Near-field Measurements Research Articles

Directional Coupling to a λ/5000 Nanowaveguide.

Silicon-based microdevices are considered promising candidates for consolidating several terahertz technologies into a common and practical platform. The practicality stems from the relatively low loss, device compactness, ease of fabrication, and wide range of available passive and active functionalities. Nevertheless, typical device footprints are limited by diffraction to several hundreds of micrometers, which hinders emerging nanoscale applications at terahertz frequencies. While metallic gap modes provide nanoscale terahertz confinement, efficiently coupling to them is difficult. Here, we present and experimentally demonstrate a strategy for efficiently interfacing subterahertz radiation (λ = 1 mm) to a waveguide formed by a nanogap, etched in a gold film, that is 200 nm (λ/5000) wide and up to 4.5 mm long. The design principle relies on phase matching dielectric and nanogap waveguide modes, resulting in efficient directional coupling between them when they are placed side-by-side. Broadband far-field terahertz transmission experiments through the dielectric waveguide reveal a transmission dip near the designed wavelength due to resonant coupling. Near-field measurements on the surface of the gold layer confirm that such a dip is accompanied by a transfer of power to the nanogap, with an estimated coupling efficiency of ∼10%. Our approach efficiently interfaces millimeter waves with nanoscale waveguides in a tailored and controllable manner, with important implications for on-chip nanospectroscopy, telecommunications, and quantum technologies.

Analysis on scattering and inner near-field characteristics of a uniaxial anisotropic sphere by an off-axis high-order Bessel (vortex) beam

Based on the generalized Lorenz–Mie theory (GLMT) and the Fourier transform method, a theoretical approach is introduced to study the scattering of a uniaxial anisotropic sphere illuminated by an off-axis high-order Bessel (vortex) beam (HOBVB). According to the orthogonality of the associated Legendre function and exponential function, a concise expression of the expansion coefficients of the off-axis HOBVB in terms of the spherical vector wave functions (SVWFs) is derived that can effectively reconstruct the HOBVB with all conical angles. The differences of scattering characteristics of a uniaxial anisotropic sphere illuminated by an on-axis and off-axis HOBVB and a plane wave are exhibited. Influences of the topological charge, conical angle, particle size, and off-axis distance on the angle distributions of the radar cross-section (RCS), scattering and extinction efficiencies, and asymmetric factor are analyzed in detail. The unique internal and near-field distributions of a uniaxial anisotropic spherical particle illuminated by an on-axis and off-axis HOBVB are demonstrated. The results provide insights into the scattering and Bessel beam–matter interactions and may find important applications in optical propagation and optical micromanipulation, microwave engineering, target shielding, and near-field measurement.

Rectangular Slot based Beam Focusing Dual-Band Phase Gradient Metasurface

This study proposes method for enhancing the performance of previously developed or produced weapon systems without developing new detection equipment by using Phase Gradient Metasurfaces(PGMS). Metasurface has been studied and utilized a lot to improve the detection performance of inorganic systems, but its utilization is low due to its narrow bandwidth and complex structure. To address some of these limitations, dual-band power focusing PGMS is proposed. Its frequency independent unit cell characteristic enables creating metasurface that concentrates signals in dual-band. The designed PGMS was validated through near-field and far-field measurement, confirming signal concentration at the specified focal length and improved gain in the dual bands.

Has the German vehicle fleet become quieter? - Results of a measurement campaign using SPB and CPX measurements

In the past, the German Environment Agency has carried out several measurement campaigns to determine the noise behavior of the German vehicle fleet. The aim of the current project was to identify any trends in comparison to previous campaigns. Autonomous pass-by measurements were carried out at 30 locations throughout Germany in accordance with DIN EN ISO 11819-1 (SPB) with the aim of recording at least 1000 vehicles per location. As the road surface influences the pass-by noise, near-field measurements according to DIN EN ISO 11819-2 (CPX) were carried out to determine the homogeneity of the road surface at each measurement location. In parallel to the CPX measurements, texture measurements according to DIN EN ISO 13473 were carried out to quantify the relevant geometric parameters of the road surface. The presentation describes the methodology of these measurements and presents the final results including a comparison with data from the 2012 and 2002 measurement campaigns.

Instrumentation techniques for the measurement of gunfire (Part 3)

This paper is the third in a series concerning near-field measurement of sound pressure from small arms gunfire. It has been long established that exposure to gunfire events can lead to permanent threshold shift for an unprotected shooter. Close to a shooter's ear, the peak sound pressure level from a rifle or pistol can be 155-to-160 decibels with a pulse rise time on the order of ten microseconds. The U.S. Department of Defense has considered two approaches for assessing exposure limits from small arms gunfire - one involves the unweighted peak sound pressure and the other a series of A-weighted sound exposure events integrated to obtain an equivalent eight-hour dose. This dose is then compared to hearing damage risk criteria established for continuous noise signals. The first approach places great demands on the measurement system since the necessary bandwidth extends well into the ultrasonic range, thereby requiring fast slewing from the amplifiers combined with high sampling rates from the digitizer. The second approach requires the A-weighting filter to have specified time and frequency domain properties in the ultrasonic region. The paper discusses some laboratory experiments using actual gunfire signals to simulate the outcome from both approaches.

Noise within hospital wards: a survey method

Noise pollution is one of the most disturbing environmental factors. It directly affects people's lives, especially vulnerable people such as children, old-age, and hospital patients. Hospitals are complex buildings with manifold spaces and facilities where different users live with different sensitivity to noise. Several studies highlight the discomfort that patients, staff, and visitors experience in hospitals due to inadequate noise levels. All this leads to stress, sleep disorders, interference in communication, aggression, and annoyance. Noise exposure in hospitals is due to several sound sources. Each varies throughout the day, and their detection could be challenging. The present study proposes a clustering technique to analyze long-term sound level meter monitoring in an Italian hospital (Pisa, Tuscany). Equivalent short-term levels obtained with an interval time of 0.1 s are used as the database to exploit the occurrences curves of different chunks of the monitoring. Then, through a validated method, the spectra of different kinds of sound sources are reconstructed via the Gaussian Mixture Model. Results are compared with near-field measurements of single sources.

Millimeter-Wave Far-Field Range Antenna Measurement System for a W-Band Monopulse Antenna

In this study, a millimeter-wave far-field range antenna measurement system is proposed for W-band monopulse antenna measurements. Since the W-band monopulse antenna installed in radar systems has many input/output ports, multiple calculations are required to measure all the ports. We propose a system structure capable of the simultaneous measurement of multiple channels to quickly and accurately measure the performance of a monopulse antenna. The proposed measurement system includes a multi-channel rotary joint, a pair of diplexers, and multiple vector network analyzer extenders. In addition, an RF sub-system is implemented to reliably measure the phase of the W-band radiation pattern. It utilizes a shared local oscillator source and external power amplifiers for the transmit and receive paths. Employing the proposed multi-channel antenna measurement system, the sum/difference channels of a monopulse antenna were simultaneously measured according to the azimuth angle. In addition, a comparison of the far-field measurements provided by the proposed system and the near-field measurements was conducted.

Near field food contamination detection technique employing a compact Antipodal Vivaldi antenna

This paper presents a simple yet robust technique for identification of the presence of contaminants in food samples by detecting the variation in the transmission coefficient (S21) in a near field measurement setup. A compact step-notched antipodal Vivaldi antenna with a frequency range of operation 3 - 18 GHz, and realized gain of 12.81 dB at 16 GHz having compact dimensions 60mm×40mm×0.51mm, i.e., 0.6×0.4×0.005λ03 (where λ0 is the free space wavelength at the lowest frequency of operation) is designed for contamination detection due to its compact size, high directivity and high boresight gain for near-field measurement. An equivalent image is constructed from the S - parameters, measured at different positions and different antenna polarizations over the frequency range of 3-18 GHz for metallic contaminants, i.e., iron ball and aluminum foil. An algorithm is developed for detection of contamination from the images using the reference case of no-contamination scenario. The proposed algorithm is validated using full-wave simulation.

Colossal Near-Field Radiative Heat Transfer Mediated by Coupled Polaritons with an Ultrahigh Dynamic Range.

Near-field radiative heat transfer (NFRHT) can exceed the blackbody limit by several orders of magnitude owing to the tunneling evanescent waves. Exploiting this near-field enhancement holds significant potential for emerging technologies. It has been suggested that coupled polaritons can give rise to orders of magnitude enhancement of NFRHT. However, a thorough experimental verification of this phenomenon is still missing. Here this work experimentally shows that NFRHT mediated by coupled polaritons in millimeter-size graphene/SiC/SiO2 composite devices in planar plate configuration can realize about 302.8 ± 35.2-fold enhancement with respect to the blackbody limit at a gap distance of 87 ± 0.8nm. The radiative thermal conductance and effective gap heat transfer coefficient can reach unprecedented values of 0.136 WK-1 and 5440 Wm-2K-1. Additionally, a scattering-type scanning near-field optical measurement, in conjunction with full-wave numerical simulations, provides further evidence for the coupled polaritonic characteristics of the devices. Notably, this work experimentally demonstrates dynamic regulation of NFRHT can be achieved by modulating the bias voltage, leading to an ultrahigh dynamic range of ≈4.115. This work ambiguously elucidates the important role of coupled polaritons in NFRHT, paving the way for the manipulation of nanoscale heat transport, energy conversion, and thermal computing via the strong coupling effect.

A Miniaturized Ultrawideband V-Shaped Tip E-Probe for Near-Field Measurements.

A sensitive, miniaturized, ultrawideband probe is proposed for near-field measurements. The proposed probe is based on a new V-shaped tip design and a slope structure resulting in better field distribution and impedance matching with a span bandwidth from 10 kHz up to 52 GHz, which is compatible with ultrawideband applications. The proposed E-probe fabrication process utilizes a four-layer printed circuit board (PCB) using Rogers RO4003 (tm) and RO4450 high-performance dielectrics, with εr = 3.55 and 3.3, respectively. The probe length is 40 mm with a minimum width of 4 mm, which is suitable for narrow, complex, and integrated PCBs. The passive E-probe sensitivity is -106.29 dBm and -87.48 dBm at 2 GHz and 40 GHz, respectively. It has a very small spatial resolution of 0.5 mm at 20, 25, 30, and 35 GHz. The probe is small and cheap and can diagnose electromagnetic interference (EMI) in electronic systems such as telemetry, UAVs, and avionics.

Far-field pattern accuracy improvement in planar near-field measurement using infinitesimal dipole modeling

This paper discusses the challenges in accurately measuring the far-field performance of array antennas, which have become more complex and larger due to recent trends in increasing the number of elements and the antenna size. The far-field measurement method, which is a general method for measuring a radiation pattern in a far-field, is difficult to be utilized at a high frequency due to limitations in physical chamber size and difficulty in diagnosing an antenna. In order to improve these problems, a near-field measurement method is used as an alternative. Even though using a near-field measurement, the accuracy of the derived far-field pattern is reduced at low elevation angles due to zero-padding of data outside the measurement area. The integration of the near-field measurement with the source reconstruction is a promising approach to enhance the measurement limits of the far-field pattern analysis. This paper aims to extend the previous work to accurately predict the far-field radiation pattern, especially in the low elevation angles. The verification of the proposed method is performed for a standard gain horn and 4 by 4 patch array antennas, with and without beam steering.

Vortex-induced noise suppression of a cylinder with blowing through porous media

To mitigate vortex shedding for flow and noise control of a circular cylinder, an experimental approach combining air blowing and porous coating was implemented simultaneously as a hybrid method. Localized air blowing was symmetrically applied through structured porous media at four angles, corresponding to different regions of the flow field: boundary layers, shear layers on the cylinder, separated shear layers, and the cylinder's base. The study involved synchronizing near-field pressure fluctuation and far-field noise measurements with flow field measurements obtained via particle image velocimetry. Near-field pressure measurements were taken around the cylinder's circumference using a remote sensing method. This comprehensive investigation revealed that vortex shedding primarily induced pressure fluctuations at the cylinder's shoulders, resulting in the propagation of acoustic waves to the far field. The hybrid method, alongside the separate application of porous coating and local blowing, showcased substantial efficacy in mitigating near-field pressure, consequently leading to a reduction in far-field noise. These techniques achieved this by strategically shifting the vortex formation region further downstream and expanding the wake region compared to the baseline. Notably, the hybrid method, particularly when local blowing was applied at the base of the porous coated cylinder, exhibited a significantly enhanced impact in this regard, resembling the behavior observed with the individual application of porous coating.

Uncertainties Introduced by the Probe in Millimeter-Wave Planar Near-Field Measurements

Uncertainties Introduced by the Probe in Millimeter-Wave Planar Near-Field Measurements

Near-Field Microwave Imaging Method of Monopole Antennas Based on Nitrogen-Vacancy Centers in Diamond.

Visualizing the near-field distribution of microwave field in a monopole antenna is very important for antenna design and manufacture. However, the traditional method of measuring antenna microwave near field distribution by mechanical scanning has some problems, such as long measurement time, low measurement accuracy and large system volume, which seriously limits the measurement effect of antenna microwave near field distribution. In this paper, a method of microwave near-field imaging of a monopole antenna using a nitrogen-vacancy center diamond is presented. We use the whole diamond as a probe and camera to achieve wide-field microwave imaging. Because there is no displacement structure in the system, the method has high time efficiency and good stability. Compared with the traditional measurement methods, the diamond probe has almost no effect on the measured microwave field, which realizes the accurate near-field imaging of the microwave field of the monopole antenna. This method achieves microwave near-field imaging of a monopole antenna with a diameter of 100 µm and a length of 15 mm at a field of view of 5 × 5 mm, with a spatial resolution of 3 µm and an imaging bandwidth of 2.7~3.2 GHz, and an optimal input microwave phase resolution of 0.52° at a microwave power of 0.8494 W. The results provide a new method for microwave near-field imaging and measurement of monopole antennas.

Efficient single-frequency millimeter-wave three-dimensional imaging for rapid target identification

Compared to broadband systems, single-frequency millimeter-wave (SFQ-MMW) imaging systems can significantly simplify system complexity, but can only obtain two-dimensional information of the target. Moreover, the position of the reference plane must be adjusted in real time during the imaging process to obtain a high-quality focusing effects. Therefore, this paper proposes a SFQ-MMW three-dimensional imaging algorithm. In the plane wave spectrum domain, the propagation operator is used as a focusing term to accurately locate the target focus. Only single-frequency echo data is required to retrieve information and reconstruct high-quality images. This method can significantly improve the efficiency of near-field millimeter wave measurement and imaging. This work facilitates applications in rapid target identification and characterization. The performance of this method was tested through numerical simulation. Finally, validation was performed on real data in an independently developed system. The results show that the test results are in line with expectations, and the imaging efficiency has been significantly improved.

Far- and Near-Field Channel Measurements and Characterization in the Terahertz Band Using a Virtual Antenna Array

Far- and Near-Field Channel Measurements and Characterization in the Terahertz Band Using a Virtual Antenna Array

TROPOMI/PlumeTraj SO2 fluxes consistent with partially degassed magma supplying the 2018 Kīlauea eruption

Volcanic sulfur dioxide (SO2) emission measurements are a key element of volcano monitoring strategies and underpin our understanding of magmatic degassing impacts on air quality, aircraft, and climate. Volcanic SO2 emission monitoring is usually performed with near-field ground-based measurements, but ever-improving satellite observations increasingly offers the capacity for volcanic emission monitoring from space. Here we examine the May–August 2018 eruption at Kīlauea, Hawai'i, which produced a voluminous low-altitude gas plume. We compare SO2 emissions calculated with three approaches: ground-based networks, dissolved sulfur (S) contents and lava effusion rates, and new satellite-derived SO2 data. The high emission rates of this eruption posed a major challenge for near-field ground-based measurements, and corrections were needed to account for attenuation of the SO2 signal from the optically thick plumes. Our satellite-derived gas emissions use Sentinel-5 Precursor (S5P)/Tropospheric Monitoring Instrument (TROPOMI) observations and PlumeTraj back-trajectory analysis, deriving a total mass of 1.3 to 3.3 Mt of SO2 compared to 10.2 Mt from ground-measured fluxes (Kern et al., 2020). Our measurements deal with uncertainties such as plume aging and SO2 oxidation as well as cloud coverage which can underestimate results. The S contents also hold their own uncertainties partly due to when sampling was possible. We find agreement between satellite-derived fluxes and those that would be produced by magma which had previously lost at least 200–750 ppm (16–85%) of its initial S content during residence in the Halema'uma'u lava lake and reservoir. Our measurements allow an estimate of the efficiency of S loss from the Halema'uma'u lava lake prior to the 2018 eruption, and this implies a lava lake magma supply rate of 0.0019–0.0072 km3 per day for 2013–2018. The 2018 eruption produced a lava volume equivalent to 5 to 19 months of magma supply to Kīlauea during 3 months of eruption.

Far-Field Estimation Method of Antennas Above Conductive Plane Using a Partially Spherical Near-Field

For evaluating the practical performance of land mobile communication antennas via near-field measurement, it is essential to measure the electromagnetic field, including the housing in which the antennas are mounted. When measuring a heavy housing, scattered waves from the supporting turntable for the antenna under test (AUT) disturb the electromagnetic field. A technique for eliminating the influence of scattered waves is required to evaluate the abnormal AUT radiation performance. This paper proposes a method for estimating the far-field when the antenna exists above an infinite conductive plane, using partially spherical near-field measurements that include the turntable. The proposed method estimates the far-field by combining the equivalent source reconstruction and the far-field estimation with the reflection coefficient method. In the proposed method, the equivalent source distribution surrounding an AUT is reconstructed from a partially spherical near-field to remove the effects of the turntable. Through numerical calculations and experiments, this paper shows that the proposed method can estimate the far-field pattern above an infinite conductive plane, with arbitrary electric constants from a partially spherical near-field. The far-field estimation error is less than -20 dB when the upper hemispherical near-field can be obtained.

Phaseless Probe-Corrected Spherical Near-Field Antenna Measurements Using Two Scan Spheres

Phaseless Probe-Corrected Spherical Near-Field Antenna Measurements Using Two Scan Spheres

Monostatic Free-Space Method for Relative Permittivity Determination Using Horn Antenna Near-Field Measurements

Monostatic Free-Space Method for Relative Permittivity Determination Using Horn Antenna Near-Field Measurements