Micah D. Gregory

Fast Optimization of Electromagnetic Design Problems Using the Covariance Matrix Adaptation Evolutionary Strategy

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.

A Broadband Monopole Antenna Enabled by an Ultrathin Anisotropic Metamaterial Coating

A new type of compact flexible anisotropic metamaterial (MM) coating is proposed, which greatly enhances the impedance bandwidth of a quarter-wave monopole to over an octave. The MM coating has a high effective permittivity for the tensor component oriented along the direction of the monopole. By properly choosing the radius and tensor parameter of the MM coating, another resonance at a higher frequency can be efficiently excited without affecting the fundamental mode of the monopole. Additionally, the similar current distributions on the monopole at both resonances make stable radiation patterns possible over the entire band. To experimentally verify the concept, an S-band MM coated monopole was designed, fabricated, and characterized, exhibiting a 2.14:1 bandwidth (2.15-4.6 GHz) with a VSWR of less than 2:1. The demonstrated MM coating has a radius of only λ/24 and extremely light weight, which renders it attractive for use in applications such as broadband arrays and portable wireless devices.

Broadband High Directivity Multibeam Emission Through Transformation Optics-Enabled Metamaterial Lenses

A new broadband two- and three-dimensional, polarization independent coordinate transformation is introduced that is capable of mapping the radiation from an embedded omnidirectional source into any desired number of highly directive beams pointed in arbitrary directions. This transformation requires anisotropic materials, yet is spatially invariant and thereby can be readily implemented by currently existing metamaterial technologies. Moreover, the performance of the transformation is not sensitive to small material parameter variations, thus enabling a broad operational bandwidth. To validate the concept, a broadband 3-D coordinate transformation metamaterial lens fed by a simple monopole antenna was designed, fabricated and characterized, achieving a quad-beam radiation pattern over a 1.26:1 bandwidth with approximately 6-dB realized gain improvement in the

Design and Experimental Investigation of a Compact Circularly Polarized Integrated Filtering Antenna for Wearable Biotelemetric Devices

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Ultrawideband Aperiodic Antenna Arrays Based on Optimized Raised Power Series Representations

-plane. In addition, the near-field coupling between the monopole and the lens was carefully tuned to accomplish a remarkable 70% broadening of the impedance bandwidth compared to the monopole antenna operating alone. It is also shown from the field simulations that the realized metamaterial lens provides both near-field and far-field 3-D collimating effects.

Nature-Inspired Design Techniques for Ultra-Wideband Aperiodic Antenna Arrays

A compact circularly polarized (CP) integrated filtering antenna is reported for wearable biotelemetric devices in the 2.4 GHz ISM band. The design is based on a mutual synthesis of a CP patch antenna connected to a bandpass filter composed of coupled stripline open-loop resonators, which provides an integrated low-profile radiating and filtering module with a compact form factor of 0.44λ0 × 0.44λ0 × 0.04λ0. The optimized filtering antenna is fabricated and measured, achieving an , an axial ratio of less than 3 dB and gain higher than 3.5 dBi in the targeted ISM band. With the integrated filtering functionality, the antenna exhibits good out-of-band rejection over an ultra-wide frequency range of 1-6 GHz . Further full-wave simulations and experiments were carried out, verifying that the proposed filtering antenna maintains these desirable properties even when mounted in close proximity to the human body at different positions. The stable impedance performance and the simultaneous wide axial ratio and radiated power beam widths make it an ideal candidate as a wearable antenna for off-body communications. The additional integrated filtering functionality further improves utility by greatly reducing interference and crosstalk with other existing wireless systems.

Matched Impedance Thin Planar Composite Magneto-Dielectric Metasurfaces

Past research has shown that application of mathematical and geometrical concepts such as fractals, aperiodic tilings, and special polynomials can provide elegant solutions to difficult antenna array design problems. For example, design issues such as beam shaping and control, sidelobe levels, bandwidth and many others have been addressed with such concepts. In this paper, mathematical constructs based on the raised power series (RPS) are utilized to provide easily controlled aperiodicity to a linear array of antenna elements in order to achieve wideband performance. In addition, recursive application of raised power series subarrays and implementation of an optimization technique based on the genetic algorithm is demonstrated to realize impressive ultrawideband performance. The technique introduced here is shown to offer bandwidths of many octaves with excellent sidelobe suppression and no grating lobes. Moreover, the ultrawideband performance for one of the optimized RPS array examples is verified through full-wave simulations which take into account the coupling environment experienced by realistic radiating elements (in this case half-wave dipole antennas for three different operating frequencies).

Exploiting Rotational Symmetry for the Design of Ultra-Wideband Planar Phased Array Layouts

Over the past few decades, much research has been invested in the exploration of wideband and ultra-wideband (UWB) antenna arrays. The goals of such array designs are to determine the best element arrangements, which yield radiation patterns possessing the highest degrees of side lobe suppression, and no grating lobes over the largest possible operating bandwidths. It has been recently shown that nature-inspired array-design methodologies can provide solutions that exhibit these ultra-wideband characteristics. This article provides an overview of two such designs: linear polyfractal arrays, and planar arrays of aperiodic tilings. Robust nature-inspired genetic-algorithm optimization techniques were utilized in the design of both types of arrays in order to obtain the best-possible UWB performance. This article also discusses the fabrication and experimental validation of two 32-element linear polyfractal array-design prototypes, which exhibited close agreement to the radiation performance predicted by simulation. These experimentally validated arrays possessed wide bandwidths with suppressed grating lobes and relatively low sidelobes for their size (-16.3 dB at f0 and -5.39 dB at f0)Additional simulations discussed in this paper showed that the benefits of these methodologies are amplified when applied to larger sized array designs (Le., arrays with larger element counts). One example exhibited a peak sidelobe level less than -19.34 dB over a 40:1 bandwidth.

Improved Electromagnetics Optimization: The covariance matrix adaptation evolutionary strategy.

A novel methodology is presented for the design synthesis of matched impedance thin planar composite magneto-dielectric metasurfaces. The design synthesis involves optimizing thin, metallo-dielectric metasurfaces comprised of a periodic array of electrically small and rotationally symmetric metallic unit cells which are sandwiched between two thin dielectric layers and backed by a perfectly conducting ground plane. Optimization of the structures is carried out with a genetic algorithm (GA) to obtain a design with electromagnetic properties that are equivalent to a desired matched-impedance homogeneous medium of the same thickness. Optimized design results demonstrate the effectiveness of this new technique in synthesizing thin planar composite matched-impedance magneto-dielectric metasurfaces (MIMDM). To validate the approach, full-wave simulations of the actual metamaterial structure were compared with results obtained by employing an equivalent homogeneous effective medium and found to be in excellent agreement. Several designs are optimized with targeted applications such as substrates for miniaturized patch antennas and electromagnetic absorbing materials.

Nature Inspired Optimization Techniques for Metamaterial Design

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.