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Ultrathin and two-dimensional (2D) metals can support strong plasmons, with concomitant tight field confinement and large field enhancement. Accordingly, 2D-metal nanostructures exhibiting plasmonic resonances are highly sensitive to the environment and intrinsically suitable for optical sensing. Here, based on a proof-of-concept numerical study, nano-engineered ultrathin 2D-metal films that support infrared plasmons are demonstrated to enable highly responsive refractive index (RI) sensing. For 3 nm-Au nanoribbons exhibiting plasmonic resonances at wavelengths around 1600 nm, a RI sensitivity of SRI > 650 nm per refractive index unit (RIU) is observed for a 100 nm-thick analyte layer. A parametric study of the 2D-Au system indicates the strong dependence of the RI sensitivity on the 2D-metal thickness. Furthermore, for an analyte layer as thin as 1 nm, a RI sensitivity up to 110 (90 nm/RIU) is observed in atomically thin 2D-In (2D-Ga) nanoribbons exhibiting highly localized plasmonic resonances at mid-infrared wavelengths. Our results not only reveal the extraordinary sensing characteristics of 2D-metal systems but also provide insight into the development of 2D-metal-based plasmonic devices for enhanced IR detection. [ABSTRACT FROM AUTHOR]

Circular loop antennas have received considerable attention over the years due to their wide range of practical applications. However, in comparison, surprisingly little research work has been devoted to the study of elliptical loop antennas, which provide designers with an additional degree of freedom to control their radiation properties. Here, a complete set of analytical expressions is derived for the vector potential and corresponding far-zone electromagnetic fields of a thin elliptical loop antenna of an arbitrary size based on a general Fourier cosine series representation of the current distribution. Furthermore, several important special cases of the general theory are considered including elliptical loop antennas with a constant and a cosinusoidal current distribution. It is also shown that for the special case when the lengths of the semimajor and semiminor axes are equivalent, the general expressions for the far-zone electromagnetic fields will reduce to well-known results for a circular loop. Finally, some example radiation pattern plots for elliptical loop antennas are generated using the exact theoretical expressions derived in this paper. [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]

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]

The design and characterization of a compact, low-profile metasurface-enabled antenna with tunable frequency and polarization characteristics is reported. This antenna is made possible by a tunable artificial magnetic conducting (AMC) substrate combined with tunable crossed end-loaded dipoles. Intertwined miniaturized periodic unit cells comprise the AMC, presenting a homogeneous interface to the dipoles above. Moreover, the AMC structure is predominantly air, making it very lightweight. Tunable capacitors on the dipoles allow the dynamic reconfiguration of their input impedances, which in turn allows for near-arbitrary dynamic adjusting of the antenna’s polarization for the left- or right-hand circular as well as linear with various orientations. Measurements of a prototype showed comparable performance with commercially available alternatives that were more than three times as thick as the new design reported here. [ABSTRACT FROM AUTHOR]

A heuristic approach is presented to quantify the broadband isolation performance of nonuniform capacitively loaded electromagnetic bandgap (EBG) surfaces embedded inside a parallel plate waveguide (PPW). The approach is based on the effective mapping of the stop-band properties of the EBG to the attenuation factor (AF) of a homogeneous material slab. Subsequently, it is demonstrated that the electromagnetic wave propagation through the EBG-loaded PPW is equivalent to the propagation through a multilayered medium where each layer is characterized by a different AF. When these AFs are engineered so that their nonzero values spectrally overlap, a cumulative broadband attenuation effect can be obtained. Based on this interpretation, the broad bandgap properties that are characteristic of nonuniform capacitively loaded EBG surfaces are fully quantified. [ABSTRACT FROM PUBLISHER]

A compact circularly polarized (CP) co-designed filtering antenna is reported. The device is based on a patch radiator seamlessly integrated with a bandpass filter composed of coupled stripline open-loop resonators, which are designed together as a system. In the proposed design, the patch functions simultaneously as the radiator and the last stage resonator of the filter, resulting in a low-profile integrated radiating and filtering module with a small overall form factor of \mathbf0.53\lambda _0 \times 0.53\lambda _0 \times 0.07\lambda _0. It is shown that the filtering circuit not only ensures frequency selectivity but also provides impedance matching functionality, which serves to broaden both the impedance and axial ratio bandwidths. The designed filtering antenna was fabricated and measured, experimentally achieving an $\mathbf{{S_{11}} < - 13.5\;dB}$, an axial ratio of less than 3 dB and a gain higher than 5.2 dBi over a bandwidth from 3.77 to 4.26 GHz, i.e., around 12.2%, which makes it an excellent candidate for integration into a variety of wireless systems. A linearly polarized version of the integrated filtering antenna was also demonstrated. In addition, further full-wave simulations and experiments were carried out to verify that the designed CP filtering antenna maintains its properties even when mounted on different positions of the human body with various body gestures. The stable impedance and radiation properties also make it a suitable candidate as a wearable antenna for off-body wireless communications. [ABSTRACT FROM PUBLISHER]

Spatial tuning of a volumetric metamaterial introduces many new design considerations that strongly affect the performance and practicality of a resulting system implementation. This paper discusses the design rules and procedures for spatially reconfigurable metamaterial systems using a tunable near-zero-index slab with adjustable shape as a representative design example. We propose a split-ring resonator-based metamaterial, where each resonator is tuned in and out of resonance using a varactor diode to change the effective material response. The periodic, hexagonal tiles may be combined and connected in a planar spiral to form a cylindrical metamaterial panel whose radius may be arbitrarily large. Multiple panels are then stacked, with appropriate cropping, to form a volumetric metamaterial slab. [ABSTRACT FROM PUBLISHER]

Chiral metamaterials have attracted significant attention in recent years due to their unique properties in both the microwave and optics regimes. However, most existing works rely on engineering the desired properties of spatial interfaces, and the concept of time-varying chirality has only recently begun to be explored. As a consequence, there is currently a lack of suitable tools for evaluating the propagation of electromagnetic (EM) waves in time-varying chiral media. As such, this paper presents the theoretical formulation for temporal reflection and transmission of EM waves in time-varying chiral media using the temporal transfer matrix method (TTMM). The developed TTMM tool will then be utilized to explore several application examples including a structure with an arbitrary temporal profile, optical activity generated from time interfaces, and anti-reflection temporal coatings. In addition, we employ a specialized finite-difference time-domain (FDTD) technique, known as BI-FDTD, in all the scenarios to validate the proposed theory. [ABSTRACT FROM AUTHOR]

Chiral nanostructures have immense potential for complete polarization control of light. Given the intrinsic 3D structure of circularly polarized light, nanoarchitectures with structural chirality in 3D space are usually required to achieve "full" chiroptical response. On the other hand, due to the lack of available active materials at optical wavelengths, examples of chiral photonic devices that can be tuned in an active manner remain rare. Here, based on the large Kerr nonlinearities of silicon, the nonlinear chiroptical response from a planar Si metasurface supporting high quality (Q)‐factor guided mode resonances (GMRs) at near‐infrared wavelengths, are numerically demonstrated. In the linear regime, the optical anisotropy of the C2‐symmetric structure associated with the observed GMRs leads to a pair of chiral resonance modes, enabling pronounced cross‐polarization transmission circular dichroism and a strong handedness‐dependent field enhancement effect. Owing to the nonlinear Kerr effect, such Si metasurfaces are seen to exhibit prominent chiral‐selective optical nonlinearity. Importantly, a pronounced co‐polarization transmission circular dichroism is observed, suggesting that the circularly polarized waves in the nonlinear regime possess asymmetric Jones matrices. Dielectric chiral metasurfaces with Kerr nonlinearities can be utilized to facilitate polarization‐state modulators with simple planar structures. [ABSTRACT FROM AUTHOR]

Low-profile and light-weight coatings that offer comprehensive manipulation of the electromagnetic scattering for finite-length objects are highly desirable, but not yet achieved, for applications including camouflaging, deceptive sensing, radar cognition control, and defense security. Here, for the first time, the theory, practical design, and experimental demonstration of quasi-three-dimensional and angle-tolerant electromagnetic illusion coatings are presented which have been enabled by ultrathin single-layer functional metasurfaces. By controlling the multiple Mie scattering coefficients using the tangential and non-vanishing radial electromagnetic responses of the metasurface, the quasi-two-dimensional coating transforms the electromagnetic perception of one object to mimic that of another which has been pre-selected by the designer. The illusion coating, which is homogeneous but anisotropic, is realized using hundreds of composite electric and magnetic sub-wavelength unit cells operating at frequencies away from their resonance. Two different prototypes of the metasurface illusion coatings were fabricated and characterized, demonstrating very good camouflaging performance for finite-length dielectric as well as conducting objects within a field-of-view up to ±10° off normal. This work paves the way for practical artificially engineered material coatings with exotic and versatile scattering control capabilities that would enable a wide range of applications throughout the entire electromagnetic spectrum. [ABSTRACT FROM AUTHOR]

An efficient methodology is introduced for rapid analysis and design of three-dimensional (3-D) doubly periodic structures over a wide frequency range based on hybrid finite element boundary integral (FEBI) methods. The 3-D doubly periodic structures can be represented as nonorthogonal lattices composed of general inhomogeneous bianisotropic media with arbitrarily-shaped metallic patches. Based on Floquet theory and periodic boundary conditions, the original stated problem that involves infinite periodic structures can be converted into a single unit cell. Using the equivalence principle, the derived BI equation formulation is applied to the top and bottom surfaces of the unit cell, which results in a perfectly reflectionless boundary condition for the FE-based approach. Then, the unit cell was meshed using triangular prismatic volume elements, which provide a great deal of flexibility in modeling complex planar geometries with arbitrary shapes in the transverse direction. The adaptive integral method (AIM) was employed to accelerate the calculation of the matrix-vector product for the BI portion within the iterative solver. Furthermore, a model-based parameter estimation (MBPE) technique was proposed for the wide-band interpolation of the required impedance matrix elements in the BI part for near field components that were used in the AIM procedure. The accuracy and efficiency of the proposed hybrid algorithms are demonstrated by the presented numerical results (e.g., in comparison with analytical solutions). Several simulation results are presented to illustrate the flexibility of the proposed methods for analysis of frequency selective surfaces with arbitrarily-shaped metallic patches, bianisotropic materials, and nonorthogonal lattice configurations. [ABSTRACT FROM PUBLISHER]

Time-varying materials bring an extra degree of design freedom compared to their conventional time-invariant counterparts. However, few discussions have focused on the underlying physical difference between spatial and temporal boundaries. In this letter, we thoroughly investigate those differences from the perspective of conservation laws. By doing so, the building blocks of optics and electromagnetics such as the reflection law, Snell's law, and Fresnel's equations can be analogously derived in a temporal context, but with completely different interpretations. Furthermore, we study the unique features of temporal boundaries, such as their nonconformance to energy conservation and causality. [ABSTRACT FROM AUTHOR]

A sub-entire domain (SED) basis function method, which was first introduced for modeling large-scale finite periodic PEC structures in free space, has been extended for fast characterization of electromagnetic scattering from an electrically large planar finite periodic microstrip patch array. The microstrip array may have a nonrectangular layout and non-orthogonal lattice configurations (e.g., hexagons or quadrangles). Based on the mixed potential integral equation, and utilizing the proposed SED basis function algorithm, the original large-scale finite periodic array of microstrip patches can be efficiently simulated by decomposing it into two problems with matrix equations of small dimensions. The first is to construct the SED basis functions for the corresponding microstrip arrays with orthogonal/non-orthogonal lattices. Three kinds of the SED basis functions are constructed, including those related to the edge patch elements, the interior patch elements, and the corner patch elements. The second is to solve the system equation with significantly reduced problem dimension as compared to the original larger problem. Based on the obtained SED basis functions, the reduced matrix equation of small size can be generated by the Galerkin procedure, and solved by use of the LU (lower-upper) decomposition-based direct solver, which results in a fast solution. The accuracy and efficiency of the developed algorithms are demonstrated by numerical tests that include the scattering from several large-scale finite periodic arrays of microstrip patches with rectangular, non-orthogonal lattices. [ABSTRACT FROM PUBLISHER]

In this paper, we report an approach for achieving near-perfect low-profile electromagnetic cloaking beyond the quasi-static limit. In contrast to previous works on metasurface cloaks where only the Leontovich surface impedance boundary condition has been considered, we employ a second-order surface impedance boundary condition to account for the radial response of the proposed anisotropic metasurfaces. This radial surface polarizability tensor parameter can be tailored to eliminate the higher order scattering modes. It is shown from analytical expressions that for a moderate-sized perfect electric conducting or dielectric cylinder (∼0.4λ0 in width), near-perfect scattering reduction, i.e. >98%, can be achieved by a single anisotropic metasurface with non-vanishing radial response, which is far superior to the conventional scalar impedance surface. A practical design of the metasurface is also presented and further validated by full-wave simulations. The physical mechanism of the metasurface cloaks is investigated in detail, revealing that the radiation cancellation of the induced surface currents is responsible for the scattering reduction. Importantly, it is shown that in addition to a ‘low-visibility coating’, the metasurface also functions to provide induced current enhancement which would be beneficial for the operation of a ‘cloaked sensor’. These findings will broaden the usage of metasurfaces to applications ranging from scattering reduction to noninvasive probing for objects beyond the quasi-static limit. [ABSTRACT FROM AUTHOR]

This paper presents a hybrid approach for efficient analysis of electromagnetic (EM) scattering from large-scale aperiodic structures (e.g., aperiodic Penrose and Danzer tilings), which integrates the characteristic basis function method (CBFM) and the adaptive integral method (AIM). By performing a domain decomposition, a series of characteristic basis functions (CBFs) that are defined on a macro block and comprised of a relatively large number of sub-domain basis functions facilitate a substantial reduction in the method of moments (MoM) matrix size, enabling the use of a direct solver for large problems. The AIM is applied to accelerate the calculation of CBFM-reduced MoM matrices, significantly decreasing the CPU time and memory required for solving large-scale problems. As the size of one block becomes electrically large, the original CBFM combined with the AIM is employed to generate the initial CBFs by solving a large problem with multiple excitations, which results in efficiently constructing the final CBFs afforded by the singular value decomposition (SVD) procedure. This methodology produces a two-level “CBFM + AIM” hybrid algorithm for efficiently characterizing large-scale objects. The numerical results presented demonstrate the accuracy and efficiency of the proposed hybrid algorithm. Then, the developed solver is applied to investigate EM scattering properties of large-scale aperiodic tilings. The numerical results show that Penrose/Danzer tilings exhibit significantly improved grating lobe suppression as compared to their periodic counterparts. [ABSTRACT FROM PUBLISHER]

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]

Metamaterials provide an unprecedented ability to manipulate electromagnetic waves and are an enabling technology for new devices ranging from flat lenses that focus light beyond the diffraction limit to coatings capable of cloaking an object. Nevertheless, narrow bandwidths and high intrinsic losses arising from the resonant properties of metamaterials have raised doubts about their usefulness. New design approaches seek to turn the perceived disadvantages of dispersion into assets that enhance a device's performance. Here we employ dispersion engineering of metamaterial properties to enable specific device performance over usable bandwidths. In particular, we design metamaterials that considerably improve conventional horn antennas over greater than an octave bandwidth with negligible loss and advance the state of the art in the process. Fabrication and measurement of a metahorn confirm its broadband, low-loss performance. This example illustrates the power of clever implementation combined with dispersion engineering to bring metamaterials into their full potential for revolutionizing practical devices. [ABSTRACT FROM AUTHOR]

The Peano-Gosper space-filling curve provides an excellent framework for designing planar antenna array distributions with modular architectures and suppressed sidelobes over a relatively wide bandwidth. The curve consists of a self-avoiding path that intersects a triangular lattice and its construction is based on the iterative application of a generating curve. There exist a number of other recently discovered curves, coined generalized Gosper curves, that possess properties similar to those of the Peano-Gosper curve but are based on larger and more complex generating curves. In this paper these generalized curves will be examined as the framework for antenna array layouts. It will be shown that arrays based on these curves have a number of excellent characteristics while offering modular array configurations and sizes that are not inherent to Peano-Gosper arrays. Moreover, when combined with a simple recursive-perturbation technique, the element distributions of these arrays can be efficiently adjusted to generate designs with bandwidths that far exceed that of standard periodic- and triangular-lattice arrays. The efficacy of this technique will be demonstrated through design examples, including one that has more than a 10:1 bandwidth. Full-wave simulations of a wideband patch array will also be used to investigate the effects of mutual coupling on these array distributions. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

An improved "smart" interpolation approach known as model-based parameter estimation (MBPE) is applied to the wide-band interpolation of periodic method of moments (PMM) impedance matrices for normal and oblique incidence cases. Prior to interpolation, easy to calculate but hard to interpolate, phase terms are removed from the impedance matrices. An efficient spectral-domain PMM formulation is introduced for the accelerated analysis of frequency selective surface (FSS) problems with a large number of unknowns, employing a one dimensional O(N log N) FFT-based method to speed up the computation of matrix-vector products within the bi-conjugate gradient (BCG) iterative, solver, which is made possible by the asymmetric multilevel block-Toeplitz structure of the impedance-matrix. The MBPE interpolation algorithm provides a faster matrix fill time than the brute force method and is comparable or even faster than the 2-D FFT-based method for a large number of unknowns. It also has the advantage that it can be applied to non-uniform gridding cases. The accuracy and efficiency of the proposed techniques for large FSS problems are demonstrated by several design examples for both the normal and oblique incidence cases. We also apply this efficient analysis tool to the design of multiband single-layer FSS filters and artificial magnetic conductors (AMC) comprised of a 2-D periodic arrangement of convoluted metallic strips in the shape of a Hilbert curve. The multiband properties of the Hilbert curve FSS filters are studied for different iteration orders (i.e., different degrees of space-filling). [ABSTRACT FROM AUTHOR]

There is considerable interest in interleaving multiple phased array antennas into a single common aperture system. Current phased array antenna technology is limited to narrowband operation, leading to the appearance of grating lobes and strong mutual coupling effects when they are incorporated into the design of a common aperture system. To overcome this obstacle, a new class of arrays, called polyfractal arrays, has been introduced that possess natural wideband properties well suited for large-scale genetic algorithm optimizations. These arrays also possess recursive beamforming properties and an autopolyploidy-based chromosome expansion that can dramatically accelerate the convergence of a genetic algorithm. In addition, a robust Pareto optimization can be applied to reduce the peak sidelobe levels at several frequencies throughout the intended operating band, leading to ultrawideband antenna array designs. Because of their lack of grating lobes, these polyfractal arrays are ideal building blocks for interleaved antenna array systems. This paper develops these concepts, first creating ultrawideband array designs based on polyfractal geometries and then interleaving these designs into a common aperture system. Several examples of interleaved systems are discussed, with one two-array system possessing a peak sidelobe level of nearly -18 dB with no grating lobes over a 20:1 bandwidth with either of the component array mainbeams steered independently up to 60° from broadside. [ABSTRACT FROM AUTHOR]

The transformation optics technique is applied to design three novel devices—a wave collimator, far-zone and near-zone focusing flat optical lenses and a right-angle bend for propagating beam fields. The structures presented in this paper are all two-dimensional (2D), however, the transformation optics design methodologies can be easily extended to develop 3D versions of these optical devices. The required values of the permittivity and the permeability tensors are derived for each of the three devices considered here. Furthermore, the functional performance of each device is verified using full-wave electromagnetic simulations. A wave collimator consists of a 2D rectangular cylinder where the fields (cylindrical waves) radiated by an embedded line source emerge normal to the top and bottom planar interfaces thereby producing highly directive collimated fields. Next, a far-zone focusing lens for a 2D line source is created by transforming the equi-amplitude equiphase contour to a planar surface. It is also demonstrated that by aligning two far-zone focusing flat lenses in a back-to-back configuration, a near-zone focusing lens is obtained. Finally, a 2D square cylindrical volume is transformed into a cylinder with a fan-shaped cross section to design a right-angle bend device for propagating beam fields. [ABSTRACT FROM AUTHOR]

Antenna arrays based on aperiodic tilings have been shown to exhibit low sidelobe levels and modest bandwidths over which grating lobes are suppressed. In addition, compared to conventional periodic arrays, these arrays are naturally thinned (i.e., mean interelement spacing is greater than λ/2). The generation of these arrays involves placing array elements at the locations of the vertices of an aperiodic tiling. To obtain a realizable design, the entire array is then scaled and truncated to achieve a desired minimum element spacing and aperture size. This paper demonstrates that it is possible to greatly extend the bandwidth of these arrays by incorporating a simple perturbation scheme into the basic array generation process. The implementation of this perturbation scheme is straightforward and it lends itself well to being combined with an optimization technique such as the genetic algorithm. It is successfully used to generate arrays that have large bandwidths (peak sidelobe level ≤ -10 dB with no grating lobes) of up to a minimum element spacing of 5λ. Moreover, the flexibility of this technique will be further demonstrated by introducing a slight variation of the basic scheme that is capable of generating arrays with extremely wide bandwidths. An example will be presented for an array design that has a bandwidth corresponding to a minimum element spacing of up to 11λ. [ABSTRACT FROM AUTHOR]

The application of global optimization techniques, such as genetic algorithms, to antenna array layouts can provide versatile design methodologies for highly directive, thinned, frequency agile, and shaped-beam antenna systems. However, these methodologies have their limitations when applied to more demanding design scenarios. Global optimizations are not well equipped to handle the large number of parameters used to describe large-N antenna arrays. To overcome this difficulty, a new class of arrays was recently introduced called polyfractal arrays that possess properties well suited for the optimization of large-N arrays. Polyfractal arrays are uniformly excited with an underlying self-similar geometrical structure that leads to aperiodic element layouts. This paper expands on polyfractal array design methodologies by applying a robust Pareto optimization technique with the goal of reducing the peak sidelobe levels at several frequencies specified over a wide bandwidth. A recursive beamforming algorithm and an autopolyploidy based mutation native to polyfractal geometries are used to dramatically accelerate the genetic algorithm optimization process. This paper also demonstrates that the properties of polyfractal arrays can be exploited to create designs that possess no grating lobes and relatively low sidelobe levels over ultrawide bandwidths. The best example discussed in this paper maintains a -15.97 dB peak sidelobe level with no grating lobes from a 0.5λ, to more than a 20λ minimum spacing between elements, which corresponds to at least a 40:1 bandwidth for the array. [ABSTRACT FROM AUTHOR]

The interaction of electromagnetic waves with an arbitrarily shaped three-dimensional (3-D) homogeneous chiral object located above a lossy half space is investigated using the method of moments (MoM) via the coupled mixed potential integral equations (MPIEs). Based on the surface equivalence principle, the equivalent surface electric and magnetic currents are used to replace the homogeneous chiral target in the presence of the half space. Two coupled MPIEs are developed for the unknown equivalent surface electric and magnetic currents by utilizing the continuity condition of the tangential total electric and magnetic field components on the chiral body's surface. The well-known Galerkin procedure with Rao-Wilton-Glisson (RWG) basis functions is applied to solve this problem. The spatial domain half-space Green's functions are obtained from the corresponding spectral domain Green's functions via the discrete complex image method (DCIM) combined with the generalized-pencil of function (GPOF) technique. The reciprocity theorem is employed to calculate the far-zone scattered field. Numerical results are presented for characterizing electromagnetic scattering by a 3-D arbitrarily shaped homogenous chiral object located above a lossy half space so as to demonstrate the accuracy and efficiency of the proposed technique. [ABSTRACT FROM AUTHOR]

There has been considerable recent interest in techniques for the optimization of large-N antenna arrays. Unfortunately, the successful development of such techniques has been hindered by the large number of independent parameters that must be optimized and the complexity of the calculations needed for the electromagnetic evaluation of large-N arrays. One promising new design methodology for large-N arrays which has recently been introduced is based on properties of a subset of fractal-random arrays called polyfractal arrays. Polyfractal arrays have many embedded self-similar structures, thereby allowing very large and seemingly complex array layouts to be described with only a small set of independent parameters. In addition, by effectively utilizing the self-similarity of polyfractal arrays, a considerable reduction can be achieved in the amount of time required to evaluate the radiation patterns of large-N arrays. This paper introduces a type of nature-based design process that applies a specially formulated genetic algorithm (GA) technique to evolve optimal polyfractal array layouts. The most unique aspect of this optimization technique is a new autopolyploidy-based chromosome expansion that maximizes the efficiency of the GAs. Simple polyfractal geometries are used in the initial stage or first epoch of the optimization because the number of independent parameters is small and the computation times are relatively fast. After the optimization converges for the first epoch, more complicated descriptions of these polyfractal arrays are introduced to provide additional independent parameters for the optimizer as it progresses through later epochs of evolution. This process has been shown to be very effective in creating optimized large-N arrays, the largest example considered here being a 1616-element linear array with a -24.30-dB sidelobe level and a 0.056deg half-power beamwidth [ABSTRACT FROM PUBLISHER]

The authors describe a novel concept for reconfigurable frequency selective surfaces based on metallic dipole and cross-dipole elements connected by switches on an exposed silicon substrate. As the conductivity of silicon can be varied over a large dynamic range by photonic excitation, it represents a good candidate substrate material for producing effective switches. The genetic algorithm was used to obtain optimal performance of the switch with respect to variations of the geometric and electrical parameters of the design for desired excitation frequencies. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 109–114, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22041 [ABSTRACT FROM AUTHOR]

Open-sleeve dipoles are versatile antennas primarily because they have several design parameters that can be varied to achieve a wide range of VSWR performance and operating bandwidth. By properly adjusting their parameters, it is possible to create versions of these antennas that have either a broadband or a dual-band response. In this paper, a new variation of the conventional open-sleeve dipole antenna is introduced, which we call an end-loaded planar open-sleeve dipole (ELPOSD). This configuration provides a degree of miniaturization while retaining a comparable bandwidth in terms of the VSWR response to that of conventional open-sleeve dipoles. Several designs are presented for the conventional and end-loaded configurations that demonstrate the VSWR and miniaturization capabilities of this class of antennas. Measured results have been compared with simulations and found to be in good agreement for an end-loaded planar open-sleeve monopole. [ABSTRACT FROM AUTHOR]

For conformal phased arrays, generally the excitation amplitude of the array elements must be adjusted in order to maintain low sidelobes as the array is scanned. While the desired phase weights for maximum gain are deterministically set by the array geometry and scan angle, the representation of optimum low sidelobe amplitude weights remains an open problem. Following up on prior work using the efficiency-constrained optimization of a modified Bernstein polynomial for low sidelobe conformal array synthesis, a Bézier surface is shown to provide a good representation of the optimized amplitude weights with a reduced number of parameters, while demonstrating ϵ-constraint multi-objective optimization of conformal aperture efficiency versus sidelobe level. These results are extended to include a Bézier volume representation for the multiobjective optimization of conformal aperture efficiency versus both sidelobe level and scan angle. [ABSTRACT FROM AUTHOR]

In this paper, micron-scale frequency selective surfaces (FSS) are presented for the first time that exhibit multiple strong stopbands (>10dB) in the far-infrared (IR). Fractal and genetic algorithm (GA) synthesis techniques are employed in the design of single-layer, multiband IR FSS. These designs have been fabricated on thin, flexible polyimide substrates and characterized using Fourier transform infrared (FTIR) spectroscopy. Measurements show excellent agreement with predictions from a periodic method of moments (PMoM) analysis technique that takes into account metallic and dielectric losses. Additional design constraints were incorporated into the GA in order to guarantee that the synthesized FSS structures could be accurately fabricated. [ABSTRACT FROM AUTHOR]

A model-based parameter estimation (MBPE) technique is introduced in this paper for efficiently interpolating periodic moment method (PMM) impedance matrices over a wide frequency band. In the model, only the Floquet harmonics that strongly affect the frequency band of interest are employed to approximate the matrix elements, while the contributions from all other higher-order harmonics are compactly represented by two additional terms. The derivation of the model is physics-based, and the objective is to find the coefficients of the terms in the model by utilizing the values of the impedance matrix elements calculated via PMM at only a few frequency points. The number and position of these fitting points can be pre-determined from the cutoff frequencies of the Floquet harmonics, which allows the MBPE interpolation process in this case to be completely automated. In other words, the number and position of the sampling points are only dictated by the periodicity of the frequency selective surface (FSS) structure and the frequency range of interest. Unlike many of the other scattering parameter-based techniques, the shape and the resonances in the response of the FSS do not have any impact on the construction of the interpolation model. This makes it particularly useful in genetic algorithm (GA) aided FSS design, since for a fixed periodicity and frequency range the MBPE interpolation is independent of the scattering response of candidate FSS designs. Several examples of the new PMM-MBPE approach are presented including one in which it is used to considerably speed up the GA-based design process for a reconfigurable FSS. [ABSTRACT FROM AUTHOR]

Recently, in order to successfully combine the positive attributes of both periodic and random arrays into one design, a novel class of arrays, known as fractal-random arrays, has been introduced. In addition, several researchers have successfully used genetic algorithms, robust global optimization techniques based on natural selection, to find solutions to complex array layout problems. This paper introduces a type of nature-based design process that applies a specially formulated genetic algorithm to evolve optimal layouts of an important subset of fractal-random arrays, which we call polyfractal arrays. Also, this paper discusses how the underlying self-similar properties of polyfractal arrays can be exploited to increase the speed of the associated array factor calculations. This speed increase dramatically reduces the time required for the genetic algorithm to converge thereby making it possible to effectively evolve optimal array configurations which are much larger than has been previously possible. Moreover, the fractal-random properties of these polyfractal arrays are shown to provide substantially wider bandwidth performance than their conventional counterparts. Finally, several design examples of genetically optimized linear polyfractal arrays with narrow beamwidths, improved sidelobe suppression and wide bandwidths are presented. [ABSTRACT FROM AUTHOR]

As various enabling technologies advance, conformal phased arrays are finding more numerous applications. Because a conformal array is curved, new far field pattern behaviors emerge and many of the traditional linear and planar phased array synthesis methods are not valid. This paper starts by reviewing the equations for the far field of a curved phased array, and provides a generalized definition of aperture efficiency appropriate for con- formal arrays. A modified Bernstein polynomial, defined with just five parameters, is introduced which provides a flexible method to specify a variety of smooth unimodal amplitude distributions that are shown to give good sidelobe levels and aperture efficiencies. By using particle swarm optimization of the modified Bernstein polynomial parameters constrained to provide a specified aperture efficiency, a family of aperture distributions and corresponding far field patterns is produced that allows aperture efficiency to be traded for sidelobe level. [ABSTRACT FROM AUTHOR]

In this paper, a new reconfigurable frequency selective surface (RFSS) design concept is introduced. A grid of simple metallic patches interconnected by a matrix of switches is proposed as the unit cell of an RFSS. The switches are independently addressable and provide significant transmission and reflection flexibility over a large range of frequencies. This flexibility is exploited by optimizing the switch settings using a genetic algorithm to produce a desired frequency response. The versatility of the design technique is demonstrated by presenting several examples of genetically optimized RFSS. The first example to be considered is a linearly polarized FSS that can be reconfigured for either single-, dual-, or tri-band operation. An HISS design is also introduced that can be optimized to have a frequency response that is polarization independent in one state (i.e., for one combination of switch settings) and polarization dependent in another state. [ABSTRACT FROM AUTHOR]

A methodology is presented for the design synthesis of metamaterial ferrites, or metaferrites, that retain their desirable magnetic properties at frequencies above 1 GHz. The design syn- thesis is accomplished by optimizing a high impedance frequency selective surface (HZ-FSS) structure via a genetic algorithm (GA) for the desired effective permeability of an equivalent magnetic substrate backed by a perfect electric conductor ground plane. The ability to optimize the design parameters of these HZ-FSS structures allows for the possibility of synthesizing low-loss dispersive metaferrites with either a positive or a negative real part of the effective permeability at the desired operating frequency band. The results presented in this paper demonstrate five possible metaferrite designs: two with the associated real and imaginary permeabilities for use as low-loss magnetic materials, and three designs for use as absorbing materials. [ABSTRACT FROM AUTHOR]

Genetic algorithms are commonly used to solve many optimization and synthesis problems. An important issue facing the user is the selection of genetic algorithm parameters, such as mutation rate, mutation range, and number of crossovers. This paper demonstrates a real-valued genetic algorithm that simultaneously adapts several such parameters during the optimization process. This adaptive algorithm is shown to outperform its static counterparts when used to synthesize the phased array weights to satisfy specified far-field sidelobe constraints, and can perform amplitude-only, phase-only, and complex weight synthesis. When compared to conventional static parameter implementations, computation time is saved in two ways: 1) The algorithm converges faster and 2) the need to tune parameters by hand (generally done by repeatedly running the code with different parameter choices) is greatly reduced. By requiring less iteration to solve a given problem, this approach may benefit electromagnetic optimization problems with expensive cost functions, since genetic algorithms generally require many function evaluations to converge. The adaptive process also provides insight into the qualitative importance of parameters, and dynamically adjusting the mutation range is found to be especially beneficial. [ABSTRACT FROM AUTHOR]

This paper introduces several different design methodologies for multiband artificial magnetic conducting (AMC) surfaces. The paper begins by investigating the multiband properties exhibited by a conventional electromagnetic bandgap (EBG) AMC that consists of a frequency selective surface (FSS) on top of a thin dielectric substrate with a PEC back plane. The higher-order resonances associated with these surfaces have not been discussed in detail to date, as previous research has been concerned only with exploiting the primary resonant frequency. However, it will be shown that by understanding and making appropriate use of these higher order resonances, it is possible to design multiband AMC surfaces that work for nearly any desired combination of operating frequencies. The first multiband AMC design approach that will be considered is based on the introduction of FSS screens that have fractal or nearly fractal unit cell geometries. This is followed by a more general and robust genetic algorithm (GA) technique for the synthesis of optimal multiband AMC surfaces. In this case, a GA is used to evolve multiband AMC surface designs by simultaneously optimizing the geometry and size of the FSS unit cell as well as the thickness and dielectric constant of the substrate material. Finally, several examples of multiband AMC surfaces are presented, including some practical dual-band and tri-band designs genetically evolved for operation at GPS and cellular frequencies, as well as an example illustrating the success in creating a multiband AMC surface with angular stability. [ABSTRACT FROM AUTHOR]

This paper presents an extension of the recently developed finite-difference time-domain (FDTD) technique for modeling electromagnetic wave interactions with bianisotropic (BI) media, known as BI-FDTD, to include the more general class of bianisotropic materials. This new FDTD formulation is called BA-FDTD. The theoretical foundation for this method is based on a wave field decomposition technique. The formulations based on the application of this wave field decomposition technique to BA media will be presented. Validations of this new model are demonstrated for the interaction of an electromagnetic wave propagating transversely through a uniaxial BA half-space. [ABSTRACT FROM AUTHOR]

Chiral media engineered for applications at microwave frequencies can be described as metamaterials composed of randomly oriented helices (with sizes typically less than a wavelength) embedded within an achiral background that is characterized by its permittivity and permeability. Chiral metamaterials embody properties of magnetoelectric coupling and polarization rotation. Chiral media are also highly dispersive and no effective full-wave time domain formulation has been available to simulate transient propagation through such an important class of metamaterials. A new finite-difference time-domain (FDTD) technique is introduced in this paper to model the interaction of an electromagnetic wave with isotropic dispersive chiral metamaterials, based on the implementation of a wavefield decomposition technique in conjunction with the piecewise-linear recursive convolution method. This formulation represents the first of its kind in the FDTD community. The FDTD model is validated by considering a one-dimensional example and comparing the simulations with available analytical results. Moreover, the FDTD technique is also used to investigate the propagation of electromagnetic waves through multilayered metamaterial slabs that include dispersive chiral and double-negative media. Hence, this model enables the investigation of complex dispersive metamaterials with magnetoelectric coupling and double-negative behavior as well as facilitates the exploitation of their unique properties for a variety of possible applications. [ABSTRACT FROM AUTHOR]

In this paper, a new class of antenna arrays are introduced, which we call fractile arrays. A fractile array is defined as any array with a fractal boundary contour that tiles the plane without gaps or overlaps. It will be shown that the unique geometrical features of fractiles may be exploited in order to make available a family of deterministic arrays that offer several highly desirable performance advantages over their conventional pen odic planar array counterparts. Most notably, fractile arrays have no grating lobes even when the minimum spacing between elements is increased to at least one-wavelength. This has led to the development of a new design methodology for modular broadband low-sidelobe arrays that is based on fractal tilings. Several examples of fractile arrays will he considered including Peano-Gosper, terdragon, 6-terdragon, and fudgeflake arrays. Efficient iterative procedures for calculating the radiation patterns of these fractile arrays to arbitrary stage of growth P are also introduced in this paper. [ABSTRACT FROM AUTHOR]

This paper introduces a design methodology for miniature multiband as well as reconfigurable (i.e., tunable) antennas that exploits the self-similar branching structure of three-dimensional (3-D) fractal trees. Several fundamental relationships, useful for design purposes, are established between the geometrical structure of the fractal tree antenna and its corresponding radiation characteristics, in particular, it will be shown that the density and elevation angle of the branches play a key role in the effective design of miniature 3-D fractal tree antennas. Several design examples are considered where fractal trees are used as end-loads in order to miniaturize conventional dipole or monopole antennas. Multiband and reconfigurable versions of these miniature antennas are also proposed, where either reactive LC traps or RF switches are strategically placed throughout the branches and/or along the trunk of the trees. Included among these designs is a miniature reconligurable dipole antenna that achieves a 57% size reduction for the center frequency of the lowest intended band of operation and has a tunable bandwidth of nearly 70%. [ABSTRACT FROM AUTHOR]

This paper presents a newly developed finite-difference time-domain (FDTD) technique, referred to as BI-FDTD, for modeling electromagnetic wave interactions with bi-isotropic (BI) media. The theoretical foundation for the BI-FDTD method will be developed based on a wavefield decomposition. The main advantage of this approach is that the two sets of wavefields are uncoupled and can be viewed as propagating in an equivalent isotropic medium, which makes it possible to readily apply conventional FDTD analysis techniques. The BI-FDTD scheme will also he extended to include the dispersive nature of chiral media, an important subclass of bi-isotropic media. This extension represents the first of its kind in the FDTD community. Validations of this new model are demonstrated for a chiral half-space and a chiral slab. [ABSTRACT FROM AUTHOR]

The self-similar geometrical properties of fractal arrays are exploited in this paper to develop fast recursive algorithms for efficient evaluation of the associated impedance matrices as well as driving point impedances. The methodo1ogy is demonstrated by considering two types of uniformly excited fractal arrays consisting of side-by-side half-wave dipole antenna elements. These examples include a triadic Cantor linear fractal array and a Sierpinski carpet planar fractal array. This class of self-similar antenna arrays become significantly large at higher order stages of growth and utilization of fractal analysis allows the impedance matrix, and hence the driving point impedance, to be obtained much more efficiently than would be possible using conventional analysis techniques. [ABSTRACT FROM AUTHOR]

A methodology is presented for the design synthesis of metamaterials that act as thin multifrequency artificial magnetic conductors. These structures are realized by placing a frequency-selective surface above a conventional prefect electric conductor, separated by a thin dielectric layer. The frequency-selective surface design is optimized using a micro-genetic algorithm to operate at multiple, narrow frequency bands. Two examples of genetically engineered multiband high-impedance frequency-selective surfaces (that is, artificial magnetic conductors) are presented and discussed. © 2003 Wiley Periodicals, Inc. Microwave Opt Technol Lett 38: 400–403, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.11073 [ABSTRACT FROM AUTHOR]

This paper investigates the radiation characteristics of a new type of array that is based on the family of space-filling and self-avoiding fractals known as Peano-Gosper curves. The elements of the fractal array are uniformly distributed along a Peano-Gosper curve, which leads to a planar array configuration with parallelogram cells that is bounded by a closed Koch curve. These unique properties are exploited in order to develop a design methodology for deterministic arrays that have no grating lobes even when the minimum spacing between elements is increased to at least one wavelength. This leads to a class of arrays that are relatively broad-band when compared to more conventional periodic planar arrays with square or rectangular cells and regular boundary contours. An efficient iterative procedure for calculating the radiation patterns of these Peano-Gosper fractal arrays to arbitrary stage of growth P is also introduced in this paper. [ABSTRACT FROM AUTHOR]

Techniques previously reported for interpolating antenna radiation patterns simultaneously in both the spatial and frequency domains are reexamined here in order to develop new algorithms with improved efficiency and speed while maintaining comparable accuracy. To this end, interpolation in the frequency domain is constrained to a windowed scheme whereby a series of reduced-order Padé rational function fitting models of only three or four sampling frequencies are used to interpolate over a relatively large bandwidth. The use of this piece-wise technique is shown to reduce the complexity of determining the unknown coefficients without any significant loss in the resulting interpolation accuracy or increase in the required number of sampling frequencies. In this paper, exact analytical expressions are found for the unknown coefficients that allow the spatial domain interpolation to be performed entirely separate from the frequency domain interpolation. The technique is applied to the case of a 0.5 meter dipole modeled from 150 to 950 MHz, and for Θ from 0° to 90°, using fitting windows with three and four sampling frequencies. The results are compared to those obtained using a previously developed generalized simultaneous interpolation procedure. In the spatial domain, spherical harmonics are introduced as model-based fitting functions for antenna radiation patterns. Specifically, interpolation procedures are developed using zonal harmonics and tesseral harmonics to model one- and two-dimensional far-field radiation patterns, respectively. The case of a z-oriented 0.5 meter dipole is considered to demonstrate the efficiency and accuracy obtained by applying these physically-based fitting models in the spatial domain. Zonal harmonics are used to interpolate far-field radiation patterns of this antenna for Θ from 0° to 180° for several different frequencies over the range from 150 to 950 MHz. This example demonstrates the attractiveness of using spherical harmonics functions to accurately and efficiently interpolate radiation patterns in the spatial domain. [ABSTRACT FROM AUTHOR]

A novel method for designing reconfigurable multiband linear and planar antenna arrays is presented in this paper. The technique is based on a generalized Fourier series synthesis approach that exploits the self-similarity of a specified fractal radiation pattern in order to achieve the desired multiband performance. The fractal radiation patterns are composed of scaled and shifted copies of an appropriately chosen generating window function that exhibits low sidelobe levels and rapid spectral rolloffs in the transform domain. A newly developed thinning algorithm will be presented which may be employed to considerably reduce both the overall physical size as well as the total number of elements in a synthesized multiband array. Finally, a band-switching scheme is introduced that is well-suited for implementation in the form of a reconfigurable common aperture array. [ABSTRACT FROM AUTHOR]

A domain-decomposition/reciprocity procedure is presented which allows the radiation patterns of microstrip patch antennas mounted on arbitrarily-shaped three-dimensional perfectly electric conducting (PEC) platforms to be computed accurately as well as efficiently. The utility of this technique is demonstrated by considering an example consisting of a nine-element conformal array of microstrip patch antennas mounted axially along a finite-length PEC circular cylinder. It is shown that the elements close to the ends of the cylinder have significantly different patterns than those close to the center of the cylinder. The results from this example suggest that the common practice where all the individual element patterns are assumed identical is not always valid and, in fact, can lead to significant performance degradation in the design of conformal phased arrays. This observation is supported by the fact that an attempt to steer the main beam of the nine-element conformal array to an angle θ[sub 0] = 60° using a standard uniform progressive phase shifting technique proves unsuccessful. Next a genetic algorithm (GA) synthesis procedure is introduced that is capable of determining the optimal set of element excitation phases required to yield a desired or specified far-field radiation pattern. The results of this GA phase-only optimization are shown to yield the desired main beam steered to the correct angle for this nine-element linear array mounted on a circularly cylindrical platform. The GA radiation pattern synthesis procedure introduced appears to be a highly effective means of correcting for platform effects on the individual element patterns of a conformal phased array. [ABSTRACT FROM AUTHOR]

This paper presents a convenient and efficient approach, based on the reciprocity theorem, for calculating the radiation patterns of arbitrarily shaped microstrip antennas with dielectric substrate and superstrate layers mounted on circularly cylindrical platforms. A detailed theoretical development followed by examples with numerical and graphical results illustrate the versatility of the technique. The reciprocity approach presented is very flexible and may be used in conjunction with any of the commonly employed computational electromagnetics modeling approaches such as the method of moments, finite-element methods, and finite-difference time-domain techniques. [ABSTRACT FROM AUTHOR]