Microwave Anechoic Chamber Research Articles

Independent dual-beam control based on programmable coding metasurface

Programmable metasurfaces incorporating with tunable materials or components are emerging as an attractive option to realize reconfigurable manipulations of electromagnetic (EM) behaviors in real-time. Many efforts have been devoted to the realization of active EM manipulations of the metasurface and significant progress has been achieved, showing their unprecedented ability to arbitrarily manipulate wavefronts in dynamic functions. However, most of the existing multi-beam metasurfaces are based on passive building blocks, only possessing one or a few functions, which cannot provide tunable and independent multi-beam control, thus limiting their further uses in wireless communications. Hence, a 1-bit coding metasurface with high-efficiency, programmable, and independent multi-beam control is proposed in this paper, providing dynamic EM responses with real-time reconfigurability, and controlled by external digital circuits through direct current (DC) bias networks. Specifically, the meta-atom loaded with PIN diodes is employed to achieve independently tunable phase characteristics, thus complex EM functions can be manipulated by redistributing the spatial phases of the metasurface. Symmetric/asymmetric independent dual- and multi-beam manipulations are analyzed theoretically and simulated by EM software. Then as an experimental verification, a metasurface consisting of 14 × 14 meta-atoms is fabricated and tested in a standard microwave anechoic chamber, and the measured results accord well with the simulations. The proposed metasurface has promising ability to generate the arbitrary and independent multi-beams, which may largely enhance the information capacity of the metasurfaces, offering untapped potentials in wireless communication systems.

A Microwave Three-Dimensional Imaging Method Based on Optimal Wave Spectrum Reconstruction

Limited by the Shannon–Nyquist sampling law, the number of antenna elements and echo signal data of the traditional microwave three-dimensional (3D) imaging system are extremely high. Compressed sensing imaging methods based on sparse representation of target scene can reduce the data sampling rate, but the dictionary matrix of these methods takes a lot of memory, and the imaging has poor quality for continuously distributed targets. For the above problems, a microwave 3D imaging method based on optimal wave spectrum reconstruction and optimization with target reflectance gradient is proposed in this paper. Based on the analysis of the spatial distribution characteristics of the target echo in the frequency domain, this method constructs an orthogonal projection reconstruction model for the wavefront to realize the optimal reconstruction of the target wave spectrum. Then, the inverse Fourier transform of the optimal target wave spectrum is optimized according to the law of the target reflectance gradient distribution. The proposed method has the advantages of less memory space and less computation time. What is more, the method has a better imaging quality for the continuously distributed target. The computer simulation experiment and microwave anechoic chamber measurement experiment verify the effectiveness of the proposed method.

Insect Multifrequency Polarimetric Radar Cross Section: Experimental Results and Analysis

The measurement of insect radar cross section (RCS) is a prerequisite for the studies such as the quantitative estimation of insect population density and the identification of insects using entomological radar. In this article, we established a multiband polarimetric RCS measurement system in the microwave anechoic chamber. The targets’ range profile at different frequencies can be obtained based on the step frequency continuous wave, and meanwhile the clutter elimination and polarimetric calibration were applied to reduce the measuring error. The multifrequency (X-/Ku-/Ka-bands) polarimetric RCSs of 169 insects belonging to 21 species were measured and reported, which is the first time to systematically present the multifrequency polarimetric RCSs of insects. The mass of all specimens range from 25.6 to 964 mg, and their ventral-aspect RCSs range from −57.47 to −32.17 dBsm at X-band, from −48.27 to −33.87 dBsm at Ku-band and from −69.76 to −36.40 dBsm at Ka-band. For small insects less than 300 mg, the HH polarization RCS increases rapidly with frequency at X-band and fluctuates with the frequency at Ku-band, while the VV polarization RCS increases monotonically with frequency at X- and Ku-band. For larger insects, the HH polarization RCS decreased slowly with frequency at X-band and fluctuates with the frequency at Ku-band, while the VV polarization RCS increases with the frequency, then reaches the maximum, finally fluctuates with the frequency. At Ka-band, the measured polarization RCS versus frequency curves are smooth and all show similar variation. The measurement results verify the effectiveness and accuracy of the established system.

Anechoic chamber measurements for circular isar imaging at Mersin University’s Meatrc Lab

Inverse synthetic aperture radar (ISAR) imaging is a reliable detection and classification technique for maneuvering targets at near and far-field ranges. In this study, we examine the near-field circular (turntable) ISAR imaging by conducting various real measurement experiments that were performed in the microwave anechoic chamber of the Mersin University’s MEATRC laboratory. The backscattered data were collected via a vector network analyzer that works as a Stepped Frequency Continuous Wave (SFCW) radar and for a number of simple and complex metal objects. The collected raw data were calibrated by using the backscattering data of a canonical object and then focused by applying a near-field backprojection image reconstruction algorithm. The resultant circular ISAR images demonstrate successful and well localized detection of various types of targets even though they are camouflaged by clothing. The obtained results reveal the preliminary efficacy of C band ISAR imaging in concealed object detection problem encountered at security checkpoints such as airports.

Wireless Communications With Reconfigurable Intelligent Surface: Path Loss Modeling and Experimental Measurement

Reconfigurable intelligent surfaces (RISs) comprised of tunable unit cells have recently drawn significant attention due to their superior capability in manipulating electromagnetic waves. In particular, RIS-assisted wireless communications have the great potential to achieve significant performance improvement and coverage enhancement in a cost-effective and energy-efficient manner, by properly programming the reflection coefficients of the unit cells of RISs. In this paper, free-space path loss models for RIS-assisted wireless communications are developed for different scenarios by studying the physics and electromagnetic nature of RISs. The proposed models, which are first validated through extensive simulation results, reveal the relationships between the free-space path loss of RIS-assisted wireless communications and the distances from the transmitter/receiver to the RIS, the size of the RIS, the near-field/far-field effects of the RIS, and the radiation patterns of antennas and unit cells. In addition, three fabricated RISs (metasurfaces) are utilized to further corroborate the theoretical findings through experimental measurements conducted in a microwave anechoic chamber. The measurement results match well with the modeling results, thus validating the proposed free-space path loss models for RIS, which may pave the way for further theoretical studies and practical applications in this field.

Parametric Synthetic Aperture Radar Image Recovery for Multiple Linear Structures: An Image Domain Approach

The linear structures in synthetic aperture radar (SAR) images can provide important geometric information regarding the illuminated objects. However, the linear structures of various objects sometimes disappear in traditional SAR images, which severely affects the application in automated scene analysis and information extraction techniques. This paper proposes a parametric SAR image recovery method for linear structures of extended targets. By extracting the spatial phase distribution feature in image domain, the proposed method is used to identify the endpoints among all of the scattered points in SAR images and reconstruct the disappeared linear structures based on the scattering model. The method can be generally divided into three procedures: endpoints and point targets classification, linear structures recognition, and linear structures reconstruction. In the first step, the endpoints of linear structures and point targets are classified by setting a threshold related to the spatial phase distribution feature. Afterwards, the linear structures recognition method is used to determine which two endpoints can be formed into a linear structure. Finally, the parametric scattering models are used to reconstruct the disappeared linear structures. Experiments are conducted on both computer simulations and the data that were acquired by microwave anechoic chamber experiment, tower crane radar experiment, and unmanned vehicle radar experiment in order to validate the effectiveness and robustness of the proposed method.

Discrimination of Parallel and Perpendicular Insects Based on Relative Phase of Scattering Matrix Eigenvalues

Current vertical-beam entomological radars record the polarization direction corresponding to the maximal ventral-aspect radar cross section (RCS) as the insect’s orientation. For so-called “parallel” insects, this direction is indeed their orientation; but for “perpendicular” insects, it is at right angles to the orientation. Current entomological radars cannot discriminate the parallel and perpendicular cases. This article shows here that discrimination is possible using the relative phase of the scattering matrix (SM) eigenvalues. Multifrequency fully polarimetric ventral aspect SM measurements of 80 insect specimens of 12 species have been made in a microwave anechoic chamber. The relationship of the polarization direction corresponding to the maximal RCS and the radar frequency has been analyzed, and from these results a method of discriminating parallel and perpendicular insects, based on the relative phase of the SM eigenvalues, is proposed. The method is applicable to X- and Ku-band observations, with a high correct-identification rate, and can be used with both fully polarimetric entomological radars and coherent rotating-polarization units, but not with the noncoherent rotating-polarization configuration used in traditional vertical-looking radars (VLRs). Finally, the performance of the method is discussed, and it is found that it has better performance for middle and large insects at X-band and small and middle insects at Ku-band.

SAR Observation Error Estimation Based on Maximum Relative Projection Matching

The existing sparse imaging observation error estimation methods are to usually estimate the error of each observation position by substituting the error parameters into the iterative reconstruction process, which has a huge calculation cost. In this paper, by analysing the relationship between imaging results of single-observation sampling data and error parameters, a SAR observation error estimation method based on maximum relative projection matching is proposed. First, the method estimates the precise position parameters of the reference position by the sparse reconstruction method of joint error parameters. Second, a relative error estimation model is constructed based on the maximum correlation of base-space projection. Finally, the accurate error parameters are estimated by the Broyden–Fletcher–Goldfarb–Shanno method. Simulation and measured data of microwave anechoic chambers show that, compared to the existing methods, the proposed method has higher estimation accuracy, lower noise sensitivity, and higher computational efficiency.

Integrated antenna and receiver system with self-calibrating digital beamforming for space-based ADS-B

This paper describes a digital beamforming (DBF) system with a self-calibrating function to receive Automatic Dependent Surveillance-Broadcast (ADS-B) signals on a satellite, which can expand the coverage range and improve the detection probability that is mainly reduced by multi-signal collision. This system integrates a 4 × 4 microstrip array antenna and a 16-channel receiver into a cuboid box that can simultaneously generate more than 19 independent beams whose size and direction can be flexibly controlled. Specifically, the on-orbit self-calibration method, signal processing algorithm, antenna and receiver design, and system test method are presented in this paper. Test results in a microwave anechoic chamber verified that the design can significantly improve the sensitivity of the receiver, reduce signal collision, and enhance the reliability of the space-based ADS-B system.

Composite polarization conversion metasurface and its application in integrated regulation radiation and scattering of antenna

The transmission polarization conversion metasurface has been widely concerned, because it has the advantage of being easy to be conformal with the antenna. Based on the reasonable arrangement of transmission polarization conversion units, various and complex electromagnetic functions can be realized. As the electromagnetic open window on the flight platform, the antenna is the bottleneck that restricts the decrease of radar cross section (RCS) of the whole flight platform. It is difficult to simultaneously realize the normal and efficient radiation of the antenna and the decrease of the RCS of the antenna. When the designed transmission metasurface is used in the antenna design, the radiation and scattering of the antenna can be regulated comprehensively. In this paper, a composite polarization conversion metasurface is proposed and verified. The unit cell of composite polarization conversion metasurface consists of two mirror symmetrical anisotropic metal patches in the upper layer, a dielectric layer and a polarization gate in the lower layer. When the polarization direction of the incident electromagnetic wave is perpendicular to the extension direction of the polarization gate and arrives at the composite polarization conversion surface, the conversion surface can realize the conversion from transmission linear polarization to right-hand circular polarization in a frequency range from 9.3 GHz to 10.9 GHz. When the polarization direction of the incident electromagnetic wave is parallel to the extension direction of the polarization gate, co-polarized total reflection can be realized. The chessboard arrangement metasurface is composed of composite polarization conversion unit and its mirror unit. A novel linearly polarized chessboard arrangement metasurface antenna is composed of the linearly polarized source microstrip antenna with a bandwidth of 9.4–10.7 GHz and the chessboard arrangement metasurface. By using the counter rotating cancellation characteristic of circular polarization, the chessboard arrangement metasurface antenna maintains linearly polarized radiation. Comparing with the source microstrip antenna, the linear polarization purity of chessboard arrangement metasurface antenna is improved from 9.5 GHz to 10.5 GHz. At the same time, the forward gain of the chessboard arrangement antenna increases and the radar cross section decreases. The maximum reduction is 39.2 dB. To further verify the practicability of the design and analysis, the chessboard arrangement metasurface antenna sample is fabricated and measured in microwave anechoic chamber with an Agilent 5230C network analyzer. The experimental results are in good agreement with the simulation results. This study has important reference value in the design of high gain, low RCS antenna and integrated regulation radiation and scattering of antenna.

Dual-Wideband High-Gain Fabry-Perot Cavity Antenna

In this paper, we proposed a novel Fabry-Perot cavity antenna with high-gain radiation over two wide bands based on a partially reflective metasurface. To satisfy the resonance condition of the dual-wideband Fabry-Perot cavity, we constructed the unit cell of the partial reflection metasurface by loading magnetic coupling enhancement layers and etching the bottom patches. The partial reflection metasurface composed of the above unit cells can generate reflection phases of positive gradient with almost the same slope over two wide bands, where the partial reflection amplitude is close to 1. Subsequently, a normal dual-band microstrip antenna has also been designed for excitation. Finally, the Fabry-Perot cavity antenna consists of the proposed partial reflection metasurface and the dual-band antenna has been simulated, fabricated, and measured in a microwave anechoic chamber. Both simulation and experimental results demonstrate that the designed Fabry-Perot cavity antenna achieves 3-dB gain bandwidths of 20.7% and 10.1% with peak gains of 12.07dBi and 13.5dBi over two wide bands of 5.7-6.9GHz and 11.7-12.95GHz, respectively.

Single-Layer Dual-Band Wide Band-Ratio Reflectarray With Orthogonal Linear Polarization

In this letter, a single-layer broadband wide band-ratio (ratio of f higher to f lower ) reflectarray that can achieve different linear polarizations at the X-band and the K-band is designed. Its elements consist of a split square loop with two square shaped phase delay lines (outer structure) and a phoenix square loop (inner structure) operating at the X-band and the K-band, respectively. The ingeniousness of the design is that the outer structure and the inner structure can achieve a 0° - 360° phase shift by adjusting the phase shift lines and the side length of the inner square ring, respectively. At the same time, they do not affect each other in space. Different polarizations corresponding to the two structures are orthogonal to each other, so their electrical performances do not affect each other. To verify our design concept, the reflectarray is fabricated and measured in a microwave anechoic chamber. The simulated and measured results are in good agreement. The measured gains are 23.6 dBi at 10 GHz (f lower ) with aperture efficiency of 48%, and 30.6 dBi at 22 GHz(f higher ) with aperture efficiency of 43%. The 1-dB gain bandwidths are 20% (9.1 GHz-11.1 GHz) at the X-band and 19% (19.8 GHz-23.9 GHz) at the K-band. The band-ratio of the reflectarray is 2.2. A single-layer dual-band wide band-ratio reflectarray with dual-linear polarization is confirmed.

Design and Characterization of Periodically Conductive Woven Fabric

Abstract In this paper, a novel kind of electromagnetic (EM) functional textiles is proposed, which show high-pass characteristics as they interact with EM waves. The periodically conductive woven fabric was designed, fabricated, and measured. Specifically, by means of unit cell model building and EM simulation, the theoretical S21 (transmission coefficient) and S11 (reflection coefficient) curves were obtained. A concrete sample was fabricated through weaving process, and its transmission characteristics were measured in the microwave anechoic chamber. The measured and simulated results were highly consistent, demonstrating the validity of design process. Compared with the aluminum foil paper sample, the S21 values of fabricated sample were a little smaller, and the reason could be attributed to yarn crimp and surface roughness. The EM characteristics of fabricated sample under two different polarization modes were slightly different, which was due to the beating-up tension of weaving process. The work could offer new research ideas, and the related products have potential advantages over rigid plates on the account of textile characteristics.

Miniaturised frequency selective surface based on fractal arrays with square slots for enhanced bandwidth

A novel approach is used to miniaturise the array size and broaden the bandwidth (BW) of a multilayer antenna-filter antenna- based frequency selective surface (FSS). Although such FSSs have broadly been investigated for higher frequency selectivity and less thickness, they still own high periodicity and lower BW. The proposed fractal FSS with centre square slots consists of three metallic layers, separated by two dielectric substrates and periodicity of the unit cell is sub-wavelength order (0.2 λ ₒ). Central square slots are added in the external Minkowski fractal patches to increase the capacitive effect whereas fractal slots are etched on the ground placed in the middle for enhanced coupling. It is demonstrated through simulations that proposed design can effectively minimise the element size and enhance the BW. Owing to minuscule array size and symmetric fractal geometry, the presented structure is insensitive to higher incidence angles (up to 30°) for both polarisations. A prototype of the proposed FSS is also fabricated and tested in a microwave anechoic chamber. An equivalent circuit model, full wave simulation and measurement results agree well and demonstrate stable response with 3 dB fractional BW approaching 87.4 and 92%, respectively, at a normal angle of incidence.

Migratory Insect Multifrequency Radar Cross Sections for Morphological Parameter Estimation

Insect migration provides major ecosystem services, and sometimes, migratory pests cause serious crop damage and yield loss. Species identification is critically important in studies of insect migration, for both entomologists and pest managers. Radar is an effective means of detecting insect migrants. Current entomological radars usually operate at X-band, and signal amplitude information is used to estimate body mass and wing-beat frequency, which can then be used to categorize migratory insects into broad taxon classes. To improve the identification performance, this paper presents a novel radar method of measuring insect mass and body length. The multifrequency radar cross sections (RCS) of insects at X-band and Ku-/K-band are fully investigated, and the comprehensive relationship between RCS and insect morphological parameters provides an improvement in the estimation of insect mass. More importantly, estimations of body length can also be realized with an accuracy of 84% based on experimental data acquired by a vector network analyzer in a microwave anechoic chamber. If multifrequency RCS measurements can be obtained by radar in the future, then highly accurate estimations of insect mass and body length will be possible, although it is currently still a challenge to build a radar capable of making the required measurements over such a wide frequency range.

Synthetic Aperture Radar Target Feature Modulation Using Active Frequency Selective Surface

Target feature control on the synthetic aperture radar (SAR) image using the passive way is a challenging task in the field of SAR jamming. The conventional passive jamming devices such as corner reflectors and chaffs cannot achieve the flexible modulation of SAR target feature duo to their fixed scattering cross section. As a passive material, active frequency selective surface (AFSS) can dynamically control their electromagnetic characteristics and has been widely applied in the radar target stealth field. In this paper, a target feature modulation method based on AFSS is proposed against SAR. The method intermittently switches the scattering intensity of AFSS material and imposes periodic or random code amplitude modulation onto the radar signal. As a consequence, multi-false targets with verisimilar characteristics or cross-shaped areas are formed when modulated signal is received and processed by the victim SAR. Thus, the protected target is difficult to detect and identify by the SAR system. Microwave anechoic chamber experiments and measured SAR data simulations with different modulating parameters are separately conducted to demonstrate the effectiveness of the proposed method.

Ultra-wideband and polarization-insensitive fractal perfect metamaterial absorber based on a three-dimensional fractal tree microstructure with multi-modes

In this paper, an attractive, ultra-wideband, and polarization-insensitive fractal perfect metamaterial absorber had been proposed based on a tree-shaped microstructure. The absorber was composed of two dielectric substrates, a three-dimensional fractal metal tree, and four lumped resistances. The absorption mechanism and different modes for the absorber had been analyzed in simulation. The characters of polarized-insensitivity and wide-incident absorption for the proposed metamaterial absorber were systematically illuminated by the near electric fields, angular absorptions, and surface current distributions. A presented perfect metamaterial absorber device had been easily implemented using the common printed circuit board method and was measured in a microwave anechoic chamber. The results showed that this proposed FPMA provided a relative bandwidth of 86.9% with the absorptivity larger than 80%. Both the simulated and experimental results adequately verified the characters of wideband, wide-incident, and polarization-insensitive absorption.

Microwave absorbing properties of a radar absorbing structure composed of carbon nanotube papers/glass fabric composites

AbstractIn this study, carbon nanotube papers were employed in fabricating thin and broadband radar absorbing structures (RAS). Different concentrations of the CNT papers have been made by using a vacuum filtration method, with 20 × 20 cm in size and 21‐27 μm in thickness. An epoxy resin was added into the CNT paper and then cured to become a composite with 1‐5 wt.% of CNTs and 83‐309 μm in thickness. The complex permittivity and permeability (ε′, ε″, μ′, μ″) of the CNT paper composites were measured using the transmission/reflection method in the frequency range of 2‐18 GHz. The results reveal that the real (ε′) and imaginary (ε″) parts of the complex permittivity are increased with the CNT concentration. The ε′ of 5 wt.% CNT sample reaches 323 at 2 GHz and then decreases to 49.0 at 18 GHz. The ε″ reaches 321 at 2 GHz and decreases to 26.0 at 18 GHz. The CNT paper composite combined with a glass fabric composite used for a dielectric spacer is fabricated for an innovative RAS and the reflection loss is measured using the arch method in a microwave anechoic chamber. The results show that the 5 wt.% CNT paper composite/glass fabric composite absorbers attain maximum reflection loss of −13.3 dB at 12.0 GHz, −13.8 dB at 10.0 GHz, and −16.0 dB at 7.5 GHz for spacer thickness of 1.5, 2.0, and 3.0 mm, respectively.

Design and Verification of an Integrated Free-Standing Thick-Screen FSS Radome

Frequency selective surface (FSS) radomes can effectively improve the stealth of flying weapons. In this letter, an integrated free-standing thick-screen FSS radome is designed using the CST simulation software. The bandpass characteristic is achieved by Y-shaped apertures with periodic distribution around the central axis of the radome. Based on a model of a round-head FSS radome with a built-in phased array antenna, we have analyzed the bandpass transmittance and boresight error of the radome. The copper shell was prepared by a spin forming process. The FSS radome was fabricated by a laser robot system with multiple degrees of freedom and a rotary platform. The results of the microwave anechoic chamber test show that the thick-screen bandpass FSS radome has a good electromagnetic performance with a passband transmittance of about 80% and a stopband transmittance of less than 10%.

WiFi Indoor Localization with CSI Fingerprinting-Based Random Forest

WiFi fingerprinting indoor positioning systems have extensive applied prospects. However, a vast amount of data in a particular environment has to be gathered to establish a fingerprinting database. Deficiencies of these systems are the lack of universality of multipath effects and a burden of heavy workload on fingerprint storage. Thus, this paper presents a novel Random Forest fingerprinting localization (RFFP) method using channel state information (CSI), which utilizes the Random Forest model trained in the offline stage as fingerprints in order to economize memory space and possess a good anti-multipath characteristic. Furthermore, a series of specific experiments are conducted in a microwave anechoic chamber and an office to detail the localization performance of RFFP with different wireless channel circumstances, system parameters, algorithms, and input datasets. In addition, compared with other algorithms including K-Nearest-Neighbor (KNN), Weighted K-Nearest-Neighbor (WKNN), REPTree, CART, and J48, the RFFP method provides far greater classification accuracy as well as lower mean location error. The proposed method offers outstanding comprehensive performance including accuracy, robustness, low workload, and better anti-multipath-fading.