Near-field Range Research Articles

Enabling two-dimensional optical subdiffraction imaging at an extended working distance: a planar antenna-array approach

We propose a practical optical probe configuration capable of two-dimensional subdiffraction imaging beyond the conventionally used near-field range. The probe consists of a planar array of plasmonic monopoles radiating with different amplitudes and phases, such that the near-field interaction of the array elements produces a subdiffraction spot size 40% smaller than the diffraction limit at a quarter-wavelength away from the probe. Although designed to operate in the visible, this topology is scalable to other spectra as well. Our proposed configuration could alleviate the “working distance” issue between the object and imaging apparatus since it enables superresolution focusing at relatively long distances while being compatible with existing near-field imaging setups, such as scanning near-field optical microscopes.

Wireless RF Sensor Structure for Non-Contact Vital Sign Monitoring

This paper describes a compact and novel wireless vital sign sensor at 2.4 GHz that can detect heartbeat and respiration signals. The oscillator circuit incorporates a planar resonator, which functions as a series feedback element as well as a near-field radiator. The periodic movement of a human body during aerobic exercise could cause an input impedance variation of the radiator within near-field range. This variation results in a corresponding change in the oscillation frequency and this change has been utilized for the sensing of human vital signs. In addition, a surface acoustic wave (SAW) filter and power detector have been used to increase the system sensitivity and to transform the frequency variation into a voltage waveform. The experimental results show that the proposed sensor placed 20 mm away from a human body can detect the vital signs very accurately.

Compact Vital Signal Sensor Using Oscillation Frequency Deviation

In this paper, a compact vital signal sensing method using oscillation frequency deviation at 2.4-GHz industrial-scientific-medical band is proposed to detect vital signals, such as heartbeat and respiration signal. The oscillation circuit of the proposed vital sensor system has been realized by a planar resonator, which functions as a positive feedback element, as well as a near-field radiator to sense vital signals, simultaneously. The periodic movement of a body by respiration exercise causes the impedance variation of the radiator within the near-field range. The impedance variation results in a corresponding change in the oscillation frequency, and this variation has been utilized for sensing of the vital signals. In addition, a surface acoustic wave filter and power detector have been used to increase the sensitivity of the system and to transform the frequency variation to voltage waveform. The experimental results show that the proposed vital sensor placed 20 mm from the body can detect the heartbeat waveform very accurately.

Wireless RF Vital Signal Sensor Using Autoregressive Spectral Estimation Method

This letter describes a compact and novel wireless vital signal sensor at 2.4 GHz that can detect heartbeat and respiration signals. The oscillator circuit incorporates a planar resonator, which functions as a series feedback element as well as a near-field radiator. The periodic movement of the body during aerobic exercise causes an input impedance variation of the radiator within the near-field range. This variation results in a corresponding change in the oscillation frequency, and this change has been utilized for detection of human vital signals. In addition, a surface acoustic wave (SAW) filter and power detector have been used to increase the system sensitivity and to transform the frequency variation into a voltage waveform. Also, the autoregressive spectral estimation method is applied to achieve precise vital sign detection. The experimental results show that the proposed sensor placed 20 mm away from a human body can detect the vital signals very accurately.

Three-Dimensional Near-Field Microwave Holography Using Reflected and Transmitted Signals

A new 3-D holographic microwave imaging technique is proposed to reconstruct targets in the near-field range. It is based on the Fourier analysis of the wideband transmission and reflection signals recorded by two antennas scanning together along two rectangular parallel apertures on both sides of the inspected region. The complex scattering parameters of the two antennas are collected at several frequencies and then processed to obtain a representation of the 3-D target in terms of 2-D slice images at all desired range locations. No assumptions are made about the incident field and Green's function, which are derived either by simulation or by measurement. Furthermore, an approach is proposed to reduce the image artifacts along range. To validate the proposed technique, predetermined simulated targets are reconstructed. The effects of random noise, number of sampling frequencies, and dielectric contrast of the targets are also discussed.

Full-waveform based complete moment tensor inversion and source parameter estimation from downhole microseismic data for hydrofracture monitoring

Downhole microseismic monitoring is a valuable tool in understanding the efficacy of hydraulic fracturing. Inverting for the moment tensor has gained increasing popularity in recent years as a way to understand the fracturing process. Previous studies utilize only part of the information in the waveforms, such as direct P- and S-wave amplitudes, and make far-field assumptions to determine the source mechanisms. The method is hindered in downhole monitoring, when only limited azimuthal coverage is available. In this study, we develop an approach to invert for complete moment tensor using full-waveform data recorded at a vertical borehole. We use the discrete wavenumber integration method to calculate full wavefields in the layered medium. By using synthetic data, we find that, at the near-field range, a stable, complete moment tensor can be retrieved by matching the waveforms without additional constraints. At the far-field range, we discover that the off-plane moment tensor component is poorly constrained by waveforms recorded at one well. Therefore, additional constraints must be introduced to retrieve the complete moment tensor. We study the inversion with three different types of constraints. For each constraint, we investigate the influence of velocity model errors, event mislocations, and data noise on the extracted source parameters by a Monte Carlo study. We test our method using a single well microseismic data set obtained during the hydraulic fracturing of the Bonner sands in East Texas. By imposing constraints on the fracture strike and dip range, we are able to retrieve the complete moment tensor for events in the far-field. Field results suggest that most events have a dominant double-couple component. The results also indicate the existence of a volumetric component in the moment tensor. The derived fracture plane orientation generally agrees with that derived from the multiple event location.

3D Power Synthesis with Reduction of Near-Field and Dynamic Range Ratio for Conformal Antenna Arrays

A simple method of power synthesis for 3D radiation patterns of conformal antenna arrays is presented. The method enables to synthesize a desired far-field pattern simultaneously controlling the dynamic range ratio (DRR) of the array excitations and the electric field amplitude in a given region of interest close to the antenna. The power synthesis problem is reduced to a field synthesis one by introducing an auxiliary phase pattern, which is incorporated in a cost function together with the desired far-field pattern, the array pattern, the near electric field amplitude, and the array excitations. Such cost function is minimized by iteratively modifying the auxiliary phase pattern and the array excitations. Applications of the proposed method to conformal arrays of different geometries show that accurate results are obtained also in presence of stringent requirements and within acceptable CPU times, even when hundreds of elements are involved.

Effects of the intermediate SiO2 layer on polarized output of a light-emitting diode with surface plasmon coupling

The variation behaviors of the output intensity and polarization ratio of InGaN/GaN quantum well (QW) light-emitting diodes (LEDs) with surface plasmon (SP) coupling by inserting SiO2 intermediate layers between the p-GaN layers and surface Ag grating structures are demonstrated. The insertion of the SiO2 layer is expected to reduce the metal dissipation of SP energy and extend the near-field distribution range of the induced SP for generating more favored SP-QW coupling effects. The Ag grating period for optimizing SP-QW coupling is increased when a SiO2 layer is added to the device, which is consistent with the simulation results of the momentum matching of SP polariton and the resonance behavior of localized SP. The almost unpolarized outputs from other LED samples fabricated with an epitaxial structure of thicker p-GaN layer, which leads to weak SP-QW coupling, indicate that the observed polarization ratios are due to near-field SP-QW coupling, instead of far-field diffraction.

Fundamental Aspects of Near-Field Coupling Small Antennas for Wireless Power Transfer

A physical limitation on the power transfer efficiency between two electrically small antennas in the near-field range is presented. By using a Z-parameter which describes the interaction between two antennas for spherical modes in connection with antenna parameters, the maximum power transfer efficiency and the optimum load impedance are shown as functions of the distance between two antennas, the radiation efficiency and the input impedance of the isolated antenna. The theory is verified by a simulation with a small helical antenna, which generates and modes, simultaneously.

Analysis of Antenna Coupling in Near-Field Communication Systems

A simple mathematical formula is proposed for computing the coupling coefficient between two arbitrary antennas that are placed within the other's near-field region. The information required by this expression consists of the associated normalized vector far-field patterns, their relative orientations, and the antenna spacing. To validate our proposed expression, coupling coefficients in several near-field scenarios are computed, including for the case of a practical near-field ultra-high frequency (UHF) radio-frequency identification (RFID) system. These results are then compared to practical measurements and to the outcome of full-wave simulations generated using Ansoft HFSS. All results are in good agreement. Additionally, in this paper, it is shown that several factors may influence the coupling coefficient, such as the impedance matching of the receiving antenna and the directivities of both antennas. With the aid of our formula, the near-field read range can be determined and near-field coupling phenomena can be investigated. The results thus obtained may be useful in near-field communication systems.

Semianalytic pulsed coherent laser radar equation for coaxial and apertured systems using nearest Gaussian approximation

We present a semianalytic pulsed coherent laser radar (CLR) equation for coaxial and apertured systems. It combines the conventional CLR equation, numerical Fresnel integration (NFI), and nearest Gaussian approximation, using correction factors that correspond to beam truncation. The range dependence of the signal-to-noise ratio obtained by this semianalytic equation was found to agree well with the precise NFI solution for not only the focal range, but also the near-field range. Furthermore, the optimum beam truncation condition depending on the atmospheric refractive index structure constant is shown. The derived equation is useful for precisely predicting the CLR performance simply by its semianalytic expression.

Coseismic displacement field of the Wenchuan Ms 8.0 earthquake in 2008 derived using differential radar interferometry

We used the radar data from satellite ALOS/PALSAR of Japan and D-InSAR technology to derive the coseismic displacement produced by the Wenchuan, China Ms 8.0 earthquake on 12 May 2008. The result shows that the coseismic displacement primarily concentrated in a near-field range about 100km width on both sides of the Yingxiu-Beichuan fault. The incoherent zone about 250km long and 15~35km wide nearby the fault suffered the largest deformation with surface ruptures. The secondary deformed areas are 70km wide on each side of the incoherent zone, where the displacements exhibit a sunk northern wall with maximum -110~120cm and an uplifted southern wall with maximum 120~130cm, respectively. In the far-field range of the fault, displacements are less than 10cm. Using the offset tracking, we found clear rupture traces and coseismic displacement of 3m along the faults. With a model of four fault sections, we retrieved slip distribution on the faults. The inversion result reveals two slips of 10m at depths 5~20km beneath the Yingxiu-Beichuan fault and one slip of 2.3m at depth 5~20km below the Guanxian-Jiangyou fault, respectively. Thrust faulting dominates the southwestern Yingxiu-Beichuan fault and the entire Guanxian-Jiangyou fault, while right-slip is the primary component along the northeastern Yingxiu-Beichuan fault.

Behaviour of steel reinforced polymer (SRP) strengthened RC members under blast loadThis paper is one of a selection of papers in the Special Issue on Blast Engineering.

During the past decade, significant research has been carried out on the strengthening of reinforced concrete (RC) slabs, beams, and columns using externally bonded carbon fibre reinforced polymer (CFRP) sheets. Steel reinforced polymer (SRP) sheets have recently been proposed as an alternative to CFRP to strengthen reinforced concrete beams. This paper reports experimental and numerical results of RC beams and beam-columns transversely wrapped with SRP and tested under blast load. A total of 10 scaled RC members were tested at a variety of blast wave intensities. Detailed observations are reported and validated against numerical models created in AUTODYN for the unstrengthened RC members. The SRP wraps were resilient in the near-field blast range and enhanced the ductility of the concrete likely through enhanced confinement. Member capacity could be increased by the wraps in failure modes dominated by concrete crushing. AUTODYN appears able to reasonably predict the behaviour of the RC members when loaded by blast.

Shape and Motion Estimation from Near-Field Echo-Based Sensor Data

This paper presents a new approach for reconstructing both shape and motion from data collected by echo-based ranging sensors. The approach is based on geometric invariant theory and exploits object-image relations for near-field (spherical-wavefront) range data. These object-image equations relate the data to a unique matrix of Euclidean invariants that completely describe the object shape. The object-image relations can be used to determine the shape of a scene viewed from unknown vantage points. Specifically, the object-image equations form a linear system of equations whose solution determines the relevant shape parameters for a configuration of features within the scene. Once the shape parameters are estimated, a single shape exemplar from the point in shape space can be used to determine the relative motion (up to an arbitrary rotation) between the sensor and the object. One advantage of this motion-estimation approach is that the geometric-invariant-based strategy allows us to uniquely solve the optimization problem without the need to introduce coordinate-system-dependent “nuisance” parameters. The theorems stated in this paper hold for any range-measurement sensor scenario. As an example of the utility of the given theorems, the object-image relations are used to augment noisy GPS measurements in a circular synthetic aperture radar geometry.

Unexpected measurement results of 94 GHz lens antenna in short far-field conditions

A hyperbolic W-band lens antenna has been designed to study the measurement limitations of a far-field range at short far-field conditions in LEAT, University of Nice, France. A reference measurement is performed at 94 GHz in a planar near-field range at TKK, Helsinki University of Technology, Finland. Near-field measurement accuracy is calculated and the measurement results of the different ranges are analysed. It turns out that unrealistically good results can be obtained if a defocused antenna is measured in short far-field conditions.

Conical near-field antenna measurement system

A probe-compensated near-field-to-far-field-transform algorithm has been developed that can generate far-field patterns from near-field measurements made on an arbitrary surface. We present the concept, the algorithm, and computer-simulated and measured test results for measurements on a conical surface. The prototype conical near-field measurements were made in a planar near-field range on a horn antenna under test (AUT), mounted on an azimuth-over-elevation positioner to produce a conical measurement surface. This system is especially applicable for producing full-hemisphere far-field patterns for antennas mounted on vehicles, where other standard measurement systems may not adapt well to the profile, may not provide full-hemisphere coverage, or may require large, mechanically complex systems.

Single-Negative, Double-Negative, and Low-index Metamaterials and their Electromagnetic Applications

Spherical near-field measurements have become a common way to assess the performance of a wide variety of antennas. However, published reports on range-error assessments for spherical near-field ranges are not very common. This is likely due to the perceived additional complexity of the spherical near-field measurement process, as compared to planar or cylindrical measurement techniques. This paper will establish and demonstrate a simple procedure for characterizing the performance of a spherical near-field range. The measurement steps and reporting can be largely automated with careful attention to the test process. We will summarize the process and document the accuracy of a spherical near-field test range at nearfield systems (NSI), using the same NIST 18 terms commonly used for planar near-field measurements

AMTA Coner

Spherical near-field measurements have become a common way to assess performance of a wide variety of antennas. Published reports on range error assessments for spherical near-field ranges however are not very common. This is likely due to the perceived additional complexity of the spherical near-field measurement process as compared to planar or cylindrical measurement techniques. This paper will establish and demonstrate a simple procedure for characterizing the performance of a spherical near-field range. The measurement steps and reporting can be largely automated with careful attention to the test process. We will summarize the process and document the accuracy of a spherical near-field test range at NSI using the same NIST 18 terms commonly used for planar near-field measurements.

Airflow velocity effects on air bearing with grooved disk surface in near-field optical disk drives

By means of an air bearing, the pickup head slider of a near-field optical disk drive flies above data tracks on a rotating disk surface to achieve a stable flying height. The influence of airflow velocities of the air bearing on lift force deserves investigation, as the airflow velocity varies during track seeking or disk speed variation. In this study, a direct simulation Monte Carlo method is used to investigate three-dimensional nanoscale gas film lubrication at the air bearing between a slider and a rotating disk. This study aims to investigate air bearing behavior at different airflow velocities. Computational results show that faster flow generates larger pressure and lift force. Lower flying height reduces force variation caused by different velocities. This study also proposes a method for maintaining the flying height within the near-field range.

Extended Edge Wave Diffraction Model for Near-Field Directivity Calculations of Horn Antennas

The edge wave diffraction model for pyramidal horn antennas is extended to include the finite radii of curvature of the primary incident wave approximated by an astigmatic ray tube in determining the edge diffraction fields contributing to the radiated fields. The model is adapted to calculate the effective near-field directivity of a pyramidal horn antenna for generating standard electromagnetic field strengths for the calibration of field probes. The predicted directivity results for a range of standard gain antennas are verified with method of moments data and measured values obtained in a near-field antenna range.