Radiative Near-field Region Research Articles

TE-polarized leaky-wave beam launchers: Generation of Bessel and Bessel–Gauss beams

The generation of focused beams in the millimeter- and submillimeter-wave ranges, with transverse-electric (TE) polarization, is investigated in the radiative near-field region. The desired field distribution is achieved through a leaky-wave beam launcher consisting of a grounded dielectric slab with an annular strip grating on top excited by a circular slot on the bottom ground plane. The latter is fed by a Marié transducer, which converts the input, fundamental TE10 mode of a standard rectangular waveguide into the higher-order TE01 mode propagating in the circular waveguide connected to the device. The generation of TE-polarized diffraction-limited Bessel and Bessel–Gauss distributions is achieved by suitably synthesizing the annular strip grating. Simulated results are in excellent agreement with those predicted by leaky-wave analysis providing a proof-of-concept for the generation of TE-polarized Bessel and Bessel–Gauss beams at 300 GHz with a beam size of 1.7 and 1.9 mm up to the nondiffractive range of about 25 and 15 mm from an aperture plane with radius of 12.75 mm, respectively.

Design of TE-polarized resonant Bessel-beam launchers for wireless power transfer links in the radiative near-field region

Abstract Resonant Bessel-beam launchers (BBLs)are radiating devices constituted by a cylindrical metallic cavity with a partially reflecting sheet (PRS) on top. Millimeter-wave resonant BBLs typically exhibit transverse magnetic (TM) polarization due to the use of coaxial probes as feeders and homogenized metasurfaces as PRS. Launchers showing either a purely transverse electric (TE) or a hybrid (quasi-TE) polarization have recently been proposed for realizing wireless power transfer (WPT) links in the radiative near-field region at millimeter waves. The former are obtained by means of a radial slot array as a feeder and a homogenized metasurface as a PRS. The latter are obtained by using a loop antenna as a feeder and an annular strip grating in the homogenization limit as radiating aperture. In this work, based on an original semi-analytical model, such a metasurface is demonstrated to show a dichroic behavior. This interpretation explains the improvement in terms of polarization purity with respect to more nondichroic conventional homogenized metasurfaces. The behavior of the annular strip grating under a pure TM polarization is tested with a coaxial feeder, whereas its behavior under a pure TE polarization is tested by means of the radial slot array feeder. Results confirm the validity of the proposed analysis, which is finally exploited to evaluate the WPT performance.

Near-Field Radar Equation Based on Fresnel Diffraction Formula to Detect Short-Ranged Targets for Automotive Radar

Although the classic radar equation has been widely used to analyze the link budget of an automotive radar, it is valid only when the targets are located in the far-field region of the transmitted field and the receiving antenna is located in the far-field region of the scattered field. This paper confirms this hypothesis using measured and simulated data for short-range radars. Furthermore, a novel radar equation based on the Fresnel diffraction formula is proposed for application in situations where the receiving antenna is located in the radiative near-field region (or Fresnel region) of a target, but the target is situated in the far-field region of the transmitting antenna. In addition, the proposed radar equation is assessed by comparing the measured and simulated data.

Spherical Wavefronts Improve MU-MIMO Spectral Efficiency When Using Electrically Large Arrays

Modern MIMO communication systems are almost exclusively designed assuming locally plane wavefronts over antenna arrays. This is known as the far-field approximation and is soundly justified at sub-6-GHz frequencies at most relevant transmission ranges. However, when higher frequencies and shorter transmission ranges are used, the wave curvature over the array is no longer negligible, and arrays operate in the so-called radiative near-field region. This letter aims to show that the classical far-field approximation may significantly underestimate the achievable spectral efficiency of multi-user MIMO communications operating in the 30-GHz bands and above, even at ranges beyond the Fraunhofer distance. For planar arrays with typical sizes, we show that computing combining schemes based on the far-field model significantly reduces channel gain and spatial multiplexing capability. When the radiative near-field model is used, interference rejection schemes, such as the optimal minimum mean-square-error combiner, appear to be very promising, when combined with electrically large arrays, to meet the requirements of next-generation networks.

Leaky-Wave Analysis of TM-, TE-, and Hybrid-Polarized Aperture-Fed Bessel-Beam Launchers for Wireless-Power-Transfer Links

Bessel beams (BBs) can be generated at microwave frequencies by means of compact, planar devices based on Fabry–Perot cavity leaky-wave resonators. Most designs are based on the excitation of a single transverse magnetic/transverse electric (TM/TE) leaky mode by means of a vertical electric/magnetic dipole source. In this work, we analyze the important case of a horizontal magnetic dipole, which excites both TM and TE modes, developing an original theoretical framework suitable for resonant radiators. Analytical expressions are provided for the dominant leaky-wave aperture field. The near-field distribution is then evaluated through an accurate numerical integration of the radiating currents and validated through full-wave simulations for the relevant case of a BB launcher when excited by a waveguide-fed slot. Different designs are presented and analyzed in order to obtain TM, TE, and hybrid polarizations, finding in all cases an excellent agreement between simulations and theoretical results. Finally, we evaluate and compare the wireless-power-transfer efficiency of two coupled TM-, TE-, or hybrid-polarized BB launchers when a wireless link is established in the radiative near-field region. Interestingly, full-wave results demonstrate the superior performance of resonant BB launchers in TM or TE polarization with respect to the nonresonant hybrid case.

Tunable Terahertz Bessel-Beam Launchers

Bessel-beam launchers are planar devices capable of focusing electromagnetic energy in their radiative near-field region in the form of Bessel beams. Bessel-beam launchers are typically designed to have certain static beam features over a given operating bandwidth in the microwave/millimeter-wave range. Here, taking cues from some recent advancements in the field, we propose innovative architectures capable of generating Bessel beams at terahertz frequencies and featuring a dynamic control of the cover distance. The ideas outlined in this Vision are expected to pave the way for the realization of the first tunable terahertz Bessel-beam launchers.

Spatial Characterization of Electromagnetic Random Channels

The majority of stochastic channel models rely on the electromagnetic far-field assumption, which allows to decompose the channel in terms of plane waves. The far-field assumption breaks down in future applications that push towards the electromagnetic near-field region, such as those where the use of electromagnetically large antenna arrays is envisioned. Motivated by this consideration, we show how physical principles can be used to derive a plane-wave scalar channel model that is also valid in the reactive near-field region. Precisely, we show that narrowband wave propagation through a three-dimensional scattered medium can be generally modeled as a linear and space-variant system. We first review the physics principles that lead to a closed-form deterministic plane-wave representation of the channel impulse response. This serves as a basis for deriving a stochastic representation of the channel in terms of statistically independent Gaussian random coefficients for spatially stationary random propagation environments. The very desirable property of spatial stationarity can always be retained in the radiative near-field region by excluding reactive propagation mechanisms confined in close proximity to the source. Remarkably, the provided stochastic representation is directly connected to the Fourier spectral representation of a general stationary spatial random field.

Design and Implementation of 5.8 GHz RF Wireless Power Transfer System

In this paper, we present a 5.8 GHz radio-frequency (RF) wireless power transfer (WPT) system that consists of 64 transmit antennas and 16 receive antennas. Unlike the inductive or resonant coupling-based near-field WPT, RF WPT has a great advantage in powering low-power internet of things (IoT) devices with its capability of long-range wireless power transfer. We also propose a beam scanning algorithm that can effectively transfer the power no matter whether the receiver is located in the radiative near-field zone or far-field zone. The proposed beam scanning algorithm is verified with a real-life WPT testbed implemented by ourselves. By experiments, we confirm that the implemented 5.8 GHz RF WPT system is able to transfer 3.67 mW at a distance of 25 meters with the proposed beam scanning algorithm. Moreover, the results show that the proposed algorithm can effectively cover radiative near-field region differently from the conventional scanning schemes which are designed under the assumption of the far-field WPT.

Efficient Wireless Power Transfer System for Implantable Medical Devices Using Circular Polarized Antennas

In this article, we present a novel approach for designing a rotation insensitive, biocompatible, circular polarization-based efficient wireless power transfer (WPT) system for implantable medical devices (IMDs) in the radiative near-field region. Initially, a wideband, biocompatible, flexible circular polarized (CP) slot antenna is designed inside a single-layer skin tissue model to use as a receiving (Rx) element. To create the WPT link, a simple square patch antenna with the truncated corners is constructed to utilize as a transmitting (Tx) element. Furthermore, a polarization conversion-based metamaterial (MTM) structure is built particularly to improve the power transfer efficiency (PTE) of the proposed WPT system. Also, to show the versatility, the performance analysis of the proposed MTM integrated WPT system is conducted inside the human torso and head model. Moreover, a detailed study of the specific absorption rate (SAR) is accomplished regarding the safety of the human body in the light of IEEE regulation. Finally, the prototypes of the proposed WPT system are fabricated and experimentally verified in skin-mimicking gel and minced pork. The experimental result confirms the feasibility of the proposed concept by enhancing the PTE of the presented system by the usage of the MTM slab.

Simultaneous Generation of Three OAM Modes by Using a RLSA Fed by a Waveguide Circuit for 60 GHz-band Radiative Near-field Region OAM Multiplexing

This article reports for the first time the design of a 60 GHz planar antenna system for radiative near-field region line-of-sight (LOS) link based on an orbital angular momentum (OAM) multiplexing. The antenna system is composed of a radial line slot array (RLSA) fed by a Butler network, made in post-wall waveguide technology, and located underneath the array. The network can launch different progressive phase states in the RLSA parallel-plate waveguide (PPW) to radiate three different OAM beams. To focalize each beam in the radiative near region, the use of a modified Laguerre–Gaussian aperture distribution is proposed to extend the range where the link is guaranteed. The design concept has been verified by prototyping and testing a 20 free-space wavelengths’ radius antenna. The measurements show that the proposed antenna can generate simultaneously the three OAM-mode states $n =0$ , −1, and +1.

Battery-Less Location Tracking for Internet of Things: Simultaneous Wireless Power Transfer and Positioning

We propose a battery-less location tracking system that enables 3-D positioning of an Internet of Things (IoT) device powered by the radio frequency (RF) wireless power transfer (WPT). In the proposed system, a power beacon is equipped with a phased antenna array that has the dual purposes of high-efficiency WPT and phase-based accurate positioning. In order to enhance the efficiency of the RF WPT, we propose a beam focusing algorithm that dynamically controls the respective phases of antenna elements to place the focal point of the electro-magnetic (EM) wave onto the target IoT device. We also propose a phase-based positioning algorithm that requires only one multiantenna anchor point for determining the distance as well as the direction from the anchor point. We analyze the Cramer–Rao lower bound (CRLB) of the phase-based positioning with a single multiantenna anchor point, and show that the distance from the anchor point can be estimated as long as the IoT device lies within the radiative near-field region. We propose a joint location tracking and WPT algorithm that performs 3-D positioning and beam-focused WPT in a unified way. We have built a real-life testbed with a large-scale antenna array with 64 antenna elements for testing the proposed algorithm. The experimental results show the effectiveness of the proposed algorithm in a real environment.

Reconfigurable Modular Antenna for NF UHF RFID Smart Point Readers

In this paper, a reconfigurable modular antenna is presented for smart point readers employed in near-field (NF) radio-frequency identification applications at the ultrahigh frequency European Telecommunications Standards Institute band (865-868 MHz). The antenna comprises two modules sharing the same 250 × 250 mm 2 aperture: a spiral-shaped traveling wave antenna and an array of four resonating slot antennas connected in series to the traveling wave antenna through a printed matching/delay network. The electromagnetic (EM) field distribution generated by the overall antenna can be controlled and shaped through an absorptive RF switch placed at the end of the spiral traveling wave antenna. Specifically, two configurations can be selected, on the basis of the particular operating scenario. In the spiral traveling-wave antenna configuration, that is, the best for tag writing operations, the EM field is maximized in the reader central area, right on its surface only (reactive NF region). In the modular antenna configuration, a field coverage is guaranteed up to a few decimeters away from the reader surface (radiative NF region). Measurements on a prototype and simulation results are in good agreement. Moreover, a system-level characterization has been carried out and compared to a numerical analysis aimed to qualitatively predict tag detection performance.

Multifunctional modular antenna for near‐field ultra‐high frequency radio frequency identification readers

A multifunctional modular antenna suitable to be embedded into low-profile ultra-high frequency radio frequency identification desktop readers is designed, prototyped and characterised at system level. Two operating modes are foreseen. The first one allows for best performance right on the reader surface (reactive near-field region), and exploits a spiral-shaped travelling wave antenna (TWA) ended on a matched load and optimised (spiral TWA configuration). The second operating mode exhibits a field coverage up to a few decimetres away from the reader surface (radiative near-field region), and takes advantage of the above spiral TWA that serially feeds a low-gain resonating antenna. An array of two circularly polarised miniaturised patches is here considered for the resonating antenna. By means of a switch at the spiral TWA end, each one of the two operating modes can be selected on the basis of the application scenario of interest. Specifically, the spiral TWA configuration is suitable for tag writing operations, when the only tag that has to be encoded is most likely placed at the reader antenna centre. On the other hand, the modular antenna configuration allows the desktop reader to detect tags up to few decimetres, even in presence of stacked tagged objects.