AN ACTIVE RETRODIRECTIVE ANTENNA ARRAY by AMY

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AN ACTIVE RETRODIRECTIVE ANTENNA ARRAY

AMY FLEISCHMANN, B.S.E.E.

A Thesis

ELECTRICAL ENGINEERING

Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of

MASTER OF SCIENCES

ELECTRICAL ENGINEERING

Approved

Dr. Mohammad Saed Chair of Committee

Dr. Changzhi Li

Peggy Gordon Miller Dean of the Graduate School

August, 2011

Table of Contents

Abstract ...... iv List of Figures ...... v Chapters

I. Introduction ...... 1 II. Background ...... 2 2.1 Methods Retrodirectivity ...... 2 2.2 The Active Retrodirective Antenna Array ...... 3 2.3 Applications of Retrodirective Antenna Arrays ...... 5 2.4 Metamaterial Motivation ...... 7 2.5 Metamaterial Background ...... 8 III. System Overview ...... 11 3.1 System Setup ...... 11 3.2 Active Components ...... 12 VI. Antenna Design and Development ...... 13 4.1 Slot Antenna Background ...... 13 4.2 Slot Antenna Design ...... 14 4.3 Slot Antenna Fabrication ...... 17 4.4 Slot Antenna Testing and Results ...... 18 4.5 Antenna Array ...... 20 4.6 PEC Reflector ...... 23 V. Retrodirective Antenna Array ...... 28 5.1 Retrodirective Antenna Array Fabrication ...... 28 5.2 Retrodirective Antenna Array Testing Setup ...... 29 5.3 Retrodirective Antenna Array Measurements ...... 31 VI. Metamaterial Ground Plane ...... 34 6.1 Motivation ...... 34 6.2 The Mushroom Structure ...... 34 6.3 Metamaterial Design and Results ...... 35 ii

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VII. Future Work ...... 40 7.1 Solar Antennas ...... 40 7.2 Additional Future Work ...... 41 VIII. Conclusions ...... 43 References ...... 44 Appendix A ...... 46

iii

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Abstract

Retrodirective antenna arrays utilize an array of antennas to redirect a signal back in the direction it originated from without the use of digital circuitry, microcontrollers, or sophisticated signal processing algorithms. Passive retrodirective arrays, like the Van

Atta array, use equal length transmission lines to redirect the signal. Active redrodirective arrays use mixers to create the phase conjugation necessary to redirect the signal. Applications include high-speed tracking, mobile communication, RADAR, and sensors. This thesis focuses on designing antennas for the retrodirective array to lay the groundwork for future enhancements. While patch and slot antennas are investigated, slot antennas are selected to provide a foundation for the implementation of solar cells.

Due to the bidirectional radiation pattern inherent to slot antennas, a high impedance ground plane is investigated for its low profile reflecting properties. This metamaterial based plane is comprised of mushroom type structures than exhibit useful properties in their respective electromagnetic bandgap. By using this high impedance plane in association with the retrodirective array, the foundation for the implementation of solar cells is set for a solar antenna based retrodirective array.

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List of Figures

2.1 Van Atta Array [3] ...... 3 2.2 Phase Conjugation Retrodirective Antenna Array [3] ...... 4 2.3 RAA Transponder Application [3] ...... 6 2.4 Retrodirective Radar versus Phased Array Radar [7] ...... 7 2.5 Elecromagetic Band Gap [8] ...... 8 2.6 EBG Antenna Radiation Pattern [8] ...... 10 3.1 RAA Block Diagram...... 11 4.1 Slot Antenna Example [11] ...... 14 4.2 Slot Antenna Design ...... 15 4.3 Overlap Matching ...... 16 4.4 S11 of Slot Antennas ...... 16 4.5 Slot Antenna Radiation Pattern ...... 17 4.6 Fabricated Antenna Front/Back ...... 18 4.7 Slot Antenna S11 Measured...... 19 4.8 Measured Slot Antenna Radiation Pattern ...... 20 4.9 Four Element Antenna Array ...... 21 4.10 S11 of Antenna Array ...... 22 4.11 Antenna Array Radiation Pattern ...... 23 4.12 Slot Antenna with Reflector...... 24 4.13 S11 of Slot Antenna with Reflector ...... 24 4.14 Radiation Pattern of Slot Antenna with Reflector ...... 25 4.15 Slot Antenna vs. Slot Antenna with Plane Reflector Radiation Pattern ...... 26 4.16 Antenna Array with QWR ...... 27 5.1 RAA Layout (in mm) ...... 28 5.2 Fabricated RAA ...... 29 5.3 Monostatic Testing Setup ...... 30 5.4 Bistatic Testing Setup ...... 31 5.5 Monostatic Testing Results ...... 32 5.6 Bistatic Testing Results...... 33 6.1 Mushroom Structure Example [8] ...... 34 6.2 Mushroom Structure Top and Side View ...... 35 6.3 Slot Antenna with EBG ...... 36 6.4 Slot Antenna with EBG Radiation Pattern...... 37 6.5 Radiation Pattern Comparison of EBG and Plane Reflector ...... 38 6.6 Antenna Array with EBG Radiaiton Pattern ...... 39

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A.1 High Impedance Plane vs. QWR ...... 46 A.2 Antenna Array with EBG S11 ...... 47 A.3 Antenna Array with EBG Pattern 2 ...... 47

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Chapter 1 Introduction

Antenna arrays are composed of multiple antenna elements that function as a single larger antenna. As more elements are added, the directivity of the array increases.

With increased directivity, the array is capable of beam steering and scanning with additional electronics. A retrodirective antenna array (RAA) is an array of antennas that receives a signal from an unknown direction and returns the signal in that same direction.

This array does not use digital circuitry, phase shifters, microcontrollers, or sophisticated signal processing to determine the direction.

Retrodirective antenna arrays are currently used for tracking, sensor, and radar applications; however, still being in a development stage, this technology has potential applications in many areas including communication systems, RFIDs, or almost anything that transmits and receives data.

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Chapter 2 Background

2.1 Methods Retrodirectivity

The retrodirective aspect of an array can be achieved in multiple ways. First, a corner reflector can be used to redirect electromagnetic waves back in the original direction. The corner reflectors are used in RADAR systems as a passive target (when the angle is 90 degrees) because the reflector will return the signal exactly in the direction it was received [1]. This concept can be applied to the antenna array achieving the retrodirective task; however, the dimensions required for these reflectors create bulky systems and, therefore, prohibit their use. In a 90 degree corner reflector the length of the sides is typically twice the feed-to-corner spacing (l= 2s), where the feed-to-corner spacing is between 0.25λ and 0.7λ [2]. This creates a large apparatus undesirable for this application. In addition, in a corner reflector the redirected signal cannot be amplified or modulated. Another way to achieve retrodirectivity uses the relative amplitude and phase of each antenna element to determine the direction. Best stated in [3], “the array excitation phase gradient determines the direction of the main beam.” Therefore, by changing the amplitude and phase of each element, the main lobe direction can be managed. The Van Atta array uses this concept to achieve retrodirectivity. The Van Atta array is composed of pairs of antennas connected with equal length transmission lines.

These transmission lines reverse the incoming gradient directing the beam back in the direction it originated, as illustrated in Figure 2.1 [3]. The gradient reversal is achieved through the transmission lines. Since the transmission lines are of equal length, each path 2

Texas Tech University, Amy Fleischmann, August 2011 length is equal. The signal is received by one antenna and radiated by the other antenna in the pair, flipping the order of the elements [3]. For example, if the right most element receives the signal first, the left most element radiates the signal first. Flipping the order of the elements reverses the gradient for retrodirectivity. While the Van Atta array is wide band (only limited by the bandwidth of the antenna), it requires that the incoming wave and array be planar which restricts its use [3]. Symmetry is also essential for the

Van Atta array. In order for this configuration to work, the antennas are required to be spaced evenly, the transmission lines must be of equal length, and a symmetric system overall is required (even number of elements) [4].

Figure 2.1: Van Atta Array [3]

The gradient reversal can also be achieved using phase conjugation through heterodyne mixing. This is the active retrodirective array and is the basis for this project.

2.2 The Active Retrodirective Antenna Array

Instead of reversing the gradient through pairs of antennas, the active RAA uses mixing to conjugate the phase at each antenna element, as shown through Figure 2.2.

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Using this technique, the array is able to phase conjugate irregular (non-planar) wave fronts, conform to surfaces, and compensate for phase distortion [5].

Figure 2.2 : Phase Conjugation Retrodirective Antenna Array [3]

For heterodyne mixing, the incoming signal (RF) mixes with the local oscillator

(LO) which is twice the RF. In resulting signal the lower sideband has the same frequency with a conjugated phase (shown in equation 1 below) [3].

푉퐼퐹 = 푉푅퐹 cos 휔푅퐹푡 + 휃푛 ∙ VLO cos 휔퐿푂푡 (1)

1 = 푉 V [cos ω − ω t − θ + cos (ω + ω t + θ )] 2 푅퐹 LO LO RF n LO RF n

Typically the RF and IF are identical; however, retrodirectivity can occur with frequency difference. If the RF and IF are not identical, the phase will be conjugated, but there will be a pointing error in the return beam, depending on the frequency difference

[3]. Since it is difficult to differentiate between the RF and IF signal if both are the same

Texas Tech University, Amy Fleischmann, August 2011 frequency, either the frequencies should be offset slightly, or the circuit should be designed such that the RF frequency is suppressed.

When using antenna arrays, it is common practice to set the distance between antenna elements to half-wavelength to prevent grating lobes. For retrodirective arrays, the antenna spacing correlates to the angle of incidence, shown in the following equation.

From equation 2, it can be shown that the array spacing should be less than half wavelength to scan the entire range (-90 degress to 90 degrees) [3].

휆 (2) 푑 < 0 1 + sin 휃푖푛

However, for this project, the distance between antenna elements is set to a half- wavelength. This will provide sufficient scanning range for a proof of concept.

2.3 Applications of Retrodirective Antenna Arrays

Applications for retrodirective antenna arrays vary depending on the configuration. From the inherent tracking nature of the RAA, they are useful for RFID aplications, microwave tracking beacons, and highspeed tracking [3]. Without the use of digital circuitry or signal processing, a RAA can track high speed objects. There is also work being done to use a RAA as a multiple target tracker. According to [6], the RAA will simultaneously track each individual target if each target operates at a different frequency. The speed of the RAA is also useful for sensor applications, especially in situations where it is difficult or hazardous to humans. In this application, the RAA would act as a transponder sending data back to the interrogator after being asked. By 5

Texas Tech University, Amy Fleischmann, August 2011 using a RAA, the interrogator would not have to remain stationary such that a helicopter or other mobile object could be used to gain information from sensors without stopping or stalling for each sensor [3]. A visual image is shown in Figure 2.3.

Figure 2.3 : RAA Transponder Application [3]

While most projects using a RAA focus on the array itself, modulation is possible making a sensor even more valuable when the signal is encoded with data.

There are applications within RADAR for RAAs. Integrating a RAA with the radar systems accelerates the detection of targets [7]. In the system described in [7], the radar system first sends out an omnidirectional pulse. As this pulse is reflected, each successive pulse become more directed toward the object, significantly improving the signal-to-noise ratio(SNR). In Figure 2.4, this system is compared to a phased array system.

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Figure 2.4 : Retrodirective Radar versus Phased Array Radar [7]

The retrodirective antenna based radar has a lower acquisition time when compared to the phased array radar [7]. In addition, due to the retrodirective nature, the radar system is also self-tracking, following the previous applications.

2.4 Metamaterial Motivation

For retrodirective antenna arrays, a secure and more efficient system can be achieved if the radiation pattern is unidirectional. Some antennas, such as patch antennas, are inherently unidirectional; however, due to their narrow bandwidth and other considerations to be discussed in later chapters, patch antennas are not always the optimum choice for antenna elements. Other antennas, such as the slot antennas used for this design, are inherently bidirectional. In order to create a unidirectional pattern, a reflector is necessary.

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2.5 Metamaterial Background

A normal plane reflector is sufficient to reflect the wave into the desired direction.

However, plane reflectors have a distance requirement of at least a quarter wavelength.

Similar to the corner reflector discusses previously, the dimensional requirements are not desirable for this application. In order to create a low profile reflector, a high impedance metamaterial surface serves as an artificial ground plane. In effect, it is a perfect magnetic conductor. The surface is composed of periodic copper patches with plated vias to the bottom copper plane. This structure can be visualized as mushrooms and can be modeled as a LC circuit, where the capacitance occurs between the patches and the inductance occurs through the via to ground [8]. This structure can also be thought of in terms of an electromagnetic band gap structure. Below resonance, the surface is inductive and supports TM surface waves while above resonance, the surface is capacitive and supports TE surface waves; however, near resonance, the surface supports neither creating a band gap [8].

Figure 2.5 : Electromagnetic Band Gap [8]

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More importantly for this application is the reflection phase. A typical reflector is an electric conductor which is a low impedance surface. The ratio of electric field to magnetic field is small and correlates to an electric field node at the surface and a magnetic field antinode at the surface [8]. The high impedance surface exhibits the exact opposite. For the high impedance surface, the electric field has an antinode at the surface, and the magnetic field has a node at the surface; this is characteristic of an artificial magnetic conductor [8]. The reflection phase within the band gap follows this pattern. While in an electric conductor the phase is –π and out of phase, the magnetic conductor has opposite findings. When in the band gap, the reflection phase falls between π/2 and –π/2 which is in phase [8]. This means that the image currents are constructive, not destructive, which is important for reflectors. This surface has been previously proved to work as a reflector with results shown below.

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Figure 2.6 : EBG Antenna Radiation Pattern [8]

These results are for a monopole antenna (shown in (a)). The normal pattern with a flat metal ground plane has significant back radiation (b). By adding the high impedance surface, the back radiation is significantly reduced within the band gap(c). (d) shows the pattern outside the band gap. With these properties and proof, the high impedance surface is ideal for use as a reflector in this application.

For this project, the high impedance plane is used as a reflector for a retrodirective antenna array. The antenna design produces an undesired bidirectional beam which a reflector can eliminate. By using a high impedance ground plane instead of a plane reflector, the overall system remains low profile which increases the applications for its use.

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Chapter 3

System Overview

3.1 System Setup

The general block diagram for the array consists of antenna elements, mixers, low noise amplifiers, and a local oscillator. There are four antenna elements in a planar array; although, this concept can support more antenna elements and configurations. The local oscillator is divided to provide for all four antennas. Each antenna element has its own mixer and amplifier. Each of these active elements requires power to function.

Figure 3.1 : RAA Block Diagram

Due to the system requirements of the LO being double the RF and the IF equal to the RF, there is limited inexpensive, readily available parts for this project. From those parts available, the maximum IF is around 3 GHz. To remain under this requirement, the

RF/IF are decided to remain at 2.5 GHz and the LO at 5 GHz. This parameter is important to begin the antenna design process discussed in Antenna Design and

Development.

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In order to prevent crosstalk issues during testing, separate antennas are designed for transmission and reflection at slightly offset frequencies. As stated before, a slight offset in frequency between the RF and IF will minimally affect the retrodirectivity by introducing a minimal beam error. In addition to offset frequencies, the antennas are polarized orthogonally. One antenna is vertically polarized while the other is horizontally polarized. Again, the differences are used to reduce the crosstalk and error in testing.

3.2 Active Components

The active components within this design are a mixer and low noise amplifier.

The mixer selected for this project is a double balanced passive mixer, part number SIM-

83+, from Minicircuits. This mixer provides a conversion loss of 6 dB while consuming no power, being input/output matched to 50 Ω, and expecting a LO power of +7 dBm for optimum results [9]. The amplifier selected is the MNA-7+ also from Minicircuits. This low noise amplifier is internally input/output matched to 50 Ω and requires a minimal bias network. The bias network for this amplifier is a resistor and capacitor only for the

DC bias because the amplifier includes an internal bias network as well. The amplifier provides 15 dB of gain at the frequency of interest with a +5 V supply and a current draw

73 mA on average and a noise figure of 6.9 dB at 2 GHz [10]. For the initial testing phase, an external signal source is used as the local oscillator. Using an external source allows flexibility during the testing phase and removes another level of troubleshooting.

Future designs plan to use a voltage controlled oscillator. The fabrication and testing of the RAA is discussed in Chapter 4. 12

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Chapter 4

Antenna Design and Development

4.1 Slot Antenna Background

Slot antennas are selected for this project for a few reasons. First, slot antennas are planar and a low profile design, which is desired for this application. Second, slot antennas have a wider bandwidth than patch antennas. In addition, slot antennas provide a platform for the future work on this project, solar antennas, to be discussed later. For this future work, a simple geometric shape is necessary so a rectangular slot antenna is designed.

A microstrip slot antenna is comprised of a geometrical slot, in this case rectangular, cut out of the ground plane. The antenna is fed by a microstrip line on the other side of the board. Typically, a matching network is required to match the slot antenna impedance at the center to the microstrip line. There are multiple ways to match the antenna and the microstrip, including off-set feed and introducing stub tuning [11].

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Figure 4.1 : Slot Antenna Example [11]

For this design, an open circuit microstrip line is used to feed the antenna with an overlap to match impedances. The slot itself is half-wavelength in length. The radiation pattern of slot antennas is naturally bidirectional.

4.2 Slot Antenna Design

The design for the slot antennas is based first on the frequency of interest. As stated previously, the two antennas, one for transmitting and one for receiving, are at different polarizations and frequencies to eliminate problems during testing. One antenna will be vertically polarized while the other is horizontally polarized. The frequency of interest for RF and IF is around 2.5 GHz, as discussed previously. The dielectric substrate selected for this project is the Rogers Duroid 6010 with a dielectric constant of

10.2 and a height of 50 mils. While a lower dielectric constant will have better radiation and less surface waves, the slot antenna would be too large in dimensions for this project.

Using these parameters, the slot antenna is designed and simulated. For the initial design, 14

Texas Tech University, Amy Fleischmann, August 2011 the antenna is determined using the dielectric constant of 10.2 as stated in the board characteristics. However, through simulation, the effective dielectric constant with the slots cut out is closer to 4 or 5 which changes the slot antenna design. The final slot antenna design of both antennas is shown in Figure 4.2.

Figure 4.2 : Slot Antenna Design

The overlap is minimal; however, it is important to match impedances and allow coupling from the microstrip feed and the antenna. A closer view of the overlap is shown in Figure 4.3.

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Figure 4.3 : Overlap Matching This overlap matching is found by optimizing the overlap and the S11 parameter.

For the finalized design, the S11 matching is below -10 dB, which is the typical criteria for a well matched system. The S11 for the slot antennas is shown below in Figure 4.4.

Figure 4.4 : S11 of Slot Antennas

The radiation pattern for a slot antenna is bidirectional, as stated previously. This bidirectional pattern is shown in Figure 4.5.

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Figure 4.5 : Slot Antenna Radiation Pattern

It is important to note that the distance between the slot antennas will directly affect the matching and the radiation pattern. Having the antenna overlap in a cross or

„x‟ style is more desirable than having a large gap between the antennas. However, due to the nature of high dielectric constants allowing surface waves, there is high coupling between the two slot antennas. There is even an additional resonance due to the coupling between the two antennas. From this information, the decision is made to distance the two antennas until the surface waves have diminished enough to reduce coupling.

4.3 Slot Antenna Fabrication

The antenna design is fabricated until a milling machine. Although the milling machine is capable of the tolerances necessary to mill the antennas, there are still fabrication tolerance errors. On the signal plane with the microstrip feed, the remaining

Texas Tech University, Amy Fleischmann, August 2011 copper is rubbed off to reduce the possibility of extraneous errors. The finalized board is shown in Figure 4.6.

Figure 4.6 : Fabricated Antenna Front/Back

In the fabricated board, there are a few minimal errors, namely in the feed to one of the antennas. However, as shown in the results, the affect of this is minimal.

4.4 Slot Antenna Testing and Results

The slot antennas are tested using a network analyzer interfaced with data acquisition software on a pc. In order to acquire a radiation pattern, the antenna is rotated on a turn table controlled by the data acquisition software. The results are shown in the following figures. There is a frequency shift in the antennas due to the fabrication tolerances.

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Figure 4.7 : Slot Antenna S11 Measured

As shown through above, the frequency shift is 3% for the lower frequency and

7.9% for the upper frequency. The shift in frequency is caused by the tolerances of the fabrication process. For the slots, a small change in the length can have large effects in the frequency. In order to reduce these effects, a different fabrication process or a milling machine with tighter tolerances is required. The radiation pattern follows the simulated pattern, as shown below in Figure 4.8.

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Figure 4.8 : Measured Slot Antenna Radiation Pattern

The radiation pattern of the measured slot antenna is similar with respect to the simulated slot antenna. The shift in degree is caused by the testing setup. Overall, the slot antennas perform as required for this project. The radiation pattern and matching characteristics are sufficient for the requirements of this project.

4.5 Antenna Array

The next step is to use these antennas in a four element array. For time and resource purposes, the antenna array is only simulated in the most elementary array configuration where the array elements lie along a straight line [2]. When an array is

Texas Tech University, Amy Fleischmann, August 2011 used, the configuration correlates to the type of array factor. The array factor directly correlates to the radiation pattern of the array.

Figure 4.9 : Four Element Antenna Array

The array is fed using the same corporate power divider feed as the entire retrodirective antenna array. This feed consists of three 3 dB power dividers to ensure the phase and power at each element is equivalent. The S11 matching for this array is sufficient, as shown in Figure 4.10.

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Figure 4.10 : S11 of Antenna Array

While the S11 shows the matching is maintained, the radiation pattern is of more interest. As shown previously, the radiation pattern of a single slot antenna is bidirectional with minimal side lobes. Now, the radiation pattern (Figure 4.11) of the array is more directional, although still bidirectional. The phi plot corresponds to co- polarization plot, and the theta corresponds to the cross-polarization plot. Note that there are now side lobes included in the pattern.

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Figure 4.11 : Antenna Array Radiation Pattern

4.6 PEC Reflector

Normally, a copper plane reflector is used to create a unidirectional beam from a bidirectional pattern. This is especially applicable with a planar array of planar antennas.

The distance for reflectors is typically a quarter wavelength away. For 2.4 GHz, the distance is 30 mm. A single slot antenna is simulated with a copper (perfect E) conductor placed 30 mm away, as depicted below.

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Figure 4.12 : Slot Antenna with Reflector

Adding the reflector increases the matching as shown through the S11. The frequency remains at 2.38 GHz as before.

Figure 4.13 : S11 of Slot Antenna with Reflector

More importantly is the affect of the reflector on the radiation pattern. As shown below in Figure 4.14, the reflector diminishes the back lobe in the radiation pattern.

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Figure 4.14 : Radiation Pattern of Slot Antenna with Reflector

When comparing this pattern to the previous pattern without the reflector, the reduction in the back radiation is evident. The next figure shows the two patterns overlapped.

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Figure 4.15 : Slot Antenna vs. Slot Antenna with Plane Reflector Radiation Pattern

After overlapping the patterns, the reduction in the back lobe is more evident.

The back lobe maximum (at 180 degrees) is reduced by 5 dB, and at the 60 degree offset points (120 degrees and 240 degrees) the pattern is reduced by 10 dB. The front lobe is increased by 4 dB at the maximum and is more directive. Both of these results are desirable changes; however, as stated before, the plane reflector sits at 30 mm below the

RAA which is undesirable.

When used with the antenna array, the PEC becomes more problematic, as shown through Figure 4.16. In this figure, the antenna array is simulated with the same reflector as previously simulated with the single slot antenna.

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Figure 4.16 : Antenna Array with QWR

As shown through the figure, the reflector does reduce the back lobe; however, the front lobe is also sacrificed. The overall reduction in the pattern is not desired for this application and, in fact, means that the reflector isn‟t reflecting properly. In order to fix this problem, the ground plane can be enlarged to extend beyond the substrate. However, this, again, creates an even larger profile for the antenna array. The required enlargement is an additional motivation for the metamaterial reflector.

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Chapter 5

Retrodirective Antenna Array

5.1 Retrodirective Antenna Array Fabrication

The new antenna designs are incorporated into the antenna array. The layout for the mixer and amplifier are found in their respective data sheets. In order to power the mixers with equal amounts, a corporate fed series of 3 dB power dividers is used. Each power divider splits the power in half, or 3 dB; therefore, the power entering the network should be 6 dB higher than the desired level which is +7 dBm. The final layout of the

RAA board is shown below.

Figure 5.1 : RAA Layout (in mm)

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The size of the board is approximately 200mm by 280 mm. The size of the board is due to the antenna design, the antenna spacing, and the number of elements in the array. The board is fabricated using a milling machine. Again, the excess copper is removed from the feed layer in order to prevent interference and noise in the system. The fabricated board is shown below.

Figure 5.2 : Fabricated RAA

The fabricated board was populated using a heat pencil and solder paste. The

MMICs and bias network require the use of a microscope or magnifying source to ensure correct application. The MMICs require a grounded pad underneath the chip which is achieved by extending the grounded pads out.

5.2 Retrodirective Antenna Array Testing Setup

The testing set up for this type of device is intricate. First, two antennas are required for transmitting and receiving in the frequency range of interest. The transmitting antenna requires a source, the LO feed requires a source, and the receiving antenna requires a data acquisition device, in this case a network analyzer controlled via a

Texas Tech University, Amy Fleischmann, August 2011 computer. The amplifiers on the RAA also require a +5 V power source. In order to test the radiation pattern, a rotation table is used. This table rotates a specified degree amount as controlled by a stepper motor and a computer. While this table rotates, the receiving antenna picks up the power level at that angle. This information is taken from the network analyzer to the computer, placed into a text file, and translated into a radiation pattern using MATLAB. For the RAA, there are two types of tests: a monostatic test and a bistatic test. In the monostatic test, the transmitting and receiving antennas are placed in the same location while the RAA is rotated, shown in Figure 5.3.

Figure 5.3 : Monostatic Testing Setup

In a bistatic testing setup, the receiving antenna is moved while the transmitting antenna and RAA are stationary. In order to achieve this testing setup, an arm is created to hold the transmitting antenna stationary in relation to the RAA. The RAA and 30

Texas Tech University, Amy Fleischmann, August 2011 transmitting antenna are rotated using the table while the receiving antenna remains stationary.

Figure 5.4 : Bistatic Testing Setup

Both of the testing setups are required to accurately demonstrate a retrodirective antenna array.

5.3 Retrodirective Antenna Array Measurements

Using these two testing setups, the RAA is tested and compared to the simulation results for a typical four element slot antenna array. A properly working RAA will have a wider front lobe in comparison to the normal array. The front lobe is tested using the monostatic testing setup. The results from this monostatic test are shown in Figure 5.5.

The front lobe is distinctively wider than the array radiation pattern. This wider front

Texas Tech University, Amy Fleischmann, August 2011 lobe implies that the array is redirecting within a range of angles. However, in order to confirm the retrodirectivity another test is required.

Figure 5.5 : Monostatic Testing Results

In order to test whether the array is exhibiting retrodirective properties, the bistatic test is used. For a properly working retrodirective array, the front lobe will be directed towards the transmitting antenna within a range of angles. From the bistatic test, the results show whether the front lobe is directive towards the transmitting antenna at different angles since the arm can be adjusted to different angles. For this test, the pattern is first tested for the transmitting antenna directly in front of the retrodirective array. Due to the testing setup, the transmitting antenna and RAA are not necessarily directly in front of the receiving antenna, so there is a slight offset from zero. Then, the transmitting antenna is moved to an offset of 20 degrees and tested again. The results from the bistatic test are shown in Figure 5.6. 32

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Figure 5.6 : Bistatic Testing Results

The results show the beam directed, in blue, slightly below zero degrees. This is the zero offset pattern, or the pattern where the transmitting antenna is directly in line with the retrodirective array. The other pattern shows the pattern when the transmitting antenna is offset 20 degrees. The pattern follows the offset of the transmitting antenna.

This figure demonstrates the retrodirectivity of the array.

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Chapter 6

Metamaterial Ground Plane

6.1 Motivation

Shown through the antenna radiation patterns, the lobe from the antennas is still a bidirectional beam when the desired beam is unidirectional. A plane reflector creates the desired results; however, being a quarter wavelength away, a plane reflector is not low profile and, therefore, undesirable for this application. Instead, it is proposed to use a high impedance ground plane made from a metamaterial mushroom structure, as stated in the Background section.

6.2 The Mushroom Structure

This mushroom structure is comprised of a copper patch and a plated via to ground.

Figure 6.1 : Mushroom Structure Example [8]

The equivalent circuit for this structure is a parallel resonant LC circuit [8].

Although this structure can reduce surface waves if in the plane of the antenna, for this 34

Texas Tech University, Amy Fleischmann, August 2011 application the mushroom structure is used as a high impedance plane. When used as a high impedance plane, the reflection phase is such that the image constructively interferes. This surface can also be placed close to the antenna which creates a low profile reflector.

6.3 Metamaterial Design and Results

The structure for this application is a square patch with a cylindrical via. The design process started at a dimension much smaller than the wavelength of interest.

From there, the dimensions were optimized using Ansoft HFSS to ensure the bandgap occurs in the frequency band of interest.

Figure 6.2 : Mushroom Structure Top and Side View

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After designing the metamaterial, the high impedance plane is applied to a single slot antenna. The high impedance plane is placed in the same plane as the antenna feed, as shown through Figure 6.3. The placement of the mushroom structure within the feed network plane reduces the height of the system, reduces the number of substrates, and allows the metamaterial structure to be more effective.

Figure 6.3 : Slot Antenna with EBG

Using this high impedance plane, the antenna radiation pattern shows a reduced back lobe.

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Figure 6.4 : Slot Antenna with EBG Radiation Pattern

This radiation pattern shows a greater reduction in the back lobe than the plane reflector.

The two plots are compared in Figure 6.5.

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Figure 6.5 : Radiation Pattern Comparison of EBG and Plane Reflector

Compared to the plane reflector, the high impedance plane decreases the back lobe radiation by 8 dB with minimal reduction in the front lobe. When compared to the original slot antenna, the overall decrease in back radiation is apparent. Directly behind the array, the reduction is around 13 dB while increasing the main lobe by 2 dB.

When used in with the antenna array, similar results are obtained. A detailed S11 matching graph is depicted in Appendix A. As shown in Figure 6.6, the back lobe is distinctively smaller than the previous simulations.

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Figure 6.6 : Antenna Array with EBG Radiaiton Pattern

The front lobe to back lobe ration is high, meaning that a majority of the signal is being radiated from the front lobe. This type of radiation is desired for this application.

This improvement is highly desirable for the RAA application in addition to the low profile advantage of the high impedance surface. These results demonstrate the mushroom metamaterial high impedance ground plane‟s ability to reduce the back radiation while maintaining a low profile system.

Texas Tech University, Amy Fleischmann, August 2011

Chapter 7

Future Work

7.1 Solar Antennas

To enhance this project, the antennas used will be solar antennas. Solar antennas are a result of the integration of solar cells and antennas. The resulting antenna is as known as a SOLANT [12]. A simple integration involves placing solar cells on top of microstrip antennas by allowing the back conductor of the solar cell act as the conductor for the antenna. With the evolution of solar cells, more integrated designs are capable.

For this project, slot antennas are chosen for the antenna design, leaving the entire ground plane to solar cells. The slot antenna provides a simple geometry and space for solar cells to be integrated into the design. The initial idea is to cover the copper ground plane with the solar cells. In this configuration, the antenna array has the capability of facing the sun and absorbing enough solar energy to power itself. The effect of the solar cell on the antenna is difficult to simulate due to the complexity of the solar cell. However, recent publications show the solar cell has minimal effect on the antenna parameters. In

[13], the author shows the table below, where the gain differences between an antenna and an integrated solar cell and antenna are at or below 10%.

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Table 7.1: Solar Antenna Differences [13]

There have already been applications of solar antennas in mobile communication applications. In one application, the solar cell and antenna are integrated on a GPS unit for a vehicle [14]. Although there a few applications fabricated, the potential applications of these solar antennas increases drastically with the research being done.

This integration of solar cell and antenna can be used in wireless communication applications, wireless sensor applications, mobile communication applications, and satellite applications. Integration into this particular system would allow for a standalone system capable of powering itself. This type of device is useful for a sensor application, especially in a rural setting such as a field or a desert.

7.2 Additional Future Work

In addition to adding solar antenna, there are a few more items for the future of this project. Although the metamaterials have been simulated for a single element, a more thorough simulation with an entire array is required. Then the metamaterials need to be fabricated and testing using the same setup as previously discussed. Also, the board

Texas Tech University, Amy Fleischmann, August 2011 will need to be tested with the solar cells attached to demonstrate the affect of the solar cells to the antenna performance. In order to create an entire stand alone board, a voltage controlled oscillator needs to be added along with filtering the output from the mixer and increasing the amplification on the board. By adding these, the signal redirected will be more powerful and contain less spurious signals. With the integration of the solar cells, power management is key. Voltage regulation and power circuitry is required to maintain an appropriate level of power for the amplifiers and voltage controlled oscillator on the board. The efficiency of all components will be crucial to ensuring the solar cells can power the entire system.

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Chapter 8 Conclusions

Overall, the retrodirective antenna array provides a basis for the future work. The slot antennas perform the task of not only transmitting and receiving but also providing a large conductor space for solar cell placement. In addition, the slot antennas are a simple geometry so that solar cells can be efficiently placed onto the conductor space. The complete retrodirective array exhibits retrodirective properties in the monostatic and bistatic testing results. Although, in order to test more extensively, a more robust testing set up is required with a spectrum analyzer. The slot antennas maintain a bidirectional beam in array form, so a reflective backing is required to make a unidirectional beam.

Although a perfect electric conductor is sufficient, the metamaterial high impedance plane demonstrates better results and is a low profile addition. The mushroom structure proves its ability to create a high impedance ground plane reflector. With these components together, the end result is a useful platform to continue a solar antenna based retrodirective antenna array.

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References

[1] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed., Steven Elliot, Ed. New York, USA: John Wiley & Sons, INC, 1997.

[2] W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, 2nd ed., Charity Robey and Ken Santor, Eds. United States of America: John Wiley & Sons, Inc., 1998.

[3] K. M. K. H. Leong, R. Y. Miyamoto, and T. Itoh, "Moving Forward in Retrodirective Antenna Arrays," IEEE Potentials, pp. 16-21, August/September 2003.

[4] W. Tseng, C. Hu, and S. Chung, "Planar Retrodirective Array Reflector Using Dual- slot Antennas," Electronics Letters, vol. 34, no. 14, pp. 1374-1376, July 1998.

[5] C.W. Pobanz and I. Tatsuo, "A Two-Dimensional Retrodirective Array Using Slot Ring FET Mixers," in 26th EuMC, Prague, 1996, pp. 217-220.

[6] S. L. Karode and V. F. Fusco, "Multiple Target Tracking Using Retrodirective Antenna Arrays," in National Conference on Antennas and Propagation, 1999, pp. 178-181.

[7] S. Gupta and E. R. Brown, "Noise-Correlating Radar Based on Retrodirective Antennas," IEEE Trans. Aerosp. Electron. Syst., vol. 43, no. 2, pp. 472-479, April 2007.

[8] D. Sievenpiper, "Review of Theory, Fabrication, and Applications of High- Impedance Planes," in Metamaterials: Physics and Engineering Explorations, Nade Engheta and Richard W. Ziolkowski, Eds. United States of America: John Wiley & Sons, 2006, ch. 11, pp. 287-311.

[9] Minicircuits. SIM-83+ Data Sheet. [Online]. http://minicircuits.com/pdfs/SIM- 83+.pdf

[10] Minicircuits. MNA-7+ Data Sheet. [Online]. http://minicircuits.com/pdfs/MNA- 7+.pdf

[11] R. Garg, P. Bhartia, I. Bahl, and A. Ittipiboon, Microstrip Antenna Design

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Handbook. Norwood, United States of America: Artech House, INC., 2001.

[12] S. Vaccaro et al., "Integrated Solar Panel Antenna," Electronics Letters, vol. 36, no. 5, pp. 390-391, March 2000.

[13] S. Vaccaro, J.R. Mosig, and P. de Maagt, "Making Planar Antennas Out of Solar Cells," Electronics LEtters, vol. 38, no. 17, pp. 945-947, August 2002.

[14] N. Henze et al., "Investication of Planar Antennas with Photovoltaic Solar Cells for Mobile Communications," IEEE, pp. 622-626, 2004.

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Appendix A Additional Radiation Patterns

This pattern is comparing only the high impedance plane with the quarter wavelength reflector. This shows the noticeable difference between the back lobe between the two types of reflectors.

Figure A.1: High Impedance Plane vs. QWR

The S11 matching graph of the antenna array with EBG mushroom structure is shown in Figure A.2. This graph depicts the matching of the antenna array. Notice that there are two resonances.

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Figure A.2 : Antenna Array with EBG S11

While one resonance is shown in the body, the second resonance is shown below in FigureA.3. The patterns are similar since the resonances are both located within the band gap.

FigureA.3 : Antenna Array with EBG Pattern 2 47