Search

Gradient Index (GRIN) lenses have the potential to tailor the radiation behavior of existing antennas for improved gain performance. Conventional methods such as transformation optics have mainly focused on two-dimensional (2-D) problems (e.g., axisymmetric or sectoral horn lenses), and often produce designs with unrealistic material parameters or poor impedance matching. In this letter, we propose a new method of designing freeform GRIN lenses using adjoint sensitivities to rapidly find optimal solutions within feasible material bounds that achieve desired performance goals. A proof-of-concept 3-D GRIN lens that achieves comparable performance to a larger 20 dB standard gain horn albeit with considerable size reduction is optimized, fabricated, and tested in order to highlight the effectiveness of the proposed approach. [ABSTRACT FROM AUTHOR]

In this article, we demonstrate that a circular advanced short backfire antenna (A-SBFA) can be optimized for high single- or dual-band aperture efficiency over a range of different aperture sizes. Optimization methods developed previously to utilize anisotropic, dispersive metasurfaces on the cavity walls of the hexagonal A-SBFA are modified and applied to achieve 100% aperture efficiency at two frequency bands for circular aperture diameters spanning a remarkably large range from $1.0\lambda _{0}$ up to $2.13\lambda _{0}$. Furthermore, a single-frequency optimization was able to achieve a design with 100% aperture efficiency for an A-SBFA with a diameter of $2.6\lambda _{0}$ , the highest known achievable efficiency for an antenna of its aperture size, and overall height. Finally, simulations revealed the first compact hard horn or open-ended waveguide design with close to 100% aperture efficiency below $1.6\lambda _{0}$. [ABSTRACT FROM AUTHOR]

Metasurfaces with independently tunable anisotropic dispersion characteristics have been designed to significantly improve the radiation efficiency of a short backfire antenna (SBFA) when placed within the antenna cavity walls. This advanced SBFA (A-SBFA) features a hexagonal aperture, which typically results in much lower aperture efficiency and increased cross polarization. However, the addition of metasurfaces leads to a higher aperture efficiency than is achievable with a conventional circular SBFA. Through careful design and optimization of the metasurface dispersion, the A-SBFA achieves high aperture efficiency across two discrete frequency bands while maintaining a relatively low height and weight. A detailed design and optimization methodology is presented here, along with the simulation studies that explore the design space and provide insight into how the antenna performance is affected as the metasurface parameters are adjusted. The A-SBFA is shown to be a promising candidate for future satellite-based antenna arrays due to its high aperture efficiency, hexagonal cavity, low weight, and low profile. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An efficient, black-box multiobjective optimization technique is presented, which is capable of simultaneously optimizing designs for performance as well as robustness when input tolerance values are not knowna priori. During the optimization process, adaptive statistical surrogate mappings between input variables and output objectives are formulated within a model selection framework. These statistical models can be evaluated in fractions of a second and serve as an efficient surrogate for a more computationally intensive process, such as an electromagnetic simulation. By exploiting the speed offered from surrogate modeling techniques, new, high-performance designs can be quickly identified. In addition, complete tolerance analysis can be conducted within the optimization loop, which provides designers with critical information regarding the robustness of designs. To demonstrate the effectiveness of this approach, it will be applied to the optimization of a capacitively loaded monopole and a wideband Vivaldi antenna. [ABSTRACT FROM AUTHOR]

Frequency-selective surfaces (FSSs) have many applications in spatial filtering of electromagnetic waves and are commonly used in antennas, polarizers, radomes, and intelligent architecture. Conventional FSS designs have ranged from canonical shapes and fractal patterns on planar surfaces to miniaturized and multilayer designs, with stable filtering responses up to 50°. Much less work has been done on 3-D FSS designs, which include multiresonant structures or cavities that offer improved angular stability, with fields of view up to 60°. Recent advances in additive manufacturing techniques have made fully 3-D FSS designs increasingly popular; however, powerful design tools to exploit such fabrication methods are currently unavailable. In this paper, multiobjective lazy ant colony optimization (MOLACO), an adaptive combinatorial optimization algorithm based on ant colony optimization, is introduced and applied to the problem of polarization and angle independent 3-D FSS design. It will be shown that the MOLACO algorithm generates several innovative and unintuitive unit cell geometries with a single-zero, single-pole response, less than 1% shift from center frequencies and -10 dB rejection and 3 dB transmission bandwidths between 6%-12% for angles of incidence up to 80° in TE and TM polarizations. Comparisons are made between designs generated by MOLACO and existing FSS designs. [ABSTRACT FROM PUBLISHER]

A differential ray tracing (DRT) formulation for the analysis and design of arbitrary optical systems is presented. An approach based on sequential rays is proposed to define a functional representation of the ray path, where the gradient information associated with the ray path is obtained using an automatic differentiation technique. The proposed DRT algorithm is neither constrained to a specific optical system geometry nor restricted to a set of surface types or material definition. [ABSTRACT FROM AUTHOR]

This paper compares three modern and two classical multiobjective optimizers (MOOs) as applied to real-world problems in electromagnetics. The behavior of sophisticated optimizers on simple test functions has been studied exhaustively. In contrast, the algorithms here are tested on practical applications, where the function evaluations are computationally expensive, making the convergence rate a crucial factor. The examples considered include the optimization of a narrowband slot antenna, a mushroom-type electromagnetic bandgap structure, and an ultrawideband Vivaldi antenna. Another popular topic in the literature is in comparing classical MOOs on electromagnetics problems. The modern optimizers chosen in this paper are state of the art and each has a distinct design philosophy. This paper introduces two unique MOOs to the electromagnetics community: BORG, an auto-adaptive genetic algorithm and the Multi-Objective Covariance Matrix Adaptation Evolutionary Strategy (MO-CMA-ES), an extension of the popular single-objective CMA-ES. These algorithms are compared to the Multi-objective Evolutionary Algorithm based on Decomposition (MOEA/D), a Chebysheff scalarization algorithm, and two classical MOOs. This paper will study the behavior of these algorithms on problems in electromagnetics with a limited number of function evaluations using five distinct metrics and will provide useful guidelines and recommended optimizer settings. [ABSTRACT FROM AUTHOR]

A method is presented that allows for the efficient design of capacitively loaded finite-size electromagnetic bandgap (EBG) structures, which can target a wide range of design objectives. The design flexibility is achieved by adding arbitrary nonuniform capacitive loading to an underlying periodic EBG structure. This system can be interpreted as having an effective aperiodic structure, which allows more design flexibility in terms of bandgap engineering. To choose the proper capacitances, a powerful global optimization technique known as the covariance matrix adaptation evolutionary strategy is employed that is aided by a fast port-reduction strategy. This approach avoids the need to carry out multiple computationally expensive full-wave simulations during the course of the optimization process by requiring only a single full-wave simulation be performed prior to initiating the optimization. To demonstrate the utility of this method, the capacitive loading of a mushroom-type EBG structure in a parallel-plate waveguide is optimized to reduce transmission from 2.4 to 7 GHz. This design was fabricated and the measured response was found to be in good agreement with the simulations. Using the same initial full-wave simulation, another structure was designed to improve isolation at the 2.4-, 3.6-, and 5-GHz WLAN bands to below -\22 dB. An additional set of structures are also designed using capacitively loaded mushroom-type EBG surfaces without placing them inside of a parallel-plate waveguide. [ABSTRACT FROM AUTHOR]

Metasurfaces offer the potential to realize large SWaP (size, weight, and power) reduction over conventional optical elements for their ability to achieve comparable functionalities in ultrathin geometries. Moreover, metasurfaces designed with phase change materials offer the potential to go beyond what is achievable by conventional optics by enabling multiple functionalities in a single reconfigurable meta-device. However, designing a single metasurface geometry that simultaneously achieves multiple desired functionalities while meeting all bandwidth requirements and fabrication constraints is a very challenging problem. Fortunately, this challenge can be overcome by the use of state-of-the-art multi-objective optimization algorithms which are well-suited for the inverse-design of multifunctional meta-devices. [ABSTRACT FROM AUTHOR]

Phase-gradient metasurfaces enable designers to tailor the behavior of electromagnetic waves at surfaces by exploiting the generalized form of Snell’s law. This ability has led to the investigation of metalenses which have the potential to significantly reduce the size, weight, and power (SWaP) of conventional optical systems. While traditional lenses are made from individual glasses, metalenses are comprised of patterned meta-atom unit cells which are arranged in such a way so as to give the metalens its desired behavior. Therefore, any metalens’s performance is ultimately determined by that of its underlying unit cell components. However, designing meta-atoms that simultaneously achieve high performance over wide frequency bandwidths and fields-of-view is an extremely challenging problem that is best addressed with powerful optimization and inverse-design techniques. [ABSTRACT FROM AUTHOR]

A new type of nature-inspired global optimization methodology based on atmospheric motion is introduced. The proposed Wind Driven Optimization (WDO) technique is a population based iterative heuristic global optimization algorithm for multi-dimensional and multi-modal problems with the potential to implement constraints on the search domain. At its core, a population of infinitesimally small air parcels navigates over an N-dimensional search space following Newton's second law of motion, which is also used to describe the motion of air parcels within the earth's atmosphere. Compared to similar particle based algorithms, WDO employs additional terms in the velocity update equation (e.g., gravitation and Coriolis forces), providing robustness and extra degrees of freedom to fine tune. Along with the theory and terminology of WDO, a numerical study for tuning the WDO parameters is presented. WDO is further applied to three electromagnetics optimization problems, including the synthesis of a linear antenna array, a double-sided artificial magnetic conductor for WiFi applications, and an E-shaped microstrip patch antenna. These examples suggest that WDO can, in some cases, out-perform other well-known techniques such as Particle Swarm Optimization (PSO), Genetic Algorithm (GA) or Differential Evolution (DE) and that WDO is well-suited for problems with both discrete and continuous-valued parameters. [ABSTRACT FROM PUBLISHER]

Two new phased array design techniques are introduced that utilize rotational symmetry to avoid grating lobes during scanning and over ultra-wide bandwidths. The first is an optimization method, which employs the covariance matrix adaptation evolutionary strategy to determine the best locations of elements inside a sector of a circle. The second technique uses standard periodic lattices inside the same circular sector. Although the segments themselves are internally periodic, the rotational symmetry of the full array eliminates the occurrence of grating lobes and is capable of generating antenna element layouts with useful ultra-wideband properties. These design techniques yield arrays with not only ultra-wideband characteristics, but properties that lend themselves to relatively straightforward modular construction, especially the semi-periodic variety. [ABSTRACT FROM PUBLISHER]

A novel methodology is introduced for the design synthesis of thin planar realizations of volumetric high-impedance or artificial magnetic conducting surfaces (AMC). The design synthesis involves optimization of two different metallic frequency selective surface (FSS) type structures printed on each side of a thin dielectric substrate material. This technique eliminates the need for a complete metallic backplane common in conventional AMC designs, making use of the same dielectric substrate for two high-impedance surfaces; one on each side. Optimization of the FSS unit cell geometries is carried out with a robust genetic algorithm (GA) technique that is combined with a full-wave periodic finite element boundary integral (PFEBI) electromagnetic simulation code for fast and accurate optimization of desired AMC performance at a single frequency or over multiple frequency bands. Several examples of thin AMC ground planes are optimized for use in the X-band. Additional design examples that provide AMC behavior on one side and absorber behavior on the other are also provided. Lastly, an example illustrating the utility of the double-sided AMC separator structure is shown for a design targeting the standard Wi-Fi frequencies of 2.4 GHz and 5.2 GHz. [ABSTRACT FROM PUBLISHER]

Micro-UAV swarm-based antenna arrays provide a novel solution for high-risk radar imaging applications. These apertures lack a single point of failure by distributing their resources and sensors across multiple platforms. However, turbulence and positional errors provide a challenging operational environment when it comes to the implementation of these systems. Turbulence can limit the aperture's ability to coherently resolve a target and cause aircrafts to collide in midair if the formation is too tightly packed with closely spaced elements. This paper introduces several techniques that can reduce the effects of turbulence on the system. First, a phase compensation algorithm is presented that can eliminate the effects of turbulence on the main beam of the array. In addition, sparse antenna apertures can be used to create flight formations that reduce the probability of midair collisions. Traditional periodic apertures are insufficient because these arrays display grating lobes at wide interelement spacings. Therefore, two aperiodic array optimization methodologies are discussed that produce sparse array configurations suitable for micro-UAV formations. These sparse arrays exhibit low peak side-lobe levels without the presence of grating lobes over wide interelement spacings. By combining phase compensation with optimized sparse aircraft formations, one can achieve high radiation pattern resolution in a micro-UAV based radar imaging application. [ABSTRACT FROM PUBLISHER]

A parallelized real-valued clonal selection algorithm (CLONALG) is successfully implemented in this paper utilizing message passing interface (MPI) to reduce the computational burden of a large clone pool. CLONALG is one of the many branches of Artificial Immune System (AIS) algorithms with unique inherent properties that make it a very efficient optimization techniques for multimodal problems such as the ones commonly encountered in computational electromagnetic design. As a demonstration of its effectiveness, a numerical study is carried out with known benchmark functions along with the optimization of multi-layered frequency selective surface (FSS) filters in the X-band. Our results show that the CLONALG can consistently outperform a standard GA implementation particularly in multi-modal optimization problems. [ABSTRACT FROM AUTHOR]

A multi-objective Pareto genetic algorithm design methodology is applied to thinned planar arrays to simultaneously minimize peak side-lobe levels and target an elliptical mainbeam with specific minimum and maximum half-power beamwidths. This new radiation pattern synthesis technique for thinned planar arrays provides antenna engineers with a set of tradeoffs between low side-lobe levels and close adherence to mainbeam design objectives (i.e., the specified half-power beamwidths corresponding to the major and minor axes of an elliptical mainbeam). One Pareto optimization example is presented for a thinned low side-lobe planar array with a desired minimum and maximum beamwidth of 8.4^\circ and 12^\circ respectively. Two designs from the Pareto front are discussed, one with a -20.92~dB side-lobe level and beamwidths between 11.5^\circ and 7.5^\circ and a second with a -18.97~dB side-lobe level with beamwidths between 12^\circ and 7.93^\circ. [ABSTRACT FROM AUTHOR]

A new method of optimization recently made popular in the evolutionary computation (EC) community is introduced and applied to several electromagnetics design problems. First, a functional overview of the covariance matrix adaptation evolutionary strategy (CMA-ES) is provided. Then, CMA-ES is critiqued alongside a conventional particle swarm optimization (PSO) algorithm via the design of a wideband stacked-patch antenna. Finally, the two algorithms are employed for the design of small to moderate size aperiodic ultrawideband antenna array layouts (up to 100 elements). The results of the two electromagnetics design problems illustrate the ability of CMA-ES to provide a robust, fast and user-friendly alternative to more conventional optimization strategies such as PSO. Moreover, the ultrawideband array designs that were created using CMA-ES are seen to exhibit performances surpassing the best examples that have been reported in recent literature. [ABSTRACT FROM PUBLISHER]