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The feasibility of a novel near-field focusing technique based on Time-Modulated Arrays (TMAs) is studied. The formulation for a near-field transmission problem is developed, showing its interesting potential for multi-user focusing at different frequencies, advantageously exploiting the characteristic TMA side frequencies radiation. Although asking for specific patterns at a set of different radiation frequencies may degrade the overall system performance, it is shown that the required radiated field distributions may still be achieved with acceptable focusing performance. This fact makes TMAs an interesting alternative to other technologies due to their simpler implementation through digital devices, also allowing for an adaptive implementation as required for many real applications. Some illustrative examples are presented., This work was partially supported by the Spanish \emph{Ministerio de Ciencia e Innovacion} and the Spanish Research Agency (AEI) under the projects ARTEINE (TEC2017-86619-R) {and ENHANCE-5G (PID2020-114172RB-C21 / AEI / 10.13039/501100011033)}, and by the Gobierno del Principado de Asturias under project GRUPIN-IDI-2018-000191. This work was supported in part by the Italian Ministry of Education and Research (MIUR) in the framework of the CrossLab project (Departments of Excellence).

A dual-polarized transmitarray antenna is proposed to generate a multi-zone coverage at 28 GHz for 5G indoor communications. The transmitarray radiates a constant power density within its near-field region over a specific area generating different spots in dual-polarization. The different spots are produced by feeding the transmitarray with a cluster of horn antennas, which are spatially distributed to obtain beams in different directions. The performance of the coverage is improved with a Phase-Only Synthesis (POS) of the transmission coefficients. The transmitarray is designed with dielectric-only elements, which provides a low-cost solution. The prototype is measured in a planar acquisition range, showing a high agreement with simulations. The proposed transmitarray together with the spatial feed distribution provides a potential alternative to generate multi-zone coverages in millimeter frequencies., This work was supported in part by the Ministerio de Ciencia, Innovación y Universidades under Project TEC2017-86619-R (ARTEINE); an d Ministerio de Ciencia e Innovación and Agencia Española de Investigación within Project ENHANCE-5G (PID2020-114172RB-C21-2/AEI/10.13039/501100011033).

In this work, a reflectarray antenna is presented as a dual-polarized plane wave generator (PWG) in mm-wave frequencies. The reflectarray produces a uniform plane wave within its near field in a volume close to the aperture. A dual-polarized pencil beam reflectarray is used as starting point to carry out a phase-only synthesis (POS) to reach a dual-polarized uniform plane wave. The synthesis aims to overcome the amplitude limitation due to the incident field while preserving the phase flatness. The phase distribution obtained in the POS is used to carry out a design based on sets of orthogonal-coplanar dipoles. This unit cell provides independent control of each polarization using a single-layer topology. The design is manufactured, and the prototype is measured in a planar acquisition range facility. The prototype shows discrepancies with simulation and further analysis is carried out. After finding out the proper value of the dielectric constant, a new design is carried out. The second prototype shows a good agreement with simulations, obtaining that reflectarray antennas are a suitable and low-profile solution to produce uniform plane waves within the near field., This work was supported in part by Ministerio de Ciencia e Innovación and Agencia Española de Investigación within the project PID2020-114172RB-C21/AEI/10.13039/501100011033, and by Gobierno del Principado de Asturias within the project AYUD/2021/51706.

In this paper, it is proposed the use of ultra-thin reflectarrays in some emerging scenarios in the new mm-wave cellular networks such as 5G, Beyond-5G (B5G), and future 6G. To that purpose, two dual-linear polarization reflectarrays are developed to provide coverages in different scenarios at the mm-waves band. Working at 28 GHz, reflectarray designs adapt the radiated field to the coverage area specified in each scenario, through an ultra-thin surface composed of simple cells. To evaluate the antenna performance, two reflectarray prototypes identical in size are manufactured and tested achieving a good trade-off between the performance and the low-cost, low losses and, low-profile characteristics of the antenna. This work demonstrates the good performance of simple topology reflectarrays, being an interesting antenna candidate in the upcoming wireless networks.

An improved reflectarray Phase-Only Synthesis technique which employs the generalized Intersection Approach (IA) algorithm is fully described. It is formulated with the First Principle of Equivalence and takes into account a dielectric frame which is usually present to screw the reflectarray breadboard to the supporting structure. The effects of the First Principle of Equivalence versus the Second Principle in the computation of the radiation patterns, as well as the dielectric frame, are assessed and taken into account in an efficient implementation of the generalized IA in order to obtain more accurate results. Different strategies to speed up the synthesis process are presented and to improve convergence. The technique is demonstrated through two examples for space and terrestrial applications: an isoflux pattern for global Earth coverage from a satellite and a Local Multipoint Distribution Service pattern for central stations of cellular systems, both with a working frequency of 25.5 GHz. In addition, experimental results validate the approach described in this work with a prototype with an isoflux pattern working at 30 GHz.

A novel ultra-thin Near-Field 2-bit 24x24-element transmitarray lens working at X-Band is presented in this letter. The aim of this antenna is to focus in a point located in the Near-Field without using any complex feeding network. In order to obtain a planar lens with a very low profile, a 2-bit-design approach is used to diminish the number of layers used in the structure. The simulations have been done using a model based on the concept that the Near-Field of an array can be calculated as the sum of the Far-Field contributions of every single one of its elements. These are considered as plane apertures over which the tangential electric field is constant. In order to validate the results of this model, they are compared with both those obtained with a full-wave commercial software and those from measurements. Thus, a Near-Field-antenna is manufactured and measured showing the measurements good concordance with the results of the model, MIRIIEM with ref. TEC2014-540005-P of the Ministerio de Econom´ıa y Competitividad and the research project of the Gobierno del Principado de Asturias / FEDER with ref. GRUPIN14-114

The aim of this paper is the development of characteristic basis functions (CBFs) for homogeneous dielectric bodies that are modeled with the surface equivalence principle. This approach contributes to generalize the characteristic basis function method (CBFM) for any kind of material. The foundations for the generation of these new basis functions are widely revised and the CBFM is adapted for this purpose. The validity of the method is illustrated with the systematic analysis of canonical geometries as well as with the computation of the specific absorption rate inside a head-like geometry. In all the cases, the error in the current distribution as well as in the near- and far-field is considered. In addition, a detailed comparison of the generation of the CBFs using plane waves and spherical waves is also carried out.

A comparison between two well-known near field to far field (NF-FF) transformation methods is presented: the spherical wave expansion (SWE) and the sources reconstruction method (SRM). The goal of this paper is to show the methods performance for NF-FF transformation pointing out the advantages and limitations of each one. First, the computational cost is studied, showing its dependence with the antenna under test (AUT) electric size. Second, the NF-FF transformation accuracy of each method is compared for a fixed AUT electric size, analyzing also the influence of the main parameters that affect the SRM and SWE accuracy. Third, the SRM capabilities are presented, with focus on the NF-NF transformation inside the minimum sphere. Practical applications of the SWE and SRM are illustrated by means of an example of NF-FF transformation and antenna diagnostics problem using measured NF data.

A technique for the determination of the equivalent currents distribution from a known radiated field is described. This Inverse Radiation Problem is solved through an Integral Equation algorithm that allows the characterization of antennas of complex geometry both for near field to far field (NF-FF) transformation purposes as well as for diagnostic tasks. The algorithm is based on the representation of the radiating structure by means of a set of equivalent currents over a three-dimensional (3D) surface that can be fitted to the arbitrary geometry of the antenna. The innovative formulation uses an integral equation involving the electric field due to the currents tangential components to the represented antenna 3D surface. For that purpose, both the magnetic and electric equivalent currents are considered in the integral equations. Regularization techniques are also introduced to improve the convergence of the proposed iterative solution. The paper concludes with several results related to the practical verification of the Equivalence Principle and the characterization of a horn antenna.

An extension of the sources reconstruction method (SRM) for antenna diagnostics using amplitude-only field measurements is presented. While previous works limit the application to canonical domain cases, the combination of the SRM capabilities for handling with arbitrary-geometry domains jointly with phase retrieval techniques, allows antenna diagnostics extension to complex geometry antenna and field acquisition domains. The consideration of the radiation inverse problem with a general integral equation formulation using arbitrary-geometry field and currents domains, and field phaseless information, supposes a challenging ill-posed problem that is solved using iterative minimization techniques for non-linear problems. Different examples are presented, from simple to general antenna diagnostics cases.