Ultrasonic beam focusing characteristics of shear-vertical waves for contact-type linear phased array in solid

We investigate the beam focusing technology of shear-vertical(SV) waves for a contact-type linear phased array to overcome the shortcomings of conventional wedge transducer arrays. The numerical simulation reveals the transient excitation and propagation characteristics of SV waves. It is found that the element size plays an important role in determining the transient radiation directivity of SV waves. The transient beam focusing characteristics of SV waves for various array parameters are deeply studied. It is particularly interesting to see that smaller element width will provide the focused beam of SV waves with higher quality, while larger element width may result in erratic fluctuation of focusing energy around the focal point. There exists a specific range of inter-element spacing for optimum focusing performance. Moreover, good beam focusing performance of SV waves can be achieved only at high steering angles.     

复合振子天线阵列的数值模拟

 用时域有限差分方法模拟了由复合振子天线单元组成的天线阵列的辐射特性;计算了天线从同轴线的馈电效率,给出了天线阵列的辐射场和能量方向图;比较了不同排列方式下阵列的辐射方向图。模拟结果表明：这种天线阵列具有宽带特性和较高的馈电效率,适合于超宽带电磁脉冲的辐射应用。     

Analytical framework of small-gap photoconductive dipole antenna using equivalent circuit model

A compact planar antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution is the prospective demand for THz imaging application. Therefore, the small-gap photoconductive dipole antennas have been explored to fulfil such applications demand. However, there are certain modalities for improving the photoconductive dipole antenna performance which need to identify to accomplish high THz average radiated power and improved total efficiency. The unit-cell small-gap photoconductive dipole antenna radiation power enhancement methods need to optimize the design parameters with photoconductive material selection from theoretical simulation. Further, the potential improvement of coupling efficiency of THz wave with air as well as femto-second laser incident efficiency is also important parameters to enhance the radiation power of small-gap photoconductive dipole antenna. In this paper, we have presented an analytical procedure employing explicit mathematical expression leading to the physical behaviour of small-gap photoconductive dipole antenna. The effects of biased lines on the antenna performance parameters are discussed with the help of proposed equivalent circuit model. We have explored the effect of gap-size on the THz radiated power and on total radiation efficiency from the proposed photoconductive dipole antennas.     

On the acoustic radiation modes of compact regular polyhedral arrays of independent loudspeakers

Compact spherical loudspeaker arrays can be used to provide control over their directivity pattern. Usually, this is made by adjusting the gains of preprogrammed spatial filters corresponding to a finite set of spherical harmonics, or to the acoustic radiation modes of the loudspeaker array. Unlike the former, the latter are closely related to the radiation efficiency of the source and span the subspace of the directivities it can produce. However, the radiation modes depend on frequency for arbitrary distributions of transducers on the sphere, which yields complex directivity filters. This work focuses on the most common loudspeaker array configurations, those following the regular shape of the Platonic solids. It is shown that the radiation modes of these sources are frequency independent, and simple algebraic expressions are derived for their radiation efficiencies. In addition, since such modes are vibration patterns driven by electrical signals, the transduction mechanism of compact multichannel sources is also investigated, which is an important issue, especially if the transducers interact inside a shared cabinet. For Platonic solid loudspeakers, it is shown that the common enclosure does not lead to directivity filters that depend on frequency.     

固体中线性相控阵列的瞬态聚焦特性

针对线性相控阵列在固体介质中的声场聚焦特性及参数优化问题,该文给出了一个线性阵列的纵波瞬态聚焦声场模型.数值结果表明,当阵元被短时脉冲信号激励时,瞬态聚焦声场中不会形成栅瓣,突破了传统稳态理论模型中对阵元间距的限制;同时由于横纵波在聚焦区域内可完全分离,声束旁瓣的幅值也得到了抑制.其次,增大阵元间距能够显著提高聚焦性能...     

Maximization of the directivity ratio with the desired audible gain level for broadband design of near field loudspeaker arrays

This paper mainly addresses design methods for near field loudspeaker arrays. These methods have been studied recently since they can be used to realize a personal audio space without the use of headphones. From a practical view point, they can also be used to form a directional sound beam within a short distance from the sources especially using a linear loudspeaker array. In this regard, we re-analyzed the previous near field beamforming methods in order to obtain a comprehensive near field beamforming formulation. Broadband directivity control is proposed for multi-objective optimization, which maximizes the directivity with the desired gain, where both the directivity and the gain are commonly used array performance measures. This method of control aims to form a directive sound beam within a short distance while widening the frequency range of the beamforming. Simulation and experimental results demonstrate that broadband directivity control achieves higher directivity and gain over our whole frequency range of interest compared with previous beamforming methods.     

水下双层圆柱壳体结构辐射噪声实时预报方法研究

根据Sommerfeld辐射条件和统计能量假设定义了具有方向性特征的声辐射系数,再结合声辐射效率特性定义了新的参数声辐射因子。在外壳体表面速度实时重构理论基础上提出了用声辐射因子实现水下双层圆柱壳体结构辐射噪声实时预报的方法,还提出了有限长水下圆柱壳体结构声辐射因子的试验测量方法。水下声学试验研究表明该方法预报的辐射声压与实测值吻合较好,能满足水下噪声预报的实时性、远场指向性和工况适应性等计算要求。      

Micromachined ultrasonic transducers and arrays based on piezoelectric thick film

This paper reports on the design, fabrication, and characterization of diaphragm-type piezoelectric micromachined ultrasonic transducer (pMUT) and arrays. A combination of piezoelectric composite thick film techniques and silicon micromachining has been proven to be a promising approach for a batch production of pMUTs, especially pMUT transmitter arrays for ultrasound radiation. In this paper, some important issues related to the pMUT element design and micromachining processes are addressed. Thanks to the well-developed processing technology, pMUTs and arrays operating at different resonance frequencies by dimension variation have been successfully fabricated with a high yield for possible mass manufacturing. The characterization of piezoelectric composite thick film will be briefly reported. The performance of the prototype devices has been characterized in terms of vibration modes, dependency of the resonance frequency on bias voltage, nonlinearity, electromechanical coupling efficiency, equivalent circuit, output sound pressure level and directivity of a two-dimensional pMUT array. PACS 85.50.-n; 85.85.+j     

Calculation of Transient Radiation Parameters of Aperture

Transient radiated field from aperture antennas is derived by using the field equivalence principle. For Gaussian pulse excited uniform and tapered planar apertures, transient radiation parameters, such as radiation energy pattern, the half-energy beamwidth and directivity, are calculated. The simulation formulations for calculating the half-energy beamwidth and directivity are presented     

天线方向系数的一类计算逼近方法

天线的方向系数是天线的核心性能指标之一,准确计算方向系数是高性能天线应用的核心要求.本文基于平面近场测试理论、实测数据和快速傅里叶变换算法,系统阐述基于近场测试来数值计算天线方向系数的原理,并进行深入的误差分析.本文选择一种方向图函数和方向系数已知的被测天线,来检验所讨论的误差评估方案.评估分两步实现,第一步,针对这一天线,采用标准的近场测试配置,仿真模拟出(相当于实际测量出)一套平面近场数据.第二步,基于这套近场数据,利用数值积分计算出天线方向系数.本文使用或提出了四种数值算法,分析了提出的后三种算法本身的误差来源,并开发出程序搜索方案,确定出后两种算法的最小误差界.随后,利用这四种数值算法分别得出天线的方向系数.结果表明,计算所得的近场方向系数都比真实方向系数大,但误差不超过0.6 d B.这一结果对实际应用中正确评估基于近场测试的天线方向系数准确性有重要参考价值.     

Analysis on effect of low dielectric permittivity on indium-doped tin oxide based optically transparent terahertz patch antenna

Indium-doped tin oxide based optically transparent rectangular patch antennas are designed to resonate at 750 GHz; one on the glass substrate and the other on the polyimide substrate. Characteristics of both the transparent antennas such as impedance bandwidth, radiation efficiency, directivity and gain are analyzed and compared. Polyimide substrate has a lower dielectric permittivity than the glass substrate. The effect of low dielectric permittivity substrate on the radiation characteristics of the terahertz transparent patch antenna is analyzed. The transparent antenna on polyimide substrate is shown to have gain greater than 3.97dB in 714–795 GHz. The proposed transparent antennas are designed and simulated by using finite element method based electromagnetic solver, Ansys–HFSS.     

Analysis of graphene based optically transparent patch antenna for terahertz communications

With and without multi walled carbon nanotube (MWCNT) loaded graphene based optically transparent patch antennas are designed to resonate at 6 THz. Their radiation characteristics are analyzed in 5.66–6.43 THz band. The optically transparent graphene is deployed as the patch and ground plane of the antennas, which are separated by a 2.5 μm thick flexible polyimide substrate. By shorting the microstrip line and ground plane of the antenna with a MWCNT via, the return loss of the antenna is improved. The peak gain of 3.3dB at 6.2 THz and a gain greater than 3dB in 5.66–6.43 THz band is obtained for antenna loaded without MWCNT. Both the antennas achieved a −10dB impedance bandwidth of 12.83%. Gain, directivity and radiation efficiency of the proposed antennas are compared with conventional transparent patch antennas and graphene based non-transparent antennas. The antenna structures are simulated by using finite element method based electromagnetic simulator-Ansys HFSS.     

阵元间互辐射对基阵指向性的影响研究及试验

针对常规基阵指向性图计算并未考虑阵元间互辐射阻抗影响这一不足,修正了常规基阵指向性图计算公式。将阵元间的互辐射阻抗引入到水声基阵的辐射声场计算中,并依据叠加原理得到包含阵元间互辐射阻抗的基阵指向性图计算修正公式。15元均匀线列阵和5×5均匀平面阵的计算机仿真和消声水池试验结果表明,该修正公式相比于常规公式更能反映基阵指向性图的真实情况。      

超宽带折线型TEM喇叭天线实验研究

基于功率容量和口径匹配的考虑,设计了一种超宽带折线型TEM喇叭天线。相比传统恒阻抗TEM喇叭,主要改进在于采用了同轴到平板过渡的馈入结构,辐射极板改为两段折线形式。通过理论模拟和实验研究对两种天线的输入、辐射特性进行了比较和分析。结果表明:采用960ps宽度的高斯脉冲激励,同轴平板过渡峰值功率传输效率达到85%;相比于恒阻抗喇叭,折线TEM喇叭辐射峰值电场提高30%,H面方向图主瓣宽度由80°压缩至60°,同时改善了700 MHz频带内的传输辐射特性,瞬态脉冲峰值功率容量达到15GW。     

Self-radiation from arrays of niobium Josephson junctions embedded in the open resonator

This paper focuses on self-radiation from arrays of Josephson junctions embedded in a quasi-optical resonator. The mechanism of coupling this radiation to the principal mode of the open resonator is illustrated using experiments and simulations with CST microwave studio software. Comparing the microstrips and dielectric resonators used as the antennas of the series arrays of discrete Josephson junctions, we demonstrate that the dielectric resonator antennas are more effective than microstrips.     

Planar superstrate for dual-frequency RHCP-LHCP array

Patch antennas are low profile, lightweight and most suitable for aerospace and mobile applications. In recent years Fabry–Pérot cavities and meta-surfaces have been used as superstrates to improve the radiation performance of planar antennas (Saenz et al., IEEE Trans. Antennas Propag. 56(4), 2008; Iriarte et al., IEEE Trans. Antennas Propag. 57(1), 2009) such as directivity, mutual coupling on array configuration and bandwidth. These structures usually work for a single polarization and frequency band, while an increasing number of applications require multifrequency operation. In this paper a Fabry–Pérot cavity that works with dual-polarized and dual-frequency patch antennas (rectangular patch excited orthogonally) is proposed in order to design high-directivity dual-frequency RHCP-LHCP Arrays.     

4G antennas for wireless eyewear devices and related SAR

In this paper, we first present a feasibility study to design 4G antennas (700–960 MHz and 1.7–2.7 GHz) for eyewear devices. Those eyewear devices should be connected to the last generation cellular networks, Wireless Local Area Networks or wireless hotspots. Three coupling element type antennas with their matching networks are evaluated in terms of reflection coefficient and total radiation efficiency when the eyewear is placed on the user's head. We also present Specific Absorption Rate (SAR) simulations when the eyewear is positioned over a homogeneous SAM phantom and over a heterogeneous VH (Visible Human) phantom: the SAR levels are compared to international limit values. In a second step, we present experimental results obtained with 3D printed eyewear and coupling elements etched on a classical PCB substrate where the matching circuits are optimized close to the feeding point of the coupling element. Simulated and measured values are in very good agreement: 7 to 16% and 9 to 35% total efficiency are respectively obtained for the low- and high-frequency bands. However, simulated SAR values are somewhat higher than authorized levels with preoccupant high electromagnetic field distribution close to the eye of the user.     

Optimum beam steering of optical phased arrays using mixed weighting technique

In this paper, we present a novel technique for high precision optical beam steering of optical phased arrays (OPA) using mixed weighting method. Optimal OPA parameters are determined to obtain the best beam directivity by minimizing the main lobe width and eliminating grating lobes of the far field diffraction pattern. The quantitative analysis for a fibre-type optical phased array is given. The calculation results demonstrate that the grating lobe level can be distinctly reduced from 80% to 20% of the main lobe level. Thus, the mixed weighting technique proposed in this paper can substantially improve the beam steering efficiency and the beam quality.     

Radiation Properties of Carbon Nanotubes Antenna at Terahertz/Infrared Range

The geometric structure and the terahertz/infrared radiation characteristics of carbon nanotubes dipole antenna arrays have been investigated by CST MICROWAVE STUDIO based on finite integral methods. In terahertz and infrared frequency span, the antenna properties such as electrical field distributions, scattering parameters, standing wave ratio, gain, and two dimension directivity patterns are discussed. Our results show that N × N antenna arrays have higher radiation efficiency than single carbon nanotube dipole antenna. This work was supported by National Natural Science Foundation of China (60571026, 10396160).     

炉内管阵列中声源辐射特性数值研究

炉内换热器结构类似于声子晶体,基于声子晶体理论,采用数值计算与软件模拟得到管阵列中声源不同方向的声传播特性,并将两种方法所得结果进行对比分析,研究声源在管阵列中不同位置处的辐射特性.结果表明：声指向性特性与声源通过管排数量,声源频率是否处在声波禁带有关.