Hossein Mosallaei

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.

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.

A substrate for small patch antennas providing tunable miniaturization factors

Magnetic properties were imparted to a naturally nonmagnetic material by metallic inclusions. A patch antenna tested the performance of the magnetic metamaterial as a substrate and validated that a single substrate can achieve a range of miniaturization values. The effective medium metamaterial substrate employed electromagnetically small embedded circuits (ECs) to achieve permeability and permittivity greater than that of the host dielectric. Geometric control of the ECs allowed mu and epsi to be tailored to the application. The magnetic metamaterial exhibited enhanced mu and epsi with acceptable loss-factor levels. Models for predicting mu and epsi are presented, the benefits of employing metamaterial substrates are discussed, and the results in this antenna experiment are presented. The metamaterial exhibits performance characteristics not achievable from natural materials. Of particular significance is that with the permeability varying strongly and predictably with frequency, the miniaturization factor may be selected by tuning the operating frequency. Simulations indicate that such performance can be extended to several gigahertz with current technology. Relative permeability values in the mur=1-5 range are achievable for moderately low-loss applications. Representative antenna miniaturization factors on the order of 4-7 over a moderate (approximately 10%) transmission bandwidth and efficiencies in a moderate range (20%-35%) are demonstrated with the possibility of higher efficiencies indicated

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.

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 (<lambda/20) and constitute artificial material molecules. Geometric control of the embedded-circuits allows epsiv and mu to be tailored to the application. A transmission line circuit model analogy is developed to theoretically investigate the behavior of designed embedded-circuit metamaterial and predict its physical parameters. In addition, a full wave analysis based on finite difference time domain (FDTD) technique is applied to comprehensively characterize the complex periodic structure. The potential advantage of magneto-dielectric metasubstrate for the design of small antennas having relatively wide bandwidth is investigated

Birefringent reflectarray metasurface for beam engineering in infrared

An infrared reflectarray metasurface with engineered birefringent behavior is demonstrated. The array reradiates incoming light into two orthogonal, linearly polarized reflections. The reflectarray is composed of rectangular metallic patch nanoantennas placed on top of a grounded dielectric stand-off layer. The patches are designed to locally manipulate the phase front of the incoming wave. They tailor the reflection phase to transform the phase front on the surface to the one desired for both orthogonal polarizations at the same time. The proposed nanoantenna metasurface can find applications in many optical devices, such as birefringent modulators, waveplates, polarizers, and splitters.

Nonuniform Luneburg and two-shell lens antennas: radiation characteristics 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 Greens 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.

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

Wave manipulation with designer dielectric metasurfaces

The concept of an ultra-thin metasurface made of single layer of only-dielectric disks for successful phase control over a full range is demonstrated. Conduction loss is avoided compared to its plasmonic counterpart. The interaction of the Mie resonances of the first two modes of the dielectric particles, magnetic and electric dipoles, is tailored by the dimensions of the disks, providing required phase shift for the transmitted beam from 0° to 360°, together with high transmission efficiency. The successful performance of a beam-tilting array and a large-scale lens functioning at 195 THz demonstrates the ability of the dielectric metasurface that is thin and has also high efficiency of more than 80%. Such configurations can serve as outstanding alternatives for plasmonic metasurfaces especially that it can be a scalable design.

Periodic bandgap and effective dielectric materials in electromagnetics: characterization and applications in nanocavities and waveguides

The main objectives of this paper are to characterize and develop insight into the performance of photonic bandgap (PBG) periodic dielectric materials and to integrate the results into some novel applications. A powerful computational engine utilizing the finite-difference time-domain technique with periodic boundary conditions/perfectly matched layers integrated with Pronys method is applied to provide an in-depth look at the physics of PBG/periodic bandgap structures. Next, the results are incorporated into two classes of applications in the areas of nanocavity lasers and guidance of electromagnetic (EM) waves in sharp bends. A two-dimensional PBG structure with finite thickness is presented to strongly localize the EM waves in three directions and design a high-Q nanocavity laser. It is shown that the periodic PBG/total internal reflections remarkably trap the EM waves inside the defect region. The effect of the number of periodic cells and defects dielectric constant on the Q of structure is investigated. It has been found that a seven-layer PBG with a dielectric impurity defect can be used in the design of a laser with a Q as high as 1050. Additionally, potential applications of the PBG structures for guiding the EM waves in sharp bends, namely, 90/spl deg/ and 60/spl deg/ channels are demonstrated. It is shown that shaping the bend by introducing small holes can noticeably improve the guidance of the waves at the bends and channel the EM waves with great efficiency. A comparative study between PBG and effective dielectric materials in controlling the EM waves is also provided and it is observed that the novel characteristics of the PBG cannot be modeled using the effective material for the frequencies within the bandgap.