Antenna Array Theory Research Articles

An Optical Super-Microscope for Far-field, Real-time Imaging Beyond the Diffraction Limit

Optical microscopy suffers from a fundamental resolution limitation arising from the diffractive nature of light. While current solutions to sub-diffraction optical microscopy involve combinations of near-field, non-linear and fine scanning operations, we hereby propose and demonstrate the optical super-microscope (OSM) – a superoscillation-based linear imaging system with far-field working and observation distances – which can image an object in real-time and with sub-diffraction resolution. With our proof-of-principle prototype we report a point spread function with a spot size clearly reduced from the diffraction limit, and demonstrate corresponding improvements in two-point resolution experiments. Harnessing a new understanding of superoscillations, based on antenna array theory, our OSM achieves far-field, sub-diffraction optical imaging of an object without the need for fine scanning, data post-processing or object pre-treatment. Hence the OSM can be used in a wide variety of imaging applications beyond the diffraction limit, including real-time imaging of moving objects.

Reflection characteristics of a composite planar AMC surface

This study investigates the reflection characteristics of a composite Artificial Magnetic Conductor (AMC) surface consisting of multiple orthogonal gradient AMC surfaces arranged in a two-dimensional periodic pattern. The gradient AMC surface in this study consists of square metal patches of variable size printed on a grounded dielectric substrate. Due to the orthogonal placement of the gradient AMC surface, the incident energy of a plane wave normally incident on the composite AMC surface will be reflected into four major lobes away from the impinging direction. To achieve a systematical design, a simple formula based on array antenna theory was developed to determine the reflection pattern of the gradient AMC surface illuminated by a normal incident plane wave. A time-domain full-wave simulation was also carried out to calculate the electromagnetic fields in the structure and the far-field patterns. The scattering patterns of the structure were measured in an electromagnetic anechoic chamber. Results confirm the design principle and procedures in this research. Since such a composite AMC surface can be easily fabricated using the standard printed circuit board technique without via-hole process, it may have potential applications in beam-steering and radar cross section reduction.

Experimental verification of subwavelength acoustic focusing using a near-field array of closely spaced elements

A linear array of closely spaced sound transducers is presented that can produce a subwavelength-focused intensity profile at a distance of a quarter wavelength. This work is related to research on super-resolution using metamaterials in both the acoustic and optical domains. It is designed using the principle of shifted beams, a near-field antenna array theory developed for the subwavelength focusing of electromagnetic waves. Once the spatial sound pattern is characterized for each source, the optimal weights for a minimum beam width can be calculated. An experiment operating at 4 kHz was able to successfully construct a super-focused beam.

An Old Tool and a New Challenge for Depicting Antenna Array Radiation Patterns (being also A Classroom Demonstration for the Principle of Wave Interference Underlying Antenna Array Theory)

This paper is intended for teachers and students dealing with antenna engineering. Graphical tools have historically been used for illustrating many basic features of radiation patterns, such as the position and number of secondary lobes, null angles, and beam steering. This paper describes one of these graphical devices, which has been successfully used by the author as a classroom demonstration in many undergraduate antenna courses. In addition, this traditional demonstration is also discussed in the context of modern computer software, such as MATLAB.

Energy Patterns of UWB Antenna Arrays With Scan Capability

A beam scanning technique is developed with a time delay for ultrawideband (UWB) arrays. In addition, we give a formulation for the time domain array factor for UWB antenna arrays. An UWB comb taper slot antenna array with large element spacing and impulse excitation is studied and four-element UWB linear array systems with element spacings of 2.5, 8.5, and 18 cm in the broadside direction and 20 degree scanning are implemented. The voltage response, spectrum response, energy pattern and energy gain for from simulation and measurement are in good agreement. The periodic grating lobes of the UWB antenna array do not occur in the energy pattern, even if the element spacing and scan angle are large. Simulation and measurement results for the energy pattern and voltage response in various directions are studied to validate the theory of an UWB antenna array with scan capability. The measured peak energy gain of arrays in the broadside direction and 20 degree beam scanning with element spacings of 8.5 and 18 cm are 10.79/10.78 dBi and 9.53/9.06 dBi, respectively.

Transmission and directionality control of light emission from a nanoslit in metallic film flanked by periodic gratings

We investigate the transmission and directional properties of light emission from a nanoslit film flanked by periodic gratings, formed on a silver film, embedded in a high index dielectric medium. Using FDTD calculations it is demonstrated that the transmission has strong dependence on dimension of the dielectric film. The directional property of the emitted beam is controlled by tapering height of output grating elements and this effect is explained by standard theory of antenna array. We propose and examine the potential of such slit grating structure as a Plasmonic antenna in axial injection of light from single mode fiber to photonic crystal waveguide.

Conformal Array Antenna Theory and Design [Reviews and Abstracts

Conformal Array Antenna Theory and Design [Reviews and Abstracts

Teaching and Designing Antenna Arrays Using Signal and Image-Processing Toolboxes of MATLAB

The goal of this paper is to apply some of the similarities between signal-processing theory and antenna-array theory to analyze and synthesize antenna arrays by using the signal and image-processing toolboxes of MATLAB <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TM</sup> . A detailed description and examples are given. The synthesis of one-dimensional and two-dimensional rectangular arrays is demonstrated using this simple method. These tools can be used for teaching antenna design.

Coherence of Photonic Crystal Vertical-Cavity Surface-Emitting Laser Arrays

We measure and compare the coherence properties of 2 times 1 arrays of photonic crystal vertical-cavity surface-emitting lasers. Antenna array theory applied to the measured far field intensity patterns is used to determine the phase of the complex degree of coherence, which is found to vary with current injection. The amplitude of the complex degree of coherence is determined by calculating the visibility from the far field patterns and making near field measurements of the relative intensities between lasing defects. We find that the amplitude and phase of the complex degree of coherence are correlated, such that coherence is maximized near in-phase and out-of-phase coupling conditions, and controllable by independent current injection to each array element

Relative phase tuning of coupled defects in photonic crystal vertical-cavity surface-emitting lasers

Evanescent coupling can be achieved by patterning the top facet of a vertical-cavity surface-emitting laser (VCSEL) with a multiple defect photonic crystal pattern. We show that for 2×1 coupled defect arrays, the phase between these emission sites changes as the current is varied. The phase variation is manifest in the far field pattern and may be determined using antenna array theory. It is found that the phase varies 81 deg between transversely coupled cavities as the current to the VCSEL is varied by 24 mA. This corresponds to a far field angular change of approximately 1.5 deg.

A Theoretical and Measurement Study of Sea Ice and Ice Shelf in Antarctica as Electrically Dense Media

Due to Antarctica's remoteness and harsh weather condition, remote sensing technology has been an attractive tool in equipping Antarctica scientists with the monitoring information and the understanding of ice physical changes in the Antarctican ecological system. Previous Antarctica researches on remote sensing of ice mainly utilize ice and snow scattering models based on independent scattering assumption. The needs of the consideration of coherent effects of the scatterers and the measurement of physical parameters affecting such effects are addressed in this study. Theoretical model based on radiative transfer theory with the incorporation of Dense Medium Phase and Amplitude Correction Theory (DM-PACT) [1] is developed. This concept of DM-PACT derived from antenna array theory requires an understanding of the distribution statistics of the scatterers in the homogeneous host medium. The ground truth measurement of multi-year period has been conducted at sea ice and ice shelf areas near Scott Base, Ross Island in Antarctica and a series of measurement of sea ice and snow physical parameters including distribution statistics of the air bubbles in sea ice are carried out. Theoretical study of the backscattering returns and scattering mechanisms in these electrically dense media is performed and analysis of important physical parameters affecting the radar returns is presented. Satellite measurements of the test areas from Radarsat have been acquired for the comparison of model predictions and measurement data with promising results.

Electromagnetic Wave Scattering from Vegetation with Odd-Pinnate Compound Leaves

Most existing EM scattering models for a vegetation medium assume that the leaves have an orientation distribution and each leaf scatters independently. For vegetations that possess compound leaves (for example leaves of a walnut tree) such an assumption is generally not valid. For this type of vegetation the leaves scatter in groups and it is each group, instead of each leaf, that scatters independently. An individual group of leaves scatters mostly coherently with a small degree of randomness in the spacing between leaves. The traditional radiative transfer approach can still be used but its phase function must be computed for a group of leaves patterned in accordance with the particular type of vegetation. To account for this group scattering effect for vegetation with odd-pinnate compound leaves we use a generalized antenna array theory. The scattered field of a group of leaves will be expressed as the product of the scattered field from a single leaf and an array factor. The corresponding power expression is averaged over the spacing between leaves which has a mean value and a random component. The vegetation medium is modeled as a half-space of randomly distributed and oriented odd-pinnate compound leaves. Theoretical analyses of the effects of the odd-pinnate compound leaves on the backscattering coefficient due to leaf size, spacing between leaves and the total number of leaves in a compound are carried out. It is found that the array factor or the coherence induced by the odd-pinnate compound leaves is important for vegetation media with a compound leave structure, especially when the size of leaves and the average spacing between adjacent leaves in the compound are comparable to the incident wavelength. Finally, the model predictions are compared with measurements from walnut trees where such a group scattering effect is expected.

Visualization cf radiation-pattern characteristics of phased arrays using digital phase shifters

This article presents graphical investigations of the array factor of phased arrays with digital phase shifters. A software program, based on basic antenna array theory, has been developed in MATLAB to obtain the main array characteristics (array factor and directivity). The array factors of linear arrays of different sizes with different types of phase shifters have been studied as a function of the number of bits and the frequency. Unconventional two-dimensional color graphical representations are used to identify some characteristics of the array factor of arrays with digital phase shifters that can not be so clearly and quickly visualized with conventional graphical representations. In particular, the effects of quantization on the array factor for arrays of different sizes and for phase shifters with different numbers of bits, over scanning, and frequency ranges, are shown using this representation. Numerous data are also provided.

Fractal antenna engineering: the theory and design of fractal antenna arrays

A fractal is a recursively generated object having a fractional dimension. Many objects, including antennas, can be designed using the recursive nature of a fractal. In this article, we provide a comprehensive overview of recent developments in the field of fractal antenna engineering, with particular emphasis placed on the theory and design of fractal arrays. We introduce some important properties of fractal arrays, including the frequency-independent multi-band characteristics, schemes for realizing low-sidelobe designs, systematic approaches to thinning, and the ability to develop rapid beam-forming algorithms by exploiting the recursive nature of fractals. These arrays have fractional dimensions that are found from the generating subarray used to recursively create the fractal array. Our research is in its infancy, but the results so far are intriguing, and may have future practical applications.

Electromagnetic Interaction of Chiral Particles in Three-Dimentional Arrays

The problem of electromagnetic mutual coupling of scatterers in the three-dimensional regular infinite arrays is considered. The arrays are infinite in the XY-coordinate plane and finite along the OZ-axis, their volume cells have rectangular shape. Each scatterer can be described by two coupled dipoles (electric and magnetic) having the same direction, i.e., the scatterers are uniaxial chiral particles. The field interaction of these objects can be an important factor in the theory of bianisotropic composite media and also concerns the antenna array theory. A simple and rigorous model of such an interaction is developed.

A Method for Accurate Calculation ofB1Fields in Three Dimensions. Effects of Shield Geometry on Field Strength and Homogeneity in the Birdcage Coil

A method for calculatingB1field strength and homogeneity as functions of radiofrequency shield geometry is presented. The method requires use of three-dimensional finite-element analysis, birdcage-coil theory, and antenna-array theory. Calculations were performed for a 12-element birdcage coil (19 cm diameter, 21 cm length) at 125 MHz. CalculatedB1field strengths and homogeneities for the coil in 25 different shields and in no shield are given. For configurations where the shield is longer than the coil, bothB1field strength and homogeneity decrease as shield diameter decreases or as shield length increases. In configurations where the shield is shorter than the coil and has a diameter of 25.6 cm,B1homogeneity is greater than in an unshielded coil.B1field strength was measured experimentally at 125 MHz in a birdcage coil of the same geometry as the model within shields of four different diameters. Calculated results very closely matched experimental measurement.

A phase matrix for a dense discrete random medium: evaluation of volume scattering coefficient

In the derivation of the conventional scattering phase matrix of a discrete random medium, the far-field approximation is usually assumed. In this paper, the phase matrix of a dense discrete random medium is developed by relaxing the far-field approximation and accounting for the effect of volume fraction and randomness properties characterized by the variance and correlation function of scatterer positions within the medium. The final expression for the phase matrix differs from the conventional one in two major aspects: there is an amplitude and a phase correction. The concept used in the derivation is analogous to the antenna array theory. The phase matrix for a collection of scatterers is found to be the Stokes matrix of the single scatterer multiplied by a dense medium phase correction factor. The close spacing amplitude correction appears inside the Stokes matrix. When the scatterers are uncorrelated, the phase correction factor approaches unity. The phase matrix is used to calculate the volume scattering coefficients for a unit volume of spherical scatterers, and the results are compared with calculations from other theories, numerical simulations, and laboratory measurements. Results indicate that there should be a distinction between physically dense medium and electrically dense medium.

Basic array theory

Basic antenna array theory is outlined with major emphasis on pattern analysis and synthesis for periodic linear and planar arrays, phased arrays, and conformal arrays. Extension is made to synthesis techniques which use computer algorithms. These include arbitrary sidelobe control, shaped beams, and phase-only null steering. The subjects of random errors and phased array quantization errors are outlined. >

Arrays: A software package for analysis of antenna arrays

This article describes a software package useful for the instruction of antenna array theory. The program provides the radiation pattern of different types of antennas, the array factor of linear, two-dimensional and three-dimensional arrays, and the total radiation pattern. The elements of the array are isotropic source, Hertzian dipole, straight thin wire dipole, and finite line source with uniform current distribution. The user has the freedom to choose one of the supported element types or to provide the pattern of an antenna element based on experimental or calculated data. This article serves as a tutorial for selected examples as well as a quick guide for the use of this software.

Analysis of the far-field output angle scanning by injection locking of a diode laser array

We present a linear theory to explain the experimentally observed emission angle scanning of an injection-locked diode laser array when the frequency of the injected light is changed. This theory is based on eigenmode analysis of the coupled waveguides and on the use of the antenna array theory for the far-field pattern. The experimental results are explained in terms of the resonant excitation of eigenmodes, which selects the far-field pattern associated with the resonant mode. The agreement with published results is good.