Dynamic structural color can empower devices with additional functions like spectrum and polarization detection beyond display or imaging. However, present methods suffer from narrow tuning ranges, low throughput, or bulky volumes. In this work, a tunable filter composed of a dichroic metagrating Fabry-Perot cavity and liquid crystal (LC) material is proposed and investigated. By modulating the polarization of the incident light with the LC, the color response can change from blue to green and deep red due to the `mode jumping' effect, with a tuning range of around 300 nm. Besides, we experimentally demonstrate the use of this device as a spectral imager in the visible range. Experimental results show that spectral resolvability can be around 10 nm, with the largest peak wavelength in accuracy of ~5 nm. This approach shows superior performance over traditional liquid crystal tunable filters in low light conditions and indicates the potential of dynamic structural color for miniaturized spectroscopic applications.

Frequency diversity array (FDA) can generate distance and angle dependent ``S'' beam patterns, but there is a problem of distance and angle coupling, which can be well solved by using nonlinear frequency offset in recent years' research. The rotational symmetry of the arc-shaped structure brings the beam scanning capability of the array antenna within a range of 360°, which can realize the all-round monitoring of the target position, and provides a more flexible method for radar communication. In this paper, a nonlinear frequency offset based frequency diversity arc array (FDAA) beam scanning method is proposed, which activates the selection matrix according to the target direction. In order to form equal phase plane beam scanning, phase compensation between array elements is carried out, and three kinds of nonlinear frequency bias are introduced to simulate beampattern synthesis. Compared with the traditional linear frequency offset FDAA, the numerical simulation results verify the feasibility and effectiveness of the scheme.

This paper presents the deep learning assisted distorted Born iterative method (DBIM) for permittivity reconstruction of dielectric objects. The inefficiency of DBIM to reconstruct strong scatterers can be overcome if it is supported with Convolutional Neural Network (CNN). A novel approach, cascaded CNN is used to obtain a fine resolution estimate of the permittivity distribution. The CNN is trained using images consisting of MNIST digits, letters, and circular objects. The proposed model is tested on synthetic data with a different signal-to-noise ratio (SNR) and various contrast profiles. Thereafter, it is verified by means of experimental data provided by the Institute of Fresnel, France. Reconstruction results show that the proposed inversion method outperforms the conventional DBIM method in terms of accuracy as well as convergence rate.

The backscattering of electromagnetic waves incident on a rotating metallic wind turbine (WT) is analyzed by using the Physical Optics method. The model developed is general and allows the computation of the spectral Doppler shift of the backscattered waves. All the parameters involved are taken into account, relative to incident wave direction, wind horizontal direction, WT geometric and electromagnetic properties. Numerical computations are carried out for various cases and presented relative to a search radar.

In this study, a novel Switched Beam Antenna (SBA) system is proposed and experimentally validated for C-Band applications. The system is made up of a 4 × 4 Butler matrix, whose outputs are connected to four square-looped radiator antenna elements. The originality of the proposed work depends on the construction of a miniaturized beamforming network with minimal complexity, low loss, and low expense. Moreover, designing a system with a broad frequency range enables its use in a variety of applications. Miniaturization is achieved by eliminating the crossover and integrating the 45˚ shifter into the 90˚ hybrid coupler, as well as tilting the antenna array (i.e., making the Butler matrix output and the feed line of the antenna element orthogonal). The simulated results of the phase difference between the suggested Butler matrix outputs closely match the -45˚-135˚ theoretical calculations. The SBA measured results show a wide bandwidth and low insertion loss of 63.64% (4.21-8.14 GHz) and -4.89 dB, respectively. Four orthogonal beams are produced by the proposed structure's input ports 1-4 when they are excited. These beams are aligned at angles of -10˚, 60˚, -60˚, and 10˚ at 5.7 GHz.

The bodies of platform staff workers in high-speed railway stations absorb induction electric field when exposed to the electric field environment of contact wires with 25 kV high-voltages. To analyze the safety of electromagnetic exposure of the station platform staff workers with different numbers of tracks, this paper establishes a model with 6 tracks, 2 platforms, and 4 staff workers on the platform simulating the actual situation. It then analyzes the distribution of induction electric field present in their human body tissues, which in the electric field environment is generated by the high voltages of the contact wires of 1 track, 3 tracks, 6 tracks, respectively. Calculation results show that the maximum induction electric field of staff worker on the platform with different track numbers appears at the scalp, and the electric field intensity levels in the skull and brain are relatively small. For example, on the two platforms with 6 tracks, the maximum induction electric field of the staff worker is found in the scalp, and the values are 58.86 times and 1688.52 times of those of the skull and brain, respectively. For the staff worker at the safety white line, the maximum induction electric field in a human central nervous system is 0.61 mV/m, which is far less than the basic limit of 100 mV/m occupational exposure in the International Commission on Non-Ionizing Radiation Protection (ICNIRP) guidelines. With the increase in the number of tracks, the maximum induction electric field of the staff also increases correspondingly at the same position. Research results can provide data reference for the formulation of electromagnetic protection and standards for high-speed railway platform staff workers.

This article focuses on the low-frequency magnetic shielding of double-layer conducting plates with periodic circular apertures. The shielding effectiveness (SE) is measured as the insertion loss of the plates when they are placed between a pair of coaxial loops, one for magnetic field emission and the other for receiving. Our experimental results show that the SE sharply increases with the layer-to-layer spacing increasing from zero to the aperture diameter. For aluminum plates with 1 mm thickness, 20 mm unit cell and 10 mm aperture diameter, the enhancement is approximately 10 dB and 20 dB for 3 mm and 9 mm spacing, respectively. In addition, the effect of the lateral deviation on the SE is evident only if the spacing is smaller than the aperture diameter.

The near and far field EM responses over layered media have long been exploited in diversified applications such as remote sensing, monitoring and communication. In this work, we utilize the near field dependence of the EM fields of a three layered structure resembling air-sea ice-sea water to estimate the thickness and dispersion characteristics of sea ice using deep learning technique. We explore two key methods of field measurement termed as the fixed and scaled sweep methods. In the fixed radial sweep method, the receiver distance and height from the source are kept constant, and in the scaled sweep method both the receiver distance and height are set as a scaled function of the operating wavelength. A synthetic training dataset has been generated (using analytical computation and FEM simulation) in the low MHz band, which is used to train a deep learning model. The model is tested on different test datasets with frequencies inside, below and above the training limits. Even though the fixed sweep method is simpler to implement, the scaled sweep appears to perform better across the wide range of test frequency, both in and outside the training range. When the test frequency is inside the training range, the percentage errors for thickness, dielectric constant, and loss tangent were found to be <2%, <10%, and <5%, respectively, for the fixed radial sweep, whereas for the scaled sweep the percentage error is < 1% for all three measurement parameters. When the test frequency deviates further from the training range, the percentage error gradually increases. Later, we investigate the problem of determining sea ice thickness assuming a priori knowledge of sea ice dielectric parameters, and results show that the model estimates the thickness of the sea ice bulk with error as low as 0.1%.

High resolution Synthetic Aperture Radar (SAR) images are affected by speckle noise, which considerably reduces their visibility and complicates the target identification. In this paper, a new Compressive Sensing (CS) method is proposed to reduce the speckle noise effects of complex valued SAR images. The sparsity of the SAR images allows solving the CS problem using Multiple Measurements Vector (MMV) configuration. Therefore, a special weighted norm is constructed to solve the optimization problem, so that the Variance-Based Joint Sparsity (VBJS) model is used to calculate the weights. An efficient Alternating Direction Method of Multipliers (ADMM) is developed to solve the optimization problem. The obtained results using raw complex-valued measurements with ground truth demonstrate the effectiveness of the proposed despeckling method in terms of both image quality and computational cost.

Studies of soil moisture with Global Navigation Satellite System (GNSS) have gained the attention of several researchers. Multipath amplitude, multipath phase, and multipath frequency are multipath observables that are utilized in the study of soil moisture. However, an inter-comparison of the performance of these parameters for soil moisture under different roughness and vegetation conditions is very much required to have a better insight so that more robust inversion algorithm for soil moisture retrieval with multipath observables can be designed. Therefore, this paper analyses the performance of these multipath observables for soil moisture over bare smooth soil, rough surface, and vegetated soil. Two different fields have been investigated to include the location variability. Navigation with Indian constellation (NavIC) multipath signal has been used in this study. Statistical parameters such as correlation coefficient (R), Root Mean Square Error (RMSE), and sensitivity have been determined to study the performance of multipath observable for soil moisture under different surface roughness and vegetation conditions.

A flexible, planar electrically small antenna (ESA) with omni directional radiation pattern is designed and fabricated for GPS and WLAN applications resonating at 1.5 GHz and 3.7 GHz. The design consists of a circular loop attached with 3 rectangular bars, and it is fed by a 50 Ω feed line. The circular loop in the antenna provides impedance matching. Generally, these electrically small antennas have narrow bandwidth. Here the antenna is fabricated on a polyimide substrate having a thickness of 0.1 mm, ε_r of 3.4 mm, and it occupies a size of 38 mm x 34 mm. Electrically small antenna is designed at 1.5 GHz, 3.7 GHz, and the parameters that are measured are S₁₁, VSWR, ka values, quality factor, and radiation patterns.

In this paper, a low profile horizontally polarized wideband omnidirectional antenna is presented, which consists of a simple single-layer dielectric substrate planar printing structure. The bottom of the substrate is loaded with six Vivaldi slits to achieve omnidirectional radiation. An equal-amplitude 1:6 power-division network is printed on the top of the substrate to provide a uniform feed. In addition, rectangular slots etched on each radiation element conduce to the enhancement of high-frequency gain and improvement of impedance matching. The antenna has 58.8% impedance bandwidth (2.8-5.13 GHz, VSWR<2) and low profile height of 0.009λ_min, and it is convenient to fix under the ceiling of buildings and could radiate well in indoor place. The radiation mode in the whole operating frequency band is stable, and the cross-polarization is less than -20 dB, which completely covers the 5G NR-n77/78/79 band.

An asymmetric coplanar waveguide (CPW) fed wideband circularly polarized monopole antenna with a slot structure is proposed in this article. Phase gradient metasurface (PGM) is placed beneath the monopole to improve the gain. Circular polarization (CP) is achieved over wide bandwidth by combining the monopole and slot modes. The asymmetric CPW-fed monopole antenna provides CP at lower frequencies, and slot mode provides CP at higher frequencies. The asymmetric ground plane in the monopole and asymmetric strips in the slot are combined to produce wide axial ratio bandwidth. The proposed design's detailed construction and operation are discussed with experimental validation. The proposed wideband CP antenna provides an impedance bandwidth of 95.46% and axial ratio bandwidth of 67.61%. The peak gain of 5.2 dBic is obtained at 2.35 GHz with 2 dB variation over operating bandwidth. The obtained radiation patterns provide good broadside radiation with better cross-polarization levels than co-polarization.

Carbon fiber wound hydrogen tanks are widely used in the field of new energy, but their complex multilayer structure makes it difficult to conduct nondestructive testing/structural health monitoring (NDT/SHM). In this paper, electromagnetic tomography (EMT) is used for noncontact in situ defect detection on a carbon fiber wound hydrogen tank. According to its structural characteristics, an open U-shaped sensor array that fits the curvature of the tank body is designed. To improve the quality of reconstructed images, an iterative image reconstruction algorithm based on a composite sensitivity matrix (CSM) is proposed. To verify the performance of the method, the method in this paper is compared with linear back projection (LBP), the Landweber iterative algorithm and the Tikhonov regularization algorithm, and the image quality is evaluated by comparing the image relative error and correlation coefficient. Both simulated and experimental results show that the method proposed in this paper is more accurate in defect localization and higher in quality than traditional image reconstruction algorithms.

This paper presents a novel theoretical and numerical approach for an infinitesimal current source (ICS) located in a planar isotropic multilayer medium. Using the mixed-potential integral equation (MPIE) formulation for depicting the electromagnetic disturbance created by the ICS, a detailed definition of Green's functions of Lorenz potentials and fields is provided in this paper. The proposed Green's functions are valid for the considered multilayer isotropic medium, which can have arbitrary layer parameters. This paper also analyzes two commonly observed special cases of the multilayer medium - the multilayer soil including air and the multilayer lossless dielectric - and the proposed equations are modified to meet the requirements of the medium. Green's functions can be obtained from the systems of linear equations proposed in this study. In comparison to other approaches, the advantage of the proposed procedure is that the solutions of the equations are immediately obtained in any field layer of the multilayer medium. In addition, the proposed system of linear equations can be solved easily using well-known numerical computation methods. Furthermore, this paper offers an alternative way of obtaining Green's functions, which are closed-form expressions for the kernels of spectral-domain Green's functions.

The safety of the electromagnetic environment of wireless power transfer (WPT) systems is one of the prerequisites for the application of wireless charging technology for electric vehicles (EVs). The electromagnetic characteristics of a wireless charging EV with a new 7.7 kW WPT system were modeled and analyzed in this paper. Firstly, a complete model of the magnetic coupler was built as a source of electromagnetic radiation, and an external excitation source was added by coupling the resonant coils to the double inductor-capacitor-capacitor (LCC-LCC) topology circuit model. Secondly, the finite element analysis software COMSOL Multiphysics was used to solve for the magneto-quasi-static values to verify the electromagnetic safety of the wireless charging process. Then, two charging scenarios were investigated when the GA and VA aligned and misaligned, involving lateral offset and longitudinal offset cases. Finally, the simulation results were compared and analyzed, showing that the values of electromagnetic fields become higher as the offset distance increases. In worst-case scenarios, the highest magnetic flux density (1.1 μT) is observed in the virtual plane of the test on the left side of the vehicle, which occupies only 17.6% of the limits specified in ICNIRP 1998 (6.25 μT), indicating a good EMF safety performance of the wireless charging system.

The equation of motion for a test particle moving in given fixed external fields is analyzed and compared to the corresponding equation of motion derived from the Darwin Lagrangian for a system of interacting charged particles. The two approaches agree as long as the part of the electric field that arises from the partial time derivative of the vector potential is taken into account. It is, however, only via the Darwin approach that the origin of this field can be understood as arising from a breakdown of the test particle approximation. Applying the formalism to an electron moving outside a long solenoid results in a classical analog of the Aharonov-Bohm effect.

A novel approach is presented to achieve improved sensing performance using a one-dimensional (1D) hyperbolic graded photonic crystal (PC). The graded structure achieves refractive index modulation that varies hyperbolically with layer depth, due to its graded index geometry. Porous materials are employed to facilitate analyte infiltration. The reflectivity and sensing performance of the proposed graded and non-graded geometry is evaluated using the transfer matrix method (TMM). The Sensing capability of the graded structure is evaluated analytically by infusing different analytes within the cavity, considering various cavity widths and incidence angles. At a 40-degree angle of incidence, the analytical results demonstrate that the suggested graded structure exhibits a maximum sensitivity of 469 nm/RIU, along with a detection limit and FOM of 9.1×10^-3 and 125 RIU^-1, respectively. The detailed electric field confinement of the graded geometry is also carried out at the interface. The proposed structure outperforms conventional non-graded structures with a 114% higher sensitivity. The bio-photonic design can easily be implemented and provides high performance compared to previous works that employ exponentially graded structures. The suggested biosensor can detect even minor fluctuations in the refractive index of blood serum samples with different cholesterol concentrations.

We present an ultra-compact modular division (de) multiplexer [(de) MUX] based on the tilted lithium niobate waveguide, an asymmetric directional coupler (ADC) composed of silica-lithium niobate waveguide (SLNW) and lithium niobate waveguide (LNW) for the modular division multiplexer. The TE0 and TE1 modes were optimized by using the finite element method (FEM). By rationally designing the size of SLNW waveguide and LNW waveguide, TE0 mode light is injected into the In1 port of LNW waveguide, TE0 mode light is converted to TE1 mode in the coupling zone, and transmitted in the SLNW waveguide, output from the Out2 port. It show that the coupling length of this MUX is only 6 μm. At a working wavelength of 1.55 um, when TE0 enters the coupling area from port In1, the mode is coupled and converted to TE1; the TE1 mode is output from Out2; the value of IL is 0.87 dB; and the value of MCE is 99.5%. When TE0 enters from port In2, the TE0 mode is output from Out2, with 0.1 dB for IL, 99.7% for MCE, and -25 dB for CT.

In this paper, a compact planar dual-band circular-shaped polarization-dependent electromagnetic band gap (DCS-PDEBG) structure operates at 2.97 GHz and 7.77 GHz in y-direction and 3.14 GHz and 10.90 GHz in the x-direction. A proposed DCS-PDEBG structure consists of a circular patch inside a square patch with a slot at the center, and the established arrangement gives additional capacitance and compact size. The simulation of the DCS-PDEBG is carried out using the Finite Element Method (FEM) of Ansys High-Frequency Simulator (HFSS) and experimentally validated. A truncated microstrip line (TML) method is used to measure the band gap of the proposed planar DCS-PDEBG structure. Experimental results agree well with simulation one. The periodic size of proposed DCS-PDEBG structure is 0.13λ_{2.97 GHz} × 0.13λ_{2.97 GHz}, which is a good candidate where compact size is highly desired.