Leila Yousefi

Enhanced-Gain Microstrip Antenna Using Engineered Magnetic Superstrates

This letter presents a novel engineered magnetic superstrate designed to enhance the gain and efficiency of a microstrip patch antenna without any substantial increase in the profile of the whole structure (the antenna with the superstrate). The modified split ring resonator (MSRR) inclusions are used in the design of the engineered magnetic superstrate. Numerical full-wave simulations as well as analytical models are used to analyze the entire radiating system. Considering as an example a microstrip antenna operating within the UMTS band, the broadside gain of the antenna was improved by 3.4 dB and the efficiency was improved by 17% when using the engineered superstrate. The total height of the proposed structure, antenna with superstrate, is lambda0/7, where lambda0 is the free-space wavelength at the resonance frequency of the antenna.

Enhanced Bandwidth Artificial Magnetic Ground Plane for Low-Profile Antennas

In this letter, it is shown that using magneto-dielectric materials as substrate can increase the in-phase reflection bandwidth of electromagnetic bandgap (EBG) structures. To show this, a compact wideband EBG structure is designed and simulated. The numerical results show that this EBG has an in-phase reflection bandwidth of 70% which is several times greater than a conventional EBG resonating at the same frequency. Additionally, the new EBG surface has a smaller cell size, an important feature in the design of small antennas. As a demonstration of the effectiveness of the new structure, a low-profile unidirectional spiral antenna is designed to operate from 8 to 18 GHz. The improvement in the voltage standing wave ratio (VSWR) and gain of this antenna is presented while comparison is made to the case when using conventional EBG surface.

Analytical Model for Calculating the Radiation Field of Microstrip Antennas With Artificial Magnetic Superstrates: Theory and Experiment

A fast analytical solution for the radiation field of a microstrip antenna loaded with a generalized superstrate is proposed using the cavity model of microstrip antennas in conjunction with the reciprocity theorem and the transmission line analogy. The proposed analytical formulation for the antennas far-field is much faster when compared to full-wave numerical methods. It only needs 2% of the time acquired by full-wave analysis. Therefore the proposed method can be used for design and optimization purposes. The method is verified using both numerical and experimental results. This verification is done for both conventional dielectric superstrates, and also for artificial superstrates. The analytical formulation introduced here can be extended for the case of a patch antenna embedded in a multilayered artificial dielectric structure. Arguably, the proposed analytical technique is applied for the first time for the case of a practical microstrip patch antenna working in the Universal Mobile Telecommunications System (UMTS) band and covered with a superstrate made of an artificial periodic metamaterial with dispersive permeability and permittivity.

Artificial Magnetic Materials Using Fractal Hilbert Curves

Novel configurations based on Fractal Hilbert curves are proposed for realizing artificial magnetic materials. It is shown that the proposed configuration gives significant rise to miniaturization of artificial unit cells which in turn results in higher homogeneity in the material, and reduction in the profile of the artificial substrate. Analytical formulas are proposed for design and optimization of the presented structures, and are verified through full wave numerical characterization. The electromagnetic properties of the proposed structures are studied in detail and compared to square spiral from the point of view of size reduction, maximum value of the resultant permeability, magnetic loss, and frequency dispersion. To validate the analytical model and the numerical simulation results, an artificial substrate containing second-order Fractal Hilbert curve is fabricated and experimentally characterized using a microstrip-based characterization method.

New Artificial Magnetic Materials Based on Fractal Hilbert Curves

In this paper, new artificial magnetic materials based on fractal Hilbert curves are proposed, modeled and numerically analyzed. It is shown that using fractal structures as inclusion in the design of artificial magnetic materials allows miniaturization of the unit cell size, thus enabling design of miniaturized devices such as antennas. Furthermore it is proven analytically and numerically that these new materials are less dispersive in comparison to conventional split rings

BROADBAND EXPERIMENTAL CHARACTERIZATION OF ARTIFICIAL MAGNETIC MATERIALS BASED ON A MICROSTRIP LINE METHOD

A broadband method is introduced to measure the effective constitutive parameters of artificial magnetic materials. The method is based on the microstrip line topology, thus making it easy to retrieve the constitutive parameters over a wide band of frequencies. To demonstrate the effectiveness of this method, artificial magnetic materials with Fractal Hilbert inclusions are fabricated and characterized. Good agreement between the experimental and numerical simulation results verifies the accuracy of the proposed method.

Metamaterial for gain enhancement of printed antennas: Theory, measurements and optimization

Metamaterials have been shown to enhance specific performance parameters of low profile and high-profile antennas. Our focus in this paper on specifically increasing the gain of low-profile antennas and in particular the microstrip patch antenna. By placing a metamaterial slab above a microstrip patch antenna (as a superstrate), we show that the gain of the antenna can be enhanced appreciably. The key advantage of using the superstrate is to maintain the low-profile advantage of microstrip patch antennas. In previous works, different types of superstrates were proposed to enhance the gain of microstrip antennas, however, to the best of our knowledge, no theory was developed to understand the mechanism behind the enhancement in the gain. In this paper, we present a simple analytical formulation that provides a very accurate prediction of the gain when a superstrate is used. In fact, our analytical technique is capable of predicting the gain when a multilayer superstrate structures is used. To validate the theory of gain enhancement, antennas and superstrates using metamaterials were fabricated and tested in an echoic chamber. The metamaterials developed were based on split-ring resonators. Strong agreement was found between the measurements and full-wave simulation using commercial tools. Finally, we present optimization results to demonstrate the maximum gain enhancement potential that can be achieved when superstrates are used.

High-Gain Patch Antennas Loaded With High Characteristic Impedance Superstrates

It is shown that, under some resonance conditions, a microstrip patch antenna can be designed to achieve the highest possible gain when covered with a superstrate at the proper distance in free space. The transmission line analogy and the cavity model are used to deduce the resonance conditions required to achieve the highest gain. The resonance conditions include the condition on spacing between the antennas substrate and the superstrate and the thickness of the superstrate. The permeability and permittivity of the superstrate are determined based on these resonant lengths and the appropriate characteristic impedance of each layer in this multilayered structure. The results are verified using both analytical and numerical methods. The effect of anisotropy of the superstrate is numerically investigated. The design criteria proposed here will reduce the total profile of the radiating system by 50% when compared to previous trends.

Characterization of Metamaterials Using a Strip Line Fixture

A method is introduced to measure the effective constitutive parameters of metamaterials having negative permittivity, negative permeability, or negative permeability and negative permittivity simultaneously. The method is based on the strip line topology, thus offering low cost and low setup complexity in comparison to other methods. The method proposed here is validated by numerically simulating the measurement setup while using different types of metamaterials. To validate the method experimentally, a metamaterial having negative permeability over a band of frequencies is characterized. Good agreement is obtained between the experimental and numerical results.

On the Fundamental Limitations of Artificial Magnetic Materials

Fundamental limitations are presented on the performance of artificial magnetic materials based on the geometrical and physical characteristics of the inclusions comprising the medium. The permeability and magnetic susceptibility of the medium are formulated in terms of newly defined geometrical and physical parameters. Based on the Lorentzian form of the effective permeability function of the medium, it is shown that the flatness of the permeability function is limited by the desired operational bandwidth. Also, by applying a specific circuit-based model for inclusions, geometric invariant fundamental constraints are derived. It is shown that inclusions with larger surface area result in higher value of permeability. Next, the magnetic loss tangent in the medium is expressed as a function of the newly defined geometrical and physical parameters. It is found that there is a tradeoff between increasing the permeability and decreasing the loss on the one hand and reducing dispersion, on the other hand.