Validation of Linear Polyfractal Arrays and the Optimization of Antenna Elements in an Infinite Array Environment

Open Access
Author:
Gregory, Micah Dennis
Graduate Program:
Electrical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 16, 2009
Committee Members:
  • Douglas Henry Werner, Thesis Advisor
  • Pingjuan Li Werner, Thesis Advisor
  • Douglas Henry Werner, Dissertation Advisor
Keywords:
  • polyfractal
  • antenna array
  • genetic algorithm
  • array factor
  • periodic array
  • planar array
  • infinite array
  • microstrip antenna
Abstract:
In recent years there has been much research into increasing the operating bandwidth of radio frequency systems. The need for wideband systems arise through demand for high data rate communications, multi-frequency radars, and other multipurpose communications devices. In addition to the circuitry of such systems, the antenna is an important component as well for energy to be effectively radiated over large operating bandwidths. Many designs and optimizations have focused on creating wideband antennas operating in isolation, but arrays are attractive because of the electronic beam steering capabilities and high gain properties that they exhibit. This thesis covers two aspects of wideband antenna array design. The first topic is the arrangement of antenna elements in an array to maintain low relative sidelobe levels over a wide bandwidth. The most common configuration, the periodic array, generates grating lobes when the elements are electrically far apart. Grating lobes are portions of the array factor with power equal to the main beam; they cause energy transmission in undesired directions. Arranging the elements in an aperiodic fashion often has the benefit of reducing sidelobe levels over extended bandwidths if designed properly. The method used for aperiodically arranging elements is important to realize arrays with sufficient sidelobe suppression over a useful bandwidth. The recently introduced polyfractal array design approach is one successful method. Using the recursive nature of fractals and a powerful optimization technique, arrays exhibiting ultra-wideband performance with very low sidelobe levels have been created. The focus of the work presented here is to validate the effectiveness of polyfractal arrays consisting of real antenna elements. To this end, two small polyfractal arrays have been optimized, fabricated, and examined for wideband performance. The second topic is the optimization of elements that are arranged in an infinite planar periodic structure. Arranging elements periodically allows easy of manufacturing and implementation. Although the upper frequency of operation is limited by the occurrence of grating lobes, large bandwidth percentages can be had if the antenna element is designed properly. Often, antenna arrays are created by designing a single element and placing it in an array configuration, though this comes with certain consequences. For instance, the antennas can shift their resonant frequencies and input return loss can become poor because of mutual coupling between elements. In addition, performance can become severely degraded when arrays are scanned, especially close to grazing angles. For these reasons, it becomes advantageous to design and optimize the antenna elements while they are in an array. In this manner, any effects due to nearby elements are automatically accounted for. Specialized simulation software along with a genetic algorithm is used for this purpose, creating effective wideband radiating elements to be used in planar microstrip phased arrays. The methods of antenna design and optimization are covered and two examples are presented and validated using a commercial electromagnetics simulation code.