Compact slot and dielectric resonator antenna with dual-resonance, broadband characteristics

The goal of this study is to improve the bandwidth of a miniaturized antenna. The proposed technique combines a slot antenna and a dielectric resonator antenna (DRA) to effectively double the available bandwidth without compromising miniaturization or efficiency. With proper design it is observed that the resonance of the slot and that of the dielectric structure itself may be merged to achieve extremely wide bandwidth over which the antenna polarization and radiation pattern are preserved. In addition, using the DRA, a volumetric source, improves the radiation power factor of the radiating slot. A miniaturized antenna figure of merit (MAFM) is defined to simultaneously quantify aspects of miniaturized antenna performance including the degree of miniaturization, efficiency, and bandwidth. Figures for various common types of antennas are given and compared with that of the proposed structures. In order to determine the effects of varying design parameters on bandwidth and matching, sensitivity analysis is carried out using the finite-difference time-domain method. Numerous designs for miniaturized slot-fed dielectric resonator antennas are simulated and bandwidths exceeding 25% are achieved. Two 2.4 GHz antennas are built, characterized, and the results compared with theory.     

Antenna miniaturization and bandwidth enhancement using a reactive impedance substrate

The concept of a novel reactive impedance surface (RIS) as a substrate for planar antennas, that can miniaturize the size and significantly enhance both the bandwidth and the radiation characteristics of an antenna is introduced. Using the exact image formulation for the fields of elementary sources above impedance surfaces, it is shown that a purely reactive impedance plane with a specific surface reactance can minimize the interaction between the elementary source and its image in the RIS substrate. An RIS can be tuned anywhere between perfectly electric and magnetic conductor (PEC and PMC) surfaces offering a property to achieve the optimal bandwidth and miniaturization factor. It is demonstrated that RIS can provide performance superior to PMC when used as substrate for antennas. The RIS substrate is designed utilizing two-dimensional periodic printed metallic patches on a metal-backed high dielectric material. A simplified circuit model describing the physical phenomenon of the periodic surface is developed for simple analysis and design of the RIS substrate. Also a finite-difference time-domain (FDTD) full-wave analysis in conjunction with periodic boundary conditions and perfectly matched layer walls is applied to provide comprehensive study and analysis of complex antennas on such substrates. Examples of different planar antennas including dipole and patch antennas on RIS are considered, and their characteristics are compared with those obtained from the same antennas over PEC and PMC. The simulations compare very well with measured results obtained from a prototype /spl lambda//10 miniaturized patch antenna fabricated on an RIS substrate. This antenna shows measured relative bandwidth, gain, and radiation efficiency of BW=6.7, G=4.5 dBi, and e/sub r/=90, respectively, which constitutes the highest bandwidth, gain, and efficiency for such a small size thin planar antenna.     

Design and Modeling of Patch Antenna Printed on Magneto-Dielectric Embedded-Circuit Metasubstrate

The design and modeling of an embedded-circuit metamaterial with epsi-mu constitutive parameters as the substrate for patch antennas is presented. The magneto-dielectric metasubstrate is constructed of periodic resonant loop circuits embedded in a low dielectric host medium, and is capable of providing both permittivity and permeability material parameters at any frequency of interest. The embedded-circuit building blocks are very small in size (相似文献   

Hybrid FDTD and single-scattering theory for simulation of scattering from hard targets camouflaged under forest canopy

A hybrid target-foliage model is developed to investigate the scattering behavior of hard targets embedded inside a forest canopy. The proposed model is composed of two existing electromagnetic-scattering models, one for the foliage and the other for the hard targets that are coupled in a computationally efficient manner. The connection between these two models, which accounts for the interaction between the foliage scatterers and the target, is accomplished through the application of Huygens' principle. Wave penetration through the forest canopy and near-field and far-field scattering from its constituents is calculated using a coherent single-scattering theory, which makes use of realistic tree structures. Defining a Huygens' surface enclosing the hard target and calculating the illuminating field (the scattered fields from the nearby vegetation scatterers and reduced incident field), the interaction between the foliage and the hard target is accounted for. Computing the scattered field from target on the Huygens' surface and using a reciprocity theorem target-foliage interaction is captured very efficiently. Calculation of scattering from a hard target is carried out using a finite-difference time-domain (FDTD) technique. For a typical vehicle dimensions, the required time and memory for the FDTD computation and exact field calculation inside the foliage limits the simulation frequency to upper very high frequency (VHF) band.     

RCS reduction of canonical targets using genetic algorithmsynthesized RAM

Radar cross section (RCS) reduction of canonical (planar, cylindrical, and spherical) conducting targets is the focus of this paper. In particular, a novel procedure is presented for synthesizing radar absorbing materials (RAM) for RCS reduction in a wide-band frequency range. The modal solutions of Maxwell's equations for the multilayered planar, cylindrical, and spherical canonical structures is integrated into a genetic algorithm (GA) optimization technique to obtain the best optimal composite coating. It Is shown that by using an optimal RAM, the RCS of these canonical structures can be significantly reduced. Characteristics of bistatic RCS of coated cylindrical and spherical structures are also studied and compared with the conducting structures without coating. It is shown that no optimal coating can be found to reduce the RCS in the deep shadow region. An in-depth study has been performed to evaluate the potential usage of the optimal planar coating as applied to the curved surfaces. It is observed that the optimal planar coating can noticeably reduce the RCS of the spherical structure. This observation was essential in introducing a novel efficient GA with hybrid planar/curved surface implementation using as part of its initial generation the best population obtained for the planar RAM design. These results suggest that the optimal RAM for a surface with arbitrary curvature may be efficiently determined by applying the GA with hybrid planar/curved surface population initialization     

Magneto-dielectrics in electromagnetics: concept and applications

In this paper, the unique features of periodic magneto-dielectric meta-materials in electromagnetics are addressed. These materials, which are arranged in periodic configurations, are applied for the design of novel EM structures with applications in the VHF-UHF bands. The utility of these materials is demonstrated by considering two challenging problems, namely, design of miniaturized electromagnetic band-gap (EBG) structures and antennas in the VHF-UHF bands. A woodpile EBG made up of magneto-dielectric material is proposed. It is shown that the magneto-dielectric woodpile not only exhibits band-gap rejection values much higher than the ordinary dielectric woodpile, but also for the same physical dimensions it shows a rejection band at a much lower frequency. The higher rejection is a result of higher effective impedance contrasts between consecutive layers of the magneto-dielectric woodpile structure. Composite magneto-dielectrics are also shown to provide certain advantages when used as substrates for planar antennas. These substrates are used to miniaturize antennas while maintaining a relatively high bandwidth and efficiency. An artificial anisotropic meta-substrate having /spl mu//sub r/>/spl epsiv//sub r/, made up of layered magneto-dielectric and dielectric materials is designed to maximize the bandwidth of a miniaturized patch antenna. Analytical and numerical approaches, based on the anisotropic effective medium theory (AEMT) and the finite-difference time-domain (FDTD) technique, are applied to carry out the analyzes and fully characterize the performance of finite and infinite periodic magneto-dielectric meta-materials integrated into the EBG and antenna designs.     

Electronically tunable magnetic patch antennas with metal magnetic films

AS new type of electronically tunable magnetic patch antennas with metal magnetic films was designed, fabricated, and tested at 2.1 GHz. The magnetic patch antennas showed an enhanced bandwidth of 50% over the non-magnetic patch antennas, a significantly enhanced directivity, and a large tunability of the radiation intensity of 4.23 dB at a low applied magnetic field of ~20 Oe     

Metamaterial Insulator Enabled Superdirective Array

Metamaterial EM insulators are shown to suppress mutual coupling between densely packed array elements. This technique allows for array element design in isolation, without consideration of adjacent elements and mutual coupling effects. Suppressing mutual coupling allows for denser packing and enhanced directivity in antenna arrays approaching the superdirective theoretical maximums. Metamaterial isolation walls 0.05lambda₀ thick exhibit 20 dB peak isolations with 10 dB isolation bandwidths of 2-6% and very low losses. These insulators achieve lower than -30 dB coupling levels for 0.2lambda₀ periodicity arrays. A simulated 1.18lambda₀ five element broadside array exhibits a superdirective 65 degree first-null beamwidth with -9.5 dB side-lobes and when physically fabricated, experiment validates theory with a squinted 75 degree first-null beamwidth and only slight degradation of sidelobe levels. The main beam of the metamaterial insulated array is steerable over a full +/-90 degree horizon with little scan loss and no instances of scan blindness, a null can be placed to any angle including broadside, and as narrow as 25 degree peak-to-null separation is possible     

Coupled Dielectric Nanoparticles Manipulating Metamaterials Optical Characteristics

In this paper, we investigate the concept and theory of all-dielectric metapatterned structures that manipulate electric and magnetic optical characteristics. A 3-D array of dielectric particles is designed, where the spheres operate in their magnetic modes and their couplings offer electric modes. An analytical solution for the problem of plane wave scattering by 3-D array of dielectric nanospheres is presented. FW multipole expansion method is applied to express the optical fields in terms of the electric and magnetic dipole modes and the higher order moments. By enforcing the boundary conditions at the surface of each sphere, with the use of the translational addition theorem for vector spherical wave functions, required equations to determine the scattering coefficients are obtained. Novel materials features in optics are demonstrated. Electric and magnetic scattering coefficient resonances around the same frequency band are obtained. It is highlighted how a metapatterned structure constructed from dielectric nanosphere unit cells can provide electric and magnetic modes resulting in backward wave phenomenon. A comprehensive circuit model based on the RLC (resistor, inductor, and capacitor) realization is presented to successfully analyze the scattering performance of a dielectric nanosphere. To better understand the physics of an array of spheres, circuit models for the interactions, and couplings between spheres are also accomplished. The engineered dispersion diagram for a 3-D array of identical highly coupled nanospheres is scrutinized, verifying that the high couplings between spheres can offer the backward wave characteristics.     

Nonuniform Luneburg and two-shell lens antennas: radiationcharacteristics and design optimization

Design optimization of radially nonuniform spherical lens antennas is the focus of this paper. In particular, special attention is given to the optimal design of nonuniform Luneburg (1964) lens antennas. One of the important engineering objectives of designing an optimal Luneburg lens antenna is to use as small number of shells as possible while maintaining an acceptable gain and sidelobe performance. In a typical radially uniform design, by reducing the number of shells, the gain is decreased and the grating lobes are increased. This deficiency in the radiation performance of the uniform lens antenna can be overcome by designing the nonuniform lens antenna. This necessitates the optimum selection of each layer thickness and permittivity. A genetic algorithm (GA) optimizer with adaptive cost function is implemented to obtain the optimal design. In this manner, the GA optimizer simultaneously determines the optimal material and its thickness for each shell by controlling the gain and sidelobes envelope of the radiation pattern. Various lens geometries, including air gaps and feed offset from the lens surface, are analyzed by using the dyadic Green's functions of the multilayered dielectric sphere. Many useful engineering design guidelines have been suggested for the optimum construction of the lens. The results have been satisfactory and demonstrate the utility of the GA/adaptive cost-function algorithm. Additionally, the radiation characteristics of a novel two-shell lens antenna have been studied, and its performance is compared to the Luneburg lens