Near-field Transformation Research Articles

Exposure Assessment of Array Antennas at 28 GHz Using Hybrid Spherical Near-Field Transformation and FDTD Method

To protect humans from overexposure to electromagnetic (EM) fields above 6 GHz, recently revised International Commission on Non-Ionizing Radiation Protection guidelines and IEEE standard specify the basic restrictions in terms of absorbed power density (APD) and epithelial power density, respectively. To assess the EM exposure resulting from the 5G mmWave array antenna, we adopted a hybrid method that combines spherical near-field measurements and numerical dosimetry with spherical near-field transformation. Four array antennas working at 28 GHz were used as the radiation sources. The incident power density (IPD), APD, and temperature rise in a stratified body model and anatomical partial body models corresponding to different body locations were evaluated for various antenna-body distances. The results were validated by comparison with the full-wave simulations. The differences in peak spatial-average IPD and APD between two methods are below 0.08 and 1.05 dB, respectively, for the simulated cases and generally decrease with the increasing antenna-body distance. The highest heating factor for the peak spatial-average APD was approximately 0.025°C/(W/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The results also show that the differences in APD and temperature rise caused by various exposed body locations are marginal.

All-metal wideband metasurface for near-field transformation of medium-to-high gain electromagnetic sources

Electromagnetic (EM) metasurfaces are essential in a wide range of EM engineering applications, from incorporated into antenna designs to separate devices like radome. Near-field manipulators are a class of metasurfaces engineered to tailor an EM source’s radiation patterns by manipulating its near-field components. They can be made of all-dielectric, hybrid, or all-metal materials; however, simultaneously delivering a set of desired specifications by an all-metal structure is more challenging due to limitations of a substrate-less configuration. The existing near-field phase manipulators have at least one of the following limitations; expensive dielectric-based prototyping, subject to ray tracing approximation and conditions, narrowband performance, costly manufacturing, and polarization dependence. In contrast, we propose an all-metal wideband phase correcting structure (AWPCS) with none of these limitations and is designed based on the relative phase error extracted by post-processing the actual near-field distributions of any EM sources. Hence, it is applicable to any antennas, including those that cannot be accurately analyzed with ray-tracing, particularly for near-field analysis. To experimentally verify the wideband performance of the AWPCS, a shortened horn antenna with a large apex angle and a non-uniform near-field phase distribution is used as an EM source for the AWPCS. The measured results verify a significant improvement in the antenna’s aperture phase distribution in a large frequency band of 25%.

Increasing the Directivity of Resonant Cavity Antennas with Nearfield Transformation Meta-Structure Realized with Stereolithograpy

A simple, nearfield transformation meta-structure is proposed to increase the directivity of resonant cavity antennas (RCA). The meta-structure is comprised of 14 × 14 meta-atoms or so called “unit-cells”, adding localized phase delays in the aperture of the RCA and thus increasing its broadside directivity. A prototype of the meta-structure is additively manufactured using the stereolithograpy process and has a profile of 0.56λ. With the meta-structure integrated with the RCA, it demonstrates a measured broadside directivity of 20.15 dBi without affecting its half-power directivity bandwidth. Benefiting from additive manufacturing, the proposed approach is a simple, light-weight, low-cost, and planar approach that can be tailored to achieve medium-to-high gains with RCAs.

Localization Based on Enhanced Low Frequency Interaural Level Difference

The processing of low-frequency interaural time differences is found to be problematic among hearing-impaired people. The current generation of beamformers does not consider this deficiency. In an attempt to tackle this issue, we propose to replace the inaudible interaural time differences in the low-frequency region with the interaural level differences. In addition, a beamformer is introduced and analyzed, which enhances the low-frequency interaural level differences of the sound sources using a near-field transformation. The proposed beamforming problem is relaxed to a convex problem using semi-definite relaxation. The instrumental analysis suggests that the low-frequency interaural level differences are enhanced without hindering the provided intelligibility. A psychoacoustic localization test is done using a listening experiment, which suggests that the replacement of time differences into level differences improves the localization performance of normal-hearing listeners for an anechoic scene but not for a reverberant scene.

Application of Nonuniform FFT to Spherical Near-Field Antenna Measurements

This article investigates on the use of nonuniform fast Fourier transform (NUFFT) to solve spherical near-field transformation problems. Traditional postprocessing algorithms make use of conventional FFT for the transformation of measured grids with uniform sampling. More recently, the processing of irregular sampling grids has been introduced by means of matrix-based algorithms. The benefits of both type of algorithms are combined in this article with the implementation of the spherical wave expansion (SWE) summation and its adjoint with NUFFT. This allows the processing of fields with the efficiency of the FFT approaches and the versatility of the matrix operators. To this end, the NUFFT will be introduced and it will be shown how it can be implemented to accelerate the multidimensional summations required in the SWE calculations. Numerical investigations will be introduced to demonstrate the computational gains achieved with negligible approximation errors. Finally, the NUFFT will be tested on three different spherical near-field postprocessing problems: far-field transformation of near fields measured with spiral scanning, phaseless measurements on spherical surfaces with different sampling grids, and antenna diagnostics.

A Broad-Band Huygens Surface Source Model for Near-Field to Near-Field Transformations

A broad-band Huygens surface method requiring only the measurement of the magnetic field for near-field to near-field transformations is presented. The proposed method is valid down to near dc and supports arbitrary shaped Huygens surfaces. The method relies on an augmented-integral equation formulation that is used to predict the electric field on the Huygens surface. The proposed formulation is discretized using a high-order locally corrected Nystrom method. The surface magnetic and electric fields can then be used to compute external electric and magnetic fields. The method is validated using electromagnetic fields radiated by a three-phase motor showing excellent results over several Fourier harmonics of the motor fields, including the zeroth harmonic. The method is also validated at high frequency for a dipole antenna array.

Simple Near-field to Far-field Transformation Method Using Antenna Array-factor

This paper presents a simple and practical method and measurement results of transformation by using the idea of array-factor for direct far-field prediction from the near-field data of antenna and RCS in cylindrical scanning system. When radiation pattern of an antenna or scattering pattern of radar cross-section (RCS) is measured, its characteristics is usually evaluated in the far-field area. However, measurement is usually done in the radio anechoic chamber to reduce unnecessary reflected wave and deviates from the condition of far-field according to the size or the measuring frequency. On the other hand, it is well known that methods such as compact-range or the transformation processing approach can transform the pick-up data from the near-field to the far-field. The measurement results by this simple near-field transformation agree very well with the conventional far region measurement method.

Light on the Tip of a Needle: Plasmonic Nanofocusing for Spectroscopy on the Nanoscale.

The efficiency of plasmonic nanostructures as optical antennas to concentrate optical fields to the nanoscale has been limited by intrinsically short dephasing times and small absorption cross sections. We discuss a new optical antenna concept based on surface plasmon polariton (SPP) nanofocusing on conical noble metal tips to achieve efficient far- to near-field transformation of light from the micro- to the nanoscale. The spatial separation of the launching of propagating SPPs from their subsequent apex confinement with high energy concentration enables background-free near-field imaging, tip-enhanced Raman scattering, and nonlinear nanospectroscopy. The broad bandwidth and spectral tunability of the nanofocusing mechanism in combination with frequency domain pulse shaping uniquely allow for the spatial confinement of ultrashort laser pulses and few-femtosecond spatiotemporal optical control on the nanoscale. This technique not only extends powerful nonlinear and ultrafast spectroscopies to the nanoscale but can also generate fields of sufficient intensity for electron emission and higher harmonic generation.

Hybrid Fast Fourier Transform-plane wave based near-field far-field transformation for “body of revolution” antenna measurement grids

[1] Near-field measurement and transformation techniques are widely applied to characterize radiation patterns of antennas. Spherical and cylindrical near-field measurements have been researched extensively and various techniques with different probe compensation capabilities and complexities exist. Among those techniques applicable for (almost) arbitrary probes, the crucial computational efficiency has been achieved through the use of Fast Fourier Transform based preprocessing of the measurement data. It is shown in this paper that the Fast Fourier Transform based preprocessing can also be utilized in conjunction with the plane wave based fully probe-corrected near-field far-field transformation with low numerical complexity. The collection of probe signals is split into smaller subsets for individual orthogonal azimuthal Fourier modes by an Inverse Fast Fourier Transform. These smaller subsets can be transformed to the far field very efficiently with full probe correction. The technique presented in this paper is applicable for arbitrary “body of revolution” antenna measurement grids, including the important cases of cylindrical and spherical measurement grids. The “body of revolution” grids are rotationally symmetric around the z-axis and the probe signals must be available equidistantly in ϕ.

Time domain near-field to near-field transformation using a spherical-multipole approach

[1] The time domain electromagnetic field of an arbitrary localized radiating structure can be efficiently obtained by means of a time domain spherical-multipole expansion valid outside a minimum sphere enclosing all radiating elements. The method is based on the Fourier transform of the frequency-domain spherical-multipole expansion and on a finite expansion of the spherical Hankel function of the 2nd kind leading to a triple sum of multipoles instead of the well-known double sum in case of the frequency-domain multipole expansion. It is shown that those time domain multipole amplitudes which are relevant only in the near-field can be recursively deduced from the time domain amplitudes dominant in the far field. The latter can be obtained by a recently proposed spherical-multipole based time domain near-field to far-field algorithm which has been shown to be particularly suited for the Finite-Difference Time Domain (FDTD) method.

Assessment of Irregular Sampling Near-Field Far-Field Transformation Employing Plane-Wave Field Representation

Near-field antenna measurements are accurate and common techniques for determining the radiation pattern of an antenna under test. The minimum near-field sampling rate is dictated by the electrical size of the antenna. Equidistant sampling is usually applied for planar, cylindrical, and spherical measurements. Certain applications either rely on or benefit from near-field sampling on irregular grids. To handle irregular measurement grids, near-field transformation algorithms such as equivalent-current methods or the multilevel fast multipole accelerated plane-wave based technique are required, which do not rely on regularly sampled data. In this contribution, the plane-wave-based near-field transformation is applied to spherical, cylindrical, and “combined” near-field measurements employing irregular sampling grids. The performance is assessed by various simulated near-field-measurement scenarios.

Hybrid multilevel plane wave based near-field far-field transformation utilising combined near- and far-field translations

Abstract. The radiation of large antennas and those operating at low frequencies can be determined efficiently by near-field measurement techniques and a subsequent near-field far-field transformation. Various approaches and algorithms have been researched but for electrically large antennas and irregular measurement contours advanced algorithms with low computation complexity are required. In this paper an algorithm employing plane waves as equivalent sources and utilising efficient diagonal translation operators is presented. The efficiency is further enhanced using simple far-field translations in combination with the expensive near-field translations. In this way a low complexity near-field transformation is achieved, which works for arbitrary sample point distributions and incorporates a full probe correction without increasing the complexity.

FDTD-PWS Scheme and Its Application to Analysis of Focusing Properties of Dielectric Lens

The focusing properties of lens are studied by a hybrid scheme that involves the finite-difference time-domain (FDTD) method for the aperture-plane field computation and the plane wave spectrum (PWS) method for the transformation from the aperture-plane field to the focal-plane field. To demonstrate the accuracy and efficiency of this new scheme, a Ka-band lens antenna is simulated and fabricated as an example. Subsequently, the near-field-region and the Fresnel-region fields of this lens antenna are measured by the planar-near-field (PNF) method. It is found that the results obtained by the FDTD-PWS scheme are in good agreement with the measured data. Furthermore, the numerical results and the computational requirements of this new scheme are compared with those of the near-field to near-field (NF-NF) transformation, the aperture integration (AI) method, the geometrical optics (GO)-physical optics (PO) method, and the full FDTD simulation. Both the central processing unit (CPU) time and memory requirement can be reduced efficiently. Finally, the effects of several fundamental issues, including spatial sample parameters, lens material, operating frequency, and designed focal length, on the accuracy of the proposed scheme and on the focusing properties of the lens are investigated.

Research on the Radiation Characteristics of Patched Leaky Coaxial Cable by FDTD Method and Mode Expansion Method

Patched leaky coaxial cable (PLCX) is proposed as an alternative to the conventional leaky cable for wireless links in a complex environment. It is expected to have the capability of adjusting the coupling between the cable and the environment and give smoother electric field coverage. In this paper, the radiation characteristics of the PLCX with general inclined patches are studied by a hybrid method that involves the finite-difference time-domain (FDTD) method for the near-field computation and the mode expansion method for the transformation of near field to far field. In the method, the space around the patched leaky cable is divided into two regions by an artificial closed cylindrical surface that is incorporated with the FDTD lattice surface when implementing the FDTD iteration. The field distribution on the artificial surface is obtained after the implementation of the FDTD method. Meanwhile, the field outside the artificial boundary is expanded in terms of the Floquet modes with coefficients to be determined. By matching the field expressed by modes and the field obtained from the FDTD method at the artificial boundary, a matrix equation with unknown coefficients is obtained. Solving this matrix equation, the expansion coefficients are known, and the field outside the artificial boundary is ready to be obtained.

RF exposure compliance boundary analysis of base station antennas using combined spherical–cylindrical near-field transformations

A measurement-based method for RF exposure compliance boundary assessment for base station antennas is presented. A full-sphere near-field measurement is used as input data, thereby avoiding spatial truncation. Combined use of spherical and cylindrical modes allows for evaluation close to the antenna. The error is compliant with current standards.

A discrete wavelet transform (DWT)‐based compression technique for the computation of Kirchhoff integrals in MR/FDTD

The multiple-region FDTD (MR/FDTD) has recently been introduced as an extension of the classical FDTD to multiple subregions within a problem domain. In MR/FDTD, the problem domain is broken down into several volumes interacting with each other by means of a near-field to near-field transformation. One of the available methods is the Kirchhoff integrals, a method that has been successfully applied to FDTD, but that suffers from a high demand for computation resources. In this paper, a new discrete wavelet transform (DWT)-based compression technique for the computation of Kirchhoff integrals is presented that leads to important savings both in memory size and computation time, while combining excellent accuracy and ease of implementation. © 2000 John Wiley & Sons, Inc. Microwave Opt Technol Lett 27: 312–316, 2000.

Computation of aperture antenna mutual couplingusing FDTD and Kirchhoff field transformation

A new method for computing the mutual coupling between aperture antennas using the finite-difference time-domain (FDTD) method together with the Kirchhoff near-field to near-field transformation is described. The method offers a reduction in computer storage, particularly for widely spaced antenna elements in an array.

Characterization of corrugated feed horns at 216 and 300 GHz

We present measurements of the near-field beam profiles of two corrugated feed horns with center frequencies of 216 and 300 GHz. Both co-polar and cross-polar components of the electric field have been probed across the operation band of the horns. A full-wave numerical near-field transformation, based on a vector Kirchhoff integral, is implemented to transform the measured field to the horn aperture for a comparison with the ideal aperture field. From the overlap integral of the transformed and the ideal field, we conclude that the field radiated by these horns is the HE11 mode, to a purity of better than 99%.

Comparison of two FDTD near-field to near-fieldtransformations applied to pulsed antenna problems

Recently, a time-domain, near-field to near-field transformation, based on the Stratton-Chu representation, was presented for use with the FDTD method. In the Letter, an alternative near-field to near-field transformation based on the Kirchhoff representation is described. Savings of both computation time and computer memory are achieved using the Kirchhoff formulation. Comparisons of the savings attained are illustrated through an example, the application of the transformation to a TEM horn antenna for pulse radiation.

Near-field to near-field transformation for usewith FDTD method and its application to pulsedantenna problems

A time-domain, near-field to near-field transformation for use with the FDTD method is given and its accuracy is verified with experimental measurements made on a metallic bow-tie antenna.