Far-field Antenna Research Articles

The Study on Near-Field Scattering of a Target Under Antenna Irradiation by TDSBR Method

An efficient time domain shooting and bouncing ray (TDSBR) method is proposed to analyze the transient near-field scattering from an electrically large complex object illuminated by a far-field antenna source. The time-domain far-field incident sources can be derived by a convolution of pulsed excitation with the inverse Fourier transform of frequency domain (FD) far-fields radiated from an antenna. In order to obtain accurate near-field scattering results, time domain physical optics (TDPO) near-field integral representations are proposed and reduced to closed-form expressions to improve computing efficiency by applying locally expanded Green function approximation. Different from the plane wave incident situation, in the case of antenna radiation, the incident fields of each individual facet in the ray path are represented by the radiation fields from the equivalent mirror antenna derived in this paper. Since the radiation fields naturally contains the time delay term, the complicated processing of the time delay problem can be avoided by solving the equivalent mirror antenna radiation fields. It is simpler and more accurate than the traditional time domain geometrical optics (TDGO) method. Numerical examples are presented to demonstrate the efficiency and accuracy of the proposed method.

All-dielectric transformation medium mimicking a broadband converging lens.

Radio waves carrying orbital angular momentum (OAM) may potentially increase spectrum efficiency and channel capacity based on their extra rotational degree of freedom. However, due to their divergence characteristics, vortex waves are not suitable to transmit over a long distance in the radio frequency (RF) and microwave domains. In this paper, a transformation optics (TO) based all-dielectric converging lens is proposed. The beam divergence angle of the vortex wave passing through the lens can be decreased from 25° to 9°. The transformed material parameters of the converging lens are determined by solving Laplace's equation subject to specific boundary conditions. Far-field antenna radiation patterns as well as near-field helical phase and electric field amplitude distributions obtained from numerical simulations are reported, demonstrating the broadband characteristics of the proposed microwave lens. Moreover, the all-dielectric compact lens design comprised by a graded permittivity profile can be fabricated by additive manufacturing technology, which greatly facilitates the potential development and application of vortex wave based wireless communications.

Modified Cross Correlation Green’s Function With FDTD for Characterization of MIMO Antennas in Nonuniform Propagation Environment

A finite-difference time-domain-based technique for evaluating envelope correlation coefficient (ECC) and mean effective gain (MEG) of multiple-input multiple-output (MIMO) antennas operating in real-life nonuniform propagation environment is introduced. The proposed algorithm is based on the formulation of a modified cross correlation Green’s function (CGF), which directly incorporates the specific angular distribution functions and cross-polarization discrimination parameters. Thus, the proposed technique based on modified CGF is able to calculate ECC/MEG for MIMO antennas in nonuniform propagation scenario, directly from the time-domain antenna currents. This avoids the troublesome determination of far-field antenna pattern at each working frequency and the consequent postprocessing, as required in the conventional approach. Numerical examples using thin-wire half-wavelength dipole antennas and printed microstrip-fed slot antennas are provided to show the effectiveness of the proposed technique for characterizing MIMO antennas in nonuniform propagation environment.

Theory, design, and experimental verification of a reflectionless bianisotropic Huygens' metasurface for wide-angle refraction

Huygens' metasurfaces are electrically thin devices which allow arbitrary field transformations. Beam refraction is among the first demonstrations of realized metasurfaces. As previously shown for extreme-angle refraction, control over only the electric impedance and magnetic admittance of the Huygens' metasurface proved insufficient to produce the desired reflectionless field transformation. To maintain zero reflections for wide refraction angles, magnetoelectric coupling between the electric and magnetic response of the metasurface, leading to bianisotropy, can be introduced. In this paper, we report the theory, design, and experimental characterization of a reflectionless bianisotropic metasurface for extreme-angle refraction of a normally incident plane wave towards 71.8$^\circ$ at 20 GHz. The theory and design of three-layer asymmetric bianisotropic unit cells are discussed. The realized printed circuit board structure was tested via full-wave simulations as well as experimental characterization. To experimentally verify the prototype, two setups were used. A quasi-optical experiment was conducted to assess the specular reflections of the metasurface, while a far-field antenna measurement characterized its refraction nature. The measurements verify that the fabricated metasurface has negligible reflections and the majority of the scattered power is refracted to the desired Floquet mode. This provides an experimental demonstration of a reflectionless wide-angle refracting metasurface using a bianisotropic Huygens' metasurface at microwave frequencies.

INPUT IMPEDANCE OF AN APERTURE OVER A LOSSY HALF-SPACE: APPLICATION TO ON-BODY ANTENNA PERFORMANCE AT 60 GHZ

This paper presents a theoretical approach to compare the performance of a directive and a quasi-omnidirectional on-body antennas. Two canonical antennas, namely, a dipole and a rectangular aperture, are considered in the 60 GHz band. We first demonstrate that for this on-body configuration, the classically-defined far-field antenna gain depends on the observation distance. Consequently, we derive results in terms of radiation efficiency and link budget. To do so, the antenna input impedance computation is a preliminary step to normalize the input power to allow a fair comparison between the two antennas. The impedance over a lossy half-plane of an aperture illuminated by a TE 10 mode normally polarized is therefore derived into a convenient easy-to-compute formulation, which to authors' best knowledge, is not available in the literature. In terms of link budget, it is obtained that the received power due to an aperture is generally higher than the one due to the dipole in the main lobe direction. A constant difference is observed along the distance and this difference increases with the aperture width for antennas touching the body. Besides, it is shown that the standard aperture waveguide WR15 exhibits a slightly higher efficiency than a vertical dipole with the same vertical size when placed at a distance less than 3 mm (i.e., 0.6 λ) from the body phantom surface. Above this distance, both the dipole and the aperture exhibit similar efficiency in the order of 60 %.

Pre-Flight SAOCOM-1A SAR Performance Assessment by Outdoor Campaign

In the present paper, we describe the design, execution, and the results of an outdoor experimental campaign involving the Engineering Model of the first of the two Argentinean L-band Synthetic Aperture Radars (SARs) of the Satélite Argentino de Observación con Microondas (SAOCOM) mission, SAOCOM-1A. The experiment’s main objectives were to test the end-to-end SAR operation and to assess the instrument amplitude and phase stability as well as the far-field antenna pattern, through the illumination of a moving target placed several kilometers away from the SAR. The campaign was carried out in Bariloche, Argentina, during June 2016. The experiment was successful, demonstrating an end-to-end readiness of the SAOCOM-SAR functionality in realistic conditions. The results showed an excellent SAR signal quality in terms of amplitude and phase stability.

Inductively Powered Pressure Sensing System Integrating a Far-Field Data Transmitter for Monitoring of Intracranial Pressure

Monitoring of intracranial pressure (ICP) provides a life-saving diagnostic tool. We present a battery-free pressure sensing system for minimally invasive ICP monitoring. It comprises a cranially concealed wireless pressure sensor and on- and off-body external units. The sensor is based on a piezoresistive element and is inductively powered through an on-body unit. It is also equipped with a far-field antenna, which conveys the pressure data to the off-body unit. We present the electromagnetic modeling of the system and report results from experiments carried out in a setting, which mimics the biological operation environment. The simulation and measurement results demonstrate the wireless pressure monitoring at a distance up to one meter over the pressure range from −3 to 33 mmHg and assess the impact of an imperfectly aligned inductive link on the operation of the system.

Application of Fourier Integrals for Determination of Antenna Far-Field Patterns from Measurements at Fresnel Distances

A method for determination of antenna far-fields, based on a measurement at Fresnel distances, is presented. The method is applicable to highly directive antennas, measured in anechoic chambers, which size is not large enough to satisfy the far-field condition for the antenna under test (AUT). The method is based on an approximation of the accurate diffraction integral at Fresnel distances by a Fourier integral. According to the method, a quasi far-field radiation pattern of the AUT is measured at a Fresnel distance R, the measured pattern is used for a reconstruction of a distorted, due to the short range, aperture excitation, and the distorted aperture excitation is corrected magnitudewise and phasewise in accordance with ${R}$ and, then, forward propagated to the far-field. An advantage of the method is that it allows an accurate treatment of the distorted phase and the magnitude of the Green kernel in the diffraction integral, with respect to the ideal far-field, in contrast to the usual quadratic approximation of the phase and expansion in an infinite row of the exponential phase term. The method offers computational precision advantages over existing methods in shorter measurement distances R and low levels of sidelobes. The accuracy of the method improves as the directivity of the AUT gets higher. The measurement distance R must not be too short to smear too much the measured Fresnel pattern and to compromise the validity of the mentioned Fourier approximation of the diffraction integral.

Multipurpose Near- and Far-Field Switched Multiband Coil Antenna for 915-MHz/2.45/5.8-GHz RFIDs

A multipurpose near- and far-field switched multiband reader antenna for radio frequency identification (RFID) applications is presented. The proposed antenna is simple, planar, and has three modes of operating frequencies: 915 MHz, 2.45 GHz, and 5.8 GHz. The prototype is realized using nonuniformly distributed turns coil for robust reactive and radiated field performance, leading to a multipurpose near- and far-field antenna in a single unit. Results show performance of the proposed multipurpose antenna in both near-field and far-field region and demonstrate its suitability for various RFID applications.

Time-Domain Methodology for the Relatively Low-Frequency Characterization of Large Antennas

Frequency-domain measurement techniques in an enclosed area suffer from some limitations when dealing with antennas of large physical dimensions or when operating in the low-frequency range. This paper presents a measurement test bench for antenna characterization based on outdoor time-domain measurements. The setup is applied to ultra-wide-band and narrow-band antennas yielding good far-field antenna characterization results as compared to standard measurement techniques. This approach employs a near-field to far-field transformation technique to obtain the radiation pattern of any radiating structure over a wide band of frequencies, from a single time-domain near-field data acquisition. The suggested approach offers significant savings in installation and material cost, in addition to a considerable reduction in measurement time.

A Simulation Framework for Multiple-Antenna Terminals in 5G Massive MIMO Systems

The recent interest in massive multiple in multiple out (MIMO) has spurred intensive work on massive MIMO channel modeling in the contemporary literature. However, current models fail to take the characteristics of terminal antennas into account. There is no massive MIMO channel model available that can be used for the evaluation of the influence of different antenna characteristics at the terminal side. In this paper, we provide a simulation framework that fills this gap. We evaluate the framework with antennas integrated into Sony Xperia handsets operating at 3.7 GHz as this spectrum is identified for the 5G new radio standard by 3rd Generation Partnership Project. The simulation results are compared with the measured terminal performance when communicating with the Lund University's massive MIMO testbed under the same loading conditions. Expressions are derived for comparison of the gain obtained from different diversity schemes computed from measured far-field antenna patterns. We conclude that the simulation framework yields the results close to the measured ones and that the framework can be used for antenna evaluation for terminals in a practical precoded massive MIMO system.

Antenna Far-Field Assessment From Near-Field Measured Over Arbitrary Surfaces

A matrix method is developed to calculate the antennas radiation pattern from near-field data measured over arbitrary surfaces. The principle of the matrix method is to determine the modal expansion coefficients by solving a system of linear equations. The number of required measured samples is deduced from the number of modes considering the cylindrical or the spherical wave expansion of antenna radiated fields. Once the modal coefficients are known, the far field of the antenna under test (AUT) is obtained. The matrix method has been implemented, and its potentialities are evaluated using two specific arbitrary surfaces (square section cylinder surrounding the AUT and a closed cylinder). Numerical (dipoles array) and experimental (base station antenna) results are provided to illustrate the efficiency and the stability of the proposed method.

Broad‐band embroidered spiral antenna for off‐body communications

An embroidered wearable spiral antenna is presented in this study. The spiral antenna is compact and flexible, yet has broad-band performance. The novelty of this study includes considering the antenna–body interaction rather than just considering the antenna alone. The antenna has been simulated and measured on a specific anthropomorphic mannequin torso phantom and a real person. The far-field on-body performance of the embroidered antenna on the phantom has been measured using a novel cylindrical near-field to far-field transformation technique. This technique allows the fast extraction of the full spherical radiation pattern and the corresponding far-field antenna characteristics on the human body without the need of rotating the phantom with expensive positioning systems. The on-body antenna performance including realised gain, directivity, radiation efficiency, radiation pattern and axial ratio have been presented.

Diagnosis Method of Radiated Emission from Battery Management System for Electric Vehicle

Electromagnetic emission from EVs has an effect on board equipment and nearby vehicles in urban transportation. Moreover, the electromagnetic emission from BMS in EVs can exceed the CISPR 12 limits of radiated emission for EVs. The radiated electric fields average for an electric car (CISPR12) is measured in semi-anechoic chamber (SAC) at a distance of 10m to study the diagnosis method for EMI. An exclusion procedure for EMI from BMS is designed to determine the component responsible for emission peaks through far-field antenna testing for vehicle. BMS and instrument are responsible for the average radiated emission exceeded the limits for the testing EV. The near-field testing is used to diagnose the location of EMI. From measurements and analysis of EMI from clock, the clock oscillator is the key component responsible for narrowband emission, and power devices’ switching in DC-DC converter BMS is the key component responsible for broadband emission.

Planar Multilayer Structure for Broadband Broad-Angle RCS Reduction

A broadband broad-angle radar cross-sectional (RCS) reduction measure applicable to planar surfaces is proposed. This novel structure contains several layers of randomly displaced metal patches and layers are spaced by a quarter wavelength. Specular reflection at normal incidence is suppressed by the phase cancellation technique. The reduction bandwidth is broadened by setting the layer surface area ratio based on binomial coefficients. Because the structure contains no absorbent material, metal surfaces are divided into small patches and displaced randomly to avoid reflection peaks beyond the boresight direction. The RCS reduction effect is validated via measurements in a far-field antenna range. Parametric studies and detailed discussions about this technique are conducted using simulation results computed from full-wave numerical tools, which include not only the specular reflection but also contributions from higher order scattering mechanisms. The proposed structure can be applied on planar target surfaces and has the potential to incorporate lossy dielectric loadings to reduce the structure thickness and suppress scatterings.

Near- and Far-Field Characterization of Planar mm-Wave Antenna Arrays with Waveguide-to-Microstrip Transition

We present the design and characterization of planar mm-wave patch antenna arrays with waveguide-to-microstrip transition using both near- and far-field methods. The arrays were designed for metrological assessment of error sources in antenna measurement. One antenna was designed for the automotive radar frequency range at 77 GHz, while another was designed for the frequency of 94 GHz, which is used, e.g., for imaging radar applications. In addition to the antennas, a simple transition from rectangular waveguide WR-10 to planar microstrip line on Rogers 3003™ substrate has been designed based on probe coupling. For determination of the far-field radiation pattern of the antennas, we compare results from two different measurement methods to simulations. Both a far-field antenna measurement system and a planar near-field scanner with near-to-far-field transformation were used to determine the antenna diagrams. The fabricated antennas achieve a good matching and a good agreement between measured and simulated antenna diagrams. The results also show that the far-field scanner achieves more accurate measurement results with regard to simulations than the near-field scanner. The far-field antenna scanning system is built for metrological assessment and antenna calibration. The antennas are the first which were designed to be tested with the measurement system.

Calculating Arbitrarily Located Sampling Data From Quiet Zone Spherical Near-Field Scanning Measurements

Amplitude and phase distribution inside the quiet zone (QZ) of a far-field antenna measurement chamber are important indicators for the quality of antenna measurements therein. Both amplitude and phase can be further subdivided into intuitive quality parameters such as linear taper, parabolic taper, and ripple. In order to retrieve these parameters from QZ spherical near-field measurements, the transmission formula is adapted to the case where the output signal of a virtually positioned output probe-antenna can be calculated on arbitrary locations inside the QZ. The output probe-antenna can be a measured or theoretically defined antenna and can be rotated in any direction of interest. This paper will show that this method provides similar results as industry standard linear field-probing by comparing amplitude taper and ripple extracted from both measurements. Further the advantages of using QZ spherical near-field scanning (QZSNFS) in combination with the adapted output transmission formula will be shown by calculating multiple projections not available by standard field-probing.

UHF-RFID Desktop Reader Antennas: Performance Analysis in the Near-Field Region

In this letter, a metric is proposed to qualitatively predict the achievable reading performance of a near-field (NF) UHF-RFID (865-928 MHz) desktop reader antenna, through a numerical analysis. Specifically, since the typical far-field antenna parameters (gain, radiation pattern, axial ratio) are not effective in the near-field region, the normalized power density is here proposed as a near-field antenna performance parameter. For a given NF antenna, the mean value of the normalized power density is evaluated on an area parallel to the antenna surface, at a set of different distances. Then, its decay rate is studied as a function of the distance from the antenna surface, which is useful to compare reading range performance of different antenna layouts. Additionally, the probability density function of the normalized power density at a given distance from the antenna surface is considered to investigate the field uniformity characteristics, the latter being important to guarantee tag reading independently on its location with respect to the reader antenna. The proposed analysis is applied to a set of NF UHF-RFID printed antennas, and conclusions are validated through a set of experimental tests.

Analysis of axially displaced elliptical antenna using hybrid method

Axially displaced elliptical (ADE) antenna is an evolved version of Gregorian antenna, bearing advantages of minimized blockage and relatively uniform field distribution. However, the analysis of the electrical performance of this type of antenna is a difficulty to be addressed properly. To enable the simulation of such type of reflector antenna, a hybrid method is derived for predicting the farfield of ADE antennas. Closed formulae are firstly derived for the calculation of the near field of ADE antennas based on vector analysis and the law of energy conservation. These formulae avoid time-consuming surface current integration and the current singularity on the vertex of the sub-reflector. In addition, the nature of vector analysis makes the hybrid method be capable of predicting vector electric field precisely. Furthermore, since these formulae are in explicit manner, no ray tracing process is required. The farfield is then predicted using Huygens equivalent method, assuring the accuracy in the farfield. Compared to published experimental results in the literature, the proposed method agrees well with the measurement in the main beam region. In comparison to physical optic method, the computational efficiency is much improved.

A plasmonic dipole optical antenna coupled quantum dot infrared photodetector

In this paper, we report a full-wavelength plasmonic dipole optical antenna coupled quantum dot infrared photodetector (QDIP). The plasmonic dipole optical antenna can effectively modify the EM wave distribution and convert free-space propagation infrared light to localized surface plasmonic resonance (SPR) within the nanometer (nm) gap region of the full-wavelength dipole antenna. The plasmonic dipole optical antenna coupled QDIP shows incident-angle-dependent photocurrent enhancement. The angular dependence follows the far-field pattern of a full-wavelength dipole antenna. The directivity of the plasmonic dipole optical antenna is measured to be 1.8 dB, which agrees well with the antenna simulation. To our best knowledge, this is the first report of the antenna far-field and directivity measurement. The agreement of the detection pattern and the directivity with antenna theory confirms functions of an optical antenna are similar to that of a RF antenna.