Search

Spatial Kramers–Kronig (KK) medium provides a new platform to achieve omnidirectionally reflectionless absorption without using any gain elements. However, impedance‐matched spatial KK media realized in previous experiments generally have flat geometries with large transverse dimensions. Realizing finite spatial KK absorbers in free space background is important for many practical applications but has not been demonstrated experimentally thus far. Here, the concept of spatial KK relation to cylindrical coordinates is extended and a double‐resonance space‐frequency KK permittivity profile is implemented in a circular geometry. Omnidirectional impedance matching to free space is achieved by truncating the spatial KK absorber at the boundary where the relative permittivity is unity. For the experimental verification, a two‐dimensional 6.6‐wavelength‐diameter doughnut‐shaped anechoic chamber is implemented using the designed spatial KK medium. The measured electric field distributions and experimental imaging confirm the omnidirectionally reflectionless property, demonstrating that this miniaturized anechoic chamber is suitable for electrically small scatterer measurements and imaging. The proposed finite spatial KK absorber and fabricated miniaturized anechoic chamber show great promises for electromagnetic shielding and compatibility. [ABSTRACT FROM AUTHOR]

Microwave imaging based on the inverse scattering problem has attracted many interests in microwave community. To obtain a satisfactory imaging quality, in most cases, a scattering-free environment is desired. In this work, to provide such an environment, we experimentally demonstrate a compact microwave imager integrated with a board-integrated, multiport vector network analyzer and a miniaturized, cylindrical anechoic chamber dedicatedly designed for inverse scattering-based microwave imaging. Based on the principle of electrically induced transparency and the effective medium equivalence, an inhomogeneous, ultrathin, dual-angle absorbing surface was designed, simulated, and implemented, to simultaneously absorb incident and scattered fields. Experimental imaging verified its effectiveness. The proposed imager would help the promotion of practical imaging, showing the potential in on-side imaging. [ABSTRACT FROM AUTHOR]

Noncontact measurements of material parameters have important applications in various fields. In this paper, we propose a noncontact measurement of complex permittivity for electrically small samples at microwave frequencies. Based on Rayleigh approximation, we propose a noncontact measurement approach that can linearly retrieve complex permittivity from the measured impedance change. Using a subwavelength resonance, far-field radiation can be effectively suppressed, and thus the measurement is not notably influenced by unwanted environmental reflection. With the features of simple calibration and noncontact measurement, our method can be widely used in repeated on-site measurements of complex permittivity for small-sized samples at microwave frequencies. [ABSTRACT FROM AUTHOR]

Coupled electromagnetic resonances have various important applications in microwave engineering. Among them, the newly discovered electromagnetically induced transparency (EIT) has attracted much interest. In this paper, we explore the use of configurable electrically induced transparency to construct a microwave “diffractive medium” whose air-like optimally matched transparency and metal-like optimally reflective opacity can be conveniently switched by a binary dc voltage. Different from EITs based on closely coupled resonances, we use decoupled electrically induced resonances to achieve a low-loss transparency along with an optimal impedance matching with air. Satisfying the condition of a lossless perfectly matched layer, such transparency and opacity can be obtained in a wide range of incident angles. We validate our approach by experimentally demonstrating two electrically controlled steering antennas based on diffractive grating effect, both with thin planar apertures and binary digital control circuits. The proposed approach can be extended to higher frequencies, achieving configurable diffractive devices at millimeter-wave and terahertz frequencies. [ABSTRACT FROM PUBLISHER]

Through‐wall imaging (TWI) is a challenging topic in the area of inverse scattering problem based microwave imaging. A TWI solution based on non‐linear inversion algorithms is experimentally investigated. Experiments are performed using a setup with the object surrounded by a closed wall. The inversion is based on a twofold subspace‐based optimisation method, and the results show that the location, shape, size, and dielectric constant of the object can be successfully retrieved from the measured scattering data. This is the first experimental demonstration of TWI of objects behind a closed wall using a non‐linear inversion algorithm. These results imply the potential of TWI for practical applications. [ABSTRACT FROM AUTHOR]

The measurement of complex permittivity of materials is of significant importance in scientific research and industrial applications. In this paper, we introduce the strongly coupled magnetic resonance previously used in high-efficiency midrange wireless power transfer to the noncontact measurement of the complex permittivity of materials. Theoretical analysis, full-wave simulation, and experimental measurements will show that the introduced high- Q-factor resonance can be used to effectively improve both the measurement sensitivity and the impedance matching to the instrument, avoiding the major weaknesses of the traditional magnetic induction method. The measurement setup can be easily calibrated for both solid and liquid materials and can provide the flexibility in noncontact measurement at a variable distance. We expect a wide range of applications to emerge from this novel approach. [ABSTRACT FROM PUBLISHER]