Near-field Antenna Research Articles

Scalable Near-Field Fed Far-Field RAIN RFID Reader Antenna for Retail Checkout Counters

This article presents a scalable ultra-high-frequency (UHF) radio frequency identification (RFID) reader antenna for retail checkout counters. The antenna’s simultaneous near-field and far-field operation enable effortless and reliable asset identification. Two meandered microstrip lines that are 90° out-of-phase form the near-field antenna rim around a far-field patch antenna. The patch antenna is fed by those meandered lines along the vertices on its orthogonal points, resulting in circular polarization. The near-field radiation is evenly distributed with no surface dead zones, and the far-field radiation pattern is symmetrical with a 90° half-power beam-width in both azimuth and elevation planes. The antenna operates in the North American UHF RFID band (902-928 MHz) with a 50 MHz impedance bandwidth. Fabricated antenna’s gain and axial ratio at the band’s center frequency of 915 MHz is 3.3 dBi, and 1.4 dB, respectively. This antenna is low-profile, low-cost and recyclable as it is constructed from polyolefin substrate and copper foils. The antenna is pragmatic, scalable with increasing checkout counter sizes and insusceptible to proximity assets and metal frames.

A Volume PEEC Formulation Based on the Cell Method for Electromagnetic Problems From Low to High Frequency

A new and general partial element equivalent circuit (PEEC) formulation based on electric and magnetic vector potentials and the cell method is presented. The purpose of this paper is manifold. First, a comprehensive review of well-established PEEC method is given and the state-of-the-art of the method with focus on the inclusion of magnetic media in the formulation is discussed. Then, a new and general formulation of the PEEC method based on integral equations is proposed. The formulation is valid for conductive, dielectric, and magnetic media and its advantages with respect to the state-of-the-art PEEC formulations are underlined. Unlike the existing PEEC methods, this novel formulation, thanks to the reinterpretation of PEEC in the context of integral equation methods, allows for maintaining a circuit interpretation for both electric and magnetic media. Thus, a standard Spice-like solver can in principle be adopted for the solution of electromagnetic (EM) problems also when magnetic media are involved. The imposition of the divergence condition for the current density vectors is guaranteed and also the case of inhomogeneous and anisotropic media is considered. A thorough discussion on the suitability of the method for the study of both low- and high-frequency EM problems is also presented. Finally, the case study of a near-field communication antenna is considered as an example of the applicability of the formulation to problems of industrial interest. Comparisons with commercial softwares, measurements, and the state-of-the-art PEEC method are given in order to demonstrate the accuracy and the advantages of the proposed PEEC formulation.

An Adaptive Data Acquisition and Clustering Technique to Enhance the Speed of Spherical Near-Field Antenna Measurements

This letter presents a new approach for adaptive spherical near-field (NF) antenna measurement. The proposed method begins with a number of initial points and sequentially focuses on the areas with a highly dynamic NF pattern. Thereafter, the source reconstruction method is utilized to calculate the equivalent magnetic and electric currents on the surface of an ellipsoid that encompasses an antenna under test. The equivalent sources are used to compute the far-field pattern of the antenna. The comparison of the adaptive algorithm with the uniform sampling indicates that the number of the required samples is decreased significantly using the adaptive method. The adaptive data acquisition can also be used in case of uniform sampling to remove the redundant samples and accelerate the source reconstruction method. Since the newly added point is not necessarily laid on the measurement points, the cubic spline interpolation technique is employed to compute the value of the field. Besides, a machine learning algorithm based on k-means clustering is applied to the uniformly sampled data to determine different clusters of data. Thus, for every new point, the cluster to which the data point belongs can be determined, and only the values of that cluster are used to calculate the value of the new point.

A Novel Near-Field UHF RFID Reader Array Antenna for Configurable Electrically Large Reading Area

In this paper, a novel configurable loop array antenna based on resonator-loaded microstrip lines (RLML) is proposed to enlarge the interrogation zone of the reader antenna for near-field ultrahigh-frequency (UHF) radio frequency identification (RFID) system. The proposed loop array antenna is composed of a number of RLML loop units. By configuration of the RLML loop units sharing a common side with its adjacent RLML loop units which carry reverse-direction flowing current in the ${x}$ -axis and ${y}$ -axis directions, loop array antennas are created to generate a strong and uniform magnetic-field distribution over a large configurable interrogation area. As an example, loop array antennas with $1\,\times \,8$ and $2\,\times \,3$ RLML loop units implemented onto a piece of an FR4 printed circuit board (PCB) are designed, fabricated, and measured. The measured bandwidth ranges from 894 to 964 MHz for a $1\times 8$ array antenna with a reading area of $570 \,\,{\mathrm{ mm}}\times 150\,\,{\mathrm{ mm}}$ and 917–932 MHz for a $2\times 3$ array antenna with a reading area of $270 \,\,{\mathrm{ mm}}\times 240\,\,{\mathrm{ mm}}$ . The reading distances of an $1\times 8$ array antenna for loop-like tag, dipole-like tag, and antimetal tag are achieved up to 22, 42, and 45 mm, respectively. For a $2\times 3$ array antenna, the reading distances of three tags are achieved up to 23, 43, and 40 mm.

Far-Sidelobe Antenna Pattern Measurement of LiteBIRD Low Frequency Telescope in 1/4 Scale

Polarization of the cosmic microwave background (CMB) has crucial information on the inflationary universe. To detect these signals, it is necessary to suppress far sidelobes of a telescope, which contaminate the CMB signals with strong foreground radiation, such as the Galactic plane. LiteBIRD is the only funded CMB observation satellite for the 2020s, and the low frequency telescope (LFT; 34-161 GHz) is one of its telescopes. We measured near-field antenna patterns of the LFT using its 1/4-scaled model and examined far sidelobes up to 60° from the peaks. To cover the 20° field of view of the LFT, we investigated the antenna patterns at the edges of the focal plane as well as at the center. The measurement frequencies were 140-220 GHz, which correspond to the lowest bands (35-55 GHz) of the full-scale LFT. The measurements were consistent with the simulated far-sidelobe patterns at least -50 dB level, and showed that far sidelobes for two orthogonal polarization directions are consistent with each other down to -40 dB level. We also measured the cross-polarization patterns, and their peak level was less than -20 dB.

Determination of Electrophysical Parameters of a Semiconductor from Measurements of the Microwave Spectrum of Coaxial Probe Impedance

We propose a method for determining electrophysical characteristics (free charge carrier concentration, mobility, and conductivity) of semiconductors from the results of measurements of the microwave spectrum of the impedance of a coaxial probe as a function of applied constant voltage U. The sought parameters have been determined by solving the corresponding inverse problem using the theory of a near-field antenna that was developed earlier. We have developed a computer program that seeks the solution by minimization of the multiparametric residual function in accordance with the Nelder–Mead algorithm. The precision of the method has been analyzed from the results of simulation in which the impedance was calculated preliminarily considering resultant concentration profile n(x, U) of the depleted layer in the vicinity of the metal–semiconductor contact. The possibility of diagnostics with a micrometer lateral resolution has been demonstrated.

Waveguide-integrated Rydberg Atom-based RF Field Detector for Near-field Antenna Measurements.

We demonstrate simultaneous amplitude and phase measurements of a radio-frequency (RF) field through the use of a Rydberg atom-based sensor embedded inside a waveguiding structure. This measurement uses the Rydberg atom-based sensor in a mixer configuration, which requires the presence of a local oscillator (LO) RF field. The waveguiding structure supplies the LO field. The combined waveguide and Rydberg atom system is used to measure phase and amplitude in the near-field of a horn antenna to extract the far-field pattern.

Near-Field Antenna Pattern Measurements in Highly Reflective Environments

The far-field pattern of an antenna under test is determined from the near-field measurements in a highly reflective environment. The proposed post-processing effectively combines time gating and spatial filtering of the radiated fields. Time gating utilizes frequency diversity, while spatial filtering makes use of probe diversity and source localization. Different probe locations are used to form a virtual array, which focuses the fields at the antenna volume. The possible scatterers are illuminated as little as possible. The weighting coefficients for the virtual array are found by solving a field synthesis problem. The effects of time gating and spatial filtering are analyzed separately. A measurement setup is described in a highly reflective chamber involving fully spherical measurements with probes in up to 120 different positions and with 1001 frequencies. Time gating yields better results, but a combination of both methods is necessary to achieve an estimated accuracy of −30 dB for the considered S-band and −40 dB for the considered Ku-band antenna.

Multipolarized Reader Antenna With Periodic Units Based on Electric Field Coupling for UHF RFID Near-Field Applications

This paper presents a multipolarized reader antenna with periodic units based on electric field coupling for near-field applications of ultra-high frequency (UHF) radio frequency identification (RFID). The antenna units are composed of two branches that are mirror-symmetric with respect to the ${x}$ -axis and produce ±45° linear polarization. A 90° phase difference is produced between the upper and lower branches by introducing a phase shifter into the upper branch, so that the near-field electric fields excited in the horizontal plane exhibit a quasi-circular polarization distribution. The proposed design is validated by simulations and experiments using prototype 4-unit and 6-unit antennas. The proposed antennas can generate strong and uniform electric fields and provide reading volumes of 400 mm $\times \,\,320$ mm $\times \,\,300$ mm and 560 mm $\times \,\,320$ mm $\times \,\,300$ mm for the 4-unit and 6-unit antennas, respectively, regardless of the orientation of the tags in the horizontal plane. In addition, the far-field gain is low with a maximum of −6 dBi, thus the misreading of tags outside the reading region is avoided. The proposed antenna achieves impedance matching over the Chinese UHF RFID band (920–925 MHz) and therefore meets the application requirements of near-field RFID reader antennas.

Valid angle criterion and radiation pattern estimation via singular value decomposition for planar scanning

Planar near-field antenna measurement techniques are addressed with particular reference to the error in the radiation pattern estimation due to the finiteness of the observation aperture. The valid angle (VA) concept is a commonly used rule-of-thumb for predicting the region of validity of the calculated far-field pattern according to observation domain, the aperture antenna size and the distance between the two. However, it is based on simple asymptotic arguments which often lead to erroneous estimations. In this contribution, a new VA criterion and related radiation pattern representation are derived by exploiting the singular value decomposition of the radiation operator. Numerical examples show the goodness of the new VA criterion and the greatest stability of the proposed radiation pattern estimation compared to the classical Fourier transform.

Antenna Design and Near-Field Characterization for Medical Microwave Imaging Applications

In medical microwave imaging (MMWI), the antennas usually operate at close distance from the body and are required to pick up weak echoes from the inside that are masked by high skin reflection. The near-field antenna behavior strongly determines how the received signals affect the effectiveness of the imaging algorithms. However, these usually simply assume the usual antenna far-field radiation characteristics. Here, we discuss three antenna effects that can deteriorate the overall imaging performance but are rarely addressed in the literature: antenna internal reflections, angular dispersion of the “near-field phase center,” and antenna frequency dispersive electric length. We propose dedicated methods to characterize these factors and present mitigation strategies to be integrated into the inversion algorithms. We demonstrate these effects for three frequently used broadband antennas, using signals from an experimental breast imaging lab setup. In fact, the antenna study and design cannot ignore that it operates in the near field of breast and interacts with its boundary. The methods and conclusions can be extended to other MMWI applications. To the best of our knowledge, this is the first systematic study of the above antenna factors in the context of MMWI.

Near-Field Antenna for RFID Applications

Near-Field Antenna for RFID Applications

Efficient spherical near‐field antenna measurement using CS method with sparsity estimation

Spherical near-field antenna measurement requires the acquisition of many points, which makes the measurement process time-consuming. Since the antenna radiation pattern has a sparse representation on the basis of vector spherical harmonics, compressive sensing (CS) can be used to reduce the number of sampling points in the near-field antenna measurement. CS requires knowledge of sparsity in advance to determine the number of sampling points. The phase transition diagram is usually used to determine the number of measurements, which is again time-consuming and requires a large number of computing data. In this study, the method for sparsity level estimation is used to estimate the sparsity level of an arbitrary antenna's near field. Using this sparsity estimation method is a novel antenna measurement which has many applied advantages. As a case in point, it estimates the sparsity level of a radiation pattern with a few numbers of measurements, which makes the CS method efficient and affordable in the near-field measurement technique. To confirm the effectiveness of the proposed method, a standard horn antenna is investigated.

Efficient and Compact Near-Field Coupled Hybrid Antenna Using a Single Radiating Subwavelength Split-Ring Resonator

Modern applications in the realms of wireless communication and mobile broadband Internet increase the demand for compact antennas with well defined directivity. Here, we present an approach for the design and implementation of hybrid antennas consisting of a classic feeding antenna that is near-field-coupled to a subwavelength resonator. In such a combined structure, the composite antenna always radiates at the resonance frequency of the subwavelength oscillator as well as at the resonance frequency of the feeding antenna. While the classic antenna serves as impedance-matched feeding element, the subwavelength resonator induces an additional resonance to the composite antenna. In general, these near-field coupled structures are known for decades and are lately published as near-field resonant parasitic antennas. We describe an antenna design consisting of a high-frequency electric dipole antenna at f d = 25 GHz that couples to a low-frequency subwavelength split-ring resonator, which emits electromagnetic waves at f SRR = 10.41 GHz. The radiating part of the antenna has a size of approximately 3.2 mm × 8 mm × 1 mm and thus is electrically small at this frequency with a product k · a = 0.5 . The input return loss of the antenna was moderate at − 18 dB and it radiated at a spectral bandwidth of 120 MHz. The measured main lobe of the antenna was observed at 60 ∘ with a − 3 dB angular width of 65 ∘ in the E-plane and at 130 ∘ with a − 3 dB angular width of 145 ∘ in the H-plane.

A Minimum Rank Approach for Reduction of Environmental Noise in Near-Field Array Antenna Diagnosis.

A method to filter out the contribution of interference sources in array diagnosis is proposed. The interference-affected near field measured on a surface is treated as a (complex-data) image. This allows to use some modern image processing algorithms. In particular, two strategies widely used in image processing are applied. The first one is the reduction of the amount of information by acquiring only the innovation part of an image, as currently happens in video processing. More specifically, a differential measurement technique is used to formulate the estimation of the array excitations as a sparse recovery problem. The second technique has been recently proposed in video denoising, where the image is split into a low-rank and high-rank part. In particular, in this paper the interference field is filtered out using sparsity as discriminant adopting a mixed minimum norm and trace norm minimization algorithm. The methodology can be applied to both near and far field measurement ranges. It could be an alternative to the systematic use of anechoic chambers for antenna array testing.

A Crane-Based Portable Antenna Measurement System—System Description and Validation

A portable near-field (NF) antenna measurement system for frequencies up to Ka -band is presented and evaluated in detail. The system follows the idea to collect measurement samples on arbitrary scan surfaces in the NF, where strong irregularities of the sample distribution may be tolerated as long as appropriate sampling criteria are fulfilled. The radio frequency (RF) equipment is integrated in a gondola, which is positioned by a standard overhead crane. During an NF scan, the exact position and orientation of the probe antenna are measured by a 6-D laser tracking system with high accuracy. The RF measurement system works with a phase-coherent measurement loop, where the signal transmission from the ground station to the RF equipment within the portable gondola is performed by an optical fiber transmission system with three optical fibers, realizing magnitude as well as phase compensation. The NF measurement samples are merged with the position and orientation data and, finally, transformed into the far-field by a flexible and highly efficient equivalent source algorithm. The complete setup of this measurement system is presented and discussed in detail, focusing on both hardware and software aspects. A series of validation and practical antenna measurement results is presented to demonstrate the feasibility of this approach.

Realization of Split Beam Antenna Using Transmission-Type Coding Metasurface and Planar Lens

In this paper, a novel compact split beam configuration of the transmission-type coding metasurface (MS) in combination with the planar lens and the patch antenna is presented. In the first stage, the spherical electromagnetic waves originating from the patch antenna are converted to plane waves by placing the planar MS lens at a height of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.55\lambda _{0}$ </tex-math></inline-formula> above the antenna aperture. This lens enhances the radiation along the broadside direction resulting in high-gain operation. Furthermore, a concept of the digital metamaterial is adopted by realizing 1 bit transmission-type coding MSs, capable of splitting the aforementioned high-gain beam pattern. These digital MSs (DMSs) have two different types of metaatoms with phase responses of 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> , corresponding to two basic digital elements 0 and 1, respectively. The assembly of these metamaterial bits (either 0 or 1) leads to a lattice of size <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$D \times D$ </tex-math></inline-formula> . The proper spatial mixture of these lattices with different sequences facilitates different elemental metamaterial-pattern resulting in multiple-lobe radiation patterns, which are angularly oriented and symmetrical to the antenna axis. The designed planar lens and the DMS with the individual thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.057\lambda _{0}$ </tex-math></inline-formula> are placed in the near field of antenna leading to an overall compact configuration with a total height of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.08\lambda _{0}$ </tex-math></inline-formula> . The aforementioned beam splitting using the proposed configuration is also validated both quantitatively (mathematically) and qualitatively (through the simulation and the measurement) for different types of DMSs.

Investigation of sparse near‐field focusing array antenna with different topologies

The method to efficiently design a sparse near-field focusing sparse array antenna is introduced here. First, the calculation model and analysis method of three array topologies including the square grid, the circular grid, and the regular triangular grid are established. The evaluation criterion is the lowest sparsity under the condition that the focusing performance of the sparse array approaches to that of a referenced focusing array with the element spacing of a half of one wavelength. After comparing the focusing performance of these array antennas with different grids, the circular-grid array is the optimal one and selected to further discuss the influence of different topology parameters. The optimal sparse near-field focusing array should have the constant distance between two adjacent rings and increase three elements compared with the last ring. Finally, the method based on the proposed selection criteria is employed to design a sparse near-field focusing sparse array. The correctness of the method has been validated by the full-wave simulated results.

Design of metasurface polarization converter for near‐field application; stacked conical horn antenna for circular polarization

In this article, a three-layer transmission mode linear to circular polarization converter (TMCP) is proposed with low-level reflection coefficient and axial ratio (AR) at 9 GHz. Each layer of the proposed TMCP consists of microstrip patches and lines, providing inductive TM impedance and capacitive TE impedance on each layer to produce 90° out of phase between TE and TM electromagnetic waves. By removing the middle layer of the proposed metamaterial, a two-layer TMCP with low-level axial ratio at 8 GHz is obtained. However, the proposed two-layer TMCP suffers from disagreements between the reflection coefficients of the TE and TM modes, which produce fluctuations on the AR results in the near-field application. Low-level reflection coefficient and AR at 9 GHz achieved from the three-layer TMCP, indeed, make it efficient for the near-field of a horn antenna. Unit cells of the two-layer and three-layer TMCPs consisting patches and lines are simulated using periodic boundary conditions. In addition, the patches and lines are printed on a FR4 substrate and cut out in circle shapes. The three substrates are arranged at the determined separations and then installed in front of a profiled conical horn antenna. The measured AR of the antenna stacked by the three-layer TMCP is below 2 dB from 8.45 up to 9.6 GHz.

Pattern Reconfigurable, Vertically Polarized, Low-Profile, Compact, Near-Field Resonant Parasitic Antenna

A vertically polarized, low-profile, compact, near-field resonant parasitic antenna with pattern reconfigurability is demonstrated. The antenna has three dynamic end-fire states facilitated with only three p-i-n diodes. The radiation pattern in each state covers more than 120° in its azimuth plane and, hence, it achieves beam scanning that covers the entire azimuth plane. The antenna height and transverse size are, respectively, only $0.048\lambda _{0}$ and $0.1\lambda _{0}^{2}$ . Measured results, in good agreement with their simulated values, demonstrate that the antenna exhibits a ~11% fractional impedance bandwidth, and a ~6.6 dBi peak realized gain in all three of its pattern-reconfigurable states. Stable and high peak realized gain values are realized over its entire operational band surrounding 2.22 GHz.