Christopher L. Holloway

An Overview of the Theory and Applications of Metasurfaces: The Two-Dimensional Equivalents of Metamaterials

Metamaterials are typically engineered by arranging a set of small scatterers or apertures in a regular array throughout a region of space, thus obtaining some desirable bulk electromagnetic behavior. The desired property is often one that is not normally found naturally (negative refractive index, near-zero index, etc.). Over the past ten years, metamaterials have moved from being simply a theoretical concept to a field with developed and marketed applications. Three-dimensional metamaterials can be extended by arranging electrically small scatterers or holes into a two-dimensional pattern at a surface or interface. This surface version of a metamaterial has been given the name metasurface (the term metafilm has also been employed for certain structures). For many applications, metasurfaces can be used in place of metamaterials. Metasurfaces have the advantage of taking up less physical space than do full three-dimensional metamaterial structures; consequently, metasurfaces offer the possibility of less-lossy structures. In this overview paper, we discuss the theoretical basis by which metasurfaces should be characterized, and discuss their various applications. We will see how metasurfaces are distinguished from conventional frequency-selective surfaces. Metasurfaces have a wide range of potential applications in electromagnetics (ranging from low microwave to optical frequencies), including: (1) controllable “smart” surfaces, (2) miniaturized cavity resonators, (3) novel wave-guiding structures, (4) angular-independent surfaces, (5) absorbers, (6) biomedical devices, (7) terahertz switches, and (8) fluid-tunable frequency-agile materials, to name only a few. In this review, we will see that the development in recent years of such materials and/or surfaces is bringing us closer to realizing the exciting speculations made over one hundred years ago by the work of Lamb, Schuster, and Pocklington, and later by Mandelshtam and Veselago.

On the Use of Reverberation Chambers to Simulate a Rician Radio Environment for the Testing of Wireless Devices

With the proliferation of wireless devices in recent years, there is a growing need to test the operation and functionality of these various devices in different multipath environments, ranging from line-of-sight environment to a pure Rayleigh environment. In this paper we discuss how a reverberation chamber can be used to simulate a controllable Rician radio environment for the testing of a wireless device. We show that by varying the characteristics of the reverberation chamber and/or the antenna configurations in the chamber, any desired Rician K-factor can be obtained. Expressions for the desired K-factor as a function of the chamber and antenna characteristics will be presented. Experimental results are presented to illustrate the validity of these expressions, to show how the reverberation chamber can be used to simulate different multipath environments, and to show the realization of a controlled K-factor test facility. We present both a one-antenna and a two-antenna test configuration approach

Shielding effectiveness measurements of materials using nested reverberation chambers

The use of reverberation chambers for determining the shielding effectiveness has the advantage over other techniques in that the reverberation chamber exposes the material to a more realistic environment. That is, in a reverberation chamber, the fields are incident on the material with various polarizations and angles of incidence. There are various reverberation chamber techniques found in the literature and international standards. These techniques can give incorrect results because aperture and cavity-size effects are not correctly accounted for. In fact, we show that for no sample in the aperture, these techniques give a nonzero result for shielding effectiveness. In this paper, we review existing techniques and present a new technique for determining the shielding effectiveness of materials from nested reverberation-chamber measurements. The new approach accounts for aperture, cavity size, and chamber loading effects. Various examples are presented to illustrate the utility of the new approach, and a discussion on edge treatments of the materials is given.

A low-frequency model for wedge or pyramid absorber arrays-I: theory

The interaction of electromagnetic waves with an array of absorbing wedges or pyramid cones is studied in the low-frequency limit; i.e., when the period of the array is small compared with wavelength. A theoretical model is obtained using the method of homogenization, which replaces the transversely periodic structure with a transversely uniform medium possessing a certain (generally anisotropic) effective permittivity and permeability. Plane-wave reflection from such structures can then be modeled using well-known techniques for one-dimensionally inhomogeneous media; a Riccati equation for the reflection coefficient is used in this work. This model is appropriate for use with absorbers found in anechoic chambers used for electromagnetic compatibility and electromagnetic interference (EMC/EMI) measurements over the frequency range of 30-1000 MHz. >

Simulating the Multipath Channel With a Reverberation Chamber: Application to Bit Error Rate Measurements

We illustrate the use of the reverberation chamber to simulate fixed wireless propagation environments including effects such as narrowband fading and Doppler spread. These effects have a strong impact on the quality of the wireless channel and the ability of a receiver to decode a digitally modulated signal. Different channel characteristics such as power delay profile and RMS delay spread are varied inside the chamber by incorporating various amounts of absorbing material. In order to illustrate the impact of the chamber configuration on the quality of a wireless communication channel, bit error rate measurements are performed inside the reverberation chamber for different loadings, symbol rates, and paddle speeds; the results are discussed. Measured results acquired inside a chamber are compared with those obtained both in an actual industrial environment and in an office.

Reverberation Chamber Techniques for Determining the Radiation and Total Efficiency of Antennas

Reverberation chambers are becoming a popular alternative testing facility for a wide range of electromagnetic applications. Because of the statistical environment created inside a reverberation chamber, they offer a unique test facility. In particular, these chambers are ideally suited for performing radiated power measurements of either an antenna or device under test, and as such, it is possible to determine the efficiency of antennas. There have been several reverberation chamber techniques proposed over the years for measuring the antenna efficiency; however, these techniques require either the use of a reference antenna (i.e., an antenna with a known efficiency) and/or require the assumption that the two antennas used in the test have identical efficiencies. In this paper, we present three different approaches for determining both the radiation and total efficiencies of an unknown antenna that overcome these limitations and assumptions. We present a one-antenna approach, a two-antenna approach, and a three-antenna approach. We present measured data for three different antennas in order to compare these three approaches. We also discuss the uncertainties related to these types of measurements.

Analyzing carbon-fiber composite materials with equivalent-Layer models

The purpose of this paper is to investigate the use of equivalent-layer models for the analysis of carbon-fiber composite materials. In this paper, we present three different models for the electromagnetic characterization (effective material properties) of fiber composites that are commonly used in aircraft and EMC/EMI shielding materials. These three models represent various orders (or levels) of detail in the fiber composite structure and, hence, capture various physical aspects of the composite. These models can be used to efficiently calculate the reflection and transmission coefficients, as well as the shielding effectiveness, of these fiber composites. We compare results of the reflection coefficient and shielding effectiveness obtained from these effective-property models to results obtained from a full numerical solution based on the finite-element (FE) method of the actual periodic fiber composite. We show that, as expected, as more of the geometric detail of the fiber composite is captured with the different models, the upper frequency limit of validity increases.

Effects of reinforced concrete structures on RF communications

The proliferation of communication systems used in and around man-made structures has resulted in a growing need to determine the reflection and transmission properties of various commonly used building materials at radio frequencies typically used in businesses and residential environments. This paper describes the calculation of reflection and transmission coefficients for reinforced concrete walls as a function of wall thicknesses and rebar lattice configuration over a frequency range of 100-6000 MHz. The transmission and reflection coefficients were calculated using a finite-difference time-domain (FDTD) solution of Maxwells equations. The rebar structures analyzed include both a two-dimensional (2-D) trellis-like structure and a one-dimensional (1-D) structure, where the reenforcing bars are all oriented in the same direction. In general, the results show that the reinforced concrete structures severely attenuate signals with wavelengths that are much larger than the rebar lattice and that the transmitted signal has a complex structure with resonances and nulls that strongly depend upon the geometry of the reinforcing structure and the concrete wall thickness.

Cross correlations and cross spectra for spaced antenna wind profilers: 1. Theoretical analysis

The presented theory ties the properties of a turbulently advected scattering medium to the cross correlation and cross spectrum of signals in a general configuration of receiving and transmitting antennas. The correlation length of Bragg scatterers and antenna diameter are the significant parameters determining the diffraction patterns correlation length. We examine how vertical anisotropy of the scattering medium affects the diffraction patterns correlation length. We demonstrate that the cross spectrum can be formulated in terms of a pair of spectral sampling functions (a one-dimensional Doppler and a three-dimensional wavenumber function), and closed form solutions are obtained. We give the conditions under which the scattering mediums statistical properties can be represented by a Gaussian correlation or spectral model, and the distance over which the diffraction pattern simply advects without significant change. We show that the diffraction pattern of a pair of scatterers can translate at the speed of the scatterers, not twice their speed as is commonly thought.

Angle and space diversity comparisons in different mobile radio environments

The angle diversity performance of two types of high-gain multibeam antennas-24 vertically polarized 15/spl deg/ beams and 12 vertically polarized 30/spl deg/ beams-were tested and compared to the space diversity performance of traditional sector antenna configurations. The antennas were tested at 850 MHz in dense urban and rural cellular mobile radio environments. A vehicle equipped with a mobile transmitter was driven in the coverage area, while the received signal strength (RSS) was recorded on multiple receiver channels attached to multibeam and sector antennas at the base site. The RSS data recorded included fast (Rayleigh) fading and was averaged into local means based on the mobiles position/speed. The fast fading was extracted from the recorded RSS and the fading distributions of the two multibeam antennas tested were studied in two distinctly different mobile environments. Fading cumulative distributions for the angular diverse antennas were compared to those of spatially diverse antennas. Diversity gain was calculated and compared to traditional space diversity in these mobile environments. Results in urban environments indicated that the angular diversity performance was comparable to the space diversity (/spl sim/8 dB improvement). Rural tests typically suggested that both space diversity and angular diversity provided little or no (<2 dB) fading reduction. A description of the experiment, data reduction and analyses, and calculation of diversity gain are presented. The motivation for this experiment is the application of fixed multiple beam antennas (FMBA) in cellular radio and digital personal communication systems.