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 and a lossy ground such earth are put into the equations, hand calculations become difficult or impractical to do.

Figure 2.1. 1: Image showing currents through objects that FEKO can calculate [1] 4 FEKO calculates vast amounts of data and three dimensional images of the current flow in objects. Figure 2.1.1 is an image from FEKO’s product overview page.

FEKO is capable of calculating other parameters such as the voltage standing wave ratio

(VSWR), S-parameters, power, gain, and directivity to name a few. VSWR and S-parameters are the two parameters that are mainly desired in this project.

2.2: Friis Transmission Equation

When using simulation software it is important that the results are accurate. Comparing the results of a simple case to hand calculations using known formulas is an accurate method to complete this check. The simple case examined was the case of two dipoles in free space. Figure

2.2.1 shows the FEKO model with a distance of 10 meters between the two antennas. Notice that the dipoles are in the same vertical z direction. Because of this, the antennas are in the same polarization state (co-polarized).

Figure 2.2. 1: 2 Half Wave Dipoles in Free Space

To simplify the calculations, the simulation frequency is 299.8 MHz. Therefore the wavelength ( λ ) is 1 meter. By making the length of each dipole a half a meter, then each antenna is a half wave dipole. For a half wave dipole in free space the gain is 1.64that is a unit-less 5 number. When the receive antenna is in a far field distance, R >> λ, the Friis transmission formula is as follows:

( )

PR and PT are the power received and transmitted, respectively, GT and GR are the power transmitted and received respectively, λ is the wavelength, and R is the distance between the two antennas. According to this formula, every time the distance between the antennas is doubled the value found in units of dB will decrease by 6 dB. The only way this will work is if the receive antenna is in the far field. To confirm that FEKO is yielding accurate results, the S21 parameter

(the forward transmission gain) will be analyzed since this is the value to be analyzed for antenna coupling.

Distance Calculated (PR/PT) Difference FEKO, S21 Difference

10 m -37.7 dB - -39.2 dB -

20 m -43.7 dB 6 dB -45.25 6.05dB

40 m -49.7 6 dB -51.25 6 dB

Table 2.2. 1: Friis Transmission Formula Data

2.3: Cross Polarization

Polarization is a way to characterize how a wave is propagating through space. As the electric and magnetic fields change it can create propagations in the shape of lines, ellipses, or circles. When something is cross-polarized it means that the waves are orthogonal to each other.

When looking at the simple dipole model the power ratio goes to -73dB at 10m when cross- polarized instead of -39.2 dB when not polarized. The value of -73 dB is effectively zero. In theory it should be exactly zero, but near field effects can still affect the cross-polarized antennas, 6 even in FEKO. Figure 2.3.1 shows the model for the cross-polarization. The receive antenna is tilted ninety degrees unlike Figure 2.2.1 which has the receive antenna straight up and down in the z-direction.

Figure 2.3. 1: Dipole Cross-Polarized Antenna Model 7

Chapter 3

Methods

The following section outlines the processes developed to analyze and optimize the antenna setup. Sample results are given in some sections, but see Chapter 4 for complete results and discussions.

3.1: Antennas under Test

A 7 MHz resonant dipole antenna that is 20.117 meters long and 9.144 meters off the ground will be used as the transmit antenna, see Figure 3.1.1.

Figure 3.1. 1: 7 MHz Resonant Dipole

8 The feed is in the middle, the other pieces hold the ends of the wires to the poles that hang the antenna. Metal hooks take the tension of the wire so the feed point does not take the tension and break. Using this kind of setup for the dipole keeps it electrically isolated from metal connectors that keep the tension on the dipole.

The receive antenna is a Barker and Williamson Folded Dipole where the wire length is

27.432 meters long and terminated with a 600 Ohm load, see Figure 3.1.2.

Figure 3.1. 2: Barker and Williamson Folded Antenna, Model #: BWDS-90N

The little wires loops seen in the image are there because the white connectors take the tension of the wire. Cardboard tubes are holding the rolled up wires. Unrolling them will yield the entire dipole as it will be used that is the Barker and Williamson Folded Antenna, Model #:

BWDS-90N.

9 3.2 Checking for Resonance

As outlined in Chapter 2, FEKO can determine the impedance of an antenna and break it down into its real and imaginary components. By finding the impedance at different frequencies, a frequency where the imaginary component is zero can be found from linear interpolation.

Figure 3.2.1 is an example of the FEKO model of the transmit antenna.

Figure 3.2. 1: FEKO Model of the Transmit Antenna

Let F1 and F2 denote the two chosen frequencies, and X1 and X2 stand for the imaginary part of the impedance. The linear interpolation formula is as follows:

, the Slope then equals

[ ]

Therefore, [ ]

10 After running two simulations at two different frequencies the following points were found.

Table 3.2. 1: Linear Interpolation Points

Frequency (MHz) Imaginary part of Impedance (Ohms)

7 46.7

6.8 5.13

[ ] [ ]

This shows that the transmit antenna is actually resonant at 6.78 MHz according to

FEKO.

3.3 Alternate Resonance Check

Another way to check the resonance and to confirm the results found in section 3.2 is by finding the voltage standing wave ratio (VSWR) of the antenna at multiple frequencies. The

VSWR is a numerical measurement that describes how well the antenna’s impedance matches the impedance of the source. The antenna’s impedance changes with frequency. When the antenna has a minimum VSWR, it means that the impedance has a best match at the frequency of interest.

It also means that the antenna is close to resonant at that frequency [2]. Using the same model found in Figure 3.2.1, one can find where the VSWR is closest to one.

11

Figure 3.3. 1: VSWR of Transmit Dipole Antenna

As shown in Figure 3.3.1, the VSWR is a minimum at 6.78 MHz with a value of 2.4. A

VSWR of 1 means that there is a perfect match of impedances and no power is reflected back.

With real word applications, a VSWR of around two is accepted to be a good match. With the use of a matching network, it is easy to get a lower VSWR. In this case we have a VSWR of 2.4 which is roughly a good match, but most importantly, it confirms that the resonant frequency of the transmit dipole antenna is close to 6.78 MHz. 12 3.4 Antenna Coupling

To measure the antenna coupling, the transmission parameter S21 is calculated using

FEKO. The subscripts stand for the ports. In this case port 2 is the receive folded dipole antenna and port 1 is the transmit dipole antenna. S21 is the effect coupling from port 1 to port 2, therefore the transmission parameter. Altogether the Scattering parameter matrix is as follows:

[ ]

S11 and S22 are the reflection coefficients at each port and S12 is the reverse transmission parameter. S11 and S21 are helpful to find out how much power will be delivered to the antenna from the source, but S22 and S12 are not needed for this project.

By using the information in Figure 3.4.1, the location of the antennas can be placed in

FEKO to create a three dimensional model. Figure 3.4.2 shows the produced model.

Figure 3.4. 1: Current Antenna Setup. 13

Figure 3.4. 2: FEKO 3D Model

Constructing the building is not just putting a box in the model. The building is made of metal, that to a good approximation a perfect electric material (PEC) is chosen. The metal the building is made out of has a high conductivity that it can be approximated by a PEC with little error. How the building is grounded needs to be taken into account. There is a foundation in between the metal and the ground with several grounding rods placed around the building.

Information given for this simulation is to use rods spaced evenly around the edges of the building that give a 0.25 meter gap between the building and the ground with 1 meter going underneath (or into) the ground plane. It is shown as a plane in the simulation, but it has the same effect as an infinite lossy solid ground.

When creating the model there are several parameters to consider. When creating the ground plane it has to mimic earth. An average relative permittivity of earth is 13 and the conductivity is .005 S/m (siemens per meter). Theses parameters are set for the infinite ground plane show in green in Figure 3.4.2. There are several options when creating a ground plane. For this simulation the ‘Homogeneous half space in region z<0 (exact Sommerfield integrals)” option 14 can handle wires that are near or penetrate the ground plane. The only other option for an imperfect ground plane uses a ‘reflection coefficient approximation’ which cannot handle wires near or in the ground plane.

The next parameters that have to be considered are triangle edge length, wire segment length, and wire segment radius. These parameters are needed when creating a “mesh.” A mesh in FEKO is how small each piece of each antenna and three dimensional objects will be when simulating. Using two tenths of the wavelength works well for the triangle edge length. Wire segment radius was chosen to be 0.4 meters. Finally the wire segment radius is 0.00328 meters.

That is 0.001 feet.

Before simulating the S-parameter configuration needs to be setup. Port one is chosen to be the transmit antenna with an input impedance of 50 Ohms. While port 2 is the receive antenna with an input impedance of 450 Ohms. Those are real world impedances for the antennas.

3.5 Alternate Setup

After finding what the antenna coupling presently is, there are alternative antenna setups that are being considered. The limited amount of space at the property has to be taken into account as well. Two extreme cases for the setup are to take the receive antenna perpendicular

(cross- polarization) and parallel (co-polarized) to the transmit antenna shown in Figure 3.5.1 and

Figure 3.5.2 respectively.

15

Figure 3.5. 1: Cross-Polarization Antenna Alignment Model

Figure 3.5. 2: Co-Polarization Antenna Alignment Model

16

Chapter 4

Results and Conclusion

Section 4.1 outlines the numerical results of the simulations. Section 4.2 explains the findings and gives recommendations for the antenna setup.

4.1 Results

The following table shows the results of the antenna coupling for all of the simulations at different frequencies. The frequencies plotted are 6.78, 7, and 7.22 MHz. 6.78 MHz is what earlier calculations and simulations showed to be the optimal frequency for the antennas while 7

MHz is the frequency that is in use now. 7.22 MHz is also used to see what the trend of the coupling is around 7 MHz. See Appendix B.2 for all of the antenna coupling graphs.

ANTENNA COUPLING Coupling at Different Frequencies in dB 6.78 MHz 7 MHz 7.22 MHz Description Building's Building's Building's S21 S21 S21 Affect Affect Affect

Current Setup -38.6752 -39.1580 -41.5905 4.9148 5.4154 4.5572 Currect Setup, No Building -43.5900 -44.5734 -46.1477

Cross-polarized -37.2130 -38.3519 -40.0648 0.1791 0.3401 0.2581 Cross-polarized, No Building -37.3921 -38.6920 -40.3229

Co-polarized (Parallel) -45.5771 -46.6229 -48.1766 -1.1121 -1.1273 -1.1181 Co-polarized, No building -44.4650 -45.4956 -47.0585

Table 4.1. 1: Antenna Coupling Results 17 4.2 Conclusion

Choosing to use the co-polarized option is the best setup to receive the least amount of antenna coupling. This finding does not agree with the Friis transmission formula. In the simple dipole model cross-polarization should decrease the antenna coupling. Increasing the distance between the antennas is not an option because of the limited space at the site or, as outlined in section 2.2, the coupling could be reduced be 6 dB every time the distance is doubled.

Using the co-polarized setup decreases coupling by 8.2 dB. Since decreasing the coupling by 3 dB is equivalent to decreasing the power transfer by 50 percent, 8 dB is a significant decrease. Looking at the effect that the building has on the different options, the co-polarized option is the only one where the building helps to decrease coupling.

Unlike the cross-polarization theory explained in section 2.3, these antennas are encountering near field effects. The near field effects can react different for every antenna. In this case cross-polarizing increases the coupling while co-polarized decreases coupling. It is important to note that the since the antennas are not perfectly perpendicular, small shifts can dramatically change the coupling as shown in the results. Since the impedances are not matched, the coupling is again not as low as the theory states.

Measured data obtained from the current antenna setup shows that for the same frequency range the antenna coupling is approximately -50dB. This is not reflected in the results. There must be other affects that are not accounted for in FEKO. Buildings around the antennas and the ground may be reducing the coupling.

18 Appendix A

FEKO Models

Figure A. 1: Transmit Antenna, Resonant Dipole

Figure A. 2: Receive Antenna, Folded Dipole 19

Figure A. 3: Combined Model for Transmit and Receive Antennas

Figure A. 4: Combined model with Simulated Earth Ground. For earth, relative permittivity ( εr) is 13 and conductivity ( σ ) is .005 S/m. 20

Figure A. 5: Complete Model. The building is modeled as a Perfect Electric Conductor (PEC).

Figure A. 6: Complete Model. The antennas are 90 deg cross polarized. 21

Figure A. 7: Complete Model. The antennas are 180 degrees cross polarized.

22

Appendix B

Data

B.1: Initial Antenna Data

Figure B.1. 1: VSWR of the transmit dipole antenna with a more zoomed in view. As found in section 3.2, the best VSWR is found at 6.78 MHz. This confirms that this antenna is resonant at 6.78 MHz. This is an alternate method to finding the resonant frequency of an antenna. 23

Figure B.1. 2: Reflection coefficient or S11 parameter. This shows that the transmit antenna has the least amount of reflections at 6.78 MHz. 24

Figure B.1. 3: VSWR of the receive folded dipole antenna. The minimum VSWR is found at 5.84 MHz. The folded dipole has a 1:9 balun on the input. Therefore the input impedance of 50 Ohms transforms to 450 Ohms. 25

Figure B.1. 4: Reflection coefficient of the folded dipole. Least amount of reflections at 5.84 MHz.

26 B.2: Antenna Coupling Results

B.2. 1 Antenna Coupling with the Current Setup 27

B.2. 2: Antenna Coupling with the Current Setup but with no Building 28

B.2. 3: Cross Polarization Antenna Coupling

29

B.2. 4: Cross Polarization Antenna Coupling with no Building

30

B.2. 5: Parallel Setup Antenna Coupling

31

B.2. 6: Parallel Setup Antenna Coupling with no Building 32

BIBLIOGRAPHY

1] "Overview of FEKO," EM Software & Systems-S.A. (Pty) Lyd., [Online]. Available:

http://www.feko.info. [Accessed 2014].

2] C. A. Balanis, Antenna Theory Analysis and Design, 3rd ed., Hoboken, New Jersey: John

Wiley & Sons, Inc., 2005.

3] F. T. Ulaby, E. Michielssen and U. Ravaioli, Fundamentals of Applied Electromagnetics,

Upper Saddle River, New Jersey: Pearson EDucation, Inc., 2007.

4] EM Software & Systems-S.A. (Pty) Ltd, FEKO Comprehensive Electromagnetic Solutions

User's Manual Suite 6.2.2, Stellenbosch, South Africa, 2013.

5] M. Steer, Microwave and FR Design A Systems Approach, 2nd ed., D. R. Kay, Ed., Edison,

New Jersey: Scitech Publcishing, 2013, pp. 420-429.

ACADEMIC VITA

Bradley Sherman

200 Bradley Ave. Apt. 2 State College, Pa. 16801

[email protected] ______

Education

B.S., Electrical Engineering, May 2014, The Pennsylvania State University, University Park, Pennsylvania

Honors and Awards

 Evan Pugh Scholar Junior Award, Spring 2013  President’s Sparks Award, Spring 2012  President’s Freshman Award, Spring 2011  William J. and Ethel Harer Madden Memorial Honors Scholarship – 2013, 2014  Fred A. Pechter Scholarship – 2013, 2014  Warren J. Cease Memorial Scholarship – 2010, 2011

Association Memberships/Activities

 Eta Kappa Nu (HKN) Electrical Engineering Honor Society– Inducted Fall 2012

 Tutoring Chair for Eta Kappa Nu – 2013, 2014

 Blue and White Society Member – 2010-2014

Professional Experience

 Phasor Corporation: Electrical Engineering Intern (Summer 2011). The overall project was to turn a hundred year old house into a modern smart house. I helped in the initial setup of the power requirements for the house for the summer. Very hands on experience geared toward learning about the power side of Electrical Engineering.