FROM AUTHOR]

A zoned fishnet metamaterial lens is designed, fabricated, and experimentally demonstrated at millimeter wavelengths to work as a negative near-zero refractive index lens suitable for compact lens antenna configurations. At the design frequency f=56.7 GHz (λ0=5.29 mm), the zoned fishnet metamaterial lens, designed to have a focal length FL=9λ0, exhibits a refractive index n=–0.25. The focusing performance of the diffractive optical element is briefly compared with that of a non-zoned fishnet metamaterial lens and an isotropic homogeneous zoned lens made of a material with the same refractive index. Experimental and numerically-computed radiation diagrams of the fabricated zoned lens are presented and compared in detail with that of a simulated non-zoned lens. Simulation and experimental results are in good agreement, demonstrating an enhancement generated by the zoned lens of 10.7 dB, corresponding to a gain of 12.26 dB. Moreover, beam steering capability of the structure by shifting the feeder on the xz-plane is demonstrated. [ABSTRACT FROM AUTHOR]

Future sub millimeter imagers are being developed with large focal plane arrays (FPAs) of lenses to increase the field of view (FoV) and the imaging speed. A full-wave electromagnetic analysis of such arrays is numerically cumbersome and time-consuming. This article presents a spectral technique based on Fourier optics combined with geometrical optics for analyzing, in reception, lens-based FPAs with wide FoVs. The technique provides a numerically efficient methodology to derive the plane wave spectrum (PWS) of a secondary quasi-optical component. This PWS is used to calculate the power received by an antenna or absorber placed at the focal region of a lens. The method is applied to maximize the scanning performance of imagers with monolithically integrated lens feeds without employing an optimization algorithm. The derived PWS can be directly used to define the lens and feed properties. The synthesized FPA achieved scan losses much lower than the ones predicted by standard formulas for horn-based FPAs. In particular, an FPA with scan loss below 1 dB while scanning up to ±17.5° (~±44 beam-widths) is presented with directivity of 52 dBi complying with the needs for future sub millimeter imagers. The technique is validated via a physical optics code with excellent agreement. [ABSTRACT FROM AUTHOR]

This study presents a beam‐steerable antenna system that uses two mechanically‐rotatable, phase‐shifting surfaces (PSSs) functioning like a pair of Risley prisms for beam scanning operation at X‐band. One of the PSSs also acts as a lens, performing beam collimation in addition to providing a phase shift gradient. Unit cells constituting the two PSSs were co‐designed to determine the distance between the two surfaces that results in high transmission coefficients and small phase errors under different beam‐scanning conditions. Both simulation and measurement results have verified that the distance of 2 mm provides the best performance regarding to peak gain. Prototypes of the PSSs having circular apertures with diameters of 11.5λ0 at 10 GHz were fabricated and illuminated by a feed antenna to characterise the beam scanning performance. The measurement results of the proposed antenna system demonstrate the capability to steer the main beam within a ±60° scanning range with a maximum gain variation of less than 3.6 dB by rotating the two PSSs with respect to each other. The fabricated prototype provides side lobe levels less than −13.1dB, polarisation purity higher than 20 dB, and small beam pointing errors of no more than 1° in all examined beam scan scenarios. [ABSTRACT FROM AUTHOR]

A dielectric lens is proposed here to broaden the half-power beamwidth (HPBW) of conventional circularly polarised (CP) patch antenna. The lens printed by the 3D technique is constructed by a dielectric cylinder with a stacked cone-shaped cavity. To illustrate the working principle, the energy distribution of the electromagnetic wave refracted by the lens is analysed. Besides, the design procedures of the lens are introduced in detail. The applicability of lens is also discussed including the changing of permittivity and dimensions. For verification, prototypes of the designed CP antenna with the lens are fabricated and measured. The performances of the CP antenna without lens are also provided for comparison. The measurements show that by using the proposed lens, the HPBW of the CP patch antenna is extended effectively from 69° to 170° while maintaining a wide 3-dB axial ratio beamwidth of >200°. [ABSTRACT FROM AUTHOR]

Millimeter-wave lens antennas will be essential for future wireless access. Conventionally, they increase the gain in the boresight direction only. In this article, cascaded Fresnel zone plate lenses are combined with a phased array to increase the gain at wide steering angles of ±52°. The side lenses are tilted to align with the maximum steering angle and cascaded to increase the focusing gain. The inner lenses increase the gain by 2.45 dB at boresight, and by 3.19 dB at the maximum steering angle. When the side lenses are repositioned, the simulated focusing gain increases to 4.69 dB. Asymmetric amplitude distributions are proposed to prevent the main lobe from splitting. An eight-element seven-lens prototype operating at 28 GHz achieved a gain from 12.96 to 15.35 dBi with a bandwidth of at least 1.3 GHz for all measured beam directions. The maximum measured azimuthal beamwidth was 27°. A design procedure and a theoretical analysis of diffraction through the lenses are provided. By increasing the SNR, this beamforming antenna could improve the coverage of three-sector 5G microcell base stations, and support gigabit wireless links for vehicular, rail, and satellite communications. [ABSTRACT FROM AUTHOR]

A millimeter‐wave radar based on active invers scattering approach for two dimensional screening of metallic and nonmetallic concealed targets is presented. The perceived challenges of detecting a nonmetallic target exhibiting poor dynamic range for measurement systems are analyzed and discussed by comparing the performance of three different antenna sensors. A short time duration pulse with frequency sweep covering 27 to 33 GHz band is used to feed the antenna sensors. In our experimental test, we buried a concealed target consists of metallic or dielectric strips under a dielectric layer that simulates the human body model. Waveguide and printed antipodal Vivaldi antennas are considered to study the target detectability and the quality of the measured millimeter‐wave images. The use of proposed AVA resulted in a better‐quality image with lower noise effect for both metallic and nonmetallic cases. [ABSTRACT FROM AUTHOR]

A transmitarray antenna (TAA) with a small-scale circular phased array antenna (PAA) feed is proposed in this article to generate orbital angular momentum (OAM)-carrying radio beams. After analyzing the operating mechanisms of the OAM-generating PAA-fed TAA, its numerical model is further developed to determine the TAA element arrangement and the PAA feed excitation coefficients. Furthermore, a three-layer element with polarization-conversion property and the traditional patch antenna element are identified as the elements of the TAA and PAA feed, respectively. Benefiting from the intrinsic multibeam capabilities of the TAA, the steering of the zero-OAM-mode beam is firstly discussed, where a conical scanning across an angular range of ±25° is realized. Meanwhile, the creation of multiple pure or mixed OAM beams with helical phase fronts is also presented, where the superposition of multiple OAM states provides more possibilities to increase the channel capacity and spectrum efficiency. To facilitate the experiments, an $8\times 8$ Butler matrix with a bandwidth over 30% is designed, which integrated with the array-fed TAA also provides an attractive solution to simultaneously generate multiple OAM modes without requiring any active modules. The experimental results agree reasonably well with the numerical ones, validating the feasibility of our idea to design a high-gain OAM-generating PAA-fed TAA. [ABSTRACT FROM AUTHOR]

A transmitarray antenna (TAA) with a small-scale phased array antenna (PAA) feed is proposed in this communication. After modeling the PAA-fed TAAs and constructing the cost functions, a three-step optimization process is introduced to determine the TAA element arrangement as well as the PAA feed excitation coefficients. Then, a novel ultrawideband TAA element is proposed, which has a pair of connected sheep-horn metallic patterns in the middle layer sandwiched by two metallic orthogonal grid polarizers on the upper and bottom layers. Furthermore, traditional Vivaldi antenna element is identified as the element of a $4 \times 8$ -element PAA feed array (central $4\times 4$ active elements), which is employed to illuminate a $25\times 25$ -element circular TAA with a focus-to-diameter ($F/D)$ ratio of mere 0.43. The simulated results show that stable radiation patterns with good main beam shapes and reasonable sidelobe levels are obtained in the interested frequency band. The measured results agree reasonably well with the simulated ones, validating the feasibility of our method to design an ultrawideband low-profile PAA-fed TAA. [ABSTRACT FROM AUTHOR]

In order to fulfill the exponential increase in the demand of high-speed wireless links, future XG wireless networks will be developed at higher carrier signal frequencies, reaching the hundreds of gigahertz. In this contribution, a leaky-wave-fed HDPE lens antenna working at ${G}$ -band which can be useful for future XG communications is presented. The proposed lens design enables the generation of up to 40 beams, with gains higher than 30 dB. Analytical tools have been applied to optimize the lens aperture efficiency, validating the results via full-wave simulations. The reached aperture efficiency for the broadside beam is higher than 80% over a 44% relative bandwidth. The measurement results for a fabricated prototype show excellent agreement with the simulated performance. [ABSTRACT FROM AUTHOR]

With the advent of small-scale satellite technologies, there has been significant interest in developing low-profile antennas. This article presents a novel collapsible flat-layered lens based on gradient-refractive-index (GRIN) metamaterials. The 3-D lens consists of multilayer double-sided microstrip square-rings units of variable sizes distributed on planar dielectric substrates to satisfy the required refractive index distribution. Adjacent layers can be stored compressed in a stacked configuration with minimal height, then deployed to an operational arrangement to their full dimensions and capabilities. The key factor in this design is the judicious choice of the discretized metamaterial unit cell such that neighboring layers are separated by an air gap, which allows the lens to collapse. A proof-of-concept prototype operating at 13.4 GHz with a 11.2 cm diameter and a height of 0.4 and 2.1 cm in collapsed and operational arrangement, respectively, was manufactured within fabrication tolerances. A resonant patch array feed was used and the lens antenna was measured in the UCLA Plane Bi-Polar Near-Field measurement range achieving a 23 dBi directivity and 22.2 dBi gain. The numerical and experimental results agree well and demonstrate that this new lens is advantageous in terms of low packaging height without compromising high-performance. [ABSTRACT FROM AUTHOR]

We present the theoretical modeling and prototype demonstration of a three-dimensional broadband, low-loss, dual-polarization, and high-directivity lens antenna using gradient index (GRIN) metamaterials, which is composed of multi-layer microstrip square-ring arrays. The elements of metamaterials, closed square-ring units of variable sizes, are distributed on the planar substrate to satisfy the radial gradient index function and the axial impedance matching layer configuration of the lens. The gradient-index metamaterials are designed to transform the spherical wave-front into the planar wave-front and to minimize the reflection loss. A prototype lens antenna, which consists of a metal conical horn and the gradient-index lens, are simulated, constructed, and measured. The resemblance of simulation and measurement results shows that the prototype lens antenna maintains low return loss and high directivity on the whole X-band (from 8 GHz to 12 GHz). Compared to the traditional horn antenna, the metamaterial GRIN lens antenna has much superior performance-for instance, the gain increases by 6 dBi at 12 GHz. These results demonstrate the feasibility of such a light weight slab metamaterial lens for broadband and high-directivity antenna applications, such as in radar and communication systems. We have used the lens antennas in the measurements of a three-dimensional invisibility cloak due to the high directivity. [ABSTRACT FROM AUTHOR]