A metasurface-based Direction of Arrival (DoA) estimation method is presented. The method exploits the properties of space-time modulated reflective metasurfaces to estimate in real-time the impinging angle of an illuminating monochromatic plane wave. The approach makes use of the amplitude unbalance of the received fields at broadside at the frequencies of the two first-order harmonics generated by the interaction between the incident plane wave and the modulated metasurface. Here, we first describe analytically how to generate the desired higher-order harmonics in the reflected spectrum and how to realize the breaking of the spatial symmetry of each order harmonic scattering pattern. Then, the one dimensional (1D) omnidirectional incident angle can be analytically computed using +1st and -1st order harmonics. The approach is also extended to 2D DoA estimation by using two orthogonally arranged 1D DoA modulation arrays. The accuracy of 1D DoA estimation is verified through full-wave numerical simulations. Compared to conventional DoA estimation methods, the proposed approach simplifies the computation and hardware complexity, ensuring at the same time estimation accuracy. The proposed method may have potential applications in wireless communications, target recognition, and identification.

This article presents a new design of diverse frequency time modulation for linear frequency-modulated (LFM) radar passive false target spoofing. To realize the passive diverse frequency time modulation, the modulated metasurface is designed accordingly, which is composed by a set of elements, each of which is independently time-modulated to realize specific false targets in the high-resolution range profile (HRRP) of the radar system. Due to the control of the modulation scheme in each element and the combined response of the overall surface, we demonstrate the capability to realize several false target patterns consisting of single and multiple targets symmetrically or, asymmetrically, distributed around the actual target. An HRRP general expression of the time-modulated diverse frequency metasurface at different observation angles is given to show the relation between the generated false targets and frequency components. Since having more flexible capacity of reflected signal manipulation compared to false target spoofing only through time modulation with consistent modulation frequency, the proposed method has enhanced performance on control of false targets at different observation angles. Both simulated and measured results demonstrate the validity of false target manipulation, demonstrating its effectiveness as electromagnetic countermeasure for achieving radar camouflaging and deception.

Space-time-modulated metasurfaces can manipulate electromagnetic waves in space and frequency domains simultaneously. In this article, an analytical design of space-time-modulated metasurfaces with modulation elements composed of two paths, In-phase (I) and Quadrature (Q), is proposed. The model is derived analytically, the space-/frequency-domain manipulations are achieved by designing the dimension and time sequence of I and Q paths. In the specular reflection direction, an objective frequency shift of the reflected first-order harmonic can be obtained. While, in other directions, the opposite first-order harmonic can be easily controlled by changing the dimension of I/Q paths, and the objective first-order harmonic remains unchanged. Furthermore, with a small dimension of I/Q paths, the first-order harmonic can be used for beam scanning by predesigning the start time of the modulation element. To realize the space-time-modulated metasurface with the required periodically time-varying responses, 2 bit unit cells loaded with dynamically switchable pin diodes are used as I/Q modulation. Both the analytical and numerical results demonstrate that space- and frequency-domain manipulations of the reflected fields by the first-order harmonics can be simultaneously obtained. The proposed designs have potential applications in wireless communications, radar camouflaging, and cloaking.

An ultra-wideband (UWB) absorber designed by equivalent circuit model (ECM) is presented in the paper. The structural parameters of multi-layer absorbers were calculated for full-wave simulation so as to verify the accuracy of this method. The simulation results illustrated that the ideal frequency response can be easily and quickly obtained by using ECM, such as single narrow absorption band, dual-band absorption peak and ultra-wide absorption band. A 20 × 20 quadruple-layer absorber was fabricated and tested, which obtained 10-dB radar cross section reduction (RCSR) from 2.1 GHz to 37.5 GHz with bandwidth ratio (BWR) of 17.62:1. The overall dimensions of the unit cell structure is 280×280×14 mm3 (1.96 λmax × 1.96 λmax × 0.098 λmax), and the absorber exhibits stable response on conformal cylinders with central angles of 30° and 60°. Moreover, in order to reduce the influence of high-order modes above U-band, the space mapping (SM) technique was introduced to optimize the absorber’s parameters, which greatly expanded the high frequency absorption bandwidth. A 10-dB RCSR was achieved from 3 GHz to 94 GHz with BWR of 31.33:1.

In this article, we propose a multifunctional space–time-modulated metasurface (STM-MTS) able to perform simultaneously a direction of arrival (DoA) estimation and radar cross section (RCS) manipulation in a single system. The metasurface consists of an alignment of unit cells, whose scattering response can be dynamically controlled to be transparent (full transmissive unit cell) or opaque (full reflective unit cell), according to the amplitude of the modulating signal applied to the loading p-i-n diodes. The DoA estimation is performed behind the metasurface, i.e., in the transmission region, where multiorder harmonics of the incident field whose complex amplitudes are proportional to the DoA are transmitted. The multiple signal classification (MUSIC) algorithm is employed to estimate DoA. Simultaneously, the monostatic/bistatic RCSs manipulation is performed in front of the metasurface, i.e., in the reflection region, where the reflected field is properly tailored by controlling the modulation scheme at unit-cell level. An $X$ -band STM-MTS is fabricated to experimentally verify the feasibility of the proposed design. A DoA estimation errors are less than 2 $^{\circ}$ in the incidence angular range of $-$ 50 $^{\circ}$ , $+$ 50 $^{\circ}$ is demonstrated, whereas monostatic and bistatic RCS manipulations of about 17 and 36 dB are achieved at operative frequency.

A electromagnetic-circuital-thermal-mechanical multiphysics numerical method is proposed for the simulation of microwave circuits. The discontinuous Galerkin time-domain (DGTD) method is adopted for electromagnetic simulation. The time-domain finite element method (FEM) is utilized for thermal simulation. The circuit equation is applied for circuit simulation. The mechanical simulation is also carried out by FEM method. A flexible and unified multiphysics field coupling mechanism is constructed to cover various electromagnetic, circuital, thermal and mechanical multiphysics coupling scenarios. Finally, three numerical examples emulating outer space environment, intense electromagnetic pulse (EMP) injection and high power microwave (HPM) illumination are utilized to demonstrate the accuracy, efficiency, and capability of the proposed method. The proposed method provides a versatile and powerful tool for the design and analysis of microwave circuits characterized by intertwined electromagnetic, circuital, thermal and stress behaviors.