This paper reports a method for bandwidth enhancement of radar cross section (RCS) reduction by metasurfaces. Scattering cancellation is one of the common methods for reducing RCS of target scatterers. It occurs when the wave reflected by the target scatterer and the wave reflected by the cancelling scatterer are the same amplitude and opposite phase. Since the bandwidth of the scattering cancellation is usually narrow, we propose a bandwidth enhancement method by metasurfaces that can control the frequency dependence of the scattering phase. We designed and fabricated a metasurface composed of a patch array on a grounded dielectric substrate. We confirmed that the metasurface gives broader bandwidth of 10dB RCS reduction than the metallic canceling scatterer.
GNSS is an important source for train location, which provides continuous train location possibilities for train operation. As GNSS positioning principle is radio localization using TOA (Time of Arrival), the radio signals are susceptible to the environment its propagated, location accuracy is dependent on the environment. According to the propagation of the radio signals, they can be divided into three categories: LOS (Line-of-Sight), NLOS (Non-Line-of-Sight) and non-visible. To quantify the ground-related satellite signal propagation error applied to typical train localization environments, this paper addresses two sets of statistical properties-based local pseudorange model for both occluded-signal and non-occluded signal. This paper takes urban canyon as an example for analysis. Firstly, the occluded-signal and non-occluded-signal are divided based on the obstacle boundary shown in the skyplot. Secondly, the difference elimination between user and stations based on CORS network is used to extract the ground-related pseudorange error. The 10-hour static observation data show that the mean and dispersion of the local pseudorange error of the occluded-signal are significantly larger than those of the non-occluded-signal, and are negatively correlated with the elevation range. The distribution of the occluded-signal is multimodal, and the GMM model is used to describe it. The distribution of non-occluded-signal shows non-Gaussian, and the steepness of the distribution changes with the elevation, which the T Location-Scale distribution model is used for reasonable description.
Quantum computers, through quantum entanglement and parallelization, offer an intriguing prospect of exponential speedup relative to classical computers for performing numerical simulations. Our interest is in the application of quantum information science to plasma physics in order to develop algorithms which can implemented on quantum computers. Concurrently, we want to test these algorithms on present conventional (classical) computers as large scale, error-correcting quantum computers with long coherence times are not yet forthcoming. Even though the constituents of laboratory and space plasmas are electrons and ions, the physics of these plasmas is dominated by collective processes. Quantum effects that are prominent at sub-atomic scales are averaged out in the statistical description of a plasma. Consequently, we need to render classical plasma physics in the language of quantum mechanics in preparation for quantum computers.
The high scanning rate and scanning linearity of leaky-wave antennas (LWA) have attracted a lot of attentions, since it can reduce both the occupation of electromagnetic spectrum resources and requirements of accuracy for transmitters and receivers, respectively [1]. As shown in Figure 1, the radiation angle of an ideal linear scanning LWA is directly proportional to its frequency, which can be described as:
The paper introduces a novel focal plane array (FPA) design aimed at enhancing the reliability of millimeter-wave backhaul communication links by compensating for the impact of antenna mounting structures’ sways. This FPA is analyzed for an offset parabolic reflector antenna with the 40-cm aperture diameter at W-band frequencies (92-96 GHz). The FPA-fed reflector antenna generates multiple closely-overlapping beams to compensate for the expected antenna tilt range, equivalent to seven beamwidths. This study addresses major challenges associated with FPA design implementation at these high frequencies, including (i) the need for a small ($\leq 0.75 \lambda$) inter-element spacing for the required beam overlap, and (ii) manufacturing complexities of an array element ensuring optimal reflector illumination and impedance matching in the presence of mutual coupling effects. An analytical approach to the FPA pattern shaping for optimal reflector illumination is proposed and successfully validated via numerical optimization and measurements. Furthermore, our analysis of the total reflector-FPA system shows promising results, i.e. 56 – 68% antenna aperture efficiency (49 – 50 dBi directivity) for all beams with compliance to the radiation emission constraints (ETSI class 3) within ±89°.
The upcoming SMART [1] (Space Measurement of A Rocket Released Turbulence) experiment will inject heavy Barium atoms at high speed (~10 km/s) across the local magnetic field into the upper ionosphere at an altitude of approximately 500 km. As these energetic heavy atoms photoionize in sunlight, the local magnetic field traps them. The ions form approximate non-gyrotropic ring-like distributions in velocity space [2], which will generate electrostatic lower-hybrid wave turbulence with perpendicular wavelengths much shorter than the electron skin depth. The lower-hybrid waves grow to a saturated amplitude that is controlled by the rate of induced nonlinear scattering of lower-hybrid waves into electromagnetic magnetosonic and whistler waves with wavelengths large compared to the skin depth, which is one of the fundamental nonlinear processes of weak turbulence. The electromagnetic waves have large group velocities, quickly escape the experimental region, and propagate into the radiation belts where the amplitude is expected to be similar to a large lightning generated whistler.
Plasma probes are simple and inexpensive diagnostic tools which now are used even in hot and hazardous plasmas, where, however, especially the probe casings, often consisting of graphite, might suffer from sputtering, evaporation and chemical reactions between plasma particles and the probe material. An additional detrimental effect frequently detected is that the sputtered-off electrically conductive graphite re-deposits on the boron nitride material usually used for electrical isolation of the probe pins, leading to unwanted and possibly dangerous shunts to ground and/or to adjacent probes, or even short circuits.
Frequency selective surface (FSS) is a kind of spatial filter, which is used in wide applications such as radomes, polarizers, and filters due to its capability of controlling electromagnetic waves [1]. Traditional FSSs consist of a two-dimensional periodic array of unit cells, however, such a single-layer surface often suffers from poor filtering characteristics. Furthermore, designing FSSs with a higher-order bandpass response to achieve a wider transmission band has also attracted intensive investigations [2].
The Magnetospheric Multiscale (MMS) four-spacecraft constellation has provided repeated opportunities to examine plasma, energetic particle, and magnetic field dipolarization behavior in the near-Earth tail region (r~20 RE) for reconnection events during magnetospheric substorms. We have examined several isolated substorm sequences using MMS data that exhibit similar behavior. Upstream solar wind data show strong southward IMF Bz turnings that initiate substorm growth phase sequences. Such signatures are normally detected in the AE index and at geostationary orbit spacecraft (GOES, LANL). All four MMS spacecraft in the near tail region detect cases of strong tailward plasma flow, consistent with near-Earth X-line (NEXL) formation just Earthward of MMS. We thus infer that substorm onset in these cases was initiated by reconnection at X ~ -20 RE. Energetic electron and proton injections along with field dipolarization are also detected at GOES and Los Alamos spacecraft. The data suggests that the primary large-scale substorm plasmoid was rapidly ejected downtail at these times from the plasma sheet at expansion phase onset and this left the thin, residual plasma sheet tailward of r ~20 RE. This allowed the four closely spaced MMS spacecraft to all pass readily back and forth across the thin current sheet. MMS sensors often subsequently saw Earthward plasma flow implying that the NEXL had moved tailward of MMS. We interpret this as the signal of tailward retreat of the X-line during the substorm recovery time. MMS data indicate fast expansion of the plasma sheet thickness with strong energetic electron (E>50 keV) flux increase (including unidirectional particle streaming). Taken together, all these data show a repeatable sequence of expected substorm dynamical features of substorm growth, expansion and recovery phases clearly driven by powerful magnetic reconnection processes. We also describe other large-scale features due to Kelvin-Helmholtz wave activity along the magnetopause boundary. Together these observations show the power and importance of the high spatial and temporal resolution of the MMS constellation.
The goal of hyperthermia treatment planning is to selectively elevate the temperature of cancerous tissue to around $42^{\circ}C$ while avoiding damage to healthy tissue. This is achieved using exterior exciting coils surrounding the affected body part, such as the thigh. However, optimizing the shape and location of these coils presents a two-part challenge. The first step involves solving for the electric field, which determines the Joule heating in different parts of the body. The second step involves determining the resulting temperature distribution, which is dependent on the material properties that change with temperature. To address this inverse optimization problem, this paper proposes the use of the genetic algorithm (GA). By adjusting the shape of the conductor as the temperature changes, the optimal coil distribution is obtained to achieve the desired temperature distribution in a human thigh.
The method of moments (MoM) is well-suited to radiation and scattering analysis of array structures. Domain decomposition strategies for array analysis, where only successive localised solutions are required to converge to the global solution, can yield high computational efficiency. Recently, such an iterative scheme has been proposed, which is denoted the direct coupling technique (DCT). In this paper, an extension to the DCT scheme is discussed, which is shown to improve its convergence. The scheme is suitable for large-scale parallelisation.
We demonstrate optically tunable control of second-harmonic generation in all-dielectric nanoantennas: by using a control beam, we modulate the amplitude and phase of the generated second-harmonic signal. The large predicted tunability of the single meta-atom response paves the way to exciting avenues for reconfigurable homogeneous and heterogeneous metasurfaces.
Indoor imaging scenarios involving complex environments are studied by utilizing a combination of ray tracing and imaging algorithms. The utilized ray tracer is based on computing equivalent sources on a Huygens surface, which can be placed arbitrarily between the antennas and the considered scattering objects. To verify the simulation results we compute synthetic aperture radar (SAR) images for several configurations by utilizing an inverse source based reconstruction algorithm and compare the results to those obtained by a standard backprojection algorithm.
Currently, one bottleneck of the Method of Moments is related to the fast and accurate computation of near-field interactions. These entail the evaluation of four dimensional integrals whose integrand may be singular. Only recently purely analytical solutions have been proposed in the literature. While showing great promises and being already competitive with state-of-the-art methods, these techniques are still perfectible, both in terms of formulation and implementation. In this paper, we build on a technique previously published by the authors and propose a different formulation. While being mathematically equivalent, the proposed formulation is conceptually more intuitive and flexible. The additional flexibility can be exploited to improve the formulation of the analytical solution, further accelerate its numerical implementation and reduce numerical noise arising from the finite double precision.
In this talk, we consider a multiple-input multiple-output (MIMO) channel in the presence of a reconfigurable intelligent surface (RIS). Specifically, our focus is on analyzing the spatial multiplexing gains in line-of-sight and low-scattering MIMO channels in the near field. We prove that the channel capacity is achieved by diagonalizing the end-to-end transmitter-RIS-receiver channel, and applying the water-filling power allocation to the ordered product of the singular values of the transmitter-RIS and RIS-receiver channels. The obtained capacity-achieving solution requires an RIS with a non-diagonal matrix of reflection coefficients. Under the assumption of nearly-passive RIS, i.e., no power amplification is needed at the RIS, the water-filling power allocation is necessary only at the transmitter. We refer to this design of RIS as a linear, nearly-passive, reconfigurable electromagnetic object (EMO). In addition, we introduce a closed-form and low-complexity design for RIS, whose matrix of reflection coefficients is diagonal with unit-modulus entries. The reflection coefficients are given by the product of two focusing functions: one steering the RIS-aided signal towards the mid-point of the MIMO transmitter and one steering the RIS-aided signal towards the mid-point of the MIMO receiver. We prove that this solution is exact in line-of-sight channels under the paraxial setup. With the aid of extensive numerical simulations in line-of-sight (free-space) channels, we show that the proposed approach offers performance (rate and degrees of freedom) close to that obtained by numerically solving non-convex optimization problems at a high computational complexity. Also, we show that it provides performance close to that achieved by the EMO (non-diagonal RIS) in most of the considered case studies.
In [1], the authors have recently introduced a circuits-based approach for modeling the mutual coupling of reconfigurable surfaces, which comprise sub-wavelength spaced passive scattering elements coupled with electronic circuits for enabling the reconfiguration of the surface. The approach is based on a finite-length discrete dipole representation of a reconfigurable surface, and on the assumption that the current distribution on each thin wire dipole is a sinusoidal function. Under these assumptions, the voltages at the ports of a multi-antenna receiver can be formulated in terms of the voltage generators at a multi-antenna transmitter through a transfer function matrix that explicitly depends on the mutual coupling and the tuning circuits through the mutual impedances between every pair of thin wire dipoles. In [2] and [3], the authors have proposed optimization methods for application of the model in wireless networks. Finally, in [4], the authors have formulated the mutual impedances in an integral form.
The “Monitoring Earth’s Evolution and Tectonics” (MEET) project is funded by the Italian “Piano Nazionale di Ripresa e Resilienza (PNRR) – Next Generation EU”, approved by the Ministry of University and Research. MEET is in the framework of the Research Infrastructure European Plate Observing System “EPOS” and aims to strengthen the observational systems dedicated to discovering the Earth’s dynamics, focusing on the Italian territory, particularly those regions more affected by natural hazards.
Fixed-frequency beam-scanning antennas have been increasing interest due to their crucial role in many 5 G and 6 G applications. Although phased antenna arrays provide a versatile approach to beam-scanning, they suffer from mutual coupling problems between adjacent elements and require a complex feed network with significant power consumption [1]. To overcome these limitations, in the last decade, conformal and flexible metasurface covers are proposed for reconfigurability and shaping modification of the radiation pattern characteristics [2], for both single antenna source scenarios [3] and antenna arrays [4]–[6]. In the latter, the cover is used to shift at the metasurface level the beamforming functionalities. Indeed, by implementing the metasurface through Huygens unit-cells ensuring zero reflections and a specific phase-insertion [4], a phase-gradient can be introduced onto the metasurface discontinuity, steering the illuminating beam toward a prescribed direction.
