Measurements In Anechoic Chamber Research Articles

Rice Height Estimation with Multi-Baseline PolInSAR Data and Optimal Detection Baseline Combination Analysis

Rice is a primary food source, and height is a crucial parameter affecting its growth status. Consequently, high-precision, real-time monitoring of quantitative changes in crop height are required for improved crop production. Polarimetric interferometric SAR (PolInSAR) has both polarization and interferometric observation capabilities. Due to the short height of crops and rapid growth changes, the large spatial and short temporal baselines of PolInSAR data are essential for effective crop height inversion; however, relevant satellite-borne SAR and airborne SAR data are currently limited. This study presents a PolInSAR rice height inversion algorithm that uses the oriented volume over ground (OVoG) mode with PolInSAR 0-time and controllable spatial baseline data from a LAMP microwave anechoic chamber. Exploiting the advantages of microwave anechoic chamber measurement data, which includes continuous frequency bands, multiple baselines, and varied incidence angles, the influences of incident angles, baseline length, number of baselines, and baseline combinations are assessed. The highest accuracy rice plant height inversion has a root mean square deviation (RMSE) of 0.1093 m and is achieved with an incidence angle of 35–55°, baseline length of 0.25°, and three to four baselines. Furthermore, an imaging geometric equivalence analysis provides reliable foundation data to guide research into new earth observation SAR systems. The results indicate that, under simulated observations from the GF3 satellite at an altitude of 755 km, the optimal spatial baseline ranges for various frequency bands are as follows: L-band: 10.93–42.59 km; S-band: 4.10–15.97 km; C-band: 2.48–9.64 km; X-band: 1.36–5.32 km; Ku-band: 0.87–3.40 km. Notably, the measurement modes corresponding to the C, X, and Ku bands are ultimately identified as the most suitable for PolInSAR rice height inversion.

Construction of a 3-m radio anechoic chamber measurement platform and its practical application

Construction of a 3-m radio anechoic chamber measurement platform and its practical application

Coupling between Cavity Resonances and Characteristic Modes on Household Appliances

Home appliances generally comprise a mechanical envelope, or enclosure, of electrically conductive material (steel, aluminum, etc.) that houses electrical and electronic circuits. That envelope typically includes all kind of apertures, openings, holes, slots, windows, etc., to connect—physically and electromagnetically—the external space with its internal space. The performance of that envelope is a key element to comply with surface current EMI/EMC (Electro-Magnetic Interference/Electro-Magnetic Compatibility) regulations for electromagnetic interference, both emissions and immunity. Here, we present a novel theoretical proposal that consists of establishing the mathematical relationship of the coupling between the resonant modes of an internal cavity (RMs) contained in a conductive body and the characteristic modes (CMs) on the external surface of its envelope through the openings that connect those external and internal spaces, and doing so by means of equivalent virtual surface currents located in those openings. The comparative results of simulations and actual measurements of immunity in an anechoic chamber (measurements originally oriented to other purposes) are presented as above-mentioned evidence of resonating modes coupling on the frequency range 40–1000 MHz. However, this theoretical proposal is novel and remains to be developed in greater depth and detail in future works.

A compact triband antenna using artificial magnetic conductor for wireless body area network communicationsFront

The proliferation of the Internet of Things devices and advancements in wireless communication have fostered the growth of Wireless Body Area Networks (WBAN). This research provides a triband antenna supported by a 4 × 4 Artificial Magnetic Conductor (AMC) array surface that has a low Specific Absorption Rate (SAR), a high Front to Back Ratio (FBR), and increased gain for use in wearable devices. For WBAN communications, the proposed antenna operates in the Industrial, Scientific, and Medical (2.4 GHz) band, the C (3.7–4.2 GHz), and the Wi-Fi 6E (5.925–7.125 GHz) bands. The dual-band AMC unit cell exhibits Double-Negative and angular stability behaviour at 2.45 GHz and 6.5 GHz. AMC-backed antenna achieved multiband functionality by incorporating slots into the unit cell and a defective ground structure into the antenna. The antenna was positioned 0.139λ0 above the AMC surface, which measured 0.556λ0 × 0.556λ0 × 0.013λ0 (at 2.45 GHz). The antenna exhibited good gain and return loss variations when mounted on curved surfaces. The proposed integrated design yielded substantial enhancements, as evidenced by the increase of 8.2 dBi in maximum gain, 25.2 dB in FBR, and over 93% in total efficiency. The AMC-backed antenna’s − 10 dB impedance bandwidth is 18.4%, 21.2%, and 22.3%, with corresponding frequency ranges of 2.25–2.66 GHz, 3.66–4.53 GHz, and 5.9–7.35 GHz. Additionally, the AMC surface showed an average reduction in SAR of 93.22%. Vector Network Analyzer and Anechoic chamber measurements proved simulation accuracy. As a result, it is strongly recommended that the integrated antenna design be acknowledged in WBAN communications.

An Electromagnetic Information Theory Based Model for Efficient Characterization of MIMO Systems in Complex Space

It is the pursuit of a multiple-input-multiple-output (MIMO) system to approach and even break the limit of channel capacity. However, it is always a big challenge to efficiently characterize the MIMO systems in complex space and get better propagation performance than the conventional MIMO systems considering only free space, which is important for guiding the power and phase allocation of antenna units. In this manuscript, an Electromagnetic-Information-Theory (EMIT) based model is developed for efficient characterization of MIMO systems in complex space. The group-T-matrix-based multiple scattering fast algorithm, the mode-decomposition-based characterization method, and their joint theoretical framework in complex space are discussed. Firstly, key informatics parameters in free electromagnetic space based on a dyadic Green's function are derived. Next, a novel group-T-matrix-based multiple scattering fast algorithm is developed to describe a representative inhomogeneous electromagnetic space. All the analytical results are validated by simulations. In addition, the complete form of the EMIT-based model is proposed to derive the informatics parameters frequently used in electromagnetic propagation, through integrating the mode analysis method with the dyadic Green's function matrix. Finally, as a proof-or-concept, microwave anechoic chamber measurements of a cylindrical array is performed, demonstrating the effectiveness of the EMIT-based model. Meanwhile, a case of image transmission with limited power is presented to illustrate how to use this EMIT-based model to guide the power and phase allocation of antenna units for real MIMO applications.

Field Deviation in Radiated Emission Measurement in Anechoic Chamber for Frequencies up to 60 GHz and Deep Learning-Based Correction

The measurement of radiated emission (RE) in an anechoic chamber becomes very challenging at high frequencies, up to 60 GHz, because the scanning plane of the receiver is in measurement standard deviation from the actual wavefront. As a result, the RE intensity of the devices may be underestimated, resulting in electromagnetic interference. The deviation between the electric field at the far-field vertical scanning point and the actual wavefront is researched. Then, in an anechoic chamber, a hybrid deep learning amendment model of convolutional neural network (CNN) and transformer is proposed to correct the RE measurement at a 3 m distance. The results indicate that the correction is reliable, with an average error of 6.35% for a 3 m distance in a semianechoic chamber and less than 4.83% for other test scenarios. The proposed method provides a promising solution for RE measurement at a millimeter wave band in an anechoic chamber.

Increasing the metrological characteristics of anechoic chambers due to a posteriori analysis based on artificial neural networks

This article considers the possibility of improving the metrological characteristics of an anechoic chamber due to a posteriori processing of measurement results based on a generative adversarial model of an artificial neural network in order to reduce the influence on the distribution of the electromagnetic field in the measuring zone of waves reflected from the outer boundaries of the chamber and the equipment located in it. The training of the neural network was carried out on a data set obtained as part of a computational experiment and including the distribution of the electromagnetic field in the anechoic region for the model of an anechoic chamber and free space for given source layouts. The distributions of the real and imaginary parts of the electric component of the electromagnetic field were encoded with colour images. On the example of two-dimensional models of anechoic chambers, the practical feasibility of the proposed approach to a posteriori processing of measurement results is shown. Methods for estimating the accuracy of a posteriori processing of measurement results based on the metrics used to assess the quality of graphic images and calculating the errors in the amplitudes of the electric component of the electromagnetic field are given. The possibility of implementing the proposed method of a posteriori analysis in the framework of natural microwave measurements in anechoic chambers is assessed.

Waveform design and processing method of pulse signal in anechoic chamber measurement

The application of pulse radar in anechoic chamber measurement is a new direction to obtain characteristics of targets. However, due to the limited size of anechoic chamber, the transmitted signal and its echo are coupled so that the target range profile can not be accurately obtained. Furthermore, the position and intensity of target scattering points can not be measured. To solve this problem, the waveform design and processing method of pulse signal is proposed in this paper. Firstly, the waveform is designed by amplitude modulation to solve the coupling between transmitted signal and its echo. Secondly, for the designed waveform, the echo processing method based on decomposition, combination, and cancellation is provided to obtain the target range profile so that the target measurement can be conducted in anechoic chamber. Considering the actual situation of multipath clutter in anechoic chamber, simulations and experiments are conducted, and the results demonstrate the validity of the proposed method.

Inverse Determination of Acoustic Properties of Acoustic Ventilation Cloth (Woven and Non-Woven) by Particle Swarm Optimization and Estimation of Its Effect on the Frequency Response of the Microspeaker

In this work, microspeaker frequency response was determined based on measurement and simulation. The vents after the rear chamber of the microspeaker were covered with ventilation cloths. Two types of ventilation cloths (acoustic cloths) are commonly used in electroacoustic products, non-woven and woven. Non-woven cloths (5 nos.) consist of irregular meshes and woven cloths (5 nos.) consist of regular meshes. The equivalent circuit model of the microspeaker was formulated by considering five types each of non-woven and woven ventilation cloths. The acoustic properties of the ventilation cloths were estimated from the measured frequency response of the microspeaker and by subsequent use of particle swarm optimization algorithm. Estimated value of the acoustic impedances of the ventilation cloths were used in an equivalent circuit model of the microspeaker for the simulated frequency responses and subsequently compared with the anechoic chamber measurements. Based on the results, the equivalent circuit adequately simulated the measured frequency response of the microspeaker and the estimations of the acoustic impedances of the ventilation cloths were in good agreement with the measured frequency responses of the microspeaker.

Far-Field Radar Cross Section Determination From Near-Field 3-D Synthetic Aperture Imaging With Arbitrary Antenna-Scanning Surfaces

In this study, we propose a generalized algorithm for far-field radar cross section determination by using 3-D synthetic aperture imaging with arbitrary antenna-scanning surfaces. This method belongs to a class of techniques called image-based near-field-to-far-field transformation. The previous image-based approaches have been formulated based on a specific antenna-scanning trajectory or surface, such as a line, plane, circle, cylinder, and sphere; the majority of these approaches consider 2-D radar images to determine the azimuth radar cross section. We generalize the conventional image-based technique to accommodate an arbitrary antenna-scanning surface and consider a 3-D radar image for radar cross section prediction in both the azimuth and zenith directions. We validate the proposed algorithm by performing numerical simulations and anechoic chamber measurements.

A compact normal-mode VHF/UHF dual-band helical antenna for lunar microsatellite

This paper has proposed a novel compact VHF/UHF dual-band helical antenna, which was used onboard Longjiang-2, the first independent Earth-Moon-transfer microsatellite. As a critical component of the first amateur radio communication system for a lunar satellite, the antenna requires small size and omnidirectional beam coverage. The structure of the resulted antenna is a combination of a helix and a loaded monopole, and has achieved a 56% size reduction for axial height compared to a conventional normal-mode helical antenna. Moreover, parametric studies were performed to explore the resonant characteristics. The radiation patterns of the proposed antenna installed on the satellite were simulated, which indicated that the antenna achieved a good beam coverage of more than 99.9% in space. Measurements in anechoic chamber and in lunar orbit were carried out to verify the effectiveness of the design.

A Low-Cost and Efficient Single Probe Based MIMO OTA Measurement Method

As the multiple-input–multiple-output (MIMO) technologies are increasingly emerging in the arena of modern wireless systems, their cost-effective and efficient over-the-air (OTA) performance verification under arbitrary fading channels, especially, for high-volume beam-reconfigurable product verifications, play a crucial role toward their technology adoption. The 2-D multi-probe anechoic chamber (MPAC) MIMO OTA measurement method has been well-developed, but requires both precise infrastructure and heavy calibration. Moreover, the extension to 3-D channel model appears impractical for the MPAC method with its significantly increased probe numbers in 3-D scenario. To address these challenges, we propose a novel single-probe anechoic chamber (SPAC) measurement method that transforms spatial synthesization of MPAC into time synthesization whereby the only source of measurement error can be restrained in time synchronization. The wireless community can be widely benefited with this proposed low-cost and efficient methodology, especially in the research and product prototyping stage. In this article, a proof-of-concept measurement system has been designed and implemented, which validates the efficiency of the proposed solution.

Microwave Anechoic Chamber Measurement and Scatter Characteristics Analysis of Typical Wetland Vegetation in South China

Microwave Anechoic Chamber Measurement and Scatter Characteristics Analysis of Typical Wetland Vegetation in South China

Dual-Band Circularly Polarized Fully Reconfigurable Reflectarray Antenna for Satellite Applications in the Ku-Band

This work presents the design and experimental validation of a dual-band fully reconfigurable circularly polarized (CP) reflectarray (RA) antenna for satellite communication applications in the Ku-band. The proposed structure operates with the downlink and uplink beams at frequency bands of [10.8–11.8] and [14–15.4] GHz, respectively. Simultaneous and independent beam control is provided over each of the two bands. The constituent unit cell is composed of two interleaved circular loops of different sizes to address the two frequency bands. Each loop is loaded using four varactor diodes. The loops are symmetrically loaded in the transverse plane to provide an isotropic response suitable for CP applications. A phase range of more than 300° is achieved in both bands as the capacitive loading varies, with an average of 2 dB loss in the lower frequency band and 3 dB loss in the higher band. In this article, the response of the unit cell is studied through full-wave simulations and verified through quasi-optical (QO) measurements, and the fully tunable performance of the reflectarray is validated through measurements in a near-field anechoic chamber.

An acoustic-wave preserved artificial compressibility method for low-Mach-number aeroacoustic simulations

An artificial compressibility (AC) method is developed for time-accurate low-Mach-number aeroacoustic simulations. The significant advantage is capturing acoustic waves in the turbulent flows at a low computational cost. An artificial equation of state with isentropic behaviour is assumed to build the relation between pressure and density fluctuations. The key measure is to rewrite the continuity equation, which is commonly expressed as ∇⋅u=0 at low Mach numbers, to a time-dependent equation for pressure. Here u is the flow velocity. Being different from the traditional AC method that was initially proposed for steady-state simulations, the present method is intended to solve transient turbulent flow at low-Mach-number conditions. Additionally, the proposed method can correctly resolve sound wave propagation, which is inherently ruled out by usual incompressible Navier–Stokes equations. Results show that the proposed method can provide fast alternatives to traditional incompressible solvers and simultaneously achieve the same time-accurate results. The capability of the proposed method is examined by benchmark aeroacoustic problems. Particularly, the challenging problem of predicting noise radiation from a propeller is considered, and the results match well with the anechoic chamber measurements.

Broadband modified-circle-shape patch antenna with H-aperture feeding for a passive radar array

In this paper, the design of a broadband modified-circle-shape patch antenna with H-aperture feeding is presented, to be used as single radiating element in the array of the surveillance channel of an UHF passive radar. Different techniques are proposed to achieve a relative bandwidth of more than 30%, and challenging radiation pattern characteristics for the defined application. The achievement of these requirements is proved through measurements in anechoic chamber. A NULA is designed using optimization techniques and considering coupling effects between elements. The NULA was integrated in IDEPAR, the passive radar demonstrator developed in the University of Alcalá, and validated through measurement campaigns. Results prove a significant improvement of the passive radar target detection and bearing estimation capabilities.

A Useful Sampling Representation for Mapping the Antenna Near-Field

A convenient sampling distribution and an optimal sampling interpolation algorithm are here presented for mapping the antenna near-field values on a planar surface. The mapping procedure is based on the theoretical background related to a non-redundant sampling representation of the electromagnetic field and uses an unconventional arrangement of the sampling points. Their position results from the standard plane-rectangular allocation by properly increasing the distance between near sampling points as their distance from the antenna grows. This permits to map the values in a given area by using the knowledge of a reduced number of samples and, therefore, the measurement time is conveniently shorted. Numerical simulations and antenna measurements in anechoic chamber have been executed to test the effectiveness of the mapping procedure, whose accuracy has been proved by means of maximum and mean-square reconstruction errors at given output points.

Single-cut Far-Field Antenna Radiation Pattern Reconstruction Accuracy Analysis in Compact Anechoic Chamber Setup

In production testing, it is of importance to measure the key radiation parameters of an antenna under test (AUT), e.g., main beam peak and direction, sidelobes, and null depth and direction in a cost-effective setup with a short measurement time. As a result, practical measurement setups are often compact and equipped with only a few probes (or probe locations). However, these system limitations would introduce errors for antenna testing. This problem has become even more pronounced for 5G radios due to utilization of large-scale antenna configurations and high frequency bands. Spherical near-field measurements are nowadays an accurate and mature technique for characterizing AUTs, which however, necessitates a full spherical acquisition, leading to a long measurement time. Singlecut near-to-far-field transformation is a promising strategy since most of the key AUT parameters are available in the single-cut pattern and it requires much reduced measurement time. In this work, a simple and flexible scheme is proposed to evaluate errors introduced by limitations in practical setups for single-cut far-field (FF) antenna radiation pattern reconstruction, where the near-field data can be easily generated and modified according to the limitations introduced in practical multi-probe anechoic chamber setups, e.g., measurement distance, truncation range, and sampling interval. The reconstructed FF pattern is obtained using a commercial near-field to far-field transformation tool, SNIFT. The proposed scheme is numerically validated via comparing the reference FF pattern of a 4 × 8 uniform planar array composed of ideal Hertzian dipoles and reconstructed FF pattern. With the proposed scheme, the impact of practical system limitations on single-cut reconstruction accuracy can be easily analyzed.

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.

Synthetic Aperture Imaging of Near-Field Scatterers Mutually Coupled With an Antenna Array

In this study, we propose a synthetic aperture imaging technique for near-field scatterers mutually coupled with an antenna array, such as supporting structures, fixtures of the antenna, and a radome enclosing the antenna. Mutual coupling is a phenomenon, in which the scattered and reradiated waves from the array elements and the near-field scatterers are received by the array elements, and it is known to cause undesired effects in array signal processing, such as direction-of-arrival estimation. Therefore, we need to design the antenna array with the goal of reducing the unwanted scattered and reradiated waves as much as possible, and we require a method to evaluate the scattering intensity for validation of the antenna design. For this purpose, we utilize the bistatic synthetic aperture radar measurement, whereby we mount the receiving antenna array under test on a turntable for azimuth rotation and irradiate this antenna with a fixed transmitting antenna. The measured signals are synthesized to image an area that contains both the array elements and scatters of interest. Because the spatial separations between the array elements and the scatterers are small, the direct waves from the transmitting antenna to the receiving elements result in serious artifacts in the reconstructed image. To suppress this effect, we apply a direct wave filter as a preprocessing method for image reconstruction. We demonstrate the proposed imaging scheme via data sets of numerical electromagnetic simulations and anechoic chamber measurements.