Abstract
Multipole expansions are an essential analysis tool in the foundations of the descriptions of the electromagnetic fields radiated by electric and magnetic sources. Nevertheless, practical antenna systems generally rely on them as an academic explanation, not as a fundamental building block. An overview of the recent surge in interest in multipole sources and their fields to achieve useful radiated and scattered fields with, for example, high directivities in preferred directions is given. Topics include Huygens sources, dielectric-based Mie-tronics, edge-singularity multipoles, and exotic metamaterial-inspired superdirective lenses and radiators. While there has been a never-ending stream of physics publications, little has happened in the engineering electromagnetics community. I will try to answer the title with examples that may stimulate interest in the field.
Highlights
As we continue to move into researching and fielding yet newer and newer generations of wireless technologies, i.e., fifth-generation (5G) [1]–[3], sixth-generation (6G) [4], [5], and even beyond [6], [7], the importance of high directivity beams to ensure meeting their anticipated performance characteristics has increased significantly
Some early MTM works from the engineering electromagnetic community considered canonical-shaped structures involving double positive (DPS), double negative (DNG) and single negative (SNG), i.e., both epsilon negative (ENG) and mu negative (MNG), media using Mie-theory multipolar expansions to understand dipole and higher order mode physics associated with negative materials [53]–[56] and their potential antenna [57], superscattering [53], [58]– [60], and cloaking [61] applications
Optimization to attain the best impedance match, maximum directivity, and front-to-back ratio (FTBR) values simultaneously when taking into consideration all of the practical material and feed issues would be required to obtain the best performance for any prototype design
Summary
As we continue to move into researching and fielding yet newer and newer generations of wireless technologies, i.e., fifth-generation (5G) [1]–[3], sixth-generation (6G) [4], [5], and even beyond [6], [7], the importance of high directivity beams to ensure meeting their anticipated performance characteristics has increased significantly. Base station systems, and user terminals must consider antennas with higher and higher directivities to compensate for the requirements for reduced physical power, increased propagation losses faced at higher frequencies, and low probability of intercept (LPI) to attain secure communications They represent the key antenna technology for supporting high data transmission rates, improved signal-to-interference-plus-noise ratios, increased spectral and energy efficiencies, and versatile beam shaping and pointing. The HOMs would have little impact on their responses if the incident beam were strictly truncated to the physical size of the scatterer, i.e., the target would have restricted access to them This connection between the reactive fields and an antenna’s directivity follows naturally if one recalls that the lower bound on the quality factor derived by Chu [20] takes the form [21]: QChu = (ka)3 + ka (4).
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