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Open Bradley Sherman Spring 2014.Pdf THE PENNSYLVANIA STATE UNIVERSITY SCHREYER HONORS COLLEGE DEPARTMENT OF ELECTRICAL ENGINEERING HIGH FREQUENCY ANTENNA COUPLING STUDY BRADLEY SHERMAN SPRING 2014 A thesis submitted in partial fulfillment of the requirements for a baccalaureate degree in Electrical Engineering with honors in Electrical Engineering Reviewed and approved* by the following: James K. Breakall Professor of Electrical Engineering Thesis Supervisor John D. Mitchell Professor of Electrical Engineering Honors Adviser Keith A. Lysiak Senior Research Associate Thesis Reader * Signatures are on file in the Schreyer Honors College. i ABSTRACT The objective of this project was to develop a methodology to accurately predict antenna coupling through the use of numerical electromagnetic modeling. A high-frequency (HF) ionospheric sounder is being developed for HF propagation studies. This sounder requires high power transmissions on one antenna while receiving on another antenna. In order to minimize the coupling of high power energy back into the receiver, the transmit and receive antenna coupling must be minimized. The results of this research effort have shown that the current antenna setup can be improved by choosing a co-polarization setup and changing the frequency to 6.78 MHz. Rotating the receive antenna so that it runs parallel to the receive antenna decreases the antenna coupling by 8 dB. Typically the cross-polarization created by putting two antennas perpendicular to each other would decrease the antenna coupling dramatically, but that does not work if the feeds are along the perpendicular access. Attaining the ideal perpendicular setup is not possible in this case due to space restrictions. ii TABLE OF CONTENTS List of Figures .......................................................................................................................... iii List of Tables ........................................................................................................................... iv Acknowledgements .................................................................................................................. v Chapter 1 Introduction ............................................................................................................. 1 Chapter 2 FEKO and Friis Transmission Equation ................................................................. 3 2.1: FEKO Antenna Simulation Software ................................................................ 3 2.2: Friis Transmission Equation ............................................................................. 4 2.3: Cross Polarization ............................................................................................. 5 Chapter 3 Methods ................................................................................................................... 7 3.1: Antennas under Test .......................................................................................... 7 3.2 Checking for Resonance ..................................................................................... 9 3.3 Alternate Resonance Check ............................................................................... 10 3.4 Antenna Coupling .............................................................................................. 12 3.5 Alternate Setup ................................................................................................... 14 Chapter 4 Results and Conclusion ........................................................................................... 16 4.1 Results ................................................................................................................ 16 4.2 Conclusion .......................................................................................................... 17 Appendix A FEKO Models ............................................................................................. 18 Appendix B Data ............................................................................................................. 22 B.1: Initial Antenna Data ......................................................................................... 22 B.2: Antenna Coupling Results ................................................................................ 26 BIBLIOGRAPHY ............................................................................................................ 32 iii LIST OF FIGURES Figure 2.1. 1: Image showing currents through objects that FEKO can calculate [1] ............. 3 Figure 2.2. 1: 2 Half Wave Dipoles in Free Space .................................................................. 4 Figure 2.2. 1: 2 Half Wave Dipoles in Free Space .................................................................. 4 Figure 2.3. 1: Dipole Cross-Polarized Antenna Model ............................................................ 6 Figure 3.1. 1: 7 MHz Resonant Dipole .................................................................................... 7 Figure 3.1. 2: Barker and Williamson Folded Antenna, Model #: BWDS-90N ...................... 8 Figure 3.2. 1: FEKO Model of the Transmit Antenna ............................................................. 9 Figure 3.3. 1: VSWR of Transmit Dipole Antenna ................................................................. 11 Figure 3.4. 1: Current Antenna Setup. ..................................................................................... 12 Figure 3.4. 2: FEKO 3D Model ............................................................................................... 13 Figure 3.5. 1: Cross-Polarization Antenna Alignment Model ................................................. 15 Figure 3.5. 2: Co-Polarization Antenna Alignment Model ...................................................... 15 iv LIST OF TABLES Table 2.2. 1: Friis Transmission Formula Data, Calulated and Simulated .............................. 5 Table 3.2. 1: Linear Interpolation Points ................................................................................. 10 Table 4.1. 1: Antenna Coupling Results .................................................................................. 16 v ACKNOWLEDGEMENTS Thank you to Dr. John Mitchell for guiding me through my academic career and setting me off in the right direction. Thanks to Dr. James Breakall and Dr. Keith Lysiak, for all of the advice and guidance both of you have given me through this thesis’s completion. Dr. Nichola Gutgold, I would simply like to say thank you. Finally I would like to thank my mom, dad, and my grandparents Jack and Barb, without you my entire college career and experience would not have been possible. Thank you for pushing me to be all that I can be. 1 Chapter 1 Introduction The Penn State University is conducting research that involves measuring the altitude of the ionosphere by way of a high-frequency (HF) sounder. An HF sounder radiates a signal into a medium through a transmit antenna. An echo is then recorded on a separate antenna. By measuring the time delay that elapses between the sent and received signal, while also knowing the propagation speed of the radiated signal in the medium, the effective altitude of the ionosphere can be calculated simply by this speed and to time delay. This sounds simple but there are complications that can occur while acquiring the measurements. One of the complications comes from overlap in the transmitted signal and the received signal. If the receive antenna picks up some of the initial signal sent out, depending on the time that elapses between then and the echo, the signals can overlap yielding useless data. When the receive antenna picks up some of the initial signal that is not the echo, this is referred to as antenna coupling. Currently there are two antennas in place for the experiment that are roughly thirty-five meters from each other. This distance is not close enough to be in the near field while not far enough to be considered in the far field. Simulations are needed to calculate how much coupling exists between the antennas. Ideally the coupling should be zero. Even though there is an existing setup, it is not permanent, and a better setup may exist. In order to receive best results, the transmit antenna should be resonant at seven megahertz (7 MHz). This occurs when the imaginary part of the impedance is zero and the Voltage Standing Wave Ratio (VSWR) is minimized. Simulations and hand calculations can verify if the dipole being used presently is resonant at the specified frequency. 2 Chapter 2 outlines the processes used in the simulation software FEKO as well as the Friis transmission formula to compute coupling. Chapter 3 explains the methods used to analyze the antenna coupling and different antenna scenarios to minimize the coupling. The diagrams, tables, and results found from the methods in Chapter 3 are presented in Chapter 4 along with observations and conclusions. 3 Chapter 2 FEKO and Friis Transmission Equation 2.1: FEKO Antenna Simulation Software According to FEKO’s webpage, “FEKO is a comprehensive electromagnetic simulation software tool, based on state-of-theart computational electromagnetics (CEM) techniques. It enables users to solve a wide range of electromagnetic problems,” [1]. FEKO is capable of using models of cars, ships, planes, buildings, and anything that can be modeled in any type of CAD program to compile data on how those structures affect the antennas that are attached to them or around them. In ideal cases antennas are modeled in free space. Once objects
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