3D‐printed cylindrical Luneburg lens antenna for millimeter‐wave applications

A 3D‐printed cylindrical Luneburg lens antenna working at 26 GHz is proposed in this article. The antenna consists of a feeding waveguide, a 3D‐printed cylinder, and a pair of printed metal grids which are stuck on the side faces of the cylinder. 3D‐printed structure ensures the convenience for processing and structural integrity of the Luneburg lens. Hole drilling technology is utilized for the design of the cylindrical lens. In the E‐plane, conversion of spherical waves into planar waves is achieved based on the gradient refractive index which is realized by the gradient equivalent relative dielectric constant. The main part of the lens contains a hole drilling region to realize the desired equivalent permittivity from 1.23 to 2, while another gradual‐thickness region realize the permittivity ranges from 1.23 to 1. H‐probe method is utilized for the optimization of the gradual‐thickness region in this article. And for the H‐plane, with the grids, H‐field distribution is optimized compared with the Luneburg lens antenna without the loading grids. Thus, the side lobe level (SLL) in H‐plane could be reduced. Meanwhile, a narrower half power beamwidth (HPBW) in H‐plane will be obtained due to the metal grids. Experiment results illustrate the feasibility and validity of the proposed 3D‐printed cylindrical Luneburg lens antenna.     

High gain substrate integrated waveguide beam scanning antenna with sub‐array elements

A method to enhance the gain of substrate integrated waveguide (SIW) beam scanning antenna is proposed in this article. 2 × 2 SIW cavity‐backed sub‐arrays are employed in array design. The antenna is constructed on two layers. The top layer places four SIW cavity‐backed sub‐arrays as radiating elements and the bottom layer is an SIW transmission line to feed the sub‐arrays. Beam scanning feature can be obtained due to the frequency dispersion. Moreover, through separating radiators to the other layer and using 2 × 2 SIW cavity‐backed sub‐arrays as radiating parts, the antenna gain is improved significantly. For a linear array, 4.1 to 6.8 dB gain enhancement is achieved compared to a conventional SIW beam scanning antenna with the same length. Then, the linear array is expanded to form a planar array for further gain improvement. A 64‐element planar beam scanning array is designed, fabricated, and tested. Experimental results show that the proposed planar array has a bandwidth from 18.5 GHz to 21. 5 GHz with beam scanning angle from ?5° to 11.5° and gain in the range of 20.5 to 21.8 dBi. The proposed high gain beam scanning antennas have potential applications in radar detection and imaging.     

A novel substrate integrated waveguide cavity‐backed self‐diplexing antenna array with frequency beam scanning characteristic

This article presented a substrate integrated waveguide (SIW) cavity‐backed self‐diplexing antenna array with frequency beam scanning characteristic. The proposed array consists of 16 SIW cavity‐backed slot antennas. The SIW cavity‐backed slot antenna can be fed by two separate ports to resonate at two different frequencies and achieve high isolation better than 20 dB between two input ports. The proposed element is a typical self‐diplexing antenna. These cavity‐backed slot antennas are shunt‐fed by a compact 1 to 16 SIW power divider and series‐fed by a set of microstrip lines, respectively. As a result, this array achieves an unidirectional radiation pattern at 10.2 GHz with high gain of 15.10 dBi, and a frequency beam scanning characteristic from 7.0 to 9.0 GHz ranging from ?50° to 46°.     

基于双焦点设计的平面波束扫描透镜天线研究

为了提升平面透镜天线的扫描性能,以实现波束扫描或多波束应用,首先研究设计了单焦点平面透镜天线,针对单焦点平面透镜天线在大角度扫描时增益降低太快的问题,引入了反射阵和传统介质透镜的双焦点设计方法。基于阵列天线合成理论,计算了双焦点透镜天线的辐射特性。采用多层金属孔阵列单元,建模了平面透镜天线模型,仿真对比了单焦点透镜和双焦点透镜天线的扫描辐射特性。结果表明,在扫描范围（0°-27°）内,双焦点透镜天线的最低增益相对于单焦点透镜天线有明显提升,而且大角度扫描方向图形状也有改善,证明了双焦点的设计方法有效提升了平面透镜的扫描性能。     

A lightweight multi‐beam cylindrical Luneberg lens antenna loaded with multiple dielectric posts

A multi‐beam cylindrical Luneberg lens antenna loaded with multiple light dielectric posts for the purpose of light weight is presented. The antenna is based on a parallel‐plate waveguide and specifically composed of 10 E‐shaped patch antennas feeds, 2 parallel plates, and 491 epoxy posts. The equivalent gradient index of the Luneberg lens antenna is realized via the positions of the epoxy posts between the parallel plates. The features of low‐profile height (0.55λ) and large radiating area (4.4 × 0.55λ²) of the cylindrical Luneberg lens result in wide beamwidth in elevation plane and high gain while operating at 4 GHz. Consequently, the 3 dB beamwidth in the elevation plane is >65°. Furthermore, the multi‐beams cover a wide scan angle of 120° in the azimuth plane. The measured aperture efficiency of the fabricated lens antenna is above 50% from 3.9 to 4.3 GHz. In addition to the good radiation performance, features of light weight and ease of fabrication have also been demonstrated for the proposed lens antenna.     

A compact wide‐band antipodal Vivaldi antenna design

A compact wide‐band antipodal Vivaldi antenna (AVA) is designed in this work. First, the traditional exponentially tapered edge of the radiator is replaced by an arc curve that makes the AVA compact. Second, the AVA is loaded with a “director” and a “convex lens” that improve its high‐frequency performance. The proposed antenna is fabricated and its dimensions measured. It occupies a small volume of 34 × 16 × 0.8 mm³ and provides an operating frequency range from 3.01 to 10.6 GHz with a higher gain in the low frequency region than that of reference antennas. The measured results perfectly coincide with the simulated ones, which demonstrates the feasibility of the novel AVA design.     

Pattern reconfigurable metasurface to improve characteristics of low profile antenna parameters

This article proposes a mushroom‐shaped electromagnetic band gap (EBG) structure for the antenna parameter enhancement of low profile antennas in 5 to 15 GHz regime. Three different type antennas including a dipole antenna, a loop antenna, and a monopole antenna are designed for the corresponding operation band, and a 8 × 8 mushroom type EBG structure is designed to obtain exotic behavior for the enhancement of antenna parameters. Bandwidth, return loss (S₁₁), main lobe gain, directivity, side lobe level, front to back ratio, and angular width of each antenna with EBG structure is examined with details. Besides, the designed EBG structure and antennas are fabricated and experimental results are obtained to support numerical ones. In addition, future study of the proposed EBG structure such as microwave imaging in cavity resonators is specified and discussed.     

Integrated Vivaldi antenna for UWB/diversity applications in vehicular environment

This article presents the design of a three‐port diversity antenna capable of producing three‐directional radiation pattern for vehicular communications. The proposed antenna consists of three uncorrelated Vivaldi antennas that are interconnected and developed on a single printed circuit board. Unlike many other antennas reported for the vehicular environment, the proposed antenna offers ultra‐wideband characteristics with end‐fire radiation pattern leading to high realized antenna gain. The integrated antenna has a footprint of 65 × 40 × 1.6 mm³ and offers 6 GHz impedance bandwidth extending from 5 to 11 GHz. The port‐to‐port isolation is greater than 20 dB within the operating bandwidth. Furthermore, the diversity performance of the proposed three‐port antenna system is evaluated and presented. The calculated envelope correlation coefficient, diversity gain, and mean effective gain are well above the minimum requirement. The prototype antenna is fabricated and the experimental results are presented.     

Design of multi‐beam antenna based on Rotman lens

A planar Rotman lens antenna that generates multiple beams is presented over a wide angular range. The proposed multi‐beam antenna consists of a Rotman lens and a ten‐element printed Yagi antenna array. By properly comparing optical aberrations, expressing as the normalized path length errors Δl, the suitable ratio of on‐axis to off‐axis focal length (g = G/F) is acquired so as to minimize phase errors for the array elements. Ten dummy ports are employed to reduce the performance deterioration caused by energy reflection. A prototype with seven input ports was fabricated and measured, covering a wide scanning angle of 60° (–30°, 30°). The measured beam patterns show that the seven beam gains are distributed from 11.9 to 13.6 dBi under operating of 8.15 GHz. Both the simulated and measured results are used to verify the design approach.     

Multi‐band pattern reconfigurable Yagi‐Uda antenna

In this study, novel designs of single‐band and tri‐band pattern reconfigurable antennas are proposed. The design of single‐band pattern reconfigurable antenna is accomplished by the use of varactor diodes with the parasitic elements placed on both sides of the driven conventional printed dipole antenna. By tuning the capacitance of varactor, the antenna operates in four different configurations of radiation pattern which include bi‐directional end‐fire, broadside, and uni‐directional end‐fire radiation patterns. The tri‐band pattern reconfigurable antenna design is achieved by the use of parasitic elements on both sides of a tri‐band driven dipole antenna. Dual‐band LC resonators are used as loading elements along the arms of printed dipole to get two lower order modes in addition to the reference dipole mode, resulting in a triband operation of the driven element. The electrical lengths of the parasitic elements with respect to the tri‐band driven element are controlled by suitably embedding varactor and PIN diodes with them. The proposed tri‐band antenna operates in ten different configurations of radiation patterns in the three operating bands. Fully functional prototypes of single‐band and tri‐band pattern reconfigurable antennas along with the DC bias networks have been fabricated to validate the results obtained in simulation.     

Metamaterial inspired fabric antenna for wearable applications

A compact and robust fabric antenna incorporated with metamaterials (MTM) at 2.4 GHz is introduced for wearable devices application where the MTM behaves as EBG/AMC. The benefit of introducing MTM in a wearable antenna is to diminish the influence of frequency detuning and reduces the backward radiation specifically when loaded on the human body. The overall size of the presented antenna incorporated with MTM is 60 × 60 × 2.4 mm3. Furthermore, the integrated design has the capability of controlling Specific absorption rate (SAR) and improved the bandwidth, front‐to‐back ratio (FBR), and gain up to 14.5%, 13.7dB, and 7.5dBi, respectively. The operations under different bending diameters on real and modelled human body are studied. Compared with conventional antennas, MTM‐inspired antennas reduce the SAR to safe levels of more than 90%. The presented integrated design can be a good candidate for incorporation into a variety of flexible systems for medical application.     

结合天线选择的空间调制及性能仿真

提出了一种分组映射空间调制方案。每个时隙中有多个发射天线被同时激活,结合幅度相位调制技术,每根选中的天线发射不同的符号,并且所激活的特定天线序列也携带相应信息。相对传统空间调制技术而言,该方案可以有效地提高空间调制系统的频带利用率和传输速率。Rayleigh衰落信道的仿真结果表明,所提方案的误比特率（Bit Error Rate,BER）性能优于传统的空间调制与多输入多输出（Multi-Input Multi-Output,MIMO）技术。方案可进一步结合发射天线选择（Antenna Selection,AS）技术,提高系统的传输质量。实际多极化MIMO信道的测量和仿真结果表明,在发端天线选择数目均相同时,分组映射调制的性能明显优于空间调制,结合AS的调制系统性能明显优于未结合AS的系统。     

Design of ultra‐wideband antenna using tapered slot for low‐end frequency extension

In order to extend the lower frequency down, resonant cavities are added to a tapered slot Vivaldi antenna. Using a full‐wave time‐domain method, the effect of the dielectric substrate on the performance of the antenna has been investigated. Permittivity was shown to play an important role in comparisons of wideband frequency range antennas with dielectric constant cases in similar geometries. When dielectric permittivity is increased, the bandwidth is improved, and when the resonant cavity is added, the low‐end frequency response is extended even lower than 500 MHz. Results from a conventional tapered slot edge compared to a tapered slot edge with resonant cavity Vivaldi antennas with different dielectric permittivity qualitatively supports the effect of the different substrate. Verification has been implemented by using the numerical method of pseudospectral time‐domain with alternating‐direction‐implicit method, and by the experiment. The simulation results show very good agreement with the experiment. Both results proved that our design is available.     

A single sided dual‐antenna structure for UHF RFID tag applications

In this article, a dual‐antenna structure is presented for UHF RFID tag. The proposed structure is made of two L‐shaped strip antennas along with a cross‐shaped slot loaded patch. One antenna is exclusively used for receiving and harvesting full energy with complex conjugate of tag chip, whereas another used as backscatter to enhance maximum differential radar cross section with purely real input impedance, which results in the enhancement of read range. Further, electromagnetic band gap structure is used around the dual‐antenna structure to increase the gain which results in improved read range. The proposed antennas are fabricated and the S‐parameters are measured with the help of differential probe technique. Simulation and measurement results are found in good agreement. The performance of the proposed antenna is also investigated when it placed on different materials such as metal, wood, glass, and plastic. The study shows that the read range of antenna increases considerably when it is mounted on a metallic surface, while the maximum performance is observed when the antenna is attached on a glass surface with highest relative permittivity. © 2015 Wiley Periodicals, Inc. Int J RF and Microwave CAE 25:619–628, 2015.     

Broadband low‐profile transmitarray antenna using three‐dimensional cross dipole elements

This article presents a novel transmitarray antenna using three‐dimensional frequency selective structures as the radiating elements. The proposed unit cell, which consists of two cascaded cross dipoles, has a thickness of 0.22λ₀ and provides a 310° transmission phase range with transmission magnitude equal or better than ?0.8 dB. Compared with those conventional transmitarray antennas, the proposed one can realize greater flexibility in the installation with less manufacturing complexity. For the purpose of validation, a transmitarray prototype using the proposed elements has been manufactured and tested at X‐band. The peak gain of 25.5 dB is achieved at the frequency of 10 GHz, resulting in an aperture efficiency of 64%. Besides, antenna bandwidth of 10% for 1‐dB gain is achieved in this design.     

A dual band dual antenna with read range enhancement for UHF RFID tags

This article presents a novel dual antenna structure for dual ultra high frequency bands (f₁ = 866 MHz and f₂ = 915 MHz) for radio frequency identification tags. The proposed structure consists of two dual band antennas, one acting as a receiving antenna and the other as a backscattering antenna at both the frequency bands. The receiving antenna is designed to have input impedance complex conjugate to the impedance of tag IC in order to maximize power transfer between the antenna and the microchip. The backscattered antenna is designed to have real‐valued input impedance at both the operating frequency bands to obtain maximum differential radar cross section leading to read range enhancement. The dual band receiving antenna is designed by embedding a pair of thin slits at a radiating edge of inset fed microstrip antenna. The backscattering antenna is comprised of two elements, one is a comb‐shaped open ring element, and the other is a meander line structure which is within the open ring element. Compared to conventional antennas, the proposed dual antenna structure provides a read range enhancement due to improved maximum differential RCS. The proposed dual antenna produced 4.3 m and 6.8 m read range at 866 MHz and 915 MHz, respectively.     

Wide‐bandwidth multifrequency circularly polarized monopole antenna for wireless communication applications

In this article, wideband circularly polarized monopole antennas with multiresonating frequencies are presented for Bluetooth, WLAN, WiMAX, and X‐band applications. The designed antennas have dimensions of 50 × 35 × 1.6 mm³. Two different substrates (FR4‐epoxy and PTFE) are used for fabricating the antennas. The antennas consist of corner truncated I‐shaped and C‐shaped strips excited by a 50 Ω microstrip feed line. The parametric analyses are performed with the help of Ansoft HFSS V.11 EM simulator. Both antennas have been fabricated and measured. The measured percentage bandwidth of the antenna made by FR4 substrate is 31.32% (centered at 1.66 GHz), and 64.85% (centered at 5.69 GHz). The percentage bandwidth of antenna made by PTFE substrate is 20.57% (centered 2.43 GHz) and 68.74% (centered at 7.39 GHz). In addition to that, there exists 3 dB AR bandwidth for LHCP of about 1050 MHz for 5.2 GHz WLAN‐band. The reflection coefficient, radiation patterns, and the gains of both the antennas are studied in detail. It is found that the measured and simulated results are in good agreement.     

A novel frequency-selective metamaterial to improve helix antenna

A novel frequency-selective metamaterial with negative permittivity and permeability for improving directivity and gain of a helix antenna is presented in this paper.The proposed metamaterial is composed of two Z-shape resonators printed on opposite sides of a dielectric substrate.Two forms of multilayered cells are found to be suitable for antennas and waveguides applications.In addition,a new method of designing a metamaterial-based helix antenna is presented with high directivity and gain.A comparison on radiation properties is given between the conventional and the new metamaterial-based helix antennas.Two comparisons on radiation properties are performed:(1) the effect of proposed Z-structure on monopole,dipole,and helix antennas;(2) the effect of OE3,split-ring resonator (SRR),and proposed Z-structure unit cells on the performance of helix antennas.The results show improvement of parameters such as directivity,gain,and radiation power of the new metamaterial-based helix antenna.Therefore,the combination of Z-structure with the helix antenna shows the best performance.     

A versatile pattern synthesis algorithm for circular antenna array

This article presents a versatile pattern synthesis algorithm for controlling the sidelobe level and nulling region for circular antenna arrays. Nonuniform fast Fourier transform using the min–max interpolation method is utilized to overcome the nonlinear feature of circular arrays. The major advantage of the proposed algorithm is low complexity, which is key for hardware implementation. Moreover, the proposed algorithm functions well in amplitude‐only pattern synthesis, which may be required for low‐cost array systems using attenuators instead of complete amplitude and phase adjustment modules. Additionally, element failure of antenna arrays can be minimized in practical operations by resynthesizing the pattern by using the remaining antennas to achieve satisfactory performance. Simulation results indicate the lower complexity and higher versatility of the proposed algorithm compared with the conventional methods.     

Reconfigurable terahertz Vivaldi antenna based on a hybrid graphene‐metal structure

This paper presents a reconfigurable terahertz Vivaldi antenna based on a hybrid graphene‐metal structure. The proposed antenna uses a novel tapered slot edge with hybrid graphene‐metal structure to improve the electromagnetic characteristics of classical metallic Vivaldi antennas. The results show that the proposed hybrid graphene‐metal Vivaldi antenna can be dynamically reconfigured via electric field bias, and has a low reflection coefficient. Moreover, this study demonstrates that the proposed antenna has excellent gain and radiation efficiency than that of the graphene Vivaldi antenna, and brings more possibilities to the realization and application of graphene antennas in the terahertz band, which is expected to use in terahertz wireless communication in the future.