Superdirective Antenna Research Articles

A multi‐objective optimisation for compact wideband and efficient superdirective antenna arrays design using network characteristic modes

This paper introduces a procedure for multi-objective optimisation, for compact wideband superdirective antenna array design, using network characteristic modes. The two main objectives considered are the maximisation of the directivity in the end-fire direction, simultaneously with the minimisation of the input reflection coefficient in a large bandwidth. The proposed method is used thereafter for designing a three-element superdirective small Yagi antenna, with an overall dimensions of 0.4λ × 0.37λ × 0.18λ. The length of this mini yagi antenna is 60% shorter than a classical one, with the same performances of gain and bandwidth. The measured results show that the antenna bandwidth, maximum gain, and efficiency are of 11.2%, 8 dBi, and 95% respectively.

Harnessing superdirectivity in dielectric spherical multilayer antennas

Small form-factor, narrowband, and highly directive antennas are of critical importance in a variety of applications spanning wireless communications, remote sensing, Raman spectroscopy, and single photon emission enhancement. Surprisingly, we show that the classical directivity limit can be appreciably surpassed for electrically small multilayer spherical antennas excited by a point electric dipole even if limiting ourselves to purely dielectric materials. Experimentally feasible designs of superdirective antennas are established by using a stochastic optimization algorithm combined with a rigorous analytic solution.

Volumetric metamaterials versus impedance surfaces in scattering applications

Artificially created media allow employing material parameters as additional valuable degrees of freedom in tailoring electromagnetic scattering. In particular, metamaterials with either negative permeability or permittivity allow creating deeply subwavelength resonant structures with relatively high scattering cross-sections. However, the equivalence principle allows replacing volumetric structures with properly designed curved impedance surfaces, ensuring the same electromagnetic properties. Here, we examine this statement from a practical standpoint, considering two structures, having a dipolar electric resonance at the same frequency. The first realization is based on arrays of inductively loaded electric dipoles printed on stacked circuit boards (a volumetric metamaterial), while the second structure utilizes a 4-wire spiral on a spherical surface (surface impedance realization). An intermediate conclusion is that the surface implementation tends to outperform the volumetric counterparts in the scenario when a single resonance is involved. However, in the case where multiple resonances are overlapping and lossy materials are involved, volumetric realization can have an advantage. The discussed structures are of significant importance to the field of electrically small antennas, superdirective antennas, and superscatterers, which find use in wireless communications and radar applications, to name just a few.

Synthesis of Super-Oscillatory Point-Spread Functions with Taylor-Like Tapered Sidelobes for Advanced Optical Super-Resolution Imaging

Recently, the super-oscillation phenomenon has attracted attention because of its ability to super-resolve unlabelled objects in the far-field. Previous synthesis of super-oscillatory point-spread functions used the Chebyshev patterns where all sidelobes are equal. In this work, an approach is introduced to generate super-oscillatory Taylor-like point-spread functions that have tapered sidelobes. The proposed method is based on the Schelkunoff’s super-directive antenna theory. This approach enables the super-resolution, the first sidelobe level and the tapering rate of the sidelobes to be controlled. Finally, we present the design of several imaging experiments using a spatial light modulator as an advanced programmable grating to form the Taylor-like super-oscillatory point-spread functions and demonstrate their superiority over the Chebyshev ones in resolving the objects of two apertures and of a mask with the letter E.

Compact Super-Directive Yagi-Uda Antenna Based on Parabolic-Shaped Reflector for Wireless Communications

This paper was undertaken to study the influence upon gain and directional characteristics caused by numerous array of elements when used in conjunction with the Yagi antenna. Antennas are beneficial for a wide range of applications such as point to point communication, radio broadcasting, radar, and wireless LAN. However, maximum gain and maximum directivity with low losses are the desired characteristics that are sought.

Enhanced superdirectivity of a multi-layered cylinder using a Huygens excitation

Superdirective antennae with magnetic/electric line source excitation have been studied in the past decade. The realization of superdirectivity suffers from large side-lobes because the form of the excitation of antenna is too simple. In this article, we introduce a technique for increasing the directivity of a superdirective antenna with a complicated excitation source. Huygens source is chosen as the complicated source due to its perfect zero-back-lobe characteristic and is modeled using numerical and analytical methods . A 10-layered cylinder is chosen as the superdirective antenna. Using the Huygens source, the directivity of the multi-layered cylinder was increased by 20% compared to the previous designs and an extremely small back-lobe is obtained. Other proposed designs mainly focus on the optimization of permittivities and permeabilities of the dielectric layers, which is usually time-consuming, while in our approach, the form of the excitation is considered first. Therefore, this method can be adapted for other complicated sources and provides a novel approach for improving the performance of the superdirective antenna.

Design of easy-manufacturing superdirective antenna: a theoretical study.

An easy-fabricating superdirective antenna with needlelike radiation in the microwave band is proposed for the first time to the best of our knowledge. The electromagnetic field of the superdirective antenna excited by an electric line source is calculated. The parameters of the superdirective antenna are obtained by optimization algorithms. A two-step optimization method is carried out to reduce the difficulty of fabrication. The mechanism of superdirectivity in the microwave band is explained briefly by traditional multilayered dielectric resonances. A machinable 10-layered superdirective antenna, using this method, is proposed. Unlike traditional superdirective antennas, the proposed antenna, after two-step optimization in this paper, has all-positive integral permittivities, which reduce the fabrication difficulty significantly.

Novel optimizing algorithm of superdirective multi-layered cylindric antenna

We propose the design of a superdirective antenna based on a multi-layered dielectric cylinder. A novel optimization goal is proposed based on the physical characteristics of the Dirac delta function. The side-lobe properties of superdirective antenna is discussed and treated as the optimization goal for antenna design for the first time. A multi-stage genetic algorithm and trust region reflective algorithm are utilized to shorten the time duration for optimization. The optimizing efficiency is higher and the cost of time is significantly reduced compared with proposed designs. The effect of weight of different algorithms is discussed. Both the dielectric loss and metallic loss are also discussed for practical cases. The optimization is of low computing cost and can be accomplished by personal computers.

Superdirective Linear Dipole Array Optimization

The theory of superdirective antenna arrays was discussed. The procedure for finding the excitation currents of fixed-geometry arrays, which maximizes the directivity, termed as superdirectivity, for a given direction and polarization, was described. The procedure was based on the solution of an eigenvalue problem including the directivity and matrices of normalized self- and mutual radiation intensities and normalized self- and mutual radiated powers of array elements. For examples of a linear dipole array located above a perfect electric conductor ground plane, this procedure was also combined with the optimization of the parameters of geometry of the array as a spacing of dipoles, or their height above the ground, for various number of dipoles in the array.

Superdirective dimers of coupled self-resonant split ring resonators: Analytical modelling and numerical and experimental validation

Superdirective antennas developed over the last century have received renewed interest in recent years from the development of metamaterials. These arrays of electromagnetic resonators (or meta-atoms) carrying short wavelength electro- and/or magneto-inductive waves support current distributions with very high spatial frequency as required by the classical conditions for superdirectivity. As meta-atoms can have both electric and magnetic dipole characteristics (and hence radiation properties), developing antennas exploiting these distributions can challenge conventional intuitions regarding the optimal configurations required. In this work we are reporting the development of a genuinely superdirective array using split ring resonators (SRRs). We provide a comprehensive analytical model characterizing the radiation from SRR dimers in which excitation of only one split ring leads to superdirective radiation via mutually coupled modes. Our model exploits simple circuit descriptions of coupled resonant circuits, combined with standard radiation formulae for curvilinear current distributions. Using this simple model we are able to map directivity against possible SRR locations and orientations in two dimensions and identify the unique optimal configuration which meets the requirements for superdirective emission. We validate the theoretical findings by comparison to both full wave simulations and experiments showing that our SRR dimer achieves endfire directivity very close to the maximum theoretical value.

Multiple scattering enabled superdirectivity from a subwavelength ensemble of resonators

An ensemble of resonators arranged on a subwavelength scale is usually considered as a bulk effective medium, known as a metamaterial, and can offer unusual macroscopic properties. Here, we take a different approach and limit ourselves to the study of only a few number of such elementary components and demonstrate that they still offer uncommon opportunities. Typically, owing to the multiple scattering and the phase shift that the resonances offer, we observe fields that vary at scales completely independent of the wavelength in free space. By smartly tuning the resonance frequencies, we can design at will the complex current distribution in those resonators. This way, we design a superdirective antenna, i.e., an antenna that is surprisingly more directive than its size would foreshadow. This approach is verified numerically and experimentally in the context of microwaves, but this applies to any wave field where subwavelength resonators exist.

Design principle of cylindrical superdirective antenna

We propose the design principle of a superdirective antenna with needle-like radiation based on multi-layered dielectric cylinder. The electromagnetic fields and far-field directivity of superdirective antenna are calculated. The radiation mechanisms of the superdirective antenna are illustrated by multi-layered asymmetric SPR phenomenon. A nonlinear searching algorithm is utilized to calculate the design parameters of the antenna. A series of constraint conditions on the design parameters are derived from the phenomenon. The constraint conditions restrict the initial values of optimization in a limited solution space, and act as a guidance on the optimizing process and improves the convergence and quality of optimization.

Coherently Driven and Superdirective Antennas

Antennas are crucial elements for wireless technologies, communications and power transfer across the entire spectrum of electromagnetic waves, including radio, microwaves, THz and optics. In this paper, we review our recent achievements in two promising areas: coherently enhanced wireless power transfer (WPT) and superdirective dielectric antennas. We show that the concept of coherently enhanced WPT allows improvement of the antenna receiving efficiency by coherent excitation of the outcoupling waveguide with a backward propagating guided mode with a specific amplitude and phase. Antennas with the superdirectivity effect can increase the WPT system’s performance in another way, through tailoring of radiation diagram via engineering antenna multipoles excitation and interference of their radiation. We demonstrate a way to achieve the superdirectivity effect via higher-order multipoles excitation in a subwavelength high-index spherical dielectric resonator supporting electric and magnetic Mie multipoles. Thus, both types of antenna discussed here possess a coherent nature and can be used in modern intelligent antenna systems.

Optimization of Far-Field Radiation From Impedance-Loaded Nanoloops Accelerated by an Exact Analytical Formulation

Impedance loading is a common technique traditionally used in the RF to enhance the performance of an antenna, but its application in the optical regime is not as rigorously studied. This is mainly due to a lack of exact analytical expressions that can be used to rapidly predict the radiation properties of loaded nanoantennas. This paper will derive a set of useful analytical expressions for the far-field radiation properties of loop antennas loaded with an arbitrary number of lumped impedances that are valid from the RF-to-optical regimes. The analytical expressions will be validated with full-wave solvers and can be evaluated more than $100\times $ faster. The ability to perform such rapid evaluations enables, for the first time, large-scale single-objective and multi-objective optimizations. A series of optimizations reveal that electrically small superdirective antennas can be achieved at a variety of far-field angles through capacitive loading, paving the way for a pattern reconfigurable antenna. In addition, gains of greater than 3 dB can be achieved for electrically small antennas over a fractional bandwidth of 28%. Finally, it is shown that impedance loading can be used to achieve circularly polarized radiation from a single loop.

Improvement of bandwidth characteristics of low-profile superdirective antenna composed of asymmetric dipole element backed by planar reflector

Improvement of bandwidth characteristics of low-profile superdirective antenna composed of asymmetric dipole element backed by planar reflector

Superdirectivity from arrays of strongly coupled meta-atoms

We explore the possibility of achieving superdirectivity in metamaterial-inspired endfire antenna arrays relying on the good services of magnetoinductive waves. These are short-wavelength slow waves propagating by virtue of coupling between resonant meta-atoms. Magnetoinductive waves are capable of providing a rapidly varying current distribution on the scale of the free space wavelength. Using dimers and trimers of magnetically coupled split ring resonators with only one element driven by an external source, we introduce an analytical condition for realising superdirective current distributions. Although those current distributions have been known theoretically for a good 60 years, this is the first time that a recipe is given to realise them in practice. Our key parameters are the size of the array, the resonant frequency and quality factor of the elements, and their coupling constant. We compare our analytical results for coupled magnetic dipoles with numerical results from CST simulations for meta-atoms of various shapes. The calculated bandwidth of 5 MHz for a dimer operating at 150 MHz indicates that, contrary to popular belief, superdirective antennas exist not only in theory but may have practical applications.

Practical Superdirectivity With Resonant Screened Apertures Motivated by a Poynting Streamlines Analysis

Fundamental limits on the degree of practical achievable superdirectivity for antennas include narrow bandwidth, large antenna loss, and sensitivity to excitation and fabrication errors. Superdirective antennas are commonly considered as transmitters, but thinking of the antenna as a receiver may help to understand and overcome some of these limits on practical superdirectivity. We directly model a superdirective antenna in receive mode by calculating streamlines of Poynting vector field near the receiving antennas. Superdirectivity is achieved by expanding the shape of the effective area beyond the antenna physical aperture area. The degree of superdirectivity is parameterized by an effective area expansion distance. The theory predicts that superdirectivity is practical for electrically small and middle size antennas and electrically large antennas with a large aspect ratio. With this motivation, a $1.5\lambda \times 2\lambda $ superdirective horn antenna with resonant screen structure in front of the antenna aperture is designed and fabricated. The superdirective horn achieves a measured antenna efficiency of 115% over a reasonable bandwidth and with moderate sensitivity to fabrication errors.

Poynting Streamlines, Effective Area Shape, and the Design of Superdirective Antennas

A receiving antenna alters an incident field in such a way that the field is concentrated at the terminals of the antenna. The Poynting power flux density vector associated with the field carries to the antenna the power dissipated in the antenna structure and the load. Streamlines of the Poynting vector field can be used to understand electromagnetic energy flow near linear and aperture antennas. Poynting streamlines provide a way to understand and guide the design of superdirective antennas. Directivity-enhancing screens that attract Poynting streamlines and increase the aperture efficiency of a horn antenna to almost 200% are studied. Superdirective antennas generally have limited practical value due to poor radiation efficiency, narrow bandwidth, and extreme sensitivity to fabrication errors. We show that these limitations can be mitigated by using metal-only structures that are optimized for broadband operation. The tradeoff between peak achievable aperture efficiency and bandwidth is explored.

Theoretical and practical limits of superdirective antenna arrays

Abstract Some applications as Wireless Power Transfer (WPT) require compact and directive antennas. However, Electrically Small Antennas (ESAs) have low efficiencies and quasi-isotropic radiation patterns. Superdirective ESA arrays can be an interesting solution to cope with both constraints (the compactness and the directivity). In this paper, the theoretical and practical limits of superdirective antennas will be presented. These limits can be summarized by the directivity sensitivity toward the excitation coefficients changes and the radiation efficiency decrement as the inter-element decreases. The need for negative resistances is also a practical limit for transforming these arrays into parasitic ones. The necessary trade-offs between the antenna total dimensions (the number of elements and the inter-element distance) and the attainable directivity and efficiency are also analyzed throughout this paper.

Small end‐fire superdirective folded meandered monopole antenna array

ABSTRACTIn this letter, we present a compact four‐element superdirective antenna array operating at 900MHz frequency band. In this antenna, only one element is excited while the others are parasitic‐loaded elements. The antenna total dimensions are 231 × 122 mm2 and it has a total directivity of 11.6dBi and radiation efficiency of 46%. This antenna is significantly compact compared with others presenting the same directivity. © 2016 Wiley Periodicals, Inc. Microwave Opt Technol Lett 58:2122–2124, 2016