For the last 20 years, the time-reversal (TR) process has been successfully applied to focus pulses in the microwave frequency range and in complex media. Here we examine the specific conditions to obtain the same results but in the sub-THz frequency range. Because of the stronger attenuation at this much higher frequency, it is more challenging to exploit the TR self-focusing property. The TR of pulses is studied in two kinds of complex media: metallic waveguide and leaky reverberating cavity. For each medium, we propose one or two models to assess the quality of the focusing. For the waveguide, we show that the angle of incidence is an important parameter. Based on these results, we perform TR experiments at 273 GHz with a bandwidth that can be as large as 2 GHz. TR experiments are successfully first conducted in a 1 m long and 10 mm diameter straight hollow cylinder and then in a 5 m long and 12 mm diameter curved waveguide. Finally, we present results obtained in a cavity of 72 cm^{3} that leaks through a copper grid. The best focusing is observed with the longer waveguide.
The marching-on-in-time electric field integral equation (MOT-EFIE) and the marching-on-in-time time differentiated electric field integral equation (MOT-TDEFIE) based on a Rao-Wilton-Glisson (RWG) spatial discretization. In both formulations we employ the Dirac-delta temporal testing functions, however they differ in temporal basis functions, i.e. hat and quadratic spline basis functions. These schemes suffer from the linear-in-time solution instability. We analyze the corresponding companion matrices using projection matrices and prove mathematically that each independent solenoidal current density corresponds to a Jordan block of size two. In combination with Lidskii-Vishik-Lyusternik perturbation theory we find that finite precision causes these Jordan block eigenvalues to split and this is the root cause of the instability of both schemes. The splitted eigenvalues cause solutions with exponentially increasing magnitudes that are initially observed as constant and/or linear-in-time, yet these become exponentially increasing at discrete time steps beyond the inverse square root of the error due to finite precision, i.e. approximately after one hundred million discrete time steps in double precision arithmetic. We provide numerical evidence to further illustrate these findings.
The advent of space exploration and space warfare along with the deployment of advanced missiles, unmanned aircraft systems, and modern nuclear reactors has reignited the field of high temperature antennas. In this context, this article surveys the field of antennas that operate under harsh environments that are often characterized by high temperature. In this context, this article surveys the field of high temperature antennas. The choice of the substrate and the conductor for antenna implementation are discussed with emphasis on their thermal and electrical properties. Further, the different fabrication techniques to realize the antenna are discussed. The performance comparison of the different types of high temperature antennas are presented. Finally, the future prospects and inherent challenges in advancing research on antennas for extreme environments are detailed. The article concludes with insights into the new developments in the field of flexible antennas operable in hostile conditions.
This paper aims to develop a new 3D analytical model devoted to the study of nonlinear transient magneto-thermal coupled problems in permanent magnet transverse flux induction heating device (PMTFIHD). Firstly, a 3D analytical solution of magneto-dynamic field problem taking into account the transverse edge effect in the workpiece is derived using variables' separation technique. This transverse edge effect allows determining the exact resulting heating power density, which is the heat source of the transient thermal problem in the work-piece. Secondly, the 3D transient analytical solution of the temperature distribution is obtained by combining variables' separation technique and Green's function method. Then, the previous models are exploited in a transient simulations procedure of the magneto-thermal process allowing the nonlinear physical properties of the part to be taking into account. Finally, the performances of the studied PMTFIHD will be calculated, in order to validate the developed 3D coupled models. The simulation results from the developed models are validated with those obtained by the finite element method and the experimental results.
In this paper, a novel miniaturized dual-band embedded near-zero index (NZI) metasurface-based patch antenna is presented. A new methodology based on loading a narrowband microstrip patch antenna (resonating at 4.6 GHz) by a metasurface embedded in the middle of the antenna's substrate is introduced. The loaded antenna has a dual-band resonance of bandwidth of 15% and 43% at 2 GHz and 4.6 GHz, respectively. The metasurface layer is an array of square holes such that there is no hole below the patch. The metasurface layer is designed as a near-zero-refractive-index material (NZRIM). By controlling the phase reflection properties of the structure, the antenna gain is increased by 5.5\,dB, original bandwidth increased ten times and the front-to-back ratio improved from 7 to 187. Also, footprint miniaturization of 56.5% with a maximum size of (1.9λ_{0})^{2} is obtained. To the best of the authors' knowledge, such enhancement is the largest to date.
Two types of quad-band millimetric-wave four-port MIMO antenna systems are proposed for the forthcoming generations of mobile handsets. A novel printed antenna is introduced to be the single-element of the MIMO antenna system. It is shown that the proposed MIMO antennas are capable to produce both spatial and polarization diversities that enhance the performance of mobile communications. A co-polarized four-port MIMO antenna is proposed to provide spatial diversity whereas another cross-polarized four-port MIMO antenna is proposed to produce both spatial and polarization diversities. It is shown that the two types of MIMO antennas can operate efficiently over the four frequency bands centered at 28, 43, 52, and 57 GHz. Prototypes are fabricated for the proposed MIMO antennas for the sake of experimental evaluation. Both the experimental and simulated results show that the achieved bandwidths, at the four operational frequency bands, are 0.6, 0.6, 1.8, and 1.5 GHz, respectively. Also, the radiation efficiencies calculated at the four operational frequencies are 86.5%, 87.5%, 89.2%, and 90.0%, respectively. The dimensions and the results concerning the performance of the proposed MIMO antennas are compared to those of other designs for MIMO antennas available in some recently published work.
This paper presents a method based on the elliptical representation of D-Q currents to detect and quantify an Inter-Turn Short-Circuit (ITSC) fault in windings of a Doubly Fed Induction Machine (DFIM). ITSC is said to be an evolving fault, so it is essential to detect it at an early stage to avoid damage on the machine. Therefore, the method should be able, on the first hand, to detect the defect and, on the second hand, to quantify its severity. Moreover, this study requires less computation time than classical methods such as harmonic analysis. In this paper, current data are acquired at a sampling frequency of 1 kHz. This method is successful with this low data sampling rate. In order to validate this study, a theoretical analysis with two models of different DFIM powers (0.3 kW, 0.25 kW and 11 kW) is carried out (healthy case and faulty case: presence of ITSC), and these results are confirmed by using platforms including Doubly Fed Induction Machines (DFIMs) and Data Acquisition (DAQ) system.
In this manuscript, plasmonic metal conductors such as Silver, Gold, Aluminum, Copper, Chromium, Tungsten, Titanium, and Nickel are investigated on a T-shaped nano dipole antenna using dielectric materials such as Silicon Dioxide, Zinc Oxide, Indium Tin Oxide, and Silicon Nitride. The optical properties of the conductors and dielectric materials are modeled using Drude and Lorentz dispersive models, respectively. It is observed that the Aluminium metal supports high quality plasmonic oscillations for a wide range of Terahertz frequencies. The Aluminium metal also shows high losses occurring at the Terahertz frequency among the other metals. The Gold and Silver can resonate in the visible region and have moderate losses compared to the other plasmonic metals. It is noticed that the near-zero permittivity point of the Silicon Dioxide substrate occurs at 2875 THz which is much greater than the other three substrates. Further, it is observed that on the Silicon Dioxide, Zinc Oxide, and Silicon Nitride substrates the Silver Nano dipole antenna shows the maximum directivity of 6.615 dBi, 5.671 dBi, and 5.709 dBi, respectively. The Aluminium nano-antenna gives the maximum directivity of 5.066 dBi on the Indium Tin Oxide substrate. The Silver-Silicon Dioxide Nano-antenna will be suitable for the terahertz optical wireless communication.