Chinwe C. Njoku

Effective Permittivity of Heterogeneous Substrates With Cubes in a 3-D Lattice

This letter studies the behavior of heterogeneous dielectric substrates created by including micro-sized cubes of different electromagnetic (EM) properties to an otherwise homogenous host medium. The letter looks at the effect of the volume fraction of cubic inclusions (dielectric or metallic) to the effective EM properties of the heterogeneous medium. It is well known that the effective permittivity of the mixture is dependent on the properties of the host and inclusions as well as the size and spacing of the cubes. EM simulations using a plane-wave excitation have been used to carry out these studies. An inversion process using a rectification algorithm was employed to correctly obtain the effective permittivity from the scattering parameters of the mixture. The analytical results of the infinite medium using canonical equations and a volume equivalence showed good agreement with the EM simulations and measurement of our samples of finite thickness. This letter shows that cubic inclusions can produce a higher effective permittivity than was previously found with spheres.

Designing multi-band and high bandwidth antennas with heterogeneous substrates

This paper investigates the concept of creating substrates with heterogeneous dielectric properties. By suitably locating areas of low and high permittivity, the second resonance can be moved closer to the 1st resonance and multiband antennas can be created. By combining, the resonances, the bandwidth of the antenna can be increased.

Microwave antennas and heterogeneous substrates using nanomaterial fabrication techniques

By exploiting the enhanced physical properties of nanomaterials and the advancement in nanotechnology, alternative methods of fabricating microwave antennas can be conceived. This paper will discuss the potential manufacturing advantages as well as different fabrication methods. By controlling the location of metallic and dielectric particles, integrated antennas and substrates can be made in one process. Electromagnetic advantages result from being able to add inclusions with different electrical properties into the host substrate and thereby create a new effective permittivity and permeability. This paper will review and analyse methods for calculating these effective properties.

Studies of permittivity and permeability of dielectric matrix with cuboid metallic inclusions in different orientations

In this paper, we investigate the possibility of using the heterogeneous materials, with cuboid metallic inclusions inside a dielectric substrate (host) to control the effective permittivity. We find that in the gigahertz range, such a material demonstrates a significantly larger permittivity compared to the pure dielectric substrate. Three principal orientations of microscale cuboid inclusions have been taken into account in this study. The highest permittivity is observed when the orientation provides the largest polarization (electric dipole moment). The detrimental side effect of the metallic inclusion, which leads to the decrease of the effective magnetic permeability, can be suppressed by the proper choice of shape and orientation of the inclusions. This choice can in fact reduce the induced current and hence maximize the permeability. The dissipative losses are shown to be negligible in the relevant range of frequencies and cuboid dimensions.

Microstrip patch antennas on substrates with metallic inclusions

This paper considers the design of microwave patch antennas using bespoke substrates. The permittivity of the substrate is controlled by inserting small scale metallic inclusions into a host medium and can be obtained using an S-parameter inversion algorithm on the results of plane-wave simulations. Electromagnetic simulations have been used to compare the performance of the patch on the heterogeneous substrate to standard homogeneous substrate.

Microstrip Patch Antennas With Anisotropic and Diamagnetic Synthetic Heterogeneous Substrates

This communication examines the performance of a rectangular patch antenna on heterogeneous substrates composed of metallic cuboid inclusions compared with homogenous substrates via simulations and measurements. The communication examines how the orientation of the inclusions affects the performance of the patch antenna. Anisotropy and diamagnetism of the heterogeneous substrate with metallic cuboid inclusions have been analyzed. As the resonant frequency of the patch is dependent on the electromagnetic (EM) parameters of its substrate(s), fixing the size of the patch and altering its substrates allows the EM properties of the substrates to be estimated and compared with predictions from theory and simulations.

Fine scale simulations of patch antennas with heterogeneous substrates

Summary form only given. As consumer and industry continue to desire wireless accessibility on an increasing range of platforms, antenna engineers are constantly required to develop new antenna systems which maximize the bandwidth-efficiency to size ratio. One possible method of achieving this is to use artificial dielectrics which allow a greater degree of freedom in the design process by enabling bespoke values of the permittivity. Furthermore, it allows the local permittivity to be varied with respect to the electric fields which can lead to bandwidth enhancements. The authors have previously addressed the idea of creating artificial dielectrics using small scale inclusions embedded in a host medium [Njoku et al. IEEE TAP, Vol. 60, No. 5, pp. 2194-2202, 2012] which contains a detailed literature review. In this paper, small scale metallic cuboid and spherical inclusions are used to control the effective permittivity of a patch antenna substrate. EMPIRE XCcel Finite-Difference Time-Domain (FDTD) commercial software was used to model the antenna which operates in the microwave spectra. The metallic inclusions were on the scale of 100 microns (see not to scale image). This creates extensive challenges for simulations as many Yee Cells are required to mesh the simulation domain. This is particularly apparent when spheres are used instead of cuboids. The simulations were carried out on a high performance computer with 96GB of RAM and took up to 20 hours to complete. The simulation software allows the dimensions and the properties of the metallic inclusions to be optimized - however, this further adds to the computational requirements.

Review of artificial dielectrics containing small scale inclusions

This paper reviews some of the latest advances in the field of artificial dielectrics. The dielectric properties (permittivity and losses) can be designed and synthesized by placing small scale inclusions in a host medium. This work will create many new opportunities for antenna designers and will also lead to applications in transformation optics. This paper highlights the progress made in understanding the analytical, practical and measurement aspects of this work.

Antenna performance on quasi synthetic media

While we seem to be experiencing a material evolution by applying unique properties of metamaterials, such as negative constitutive parameters and to some extent cloaking phenomena, not much attention has been paid in the practical suitability of synthetic materials towards antenna designs. The antenna designer is often faced with a judicious choice of:- complexity in the conducting/radiating shape, substrate and radome parameters, cost as well as ever increasing environmental effects both in the construction but also in the disposal of the antenna as part of a recycling process. This paper will outline some of the hypotheses and processes that underpin our terminology of quasi synthetic media and will proceed to illustrate how one can obtain a variety of dielectric (and magnetic) effective contrasts from 3-D structures containing either dielectric or conducting micro particles. Some representative patch designs are considered to indicate how one could replace cumbersome conventional design and manufacturing processes by using nanotechnology and additive manufacturing.

Printable windscreen quad-band GSM antenna

The design and simulation of a suitable GSM antenna for integration with the rear windscreen of a car is presented in this paper. A bent asymmetric dipole antenna was chosen to minimise the surface area and thus cause minimal effect to the aesthetics of the car as well as to the drivers visibility. The antenna has been simulated on a realistic three-layered windscreen. Parasitics were added to the design to improve the bandwidth at 900 and 1800MHz at both European and American bands. The bent asymmetric dipole was advantageous compared to a straight asymmetric dipole because i) the radiation pattern was more omnidirectional and ii) the parasitics at each band could be designed to work independently of each other.