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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).

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.