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There is today a growing need to accurately model the angular scattering response of metasurfaces for optical analog processing applications. However, the current metasurface modeling techniques are not well suited for such a task since they are limited to small angular spectrum transformations, as shall be demonstrated. The goal of this work is to overcome this limitation by improving the modeling accuracy of these techniques and, specifically, to provide a better description of the angular response of metasurfaces. This is achieved by extending the current methods, which are restricted to dipolar responses and weak spatially dispersive effects, so as to include quadrupolar responses and higher-order spatially dispersive components. The accuracy of the newly derived multipolar model is demonstrated by predicting the angular scattering of a dielectric metasurface placed in a vacuum. This results in a modeling accuracy that is substantially better than the standard dipolar model.

In this paper, a fast high-order volume surface integral equation (HOVSIE) formulas solver will be presented to fast calculate the wideband electromagnetic (EM) scattering of anisotropic medium-metallic composite objects. Firstly, in the HOVSIE, the equivalence volume and surface current vectors are discretized by the high-order hierarchical vector basis functions (HVBFs), which can reduce unknowns and facilitate the calculation of the broadband EM scattering. Secondly, a fast wideband algorithm, i.e., the hybrid multilevel accelerated Cartesian expansion-multilevel fast multipole algorithm (MLACE-MLFMA), is integrated to accelerate the matrix-vector multiplication in the iterative solution. At last, in order to improve the iterative convergence, a simple overlapping domain decom-position method (ODDM) based preconditioner is also employed in the proposed HOVSIE enhanced by the MLACE-MLFMA. Summarily, three advantages can be found here: 1) more flexible calculation of the wideband EM scattering; 2) less memory occupation; 3) better iterative convergence. The accuracy, efficiency and flexibility will be demonstrated in the numerical examples.

A novel miniature THz resonator consisting of a finite-length slotted graphene cylinder wrapped around an infinite circular dielectric rod is proposed. A full-wave 3D electromagnetic scattering model is built using the method of moments (MoM) solution of the electric-field integral equation (EFIE) in the spectral domain. In order to gain better understanding of the associated physical effects, the obtained results are compared with a 2D model where the slotted graphene cylinder is assumed infinite. A good agreement between the 3D and 2D models if found. The dependence of the backscattering radar cross-section on the frequency is analyzed. It is found that in both cases this scatterer displays quasi-static Helmholtz resonance response in the sub-THz frequency range, a behavior previously known for perfectly electrically conducting (PEC) slotted cylinders. For both models, in-resonance surface current distributions and far-zone radiation patterns are given. The most important innovation is that due to the use of graphene the Helmholtz-mode resonance becomes electrically tunable.

A flexible, dual-broadband and polarization insensitive coding metasurface is proposed to manipulate electromagnetic (EM) scattering in microwave frequency band. The 1-bit coding units “0” and “1” are formed by the Pancharatnam-Berry (PB) phase based on the same-sized meta-atoms with different orientations. The layout of the coding metasurface is obtained through genetic algorithm (GA). The simulation results indicate that the proposed coding metasurface in this paper can achieve more than 10 dB radar cross section (RCS) reduction in the range of 9.26-12.87 and 14.84-19.35 GHz in the planar case, which is attributed to the reorientation of the reflected energies into different directions by optimizing the coding sequence. The diffuse scattering performance in the dual-wideband is well maintained while the coding metasurface is conformal on metallic cylinder with diverse curvature radii. Moreover, the scattering characteristics of conformal metasurfaces become better with the decrease of curvature radius under the case of the certain size of the flexible metasurface. A flexible coding metasurface prototype is prepared and measured. The experiment results coincide with the numerical simulation ones, demonstrating the outstanding capacity of RCS reduction. The proposed design has potential application value in the field of antenna and stealth of more complex objects.

Funding Information: This work was supported in part by Saab AB. The work of Mikko K. Leino was supported in part by the Walter Ahlstr?m Foundation, in part by the Jenny and Antti Wihuri Foundation, and in part by the Nokia Foundation. Publisher Copyright: © 1963-2012 IEEE. A periodically loaded, low-scattering dipole suitable for the secondary surveillance radar, particularly for identification-friend-or-foe antenna, is presented along with the methods to design such an antenna. The dipole is chopped into the pieces with length that minimizes scattering at 10 GHz, i.e., at the operation frequency of the primary antenna. In addition, optimal inductive loading is found to connect the pieces together so that the formed entity can radiate at 1 GHz, i.e., at the operation of frequency of the secondary antenna. Two meander-line designs are presented to realize the required inductances, and the proposed structures are realized on a printed circuit board. Designed prototypes are analyzed through simulations and measurements, which are in good agreement. The reduction in radar cross section at 10 GHz with the inductively loaded dipoles is at best more than 15 dB, when compared to the reference half-wave dipole. As a consequence, the radiation efficiency at 1 GHz decreases by 0.4 dB withthe inductively loaded dipoles.

This paper presents a design concept of integrative transmitarray antenna (TAA) with gain-filtering and low-scattering characteristics. The integrative TAA element combines a bandpass frequency-selective surface (FSS) element and a square dielectric post with antireflection pyramidal structures (SDPP), achieving the simultaneous control of in-band transmitted waves and out-of-band reflected waves. The FSS element provides the phase-shifting and gain-filtering capabilities around the center operating frequency f0. Meanwhile, the SDPP elements with different heights are utilized to reduce the scattering cross section (SCS) based on the phase cancellation principle. To realize wideband and wide-angle SCS reductions, also to mitigate the in-band radiation degradation, the optimal SDPP arrangement is determined with subarray segmentation. In the experiment, a 42×42-element integrative TAA is designed. Against only the FSS TAA, the simulated results show that loading the optimal SDPP array can realize the scattering reduction in a 10:1 bandwidth up to 60°, including ~5 dB/~10 dB SCS reductions in 0.5~1.5f0/1.5~5f0, respectively. Moreover, the integrative TAA achieves gain-filtering responses with out-of-band suppression levels higher than 20 dB. After fabricating the FSS TAA and the SDPP array by the technologies of standard PCB fabrication and 3-dimensional printing, respectively, the proposed design is further demonstrated by experiments.

Near-field radar cross-section (NF-RCS) of a target depends on antennas’ radiation and can be maximized by adjusting the radiation patterns. It is justified by the power return ratio and can be computed by a comparison method. A theoretical foundation of operational mechanism for phased arrays of antennas is presented to produce conformal radiations for NF-RCS enhancement. The target’s scattering coefficients are extracted from operating the antenna array’s digital phase shifters to steer the resulting beams in this work. The scattering matrix is then analyzed by the singular value decomposition (SVD) to determine the antennas’ excitations for maximum NF-RCS with estimation stability. It results in orthogonal beams for the two-way radiations of antenna arrays to interpret the scattering mechanisms. The feasibility is validated by HFSS full-wave simulations and experimental measurement data.

This work continues the development of the raytracing method of [1] for computing the scattered fields from metasurfaces characterized by locally periodic reflection and transmission coefficients. In this work, instead of describing the metasurface in terms of scattering coefficients that depend on the incidence direction, its scattering behavior is characterized by the surface susceptibility tensors that appear in the generalized sheet transition conditions (GSTCs). As the latter quantities are constitutive parameters, they do not depend on the incident field and thus enable a more compact and physically motivated description of the surface. The locally periodic susceptibility profile is expanded into a Fourier series, and the GSTCs are rewritten in a form that enables them to be numerically solved for in terms of the reflected and transmitted surface fields. The scattered field at arbitrary detector locations is constructed by evaluating critical-point contributions of the first and second kinds using a Forward Ray Tracing (FRT) scheme. The accuracy of the resulting framework has been verified with an Integral Equation based Boundary Element Method (BEM)-GSTC full-wave solver for a variety of examples such as a periodically modulated metasurface, a metasurface diffuser and a beam collimator., Comment: 12 pages, 9 figures

As a practical microwave absorber, besides satisfying the basic broadband absorption performance, the density, cost, and process complexity are considerable factors in radar stealth and electromagnetic protection field as well. Here, a simple and economical structural absorber (SA) is proposed to remarkably broaden the absorption bandwidth of biochar from banana peel. By optimizing the size parameters of the unit cell, the optimal SA shows more than 90% absorption in the frequency range from 3.4 to 18 GHz by employing a lightweight porous carbon derived from sustainable biomass. The lightweight porous biochar endows SA low-density properties with surface density of only 0.574 g cm-2. In such SA, the greatly enhanced absorption performance is mainly originated from the synergistic effect of microscopic and macroscopic scales. By optimizing the pattern design of SA, the effective impedance is matched with free-space in a wide frequency, thus dissipating more electromagnetic waves through multiple resonances and edge scattering effects. Additionally, whether transverse electric or transverse magnetic polarizations, stable broadband absorption can be maintained in a wide incidence angle. Thus, this work provides a new prospect for waste utilization of banana peel and a facile low-cost route to greatly extend the microwave absorption bandwidth.

A 3.4–3.7 GHz dielectric rod waveguide probe for near-field measurements is proposed for applications on the 5G sub-6 GHz band. By implementing a dielectric filled waveguide, the probe achieved a higher resolution and low scattering characteristics through a miniaturized aperture compared to the open-ended waveguide (OEWG) used for conventional near-field probes. An 8 × 8 pixelated grid structure in the dielectric launcher section of the probe is implemented to further reduce the scattering characteristics of the probe. The proposed probe is fabricated and experimentally verified. A coating of silver is applied to the dielectric during a production process. Measurement shows that the –10 dB reflection coefficient bandwidth meets 3.4–3.7 GHz requirement and that the radiation pattern shows a broadside gain of about 3.8 dBi with no null at the forward hemisphere. The results of a scattering characteristic experiment indicate decreased interferer level of 7.3 dB compared to an OEWG.

An efficient EM scattering solution of geometrical uncertainty is fabricated to analyze the scattering from electrically large metallic/dielectric bodies of revolution (BoR). The algorithm started by describing the geometrical shape of the body of revolution with the two dimensional non-uniform rational B-spline curves. Then the relationship can be constructed between the local random variables and the spatial BoR basis functions. Thus, the surface integral equations were represented in terms of independent random variables. Radar cross section can be quickly obtained for perturbed geometrical shapes and the proposed algorithm can save computational resources when compared with the Monte Carlo method. The electrical field integral equation is applied to calculate the metallic BoR targets while the Poggio-Miller-Chang-Harrington-Wu-Tsai integral equation is used to analyze the dielectric BoR targets. Numerical results are given to demonstrate the accuracy and efficiency of the proposed method.

This communication presents a low-profile and low scattering wideband planar phased antenna array with enhanced coupling (PAA-EC). The proposed chessboard-like PAA-EC consists of two different types of low-profile planar antenna elements that have similar radiation performance in the X-band. In addition, the two types of antenna elements have a 180° ± 37° reflection phase difference over the X-band, which guarantees the cancelation of scattering fields for low scattering purpose. On the other hand, an 8×8 reference PAA-EC and a low scattering PAA-EC are developed and fabricated to validate the radiation performance and scattering characteristics. The simulated and measured results show that the proposed low-profile planar PAA-EC operates in the 8-12 GHz band with ±45° scanning range. It also achieves at least 9-dB scattering cross section (SCS) reduction in the 8-15.2 GHz band as compared with the reference PAA-EC.

A filtering transmitarray antenna (TAA) with high-gain in-band radiations at the $X$ -band and an out-of-band low-scattering characteristic is presented in this communication. The proposed composite TAA element consists of a dual-polarized resistive sheet, a fixed bandstop frequency-selective surface (FSS) element, and an adjustable bandpass FSS element also serving as a phase shifter. The resistive element is constructed by inserting a parallel inductor–capacitor (PLC) resonant structure into the center of a lumped-resistor-loaded metallic dipole. A transparent frequency-domain window at the center operating frequency ( $f_{c}$ ) is created by the PLC resonance and is further shared by both FSS elements, hence allowing for high in-band transmission herein. At frequencies below or above $f_{c}$ , the whole composite TAA element acts as an absorber with the bandpass or bandstop FSS elements as the ground planes for the resistive sheet, respectively. The measured results show that the composite TAA can achieve gain-filtering responses with out-of-band suppression levels ≥25 dB, ~8 dB scattering cross section (SCS) reductions in 4–7 and 14–20 GHz, and the radiation gain of 25.3 dBi simultaneously. Because of these merits, the proposed design can relieve the stress on filtering circuit designs, also offering an attractive solution in stealth technology.

Determining the total reflectivity of a manufactured microwave blackbody is a significant problem for the microwave radiometer calibration. In this work, the authors report the progress in testing the standard facility for microwave blackbody reflectivity determination in China. Following a bistatic configuration, the facility is designed to measure the Floquet scattering properties of the array-shaped microwave blackbody, and then based on that, the total reflectivity is extracted, covering a vast frequency range from 6 to 500 GHz. In this work, the key configurations of the facility design, the testing procedures, and the data processes are demonstrated. And the measured results validate the ability of the standard facility in capturing weak Floquet scattering from blackbody targets. Two typical types of array-shaped blackbodies, coated cones, and coated pyramids are included in this study. In the results, the effects of manufacturing imperfections on the measured scattering as well as the total reflectivity are investigated. Furthermore, it is shown in the results of 364, 380, and 452 GHz that the issue of referencing target is worthy of further investigation at very high frequencies. The presented design, testing procedure, and measured scattering properties provide direct reference for the blackbody reflectivity determination in the microwave radiometer calibration applications.

The complex-source (CS) method in conjunction with the spherical-multipole analysis is used to explore and demonstrate the different local phenomena associated with beam-wave scattering by acoustically hard or soft circular cones. The illumination by a beam allows a selective excitation of the scattering phenomena which include reflections, tip diffraction, and creeping beam-waves, where all of them coalesce in the vicinity of the tip. These phenomena depend on the beam’s direction and width, on whether the incident beam is converging or diverging as it hits the scatterer, and on whether the scattering point is within or beyond the beam collimation zone. A generalization of the CS method is introduced that can address converging beams. Special emphasis is given to identifying and explaining the footprint of the local diffraction phenomena. All the numerical details are provided so that the reader can readily explore the problem further. For interested readers, supplementary videos are available on IEEE DataPort that give further insight into wave physics and clarify the direction of wave propagation.

Coverage in mobile communications systems requires an acceptable link gain over a maximum service area, with areas of low link gain being problematic. A standard approach to maximize link gain is to deploy multiple-input–multiple-output (MIMO) antennas that adapt to the changing multipath. There is also recent interest in using active surfaces for adaptive reflections within the multipath environments, and this calls for new ideas at the system and component levels. A critical mechanism in urban propagation is corner diffraction, which illuminates shadow areas but with low link gain. We present a simple add-on system for corners, comprising a simple scattering dipole or scattering array, which can significantly improve link gain in the shadow region over a wide bandwidth. The concept is demonstrated through analysis, simulation, and physical measurement. Only shadowed areas are boosted, and the line-of-sight coverage is not significantly affected. Our demonstration is for a passive, fixed configuration, which is suitable for retrofitting to existing systems.

In this article, a 2-D bistatic attributed scattering center (BASC) model is developed to represent the bistatic return from a perfect electric conducting (PEC) target, based on the derived analytic scattering solutions of seven canonical primitives. Because of its unified and concise form, the BASC model is useful for extracting the geometrical features of targets from synthetic aperture radar (SAR) echoes. However, estimating the parameters of the BASC model is an ill-posed inverse problem. To reduce the ill-posedness, we present a sparse method for bistatic inverse scattering that incorporates differential evolution (DE) into doubly orthogonal matching pursuit (DOMP). The experimental results not only verify the validity of the parameter inversion method but also demonstrate the applicability of the proposed model.

A novel time-domain (TD) integral-equation (IE) technique for analyzing the pulsed electromagnetic (EM) scattering by a 2-D metasurface, represented by a thin highly contrasting screen with combined dielectric and magnetic properties, is presented. The problem under consideration is formulated with the aid of the EM reciprocity theorem of the time-convolution time. Subsequently, upon discretizing the space–time solution domain, the reciprocity relation is cast into two independent matrix forms whose elements are derived analytically using the classic Cagniard–DeHoop (CdH) technique. The thus obtained convolution-type systems of equations are solved for induced electric and magnetic currents through a stable step-by-step updating procedure. The presented numerical method is finally validated using alternative TD solutions.

A novel method based on the combination of scattering-matrix approach and microwave network theory is proposed for the realization of in-band scattering and radiation tradeoff of broadband phased arrays. In particular, the proposed method is able to efficiently predict both radiation performances and scattering characteristics of finite phased arrays with element-independent matching networks (EIMNs)/loads. The in-band scattering and radiation tradeoff of finite arrays can be achieved by the optimization of the loaded EIMNs/loads. Moreover, the accuracy of this method is guaranteed by considering some factors, such as the EM coupling and the edge effects of finite arrays. To verify the effectiveness of the proposed method, a $1\times16$ linear array operating at 8.0–10.0 GHz for scanning up to 45° is designed as the reference prototype and the quasi-coaxial lines are served as the EIMNs. The optimized results show that an obvious scattering reduction is achieved throughout the operating bandwidth in comparison with the reference array. Finally, the optimized linear array is fabricated and measured, and good agreement is obtained to further demonstrate the effectiveness of the proposed method.

We present an application of adjoint analysis for efficient sensitivity analysis and estimation of quantities of interest in the presence of uncertain model parameters in 3-D finite element method (FEM) scattering problems. We demonstrate that the adjoint solution may be leveraged to expedite quantification of uncertainty in the scattering model with extremely high accuracy and vast efficiency improvements in comparison to classic gradient approximation techniques and Monte Carlo (MC) methods. The proposed method is demonstrated for low- and high-dimensional parameter spaces for scattered electric field quantities of interest (QoIs). The results indicate strong agreement with equivalent Monte Carlo simulations for QoI responses and probability densities.

An efficient electromagnetic (EM) optimization technique for radar cross section (RCS) reduction is proposed to achieve stealth. At first, two optimization models, namely coarse and fine models, are constructed to describe the scattering problem. More specifically, the coarse model of shooting and bouncing ray (SBR) method provides worse accuracy results than the fine model of the method of moment (MoM). However, less computational resources are needed for the coarse model when compared with the fine model. Then the SBR method is implemented into the Hummingbird optimization algorithm to obtain the solution of the coarse model. Third, the solution of the coarse model is verified for the fine model by using the MoM. Noted that the impedance boundary condition is applied to accelerate the calculation. At last, the trust region aggressive space-mapping (TRASM) algorithm is used to find the optimal solution between the SBR and MoM methods. By building the mapping relationship between these two models, the optimization efficiency can be significantly improved with high accuracy. Numerical results are given to demonstrate that the proposed optimization technique can be used as a fast tool for the cloaking design of RCS reduction. It is found that the RCS can be reduced largely by using the proposed optimization technique.

In this article, we propose a software–hardware system to improve the quality of image recovery in millimeter-wave (MMW) imaging systems, which aims to benefit from copolarization and cross-polarization scatterings from the target. The proposed dual-polarized antenna array structures consist of two orthogonal subarrays with the same phase center, where each subarray is comprised of $2\times 1$ or $4\times 1$ rectangular edge fed patches and the optimized ground defects. These compact structures, which support radiation of two orthogonal linear polarizations, are designed on a single-layer board using crossover technique. The dual-polarized scattering from the target is captured by the orthogonal subarrays. Furthermore, we introduce a method to combine data obtained from measured dual (co- and cross-) polarized fields to improve the recovery of edge information and increase image quality. The proposed single-layer designs have been fabricated and measured, which exhibits high port isolation and high polarization purity. The cross-polarization level is at least 22 and 25 dB lower than the copolarization pattern for $2\times 1$ and $4\times 1$ arrays, respectively. The measured 10 dB impedance bandwidth of the $2 \times 1$ and $4\times 1$ subarrays ranges from 27 to 29.3 GHz and from 27 to 29.5 GHz, respectively. Moreover, the radiation gains of the $2 \times 1$ and $4\times 1$ subarrays are 9.7 and 12 dBi, respectively. The simulated and measured imaging results show that the cross-polarized data preserve the edge information. Moreover, by using the proposed data combination technique, the quality of the recovered image is improved.

A method to estimate the antenna mode radar cross section (AM-RCS) of a large-scale array antenna is proposed. The scattering pattern multiplication method (SPMM) indicates that the antenna mode scattering field of an array antenna can be obtained by the superposition of all the element mode scattering patterns (EMSPs). However, the mutual coupling and edge effects are ignored in the SPMM, which degrades the accuracy. The active element mode scattering pattern (AEMSP) is the EMSP affected by the mutual coupling and edge effects, which is used to improve the SPMM. Furthermore, based on the assumption that the interior elements of a large-scale array antenna have the same array environment as the middle element of its subarray, the proposed method can deduce the AM-RCS of a large-scale array antenna from the subarray. In addition, the accuracy of the proposed method can be improved with the increase of the subarray size. For verifying the generality and reliability, the proposed method is used to estimate the co-polarization AM-RCS of a 21-element microstrip linear array and $15\times13$ Vivaldi planar array. Compared with the reference method, the proposed method has a low calculated error and saves a lot of memory requirement.

The modification of a conventional plasma window antenna (PWA) into a novel endfire plasma array antenna (E-PAA) and the comparison of their performances are first made. It will be shown that the E-PAA provides better performance than the PWA does. Then, the gain improvement and more importantly the radar cross section (RCS) reductions for stealth properties of the E-PAA are described. A type of feeding and generating system of plasma antenna is given. Simulated and measured results at 300 MHz are discussed, indicating the potential applications of the E-PAA in high-level integrating of radio transceivers onboard for wireless communications.

In this article, the previously introduced finite-integration technique/method-of-moments (FIT-MoM) hybrid method, which is suited for the analysis of electromagnetic scattering by inhomogeneous bodies of revolution (BoRs), is generalized to problems involving multilayered environment. Due to the fact that BoR-typical azimuthal mode decomposition is used, appropriate modal Green’s functions for multilayered media are given. They are responsible for source-field interactions within the region that is external with regard to the FIT part. Formulas for modal decomposition of incident plane wave fields as well as the approximation enabling computing fields produced by sources associated with individual modes in the far-zone are also provided.

Perfect electric conducting (PEC) sectors illuminated by transverse magnetic (TM) and transverse electric (TE) field excitations are studied using the subdomain approach. The subdomain approach, which is similar to the method of overlapping regions (MOR) and the T-block method, requires field expansions in overlapping subdomains and subsequent mode matching. Results for surface current densities are compared with numerical results from COMSOL for problems involving wave scattering from PEC sectors of finite dimension. Results show that the subdomain approach can be as accurate as other numerical solutions and maybe more computationally efficient.

In this article, a new inversion model for 2-D microwave imaging is introduced by means of a convenient rewriting of the usual Lippmann–Schwinger integral scattering equation. Such a model is derived by decomposing Green’s function and the corresponding internal radiation operator in two different contributions, one of them easily computed from the collected scattered data. In the case of lossless backgrounds, the resulting model turns out to be more convenient than the traditional one, as it exhibits a lower degree of nonlinearity with respect to parameters embedding the unknown dielectric characteristics. This interesting property suggests its exploitation in the solution of the inverse scattering problem. The achievable performance is tested by comparing the proposed model with the one based on the usual Lippman–Schwinger equation in both cases of linearly approximated and full nonlinear frameworks. Both numerical and experimental data are considered.

The plane-wave synthesis (PWS) methods are proposed to predict the 3-D and 2-D RCS accurately with multiple scattering effects. The 3-D method employs the propagating plane-wave expansion and can be implemented identically as the near-field far-field transformation (NFFFT). Either the single-level or multilevel implementation can be applied, and in the single-level implementation, a single pseudo-inverse matrix can be used to synthesize plane waves propagating various incident directions. In addition to the 3-D formulation, the single-cut variation of the method (SC-PWS) is also presented for the 2-D RCS prediction. This method is based on the 2-D propagating plane-wave expansion and formulated employing the approximations on the 3-D Green’s function. The numerical and experimental results are presented for the PWS and the SC-PWS.

This work addresses the issue of volume scattering effects within the context of the physical optics (PO) approach. This decreases the modeling and computational effort to simulate scattering from complex material compositions. It is shown that there is a natural progression from the classical PO for perfect electric conductors over the PO for dielectric scatterers toward the proposed formulation. Four specializations of the general algorithm are presented to emphasize the versatility of this approach. Details regarding the implementation of the proposed examples are described. Results for each of the special cases are shown and compared to commercially available full-wave solvers of CST and FEKO.

This communication presents the design of a Huygens’ metasurface (HMS) coating aiming to achieve strongly enhanced invisibility. An analytical formulation for obtaining the required electric surface admittance and magnetic surface impedance is presented. The proposed unit cell consists of a pair of split-rin resonators in the top layer and a metal capacitor in the bottom layer of the same substrate. The geometries are properly designed to provide the required electric surface admittance and magnetic surface impedance for maximum scattering reduction at the operational frequency of 4 GHz. The designed HMS is optimized to realize the required electric surface admittance and magnetic surface impedance for remarkable cloaking purposes. Scattering-cross section (SCS) of uncloaked and cloaked conducting cylinders is obtained with CST Microwave Studio simulation which matches the analytical results. The results show robust scattering reduction with considerable bandwidth for the covered cylinder by the HMS. Furthermore, the obtained results with HMS are compared with the results presented in the literature for cloaking with ordinary metasurface. This comparison emphasizes a much better cloaking performance of the HMS. Considerable cloaking obtained in this communication can be applied for invisibility purposes, sensing applications, antenna isolation, radiation blockage reduction in antennas, etc.

The composite electromagnetic scattering characteristics of movable/rotatable targets coexisting with a randomly rough sea surface are studied by the extended propagation-inside-layer expansion (EPILE) combined with the generalized forward–backward method (GFBM). The composite scattering formulation is given. The multiple scattering characteristics related to the self- and mutual coupling of the targets and sea surface under different parameters are numerically implemented and analyzed.

The WS time delay matrix relates a lossless and reciprocal system's scattering matrix to its frequency derivative, and enables the synthesis of modes that experience well-defined group delays when interacting with the system. The elements of the WS time delay matrix for surface scatterers and antennas comprise renormalized energy-like volume integrals involving electric and magnetic fields that arise when exciting the system via its ports. Here, direct and indirect methods for computing the WS time delay matrix are presented. The direct method evaluates the energy-like volume integrals using surface integral operators that act on the incident electric fields and current densities for all excitations characterizing the scattering matrix. The indirect method accomplishes the same task by computing scattering parameters and their frequency derivatives. Both methods are computationally efficient and readily integrated into existing surface integral equation codes. The proposed techniques facilitate the evaluation of frequency derivatives of antenna impedances, antenna patterns, and scatterer radar cross sections in terms of renormalized field energies derived from a single frequency characterization of the system., Submitted to the IEEE Trans. on Antennas and Propagation

This article presents an active cylindrical metasurface with spatial reconfigurability to achieve dynamic control of the radar cross section (RCS) reduction. The proposed metasurface is designed based on the combination of microwave transmission network and Mie scattering. In particular, by modulating the spatial coding sequence of p-i-n diodes in meta-atoms, the surface impedance of the metasurface can be dynamically changed, which enables an active metasurface to operate in an expanded frequency band via time-division multiplexing technique. Both simulated and measured results demonstrate that the proposed cylindrical metasurface with an ultrathin thickness of $0.018\lambda $ can be continuously tuned across a wide fractional bandwidth over 32% for reducing backward scattering by 10 dB. The proposed metasurface with ultrathin thickness, tunability, conformal profile, and high performance may be applied to wideband RCS reduction, as well as advanced noninvasive detection, communication, etc.

Design, realization, and measurements of an $X$ -band antenna for data downlink (DDL) from low Earth orbit (LEO) satellites are presented. This antenna has an isoflux radiation pattern and operates in dual circular polarization with cross-polar discrimination (XPD) higher than 18 dB on the operational bandwidth. Two solutions are first proposed: one realized by a metasurface with circular strips printed on a dielectric slab and another realized by concentric metallic corrugations. Starting from a basic scattering mechanism of a self-complementary ring-type metasurface excited by a perfectly polarized source, both these antennas are designed by optimizing an equivalent anisotropic impedance surface excited by a short hat feed. The corrugated antenna is realized and measured, confirming that the performances are compliant with the requirements for next-generation DDL antennas.

A linear polarization conversion coding metasurface (MS) is proposed for the radar cross section (RCS) reduction of an object. The designed MS has both wideband RCS reduction and high efficiency of mirror reflection in different frequency bands. The cells are arranged according to different 1 bit coding sequences, and the characteristics of RCS reduction are verified by emulations and experiments. The simulation results indicate that the 01/10 coding MSs possess RCS reduction of greater than 10 dB in 9.5–13.9 and 15.2–20.4 GHz, and the 01/10 coding MS is less sensitive to polarization under normal incidence. The experiment results show that 10 dB RCS reduction is achieved in 10.2–14.0 and 15.3–20.7 GHz under normal incidence, and the relative bandwidth (BW) is 32% and 30%, while high efficiency of mirror reflection is obtained from 14.0 to 15.3 GHz. The experimental results are in good agreement with the numerical simulations. Additionally, for the 01/10 coding MS, the dual-broadband performance is also well maintained under 0°–45° oblique incidence. It is a new and practical method to suppress the scattering of metal objects by the combination of scattering and reflection, which has significant potential in the applications of antenna designs or stealth technology fields.

In the past, most of the transient electromagnetic (EM) analyses focused on the scattering from the target. The transient scattering from rough surface was rarely studied, especially for the layered rough surface. When the target is above the rough surface, it needs to solve the composite transient scattering from the rough surface and the target above it. In this article, we mainly focus on the time-domain (TD) analysis for transient scattering from the target and layered rough surface based on time-domain ray tracing (TDRT) modeling. Applying geometrical optics (GO), the multiple bounces of the transient EM waves between layered rough surfaces and target are traced to obtain the incident field of the irradiated surfaces. Then considering the particularity of the layered rough surface, we modify TD Gordon integral and reduce the TD physical optics (TDPO) integral to closed-form expressions to improve the computing efficiency. To demonstrate the efficiency and accuracy of the proposed algorithm, the transient scattering simulations of the composite model involving target and layered rough surface are compared with frequency-domain ray tracing (FDRT) with inverse fast Fourier transform (IFFT), as well as the results obtained by the multilevel fast multipole algorithm (MLFMA) of FEKO in conjunction with IFFT (IFFT-MLFMA). The characteristics of transient scattering from the composite model are further studied to show the physical phenomenon of scattering mechanisms. Specifically, the transient scattering from the composite model for different parameters is simulated for sensitivity investigation of the transient scattering to the variations in the related parameters.

In this article, a frequency-selective surface (FSS) rasorber based on four metallic FSS functional layers is proposed. The rasorber consists of two metallic FSS absorption layers, two metallic FSS transmission layers, and four corresponding dielectric isolation layers. The top absorption layer and third absorption layer are applied to generate a wide absorption band at high frequency and low frequency, respectively. The second and bottom transmission layers are adopted to generate a low-pass transmission band and bandpass transmission band, respectively. The multilayer structure of the rasorber guarantees the great span, broad bandwidth, and stable scattering characteristics. Moreover, we adopt the equivalent circuit model (ECM), surface current distribution, and loss distribution to comprehend the physical mechanism of the rasorber. The simulated results show that the transmission is −0.67 dB at 6.5 GHz, and the reflection below −10 dB ranges 1.5–4.6 and 9.2–13.7 GHz. To validate our design, we fabricated and measured the prototypes, and the measurement results show reasonable agreement with the simulated results.

The performance of different chipless RFID tag topologies are analysed in terms of Radar Cross Section (RCS) and Bit Error Rate (BER). It is shown that the BER is mainly determined by the tag Radar Cross Section (RCS) once that a standard reading scenario is considered and a fixed size of the tag is chosen. It is shown that the arrangement of the resonators in the chipless tag plays a crucial role in determining the cross-polar RCS of the tag. The RCS of the tag is computed theoretically by using array theory where each resonator is treated as a separate scatterer completely characterized by a specific reflection coefficient. Several resonators arrangements (periodic and non-periodic) are compared, keeping the physical area of the tag fixed. Theoretical and experimental analysis demonstrate that the periodic configuration guarantees the maximum achievable RCS thus providing a global lower BER of the chipless RFID communication system. We believe that the BER is the more meaningful and fair figure of merit for comparing the performance of different tags than bit/cm2 or bit/Hz since the increase of encoded information of the tag is useful only if it can be correctly decoded., Comment: IEEE Transactions on Antennas and Propagation

A volume equivalent shooting and bouncing ray (VESBR) method is proposed to analyze the electromagnetic scattering of dielectric and composite objects with thick dielectric medium. Different from traditional high-frequency methods which utilize surface sources, this VESBR considers the propagation of electromagnetic waves inside the dielectric media and introduces volume equivalent sources to calculate the scattering fields. Compared with traditional methods, two different problems need to be solved in this method, i.e., the ray tracing on dielectric boundary and the volume integral in ray tube. For ray tracing process, a refined formula is proposed to acquire the information of reflection waves and transmission waves on dielectric boundary. For volume integral in ray tube, the integral is transformed to five surface integrals based on Gauss’s law. The simulations of four cases show that the results of VESBR agree well with those of surface integral equation (SIE), while the computation time and the required memory of VESBR are much less than those of SIE.

Reconstructing the exact electromagnetic property of unknown targets from the measured scattered field is challenging due to the intrinsic nonlinearity and ill-posedness. In this article, a new scheme, named the modified contrast scheme (MCS), is proposed to tackle nonlinear inverse scattering problems (ISPs). A local-wave amplifier coefficient is used to form the modified contrast, which is able to alleviate the global nonlinearity in original ISPs without decreasing the accuracy of the problem formulation. Moreover, the modified contrast is more suitable to be the input of the deep learning scheme, due to the unity bound of the modified contrast. The numerical results show that MCS with the modified contrast input performs well in both 2-D and 3-D testing examples in real time after offline training process, even in high-relative-permittivity cases. Compared with the dominant current scheme, a significant improvement is achieved in reconstructing high-contrast scatterers.

A novel dual-polarized frequency-selective polarization converter (FSPC) with low-backward scattering is proposed by combining a bandpass frequency-selective surface (FSS) with a polarization converter. The FSPC, comprised of the top polarization conversion layer and the bottom multilayer bandpass FSS, is formed by introducing two polarization converter bands on both sides of the broadband passband. The four-port equivalent circuit model cascading FSS and polarization converter is proposed and investigated. It is numerically demonstrated that a flat 1 dB transmission window can be obtained from 8.5 to 11 GHz with a 25.6% fractional bandwidth (FBW), and the bandwidth of reflection below −10 dB is up to 92% from 5.6 to 15.13 GHz. Moreover, the proposed device can achieve two polarization conversion bands (5.66–6.9 and 12.8–15.2 GHz) with the polarization conversion ratio over 90%. Besides, by arranging the proposed structure in a checkerboard-like distribution, the backward scattering energy can be reduced in a wide frequency band ranging from 4.5 to 16 GHz. Both simulation and experimental results are in good agreement, which demonstrates our design strategy. Compared with the conventional polarization converters, the proposed design presents an extra frequency-selective performance and, hence, can be a great substitute for the frequency-selective rasorber (FSR).

The difference scattering field has been defined for studying the scattering from a target above a randomly rough surface, which is independent of the selection of rough surface’s length theoretically. It is feasible to extend the conventional finite-difference time-domain (FDTD)-based method for calculating composite scattering field from the target/surface model to the calculation of the difference scattering field. However, the dependence of its solution on the choice of rough surface’s length is significant, which is inconsistent with the intention of the proposal of difference scattering field. In this communication, a novel FDTD-based method is proposed for difference scattering calculation, which adopts different output boundary and incident wave introduction methods compared with the conventional method. The simulation results are given to verify that the proposed method can effectively reduce the dependence on the choice of the rough surface’s length in the difference scattering calculation.

Traveling waves can be supported by long, smooth surfaces under horizontal polarization, such as the wings or bodies of aircraft. A 3-D scattering center (SC) model for traveling waves is proposed in this work. The equivalent location of the SC and the dependence function of scattering amplitude on aspect angles is derived based on traveling wave antenna theory and modified according to real scattering characteristics. Chamber-measured data and full-wave numerical results of a series of geometries are presented to verify the validity of this model in imaging simulation. We find that the equivalent position of this SC is located along a circular arc. In consideration of its polarization characteristics and inverse synthetic aperture radar (ISAR) image signatures, the proposed SC model is promising for used in the model-based automatic target recognition (ATR) to distinguish traveling waves from other scattering components.

Radar cross section (RCS) reduction in the working band of antennas, likely leading to radiation performance change or deterioration, is quite difficult for both antenna elements and arrays. Aimed at this problem, this article presents a novel design strategy of microstrip patch antennas with low in-band RCS. This strategy combines metasurface with antenna from the perspectives of both structure and electrical performance. To be specific, anisotropic metasurface is proposed to replace the conventional patch and behave as the radiating structure of the antenna directly, which does not increase the original antenna size. Through proper connection of the metasurface units, the novel antenna has the same radiation performance with conventional microstrip antenna but low in-band RCS for any polarized incidence. The radiation and scattering mechanism of the novel antenna is carefully elaborated, from which a frequency reconfigurable antenna is also derived by loading varactors. Furthermore, different design strategies of low in-band-RCS antenna element and array are clarified. On the basis of which, a $4 \times 4$ array is developed to achieve in-band-RCS reduction with radiation performance kept. Both numerical and experimental results are demonstrated, and the reasonably good agreements verify the effectiveness of the proposed strategy.