Near-field Measurements Research Articles

Experimental Characterization of Superheated Ammonia Spray from a Single-hole ECN Spray M Injector

Abstract The growing recognition of ammonia as a carbon-free alternative fuel for both transportation and power generation is underscored by numerous studies. While the use of gaseous ammonia injection has been extensively considered, research on the direct injection of liquid ammonia remains relatively scarce, especially in the case of internal combustion engines (ICE). The application of liquid ammonia in gas turbines offers economic and size-related advantages over gaseous ammonia injection. But the liquid ammonia's instantaneous spray flash-boiling and its high latent heat of vaporization induce the necessity of employing preheated swirling air to enhance flame stability, mitigating the strong cooling effect induced. The preference for direct injection of liquid ammonia is driven by its efficacy in controlling in-cylinder air-fuel ratios and optimizing thermal efficiency. This study presents macroscopic and near-field Schlieren measurements of a 1-hole ECN Spray M, representative of direct liquid injector for engine, with ammonia for different degrees of superheating. Spray angles and penetration length are provided as a function of the superheat degree with also a focus near the injector nozzle. Additionally, in-spray temperature measurements are presented, providing the first valuable information for modeling purposes.

Near-field imaging of synthetic dimensional integrated plasmonic topological Harper nanochains

Topological photonics offers immense potential for applications in integrated photonic devices and information processing chips. Aubry–André–Harper model provides a platform for exploring new physics and practical applications. However, the on-chip integration of an ultracompact Aubry–André–Harper plasmonic topological insulator has encountered two limitations: the strict precision requirements for coupling parameters during sample preparation and the presence of hotspots in the nanogaps between plasmonic nanostructures, which impede direct near-field measurements. In this work, we propose a novel approach to address these challenges by integrating gold nanodisks with connecting waveguides. The topological properties of the Aubry–André–Harper configuration are directly characterized using photoemission electron microscopy. Connecting gold nanodisks with short gold waveguides of varying widths ensures compliance with the stringent precision requirements for sample nanofabrication and minimizes the impact of plasmon hotspots. We also successfully excite nanodisks in odd or even positions of trivial staggered nanochains by using incident left- or right-circularly polarized light. This approach effectively enables polarization-multiplexing control, offering a promising method for further manipulating and refining plasmonic nanochains and their potential applications. This work provides direct in-situ measurements of topological states at the nanoscale, advancing the foundational research and practical applications of controlling synthetic dimensions in integrated plasmonic topological photonics.

Single-Bacterium Diagnosis via Terahertz Near-Field Dielectric Nanoimaging.

Single-bacterium diagnostic methods with unprecedented precision and rapid turnaround times are promising tools for facilitating the transition from empirical treatment to personalized anti-infection treatment. Terahertz (THz) radiation, a cutting-edge technology for identifying pathogens, enables the label-free and non-destructive detection of intermolecular vibrational modes and bacterial dielectric properties. However, this individual dielectric property-based detection and the mismatched spatial resolution are limited for the single-bacterium identification of various species of pathogens. Here, we demonstrate a single-bacterium THz dielectric nanoimaging (STDN) strategy by customized THz scattering-type scanning near-field optical microscopy. The THz nanoimages of bacteria are explained and confirmed by theoretical modeling and near-field measurement. By synchronously tracking the bacterial intrinsic dielectric property and extrinsic morphology, the strategy achieved 99.3% and 91.6% accuracy in species identification and antibiotic susceptibility testing with the trained classifier within 2 hours. This proof-of-concept STDN strategy may propel precise bacterial infection management and help to counteract antibiotic resistance.

Efficient sampling strategies based on a reduced-order model for antenna planar measurements

Abstract The antenna characterization from planar near-field (NF) measurements is generally realized by using the classical NF to far-field transform technique of plane wave expansion (PWE). This approach imposes strong constraints on NF sampling. To overcome these limitations, an equivalent model of the antenna under test (AUT) is created based on a distribution of infinitesimal dipoles. A reduced-order model (ROM) of the problem is constructed to obtain a decomposition basis defining the radiated field. The powerful ability of the ROM in determining the number of points needed for accurate NF measurements is demonstrated. Also, efficient non-conventional sampling strategies are applied to the case of planar NF measurements and the influence of these distributions on the reduction of the number of samples is studied. The global analysis of our approach on simulated and measured NF data shows that only 20% of the total number of points are needed with respect to the classical PWE technique to achieve an accurate characterization.

Deep learning improved source reconstruction method for phaseless planar Near-Field antenna measurement

Deep learning improved source reconstruction method for phaseless planar Near-Field antenna measurement

A comparison of near-field to far-field transformation techniques for use with industrial multi-axis robotic antenna measurement systems

Abstract This invited, extended, paper compares and contrasts a number of different near-field (NF) to far-field (FF) transformation algorithms that can be used for the purpose of processing NF data acquired using multi-axis industrial robots. The merits and limitations of these various, commonly encountered algorithms are highlighted with comparison FF data presented across a frequency range spanning 3–15 GHz. Crucially, the paper explores the viability of using mixed mode acquisition geometries when performing antenna gain measurements where, prior to this work, several of the transforms yielded different transform gains, and electrical lengths. Here, we verify that at 8 GHz and above, where truncation effects were minimal, for a circa 30 dBi gain (at 8 GHz) test antenna the FF peaks were in agreement to better than ±0.02 dB, at 3σ irrespective of the acquisition geometry and transform algorithm used. In this invited, extended work, the existing simulation results are augmented with experimental results obtained from planar and spherical NF measurements of a pyramidal horn taken using a dual robotic antenna measurement system and a consistent distributed RF subsystem.

High-Precision Calibration of Probe Frequency Response for Near-Field Wideband Measurements

Abstract Nonlinear calibration of probe frequency response is a common problem in near-field measurement. In this paper, we present an over-the-air(OTA) calibration technique for near-field measurements, predicated on our proposed measurement model. A detailed discussion is provided on the use of metal sphere, plane, and innovative dihedral calibrators, which exhibit scattering properties analogous to those of point target. The effectiveness of the proposed technique is validated through both full-wave simulation and experiment. The calibration method ensures the consistency of the broadband equivalent radiation center and significantly improves the imaging quality and range profile clarity.

Near-field optical measurement and analysis of Talbot effect in terahertz band

Direct experimental observation of the near-field images in grating structures for terahertz band is studied by scanning near-field microscope. It is shown that self-images and anti-imaging can be separately achieved near integer and half-integer multiples of the Talbot distance. Particularly, non-paraxial approximation is studied for cases where the lattice constant of the grating is marginally greater than the incident wavelength. These results provide guidance for manipulation terahertz wave propagation and facilitate the potential applications in imaging, sensing and communication systems.

Modified Linear Aperture Imaging Technique for Concave Structures in Near-Field Measurement Applications

Abstract This manuscript addresses the challenge of imaging artifacts in millimeter-wave (MMW) systems caused by concave structures due to multiple internal reflections. We introduce an modified linear aperture imaging technique specifically designed for concave structures, which effectively suppresses high-order scattering artifacts and achieves precise reconstruction of multi-reflection signals. Unlike previous cylindrical measurement methods, this approach does not require the dihedral cavity to be placed in a specific position, instead, it utilizes a linear measurement aperture for a more versatile solution. The paper presents the derivation of a reconstruction model for dihedral structures and compares this method with traditional imaging techniques through simulations. The results indicate that the proposed method is effective in accurately reconstructing the internal contours of concave structures.

A spherical near-field measurements method for electrically large high-gain antennas

Abstract In this paper, a spherical near-field to far-field transformation method (SNFTM) based scattering matrix algorithm (SMA) is presented to predict accurately far-field radiation patterns of electrically large high-gain antennas. The associated simulated and measured results are provided to the accuracy and validity of the SNFTM. It implies that this method makes it possible to calculate complete far-field radiation patterns for electrically large high-gain antennas.

Near-field luminous intensity distribution measurement with ex-situ error correction

Near-field luminous intensity distribution measurement with ex-situ error correction

Spatial Acoustic Loudspeaker Measurements With a Moving Microphone

Acoustic loudspeaker measurements are an essential aspect in loudspeaker development and design. Established setups usually require acoustic measurement rooms or chambers where the speaker under test has to be installed for conducting the tests. Furthermore, typical spatial measurements require that the speaker has to be rotated on one or two axes. In addition to the traditional approach, there are different methods that use near-field measurements where the microphone is usually moved and the loudspeaker remains at a fixed position. Both techniques commonly make measurements at discrete points or angles in the spatial domain. In this article, the idea of making impulse response measurements in the near-field while the microphone is in movement is proposed and described. This approach leads to much faster results that can be comparable to the results of fixed-point measurements. The focus here is on the properties and the resulting processing of appropriate test signals and on the experimental verification of the method. This method will be applied to the measurement of line arrays, where directivity is of high interest, but it can also be used for other loudspeaker configurations.

Compact mmWave measurement system for array antennas in a production environment

Abstract Millimeter wave antenna arrays are essential components of modern communication and radar systems. To produce these devices in large quantities, manufacturers require fast and reliable measurement equipment. The measurement equipment needs to ensure the quality, interoperability, and adherence to regulatory norms of the produced devices. In this work, we present an active probe array structure (PAS), which enables fast, compact, and reliable over-the-air (OTA) measurements of radiation characteristics. No relative movement between the antenna under test (AUT) and the active PAS is required, making the system very suitable for cost-effective large-scale characterization and commercial production test scenarios. We demonstrate and discuss how a near-field (NF) OTA measurement performed by this active PAS system can be used to reconstruct the far-field (FF) antenna radiation behavior of AUTs using an NF to FF correlation approach.

Transformation-Invariant Laplacian Metadevices Robust to Environmental Variation.

As one of the typical applications of metamaterials, the invisibility cloak has raised vast research interests. After many years' research efforts, the invisibility cloak has extended its applicability from optics and acoustics to electrostatics and thermal diffusion. One scientific challenge that has significantly restricted the practical application of the invisibility cloak is the strong background dependence, that is, all passive cloaking devices realized thus far are unable to resist variation in the background refractive index. To tackle such a challenge, the concept of transformation-invariant metamaterials (TIMs) is applied to static-field systems and shows that, for any physical fields governed by Laplace equation, judiciously designed TIMs can be used to realize invisibility cloaks robust to the environment variation. As an experimental proof, an ideal direct current (DC) cloak-is implemented based on TIMs and near-field measurement results demonstrate that such a cloak can successfully conceal a large-scale object when the background conductivity varies from 22 to 859 kSm-1. Moreover, the background-immune cloaking effect is observed under arbitrary electric sources. The approach proposed in this work can be also applied to static magnetics, thermal diffusion, and beyond, enabling robust isolation of the target from the external field in versatile application scenarios.

OTA Near-Field Measurement Approach for Electrically Steered Antenna Arrays

OTA Near-Field Measurement Approach for Electrically Steered Antenna Arrays

Effect of Number of Probes on Spherical Near-Field Measurement System

Effect of Number of Probes on Spherical Near-Field Measurement System

Quasi-BIC realized in a subwavelength volumetric split ring-based resonator

In recent years, resonant structures with quasi-bound states in the continuum (quasi-BICs) have significantly expanded the practical possibilities in optics and nanophotonics. A similar mode, the so-called supercavity mode, observed in single dielectric cylinders with high permittivity and low material losses, allows achieving extremely high quality (Q) factors. Resonators supporting quasi-BICs are also promising for applications in the radio frequency range. However, creating compact structures using high-permittivity materials at frequencies below 300 MHz is challenging. This study introduces a subwavelength (∼λ/13) volumetric structure composed of two arrays of coupled split ring resonators, with one array located inside the other, which provides a supercavity mode. The numerical Q factor of this mode is increased by approximately 100 times under lossless conditions and by about 1.5 times when accounting for material losses compared to that of non-interacting modes of the two arrays. The Q factor enhancement is confirmed experimentally by near-field measurements. The advantages of the proposed resonator include its hollow cavity, ease of fabrication, and frequency tunability within the radio frequency range.

Optical field modulation-enhanced cantilever sensor-based photoacoustic spectroscopy gas detection system

Abstract This study presents an approach to enhance the sensitivity of cantilever sensor-based photoacoustic (PA) spectroscopy gas detection systems through optical field modulation. Theoretical investigations into the mechanism and method of optical field modulation are followed by experimental validation. Acetylene is selected as the target molecule, with a near-infrared DFB LD laser serving as the light source. A dual-lens configuration was constructed to yield a better optical field distribution compared to the single-lens configuration. In the near-field of the laser, the setup of the dual-lens configuration has a better sensitivity than that of the single-lens setup, with a limit of detection reaching 0.52 μl l−1. In the far-field of the laser, however, the PA signals are significantly lower than that of near-field measurements, indicating that far-field measurement is not preferred. The results can provide references to the sensitivity improvement of the PA spectroscopy-based gas sensing system.

Planar Near-Field Antenna Measurements With a Uniform Step Larger Than Half-Wavelength

Planar Near-Field Antenna Measurements With a Uniform Step Larger Than Half-Wavelength

Inverse modeling of electromagnetic emissions using grey wolf optimization

The paper proposes an inverse modeling approach for evaluating radiated electromagnetic compatibility (EMC) in electronic components and circuits. To imitate the electromagnetic behavior of real devices, the suggested methodology generates an equivalent model using elementary dipoles. The Grey Wolf Optimization (GWO) algorithm solves an inverse problem to identify dipole parameters such as locations, intensities, and orientations. This technique improves model accuracy and robustness through a two-phase exploration and exploitation process that allows for effective parameter tuning while reducing simulation time and computational resources. Recent research in near-field measurement techniques illustrates the importance of reliable predictive methodologies in EMC evaluation. This method provides significant information on the impacts of power systems on surrounding electronic circuits, making it a useful tool for EMC testing. It also promotes the development of immunity models for high-frequency applications. Overall, this methodology simplifies the examination procedure, resulting in higher design efficiency for a wide range of electronic systems.