April 6, 2010

Ray, Beam and Hybrid Techniques for Analysis of Electrically Large Antenna Configurations

Professor Prabhakar H. Pathak

At sufficiently high frequencies, for which radiating objects become quite large (or even moderately large) in terms of the wavelength, conventional numerical methods employed for the solution of practical antenna radiation problems become strained or even intractable. It then becomes natural and far more efficient to employ asymptotic high frequency ray and beam methods, and also hybrid methods which combine high frequency and numerical techniques, respectively, to analyze electrically large antenna problems. A uniform version of the geometrical theory of diffraction (GTD) [ 1 ], which is referred to as the UTD [ 2,3 ], is one such ray method which can be used effectively. A significant advantage of the UTD is that it offers a physical insight into the antenna radiation mechanisms involved in terms of diffracted rays together with conventional geometrical optics (GO) rays. Such a direct and vivid picture based on a ray description of the radiation of electromagnetic (EM) waves is typically not shared by any other methods of solution. The power of UTD will be illustrated through applications to antennas in complex environments. In some special situations involving a confluence of ray caustics (or foci) with ray shadow boundaries the UTD alone may become inapplicable. In the latter instance, the high frequency physical optics (PO) method is often employed; however, PO generally needs an integration of the assumed currents on the large radiating object thus making it rapidly inefficient with increase in the operating frequency. A useful and more accurate alternative is the use of beam methods, e.g., the one based on the complex source beam (CSB) approach [4,5,6]. An extension of the UTD for CSB illumination is developed and will be described for specific applications. In many practical applications, a radiating object can contain both electrically large and small parts; in such cases it becomes necessary to hybridize high frequency and numerical methods in order to combine the best features of both methods and hence also to overcome the limitations of both. In particular, examples involving the application of UTD, CSB, and hybrid methods, respectively, will be presented highlighting the power of each. Examples will include the treatment of large satellite antenna reflector systems, and other antennas including large complex and conformal phased arrays on complex airborne, space borne or ship platforms, and of antennas in urban/rural environments. In addition, an analysis of near field antenna measurements for far zone pattern predictions will also be considered.

 

June  20, 2010

 

ULTRA-WIDEBAND AND INTERLEAVED POLYFRACTAL ANTENNA ARRAYS

Dr Joshua S. Petko

Recently, in order to successfully combine the positive attributes of both periodic and random arrays into one design, a novel class of arrays, known as fractal-random arrays, has been introduced. In addition, global optimization techniques, such as genetic algorithms, have been applied to antenna array layouts to provide highly directive, thinned, frequency agile, and shaped-beam antenna systems. However, these methodologies have their limitations when applied to more demanding design scenarios. Global optimizations are not well equipped to handle the large number of parameters used to describe large-N arrays, and fractal-random arrays lack the recursive properties needed to reconstruct their geometries exactly from a small set of parameters. To overcome these difficulties, a new class of arrays, called polyfractal arrays, is introduced in this dissertation that possess properties well suited for the optimization of large-N arrays. These polyfractal arrays possess underlying self-similar properties that can be exploited to exactly reconstruct the array geometries from small sets of parameters and to increase the speed of the associated array factor calculations. In addition, an autopolyploidy-based chromosome expansion native to polyfractal arrays is introduced that can dramatically accelerate the genetic algorithm optimization process. This process allows the genetic algorithm to first evolve simple designs very quickly and then add increasing levels of complexity when they are needed to continue the optimization. The entire procedure has been shown to be very effective in creating optimized large-N antenna layouts. For example a 1616-element linear array can be optimized to possess a -24.30 dB sidelobe level with a 0.0056◦ half-power beamwidth. In addition, robust Pareto optimization techniques can be applied to reduce the peak sidelobe levels at several frequencies specified over a wide bandwidth. This procedure can lead to ultra-wideband antenna array designs, with one example discussed in this dissertation maintaining a −19.34 dB peak sidelobe level with no grating lobes from a range covering 0.5λ to 20.0λ minimum interelement spacings, corresponding to a bandwidth of at least 40 : 1. These powerful array designs can be utilized as building blocks in robust, multi-beam, ultra-wideband antenna array systems.

      

 

November  18, 2010

Time Domain Finite-Element Finite-Difference Hybrid Method and Its Application to Electromagnetic Scattering and Antenna Design

Shumin Wang . 

 A hybrid method that combines the Finite-Difference Time-Domain (FDTD) and the Finite-Element Time-Domain (FETD) methods is presented for high-fidelity and high-efficiency electromagnetic simulations. The FETD method is applied to model curved conductor structures while low-cost FDTD method is applied to model inhomogeneous dielectric and truncate computational boundaries. In order to simplify the treatment of highly irregular FDTD/FETD interface, composite elements are presented that encapsulate most interfacing detail. Unstructured tetrahedral meshes are generated by the advancing front technique with ``sweep-and-retry" and smoothed by a combined quality improvement procedure. Due to the low profile of the resulting tetrahedral mesh, the sparse Cholesky decomposition can be applied effectively to solve the FETD matrix. Examples demonstrate the validity and effectiveness of the hybrid method in electromagnetic scattering and near field antenna design for magnetic resonance imaging.

Presentation