NUMERICAL AND EXPERIMENTAL EVALUATION OF SINUOUS ANTENNAS FOR REMOTE SENSING APPLICATIONS A Dissertation Presented to The Academic Faculty By Dylan Andrew Crocker In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in Electrical and Computer Engineering School of Electrical and Computer Engineering Georgia Institute of Technology December 2019 COPYRIGHT © 2019 Dylan Andrew Crocker NUMERICAL AND EXPERIMENTAL EVALUATION OF SINUOUS ANTENNAS FOR REMOTE SENSING APPLICATIONS Approved by: Dr. Waymond R. Scott Jr., Advisor School of Electrical and Computer Dr. Brett T. Walkenhorst Engineering School of Electrical and Computer Georgia Institute of Technology Engineering Georgia Institute of Technology Dr. Andrew F. Peterson School of Electrical and Computer Dr. Zhigang Peng Engineering School of Earth and Atmospheric Georgia Institute of Technology Sciences Georgia Institute of Technology Dr. Gregory D. Durgin School of Electrical and Computer Dr. Bernd H. Strassner Engineering Electronic Systems Center Georgia Institute of Technology Sandia National Laboratories Date Approved: November 1, 2019 To my wife Grace, our children Karis, Andrew, David, and Hope, and the memory of our baby Haven. ACKNOWLEDGEMENTS I would like to thank my Advisor Dr. Waymond Scott Jr., for all his encouragement, guidance, and support. His mentorship has meant a great deal to my professional and personal growth, for which I am very grateful. I would also like to thank my committee members, Dr. Andrew Peterson and Dr. Gregory Durgin, for their valuable work serving on my proposal, reading, and defense committees. Thank you also to Dr. Brett Walkenhorst, Dr. Zhigang Peng, and Dr. Bernd Strassner for their service on my final defense committee. Also, I would not be where I am today without the mentorship of Dr. Kristen Donnell, my MSEE thesis advisor. A special thank you to Dr. Strassner, who has also been a great mentor to me during my time at Sandia National Laboratories. Thank you to Sandia National Laboratories for making it possible for me to attend graduate school at Georgia Tech through their Doctoral Studies Program. I also want to thank my co-workers at Sandia for their support and encouragement throughout my Ph.D. journey. Thanks to Wade Freeman, Ward Patitz, Troy Saterthwaight, Tyler LaPointe, and Dianna Frederick for their help with my “mad scientist” measurement setups in their labs and work-spaces as well as my antenna fabrication efforts. A thank you to Doug Brown for loaning me the use of his computing equipment. Thank you to my fellow student and co-worker Zach Silva for helping me to navigate Georgia Tech. Also, a special thanks to John Borchardt and Dr. Strassner for their time reviewing my publication drafts and pro- viding valuable feedback. Additionally, I would not have been able to pursue the Ph.D. through Sandia’s doctoral studies program without the support of my manager Kurt Soren- son. Thank you also to Janea Gomez and Bernadette Montano, in student programs, for helping me to get accepted and successfully complete Sandia’s doctoral studies program. Finally, thank you to the many other colleagues who provided simple words of encourage- ment or helpful conversations that have meant much more than it might have seemed. Thank you to my fellow lab-mates, Mark Reed and Dr. Yoni Gabbay, for their help and iv encouragement. Also, thank you to Dr. Scott’s past students Dr. Mike McFadden and Dr. James Sustman, whose Ph.D. work I relied upon heavily at times. Of course, I would not have been able to graduate on time without those in the ECE advising office—especially Dr. Daniela— helping me to keep all my academic “ducks in a row.” I want to thank my family and friends without whom I could not have accomplished this work. Thank you to my wife, Grace, who spent many long hours single-handedly caring for our family while I was busy with my research deadlines as well as providing support and encouragement when I really needed it. Without her, I could not have survived all the work and stress. Thank you to my four amazing children who have provided so much happiness and relief with their smiles and hugs! I would also like to thank my parents for their encouragement and all the hard work starting my academic journey with homeschooling 25 years ago. Also, my in-laws have given my family much support—especially traveling 14 hours to watch the kids while Grace and I flew to Atlanta for my Ph.D. proposal. Thank you to my siblings and siblings-in-law (all 17 of them!) for their support throughout my life and career. A special thank you to my brother-in-law Josh Mitchell who has been a friend, colleague, and fellow Georgia Tech grad student! Thank you to all our friends at Desert Springs Church for their prayers and support of my family during our time getting this degree. Most importantly, thank you to Jesus, who has saved me from myself and continues every day to make me in all ways a better person. v DISCLAIMER This work was supported in part by Sandia National Laboratories, a multimission labo- ratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly-owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under Contract DE-NA0003525. This dissertation describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the dissertation do not necessarily represent the views of the U.S. Department of Energy or the United States Government. vi TABLE OF CONTENTS Acknowledgments . iv Disclaimer . vi List of Tables . xiii List of Figures . xiv Chapter 1: Introduction and Background . 1 1.1 GPR Antennas . 3 1.2 Sinuous Antenna Background . 5 1.2.1 Design . 7 1.2.2 Radiation Characteristics . 10 1.2.3 Feeding Mechanism . 12 1.2.4 Summary . 13 1.3 Outline . 14 Chapter 2: Mitigating Unintended Resonant Modes in Sinuous Antennas . 16 2.1 Unintended Resonances . 17 2.2 Previously Reported Log-Periodic Resonance Mitigation Techniques . 18 2.2.1 Ends Trimming Technique Applied to Single Pair of Arms . 29 vii 2.2.2 Ends Trimming Technique Applied to Sinuous Antennas with Larger Angular Width . 30 2.2.3 Summary . 31 2.3 Parametric Study . 33 2.4 Outer Truncation . 47 2.4.1 Clipping Truncation Method . 48 2.4.2 Novel Truncation Method . 49 2.5 Improved Design . 49 2.5.1 Frequency-Domain Analysis . 51 2.5.2 Time-Domain Analysis . 52 2.6 Measurements . 54 2.6.1 Antenna Fabrication . 54 2.6.2 Results . 56 2.7 Summary . 57 Chapter 3: Dispersion in Sinuous Antennas . 61 3.1 Sinuous Antenna Dispersion . 62 3.2 Log-Periodic Dispersion Model . 65 3.2.1 Off-Boresight Angles . 68 3.2.2 Effectiveness for Different Soil Environments . 69 3.3 GPR Simulations . 70 3.4 Limitations of the Log-Periodic Dispersion Model . 74 3.5 Experimental Validation . 76 3.6 Summary . 79 viii Chapter 4: Other Sinuous Antenna Design Considerations . 81 4.1 Sinuous Antenna Input Impedance . 81 4.2 Substrate Effects . 85 4.3 Absorber Loaded Cavity . 89 4.3.1 Reducing the Profile of the Antenna with Cavity . 94 4.4 Summary . 100 Chapter 5: Unbalanced Sinuous Antennas . 102 5.1 Unbalanced Four-Arm Sinuous Antenna . 102 5.1.1 Driving Port Impedance . 103 5.1.2 Far-Field Radiation . 107 5.1.3 Near-Field Sensitivity . 115 5.1.4 Polarization Performance . 120 5.1.5 Mutual Coupling . 126 5.2 Unbalanced Eight-Arm Sinuous Antenna . 131 5.2.1 Driving Port Impedance and Mutual Coupling . 133 5.2.2 Radiated Fields . 136 5.3 Constructed Antenna . 141 5.3.1 Coaxial Cable Feed . 141 5.3.2 Absorber-Loaded Cavity . 142 5.3.3 Measurements . 144 5.4 Summary . 149 ix Chapter 6: Experimental Evaluation of the Unbalanced Sinuous Antenna in a GPR Testbed . 150 6.1 GPR Testbed . 150 6.2 Targets in the Air . 151 6.2.1 Background Subtraction . 152 6.2.2 Dispersion Correction . 155 6.2.3 Circular Polarization . 158 6.2.4 Target Discrimination . 160 6.2.5 Other Targets . 161 6.3 Targets Buried in the Sand . 164 6.3.1 Data Processing . 164 6.3.2 B-scan Measurement Results . 169 6.3.3 C-scan Measurement Results . 169 6.4 Summary . 172 Chapter 7: Conclusion and Future Work . 174 7.1 Contributions . 174 7.1.1 Detailed Analysis of the Excitation and Mitigation of Unintended Resonant Modes . 174 7.1.2 Novel Sinuous Antenna Outer Truncation Technique . 175 7.1.3 Simplistic Model for the Compensation of Dispersion in Sinuous Antennas . 175 7.1.4 Development of an Unbalanced Sinuous Antenna for Close-in Sensing . 175 7.2 Future Work . 176 x 7.2.1 Sinuous Antenna Design Guidance . 176 7.2.2 Improved Sinuous Antenna Dispersion Model . 176 7.2.3 Improved Feed for the Unbalanced Sinuous Antenna . 177 7.2.4 Unbalanced Eight-Arm Sinuous Antenna . 177 7.2.5 Improvement of Off-Boresight Performance . 177 7.2.6 Inversion with Full-Wave Forward Model . 177 7.3 Publications . 178 7.3.1 Peer-Reviewed Journal Articles . 178 7.3.2 Refereed Conference Proceedings and Presentations . 178 Appendix A: Full-wave simulation tools . 181 A.1 Drawing the Sinuous Antenna Geometry . ..