Near-field Scanning System Research Articles

Electric near-field scanning for electronic PCB electromagnetic radiation measurement

This paper introduces a design and test of an electric (E) near-field scanning (NFS) system by using innovative 4-layer miniature probe for electromagnetic compatibility (EMC) application. The probe works in the challenging frequency band up to 12 GHz with spatial resolution lower than 2 mm. The NFS is designed to operate by automation based on PC driven by LabVIEW® interface controlling a vector network analyzer (VNA). The PC communicates with the VNA through the local area network to read and save the measured data emitted by the device under test (DUT). The LabVIEW® interface is designed to control the E-NF positioning motion core by means of STM32® microcontroller. A stepper motor is used to move the E-NF probe along the scanning surface plan. Finally, the NFS is validated by visualizing the E-NF intensity maps generated in real time after calibration with a reference device under test represented by microstrip line and a microwave circuit.

On-demand multiplexed vortex beams for terahertz polarization detection based on metasurfaces.

The manipulation of polarization states is crucial for tailoring light-matter interactions and has great applications in fundamental science. Nevertheless, conventional polarization measurement approaches are extremely challenging to determine the polarization state of incident terahertz (THz) beams. The combination of metasurfaces and inhomogeneous vector vortex beams (VVBs) provides a new solution for integrated polarization-related functional devices. Herein, a general design strategy for spin-multiplexing all-silicon metasurfaces is presented and demonstrated in THz polarization detection. The employment of basic building blocks with a high aspect ratio (AR) imparts a greater degree of freedom for generating vector beams, and those basic blocks are subsequently utilized to explore the visualized polarization state. With the assistance of a THz near-field scanning system, we evaluate the capability of reconstructing the incident polarization state from the longitudinal polarization component multiplexed by vortex beams with tight focusing characteristics. Not only that, we also utilize the polarization with dynamically varying behavior as the illumination method to elucidate the evolution trend of the polarization state under a single snapshot and establish a visualized parametric model. This work paves the way to realize ultra-compact THz polarization detection-related devices for future applications in remote sensing, high-resolution imaging, and communications.

An S-Band-Receiving Phased-Array Antenna with a Phase-Deviation-Minimized Calibration Method for LEO Satellite Ground Station Applications

This study presents a new S-band-receiving phased-array antenna with a phase-deviation-minimized calibration method for the ground station of a low Earth orbit (LEO) satellite. The proposed antenna consists of 16 subarrays, 16 beamforming receiving RF modules (BF-RFMs), a power/control board, and a 16-way feed network. The subarray was achieved by joining two 8 × 1 arrays with a two-way power combiner. The 16-element antenna subarrays showed a gain of 16.1 dBi and a reflection coefficient of less than −10 dB from 2.12 GHz to 2.45 GHz. The BF-RFM, which consists of three low-noise amplifiers (LNAs), a power combiner, a phase shifter, and a digital attenuator, was designed and fabricated. The BF-RFMs were provided by the power/control board and showed a gain of 30.8 ± 0.8 dB, an amplitude root-mean-square (RMS) error from 0.25 dB to 0.28 dB, and a phase RMS error from 1.8° to 2.5° over the Rx frequency range. The arrangement procedures of the 16 BF-RFMs are presented to increase beam pointing accuracy at the desired angle. A commercial 16-way feed network was employed to combine all the output ports of the 16 BF-RFMs. The assembled antenna, which has dimensions of 1.58 m × 1.58 m × 0.2 m, was measured by partial and full scans in the near-field scanning system. The back-projected algorithm was employed to calibrate the antenna’s gain patterns in the partial scan. The implemented phased-array antenna had a gain greater than 28.14 dBi, sidelobe levels less than −17.1 dB, and beam pointing errors less than 0.07° over the beam pointing angle of −20~+20°. Based on the implemented antenna system, we conducted a field test using KOMPSAT-5, which is actually operating in South Korea, in order to verify the performance of the low Earth orbit (LEO) satellite ground station system.

3D printed metasurface for generating a Bessel beam with arbitrary focusing directions.

In this Letter, a metasurface combined with emerging 3D printing technology is proposed. The proposed metasurface regards the simple cube as the unit cell, and the height of the cube is the only variable. A nearly linear transmission phase range covering 360° operating at 20 GHz is obtained when the height is regulated in [2.26 mm, 11.20 mm]. Therefore, the proposed unit cell can be adopted to any metasurface with various functions. Taking the generation of a non-diffractive Bessel beam as an example, two metasurfaces composed of 30×30 units with different focusing directions are designed based on non-diffractive theory and the generalized law of refraction. Two prototypes are 3D printed and measured by a near-field scanning system. The measured results validate our design with satisfactory focusing and beam deflection performance. Additionally, the 3D printed metasurface has lower cost and a shorter processing cycle, and avoids metal loss. Therefore, a 3D printed metasurface is an excellent candidate that can be applied in millimeter wave or even higher frequency bands.

Fast Power Density Assessment of 5G Mobile Handset Using Equivalent Currents Method

As the fifth-generation (5G) mobile communication is utilizing millimeter-wave (mmWave) frequency bands, electromagnetic field (EMF) exposure emitted from a 5G mmWave mobile handset should be evaluated and compliant with the relevant EMF exposure limits in terms of peak spatial-average incident power density (PD). In this work, a fast PD assessment method for a 5G mmWave mobile handset using the equivalent current (EQC) method is proposed. The EQC method utilizes the intermediate-field (IF) data collected by a spherical measurement system to reconstruct the EQCs over a reconstruction surface and then computes the PD in close proximity of the mobile handset with acceptable accuracy. The performance of the proposed method is evaluated using a mmWave mobile handset mock-up equipped with four quasi-Yagi antennas. The assessed PD results are compared with those computed using full-wave simulations and also those measured with a planar near-field (NF) scanning system. In addition, three influencing factors related to the accuracy of the EQC method, namely, the angular resolution, the phase error, and the handset position in the IF measurements, are also analyzed. The proposed method is a good candidate for fast PD assessment of EMF exposure compliance testing in the mmWave frequency range.

Autocorrelation Analysis and Near-Field Localization of the Radiating Sources With Cyclostationary Properties

Stochastic electromagnetic fields caused by the data transferring signals along the printed circuit boards are potentially harmful interference sources for modern digital electronic devices. The characterization of the near-field electromagnetic emissions from the electronic device in time domain was implemented by real-time digital oscilloscope in conjunction with the sequential scanning system of the near-field probes. Excluding of the periodic sample mean function reveals the hidden cyclostationarity of the data transferring process measured by the near-field scanning system. In this article, the quantitative autocorrelation analysis of the hidden cyclostationary properties of the random nonperiodic data sequences is presented. The time-domain evolution of the autocovariance cross sections estimated at each scanning point were exploited for spatial-time localization of the transmission line.

A Miniature Multi-Component Probe for Near-Field Scanning

In this paper, an electromagnetic probe is proposed for the near-field scanning. Its detection section is made of a miniature loop-wire hybrid structure, which can measure ${H} _{x}$ , ${H} _{y}$ , and ${E} _{\textit {z}}$ simultaneously. It is optimized to achieve a wide working band from 0.1 to 15 GHz. The calibration and measurement error of the probe are discussed in detail. The probe is fabricated as a four-layer printed circuit board (PCB) with the RO4350B dielectric. A near-field scanning system with the manufactured probe is set up to test the near fields above a Z-type microstrip line and an inverted-F antenna. By comparing the measurement and simulation results, the accuracy of the proposed multi-component probe is verified. It greatly improves the near-field scanning efficiency compared to the traditional single-component probe.

Measurement of Near-Field Electromagnetic Emissions and Characterization Based on Equivalent Dipole Model in Time-Domain

In this article, a method for representing electromagnetic emissions from a device under test (DUT) using an equivalent time dependent dipole array model deduced from the time-domain near-field scanned tangential magnetic fields is proposed. First, a three-dimensional (3-D) time-domain near-field scanning system is established to measure the tangential magnetic fields emitted from DUTs which are a transmission line above a ground plane and a printed circuit board (PCB) with several microstrips, respectively. For time-domain measurements, two magnetic field probes are calibrated over a broad bandwidth for both amplitude and phase to obtain their complex probe factors. Then, the measured magnetic fields are utilized to construct an equivalent time dependent dipole array model to represent the electromagnetic sources of the DUT. Parameters of the time dependent equivalent dipoles are directly calculated by fitting to the measured magnetic fields. The effects of different number of dipoles on the accuracy of the reconstructed magnetic fields from the PCB are studied. The reconstructed equivalent dipoles of the DUTs can be used to predict the electromagnetic fields at other observation levels. The results predicted by the equivalent dipole model are in agreement with the simulated and measured results.

A High-Frequency and High Spatial Resolution Probe Design for EMI Prediction

In this paper, a simple and accurate equivalent circuit of the magnetic probe is proposed. Based on the equivalent circuit, a high-frequency probe for near-field measurements is proposed, manufactured, and tested. To increase its working frequency, its parasitic inductance and capacitance are reduced by using a small loop aperture and a tapered transition. The working frequency of the probe is up to 30 GHz. At the same time, sidewall metallization and bottom-vias are used in the probe, which result in a good performance of electric field suppression. The transmission section of the probe is optimized at high frequencies by using via fence and coax-thru-hole vias. The magnetic field probe is built on a four-layered printed circuit board with Rogers substrate. The loop aperture size of the probe is $250\,\,\mu \text{m}\,\,\times 250\,\,\mu \text{m}$ , so it has a high spatial resolution and is helpful to measure the detailed magnetic field from a small device. The probe is calibrated by a standard coplanar waveguide with a backside ground (CPWG). Finally, a near-field scanning system with the fabricated probe is set up to test the magnetic field above an unknown CPWG and microstrip line. From the near-field scanning results, it can be concluded that the probe can accurately measure the magnetic field over a wide frequency band.

Modeling of Wet Antenna Attenuation for Precipitation Estimation From Microwave Links

This letter presents an analysis of wet antenna attenuation (WAA) from an electrodynamic perspective. The WAA is a relevant part of the path attenuation determined from the received signal level (RSL). In the case of extracting rain rates from path attenuation, as it is done with data from country-wide commercial microwave link (CML) networks, the WAA is a significant error source and leads to overestimation of rainfall. To handle this effect, a model, based on the three antenna parameters, reflectivity, efficiency, and directivity, is developed. The model allows the variation of those three parameters depending on the degree of wetness and shows that WAA is composed of variations of antenna impedance, antenna efficiency, and antenna directivity. The suitability of the theoretical model was assessed by a full-wave numerical simulation of a pyramidal horn antenna at 18 GHz with homogeneous water film on the surface of the antenna cover. The result of this simulation is a quantification of these three subeffects. For verification of the simulation, an antenna, similar to the antenna in the simulation, was manufactured and a controlled sprinkler device to generate a water film was built. The adjustable water flow rate determines the thickness of the water film. The parameters were generated by measuring the near field and the reflectivity of the antenna in an anechoic chamber, with a near-field scanning system and a vector network analyzer, under different flow rates equaling different water film thicknesses. From these results, the reflectivity, directivity, and efficiency were computed and compared with the results of the full-wave simulation. It is found that the reflectivity is linked to water film thickness in a such way that the WAA can be deduced from monitoring the antenna reflectivity.

SNR Analysis and Optimization in Near-Field Scanning and EMI Applications

In a near-field scanning system, each element of the measurement chain contributes to the thermal noise power density: probe, cables, amplifiers, and the measuring instrument. The signal-to-noise ratio (SNR) is strongly affected by the source output impedance, source temperature, the lossy transmission lines between probe and amplifiers, amplifier noise, amplifier temperature, and amplifier gain. By minimizing the loss between the probe and by using ultralow-noise amplifiers (noise figure (NF) 10 dB, compared to a setup using a 1-m cable and a 3-dB NF amplifier. A resonant probe that is cooled with liquid nitrogen improves measurement SNR by an additional 10–12 dB, as compared to a broadband probe of similar loop size. To combine the advantages of a resonant probe, without sacrificing the ability to measure broadband, a proof of concept is demonstrated that uses a tunable resonant probe which is synchronized to the frequency sweep of the spectrum analyzer.

An ultra-compact rejection filter based on spoof surface plasmon polaritons

In this paper, we propose a scheme to construct a new type of ultra-compact rejection filter by loading split-ring resonators (SRRs) on the transmission line of spoof surface plasmon polaritons (SPPs). From the dispersion analysis of the spoof SPP transmission line with and without the SRR loading, we clearly reveal the mechanism of the rejection characteristic for this compact filter. Meanwhile, we fabricate two spoof SPPs waveguides loaded with different amounts of metamaterials particles, and experimentally test them using an Agilent Vector Network Analyzer (VNA) and a homemade near-field scanning system. Both the simulated and measured results agree well with our theoretical analysis and demonstrate the excellent filtering characteristics of our design. The isolation of both filters can be less than −20 dB, and even reach −40 dB at rejection frequencies. The proposed rejection and stop-band filters show important potentials to develop integrated plasmonic functional devices and circuits at microwave and terahertz frequencies.

Negative and near zero refraction metamaterials based on permanent magnetic ferrites

Ferrite metamaterials based on the negative permeability of ferromagnetic resonance in ferrites are of great interest. However, such metamaterials face a limitation that the ferromagnetic resonance can only take place while an external magnetic field applied. Here, we demonstrate a metamaterial based on permanent magnetic ferrite which exhibits not only negative refraction but also near zero refraction without applied magnetic field. The wedge-shaped and slab-shaped structures of permanent magnetic ferrite-based metamaterials were prepared and the refraction properties were measured in a near-field scanning system. The negative and near zero refractive behaviors are confirmed by the measured spatial electric field maps. This work offers new opportunities for the development of ferrite-based metamaterials.

Experimental demonstration in X-band of dual-negative refraction in photonic crystals

The effect of dual-negative refraction (DNR) in a photonic crystal (PhC) is demonstrated experimentally in X-band (8–12 GHz) for the first time. The two-dimensional PhC sample is made of highly pure Al2O3 ceramic cylinders close packed in air with a triangular lattice structure. The frequency regions of DNR are predicted by the analyses of wave vector diagrams. Experiments of anomalous refraction in PhCs are carried out in a near-field scanning system to demonstrate the theory and numerical results. The experimental measurements of electric field (real part and spatial intensity) for DNR are in good agreement with the numerical simulations. This work demonstrates that the unique feature of photonic crystals can be realized experimentally in different frequency regions which are proportional to the lattice constant of the PhC. Due to the large design flexibility, we believe this result will be promising for application in the range of optical devices.

Anomalous Refractions Based on Photonic Band Modulation of Photonic Crystals

Due to the existence of photonic bandgap derived from the periodical dielectric structure in wavelength scale, many novel optical phenomena can be demonstrated in photonic crystals (PhCs). Anomalous propagation of electromagnetic waves in X-band is demonstrated experimentally in a two-dimensional triangular lattice PhC. The experimental measurements of electric field are carried out in a near-field scanning system. The phenomena of anomalous refractions are observed and analyzed in respect of anisotropy of equal frequency surfaces in the PhC. The analysis results of numerical simulation are compared with the experimental measurements and good agreements are obtained.

Improved near-field scanning system for encircled flux measurement

The authors propose an improved near-field scanning system for measuring the mode distribution exiting a multimode fibre (MMF), notably for evaluating conformity to the encircled flux (EF) standard of a measurement source used for MMF loss/attenuation characterisation. This system offers advantages over both charge coupled device (CCD)-based and traditional scanning approaches. EF measurements taken with this new system are in excellent agreement with CCD-based measurements.

A Compact Multiband Antenna Based on DNG ZOR for Wireless Mobile System

A compact multiband antenna using a double-negative (DNG) zeroth-order resonator (ZOR) for global system for mobile communications (GSM850/1800/1900, 824894/17101880/18501990 MHz), wideband code division multiple access (WCDMA, 19202170 MHz), and wireless broadband (WiBro, 2.32.4 GHz) operations is presented. The proposed antenna having a total volume of 40 times 6 times 3 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> comprises a lower-band ZOR antenna part and an upper-band ZOR antenna part. The zeroth-order resonant properties are analyzed by magnitude and phase distributions of the surface magnetic field using near-field scanning system. The performance of the antenna is verified by experimental data obtained from fabrication and measurement. The antenna offers omnidirectional radiation patterns and measured radiation efficiencies exceeding about 53% and 41% for lower and upper bands, respectively.

A cryogenic microwave scanning near-field probe: Application to study of high-Tc superconductors

We report a vacuum cryogenic (80 K&amp;lt;T&amp;lt;350 K), near-field microwave scanning system based on a 90 GHz transmitting/receiving resonant slit antenna with a capacitive measurement of the probe-sample separation. The probe allows local measurement of resistance as a function of temperature with the spatial resolution of 20-50 μm. The mm-wave probe is integrated with the eddy-current probe which allows global measurement of resistance of conducting and superconducting samples. This integrated probe is used for local study of the superconducting transition in high-Tc superconducting thin films. The sensitivity of our present mm-wave probe is sufficient for probing conductivity in the normal state and in the superconducting state close to Tc however not yet sufficient for probing conductivity variations far below superconducting transition temperature.

Linear behavior of a near-field optical scanning system

A hybrid finite-difference time-domain and angular-spectrum propagation modeling technique is used to study the imaging properties of a near-field optical scanning system with dielectric samples. The model is used to calculate system transfer functions based on scanning sinusoidal gratings of various spatial periods or on scanning a straight edge and then taking a derivative and a Fourier transform. Results from these two methods are in good agreement. A square-wave grating is simulated by linear addition of component sine-wave grating images that are weighted by the transfer function. The image generated by this method agrees well with an image generated by direct use of the hybrid model. In the region of parameter space investigated with the model, the near-field optical scanning system exhibits nearly linear behavior. The region of linear operation depends on the index of the sample and on the probe-to-sample spacing.