Spiral Antenna Miniaturization with High-contrast Dielectrics A Thesis Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Bradley Allen Kramer, B.Sc. ***** The Ohio State University 2004 Approved by Master's Examination Committee: Prof. John L. Volakis, Adviser Adviser Adj. Asst. Prof. Chi-Chih Chen Department of Electrical and Computer Engineering ii ABSTRACT There is a great interest in the automotive and military sectors for small broadband antennas that meet modem communication needs. These needs require ultra- wide bandwidth (>10:1) broadband antennas, such as the spiral antenna. However, the physical size of the antenna at the low-frequency end becomes prohibitively large for many applications. To reduce the size of the antenna, miniaturization techniques such as high-contrast (εr>l and/or μr> 1) material loading or reactive loading must be employed. The concept of antenna miniaturization is to increase the electrical size of the antenna without increasing its physical size by slowing down the current flowing on the antenna structure. A common approach is to use dielectric materials to reduce the propagation constant of the current. This approach has received considerable attention in the past but has been limited by the lack of quality dielectrics which exhibit low-loss and high permittivity. However, there has recently been a renewed interest because of the currently available low-loss high-contrast dielectrics. In addition, size reduction using high- contrast dielectrics has already been demonstrated for narrowband antennas, such as patch antennas, but not for broadband antennas. Therefore, the concept of miniaturizing a broadband spiral antenna using dielectric materials is investigated experimentally and numerically. To investigate the miniaturization of the spiral antenna, a previously developed shallow cavity backed spiral antenna is used as the starting point. This design was then altered to facilitate the dielectric loading by using only a single resistor to terminate the spiral arm instead of multiple resistors. In addition, the previous balun which occupied the surface of the antenna was removed in favor of a hybrid balun. The performance of iii the new balun and termination are evaluated and compared to the previous balun and termination methods. Following the discussion of these modifications and improvements the miniaturization of a spiral antenna is studied using a 2” square spiral. It is shown that by loading the spiral with high-contrast dielectrics it can achieve sufficient operation down to 574 MHz where its size is only λ/10. As a consequence of dielectric loading there are issues that arise such as impedance reduction. This issue is examined using simulations and is addressed by introducing the concept of dielectric tapering. In addition to demonstrating the practicality of broadband antenna miniaturization using high-contrast materials, it is also observed that there are limitations associated with the miniaturization and that these limitations require further examination. iv ACKNOWLEDGMENTS I wish to thank Prof. John. L. Volakis and Dr. Chi-Chih Chen, for their advice, intellectual support, encouragement, and enthusiasm during the course of this research. Thanks is also due to Ming Lee for the valuable discussions and his alternative approach to antenna miniaturization which led to further and invaluable insights into the problem. v VITA August 19, 1978 .......................... Born - Newark, Ohio, United States of America 2002 .............................................. B.Sc. Electrical and Computer Engineering The Ohio State University 2002 - present ................................................... Graduate Research Associate, The Ohio State University PUBLICATIONS Research Publication 1. Brad A. Kramer, Ming Lee, Chi-Chih Chen and John L. Volakis, "Design and Performance of an Ultra Wideband Ceramic-Loaded Slot Spiral," Accepted by IEEE Transactions on Antennas and Propagation. 2. Brad A. Kramer, Chi-Chih Chen and John L. Volakis, "The Development of a Mini-UWB Antenna," AMTA 2004 Conference Proceedings, Oct. 2004. FIELDS OF STUDY Major Field: Electrical and Computer Engineering vi Table of Contents ABSTRACT ...................................................•.........•.•......•.......•....•.......•.•.••..••.•.••.•.••.••••.••.•.•..•.•................ III ACKNOWLEDGMENTS •.•.••.•..•.•.••.•.••.•.••.•.•.••.•.••...•..•.•..•.••.•.••....•..•......•..•...................................•.....•••.••• V VITA ...........................................................................................................................••.•.••.••••.•.••••.•.••••.•.•.•. VI PUBLICATIONS ••.•.••.•..•.•.•.•..•.••.•.••.•.••.•.•..•.•.••.••••.••••••.••.••.•.••••.•..•.••.•.••.••.•.••••.••.••.••••.•.••.•...............••.....•• VI FIELDS OF STUDY .......................................................•.........•..........•..•...•..•....•.••.•.......................•.......... VI TABLE OF CONTENTS .•....•....•.•....•.••.•......•................•.....•.•.•..••....•...•..•..••••..•...........•..........................• VII LIST OF TABLES ......•..•.......•.....•.......................•.................•...•............•..........•..•.....•••..•••.••.•.••.•.••••••.•••.•. IX LIST OF FIGURES ...•••••.•.••.••••.•.••.•.•.••••.•.•.••.•.•••••••.•.•••••••.••.•.••.•.••.•..•.••.•.••.••.••••.•••••••.••.•.•••...•.•.••..........•.... X LIST OF FIGURES .....................................................•......•........•...•..•..•..........•.•.......•............................•...• X CHAPTER I •.•..••.•.•..••...........•....•.••.•......•.....•....••.•...•..•.•.•.•.•.•.....•.......•.....•........••......•..•.••.•.•..••••.•.••••.•.•.•.•.• 1 INTRODUCTION ..•.••.•..•......•..•.••.•.•.••......•.....•....••.•.•..•.•.........................................................•.•....•.••.•...••.• 1 1.1 MOTIVATION ......................................................................................................................................... 1 1.2 OBJECTIVE ............................................................................................................................................. 2 1.3 TECHNOLOGY DESCRIPTION .................................................................................................................. 3 1.4 TECHNICAL ISSUES ................................................................................................................................ 7 1.5 TECHNICAL APPROACH ......................................................................................................................... 8 CHAPTER 2 ...............................................................•...........•.••.••••.••.•..•.••.••.•.••.•..•.•••••.•.•.•••....•.•...•.••....•.• 10 BASELINE DESIGN - ARCHIMEDEAN SLOT SPIRAL ANTENNA .....•........................•................ 10 2.1 SPIRAL ANTENNA ................................................................................................................................ 10 2.2 GENERAL SPIRAL ANTENNA DESIGN ISSUES ....................................................................................... 14 2.3 BASELINE DESIGN ............................................................................................................................... 20 2.4 DESIGN IMPROVEMENTS ...................................................................................................................... 22 CHAPTER 3 •.••.•.•.••.•..••.•..•.•.••.••.•.••.•.••••..•...•.••.•..•.•.......•.•..•.•.•...........................•....................................... 30 ANTENNA MINIATURIZATION USING DIELECTRICS .......................•........•.•.•....•........•.............. 30 3.1 CONCEPT ............................................................................................................................................. 30 vii 3.2 MINIATURIZATION CRITERIA FOR BROADBAND ANTENNAS ................................................................ 34 3.3 DIELECTRIC LoADING OF THE SPIRAL ANTENNA ................................................................................. 35 3.4 DIELECTRIC LoADING ISSUES .............................................................................................................. 38 CHAPTER 4 ............................................................................................................................................... 50 SUMMARY................................................................................................................................................. 50 CHAPTER 5 ............................................................................................................................................... 52 FUTURE WORK ....................................................................................................................................... 52 5.1 DIELECTRIC LoADING ......................................................................................................................... 52 5.2 AN ALTERNATIVE APPROACH TO Low FREQUENCY GAIN IMPROVEMENT .......................................... 53 BIBLIOGRAPHY ...................................................................................................................................... 57 Vlll List of Tables TABLE 1.1 FREQUENCY BANDS OF MILITARY COMMUNICATION SYSTEMS ....................................................... 2 TABLE 3.1 Two INCH SQUARE