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UNIVERSITY OF CINCINNATI

Date:______

I, ______, hereby submit this work as part of the requirements for the degree of: in:

It is entitled:

This work and its defense approved by:

Chair: ______

Slotted Spiral Antennas and Widebandwidth Array Systems

A thesis submitted to the

Graduate School

of the University of Cincinnati

In partial fulfillment of the

requirements for the degree of

MASTER OF SCIENCE

In the Department of Electrical and Computer Engineering

of the College of Engineering

Piyou Zhang

MS, Physics, University of Miami, FL, 2005

MS, Physics, Nankai University, China, 2003

BS, Physics, Nankai University, China, 2000

Committee Chair: Professor Altan Ferendeci Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Abstract

The conventional 2-dimension monolithic microwave integrated circuits (2D-MMICs) technology has reached its limitation and cannot address the needs of the next generation of communication systems and microwave radars. A 3D multilayer MMIC is needed to increase the functionality and compactness of microwave circuits.

The major challenges lie in the design of efficient antenna structures on very thin substrates with closely spaced bottom ground planes. An Archimedean slotted spiral antenna was presented with a relatively thin dielectric substrate on a ground plane.

The requirement is that the slot width should be less than or equal to the substrate thickness. In this research, the slotted spiral antennas were as thin as 400um (0.005λg).

The Marchand balun was also introduced to provide the mode 1 input for the slotted

spiral antenna as well as to increase the frequency response of the antenna.

Furthermore, a 4-element linear antenna array was built using the slotted spiral

antennas and loaded line phase shifter to introduce the problems that are encountered

in wideband phased array antenna systems.

1 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Piyou Zhang 2008

2 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Acknowledgements

I would like to express my sincere gratitude to my advisor, Dr. Altan Ferendeci, for his guidance, support and encouragement. Dr. Ferendeci has been an exceptional model with his academic insights and his accessibility to students. Dr. Ferendeci is one of the most important people in my life, because he showed me the door to the RF

world. Appreciation is also extended to my thesis committee: Dr. Peter B. Kosel, and

Dr Joseph T. Boyd. I truly appreciate Dr. Peter Kosel for allowing me to expose the

samples in his lab.

Special mention is given to my wife Yurong, for her support and encouragement

throughout these years. Furthermore, weekly conversations with my parents have

given me extraordinary motivation and inspiration.

I would also like to thank Mr. John Phillips for setting up the milling machine. Many

thanks also go to my colleagues, faculty and staff at the ECE department of the

University of Cincinnati, especially Jian Xu, Yuanzhi Lin, Ruirong Shi, Qingyi Wu,

Weiqun Chen. Many thanks to Kaichang Zhou, Hua Tan for their help for the

fabrication process. Appreciation also with to express to Rogers Corporation for the

sample of the RT6002 PCB. I apologize in advance for anyone that I may have missed.

1 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Table of Contents

Abstract...... 1 Acknowledgements...... 1 Chapter 1 Introduction ...... 1 Chapter 2 Wideband Antenna ...... 5 2.1 Wideband Antennas ...... 5 2.2 Archimedean Spiral Antenna ...... 7 2.3 Wire Spiral and Slot Spiral ...... 11 2.4 Cavity-backed Slot Antenna ...... 11 2.5 Slotted Archimedean spiral antenna...... 12 2.6 Shorting Pins...... 15 Chapter 3 Slotted Spiral Antenna...... 17 3.1 3D Broadband Balun for 2-arm Slotted Spiral Antenna ...... 17 3.1.1 Marchand Balun...... 18 3.1.2 Simulation Results of Marchand Balun ...... 23 3.2 Conductor Backing Slotline...... 27 3.3 Terminations of the arms...... 30 3.4 Final Design...... 32 Chapter 4. Fabrication Procedure ...... 36 4.1 Milling Method ...... 36 4.1.1 Introduction...... 36 4.1.2 Bonding...... 38 4.1.3 Soldering...... 40 4.2 Photolithographic Techniques...... 40 4.2.1 Introduction...... 40 4.2.2 Spin Coating...... 41 Chapter 5 Results of Antenna Measurements ...... 46 5.1 Antenna Measurement ...... 46 5.1.1 Far Field Measurements...... 47 5.1.1 Polarization Measurements...... 50 5.2 Experimental Results ...... 51 Chapter 6 Phased Array Antenna ...... 55 6.1 Array Theory...... 55 6.1.1 Linear Array...... 56 6.1.2 Feed Network for Phased Array Antenna ...... 58 6.2 Linear Slotted Spiral Antenna Array...... 58 6.2.1 Loaded line phase shifter design...... 59 6.2.2 Phased Array with Loaded line phase shifter...... 65 Chapter 7 Conclusion...... 73 7.1 Thesis Summary...... 73 7.2 Future Work ...... 73

2 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

List of Tables

Table 3.1 The parameters of the PCB RT6002 …………………………………….. 25 Table 3.2 The parameters of the Marchand Balun …………………………………. 25 Table 3.3 Effective wavelengths of conductor backing slotline from HFSS …….… 29 Table 3.4 The parameters of the slotted spiral antenna ...... …... 34 Table 6.1 The parameters of the PCB ULTRA 2000 ………………………………. 62 Table 6.2 The parameters of the 450 loaded line phase shifter (unit: mm) ………… 65

3 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

List of Figures

Figure 1.1 Vertically interconnected 3D passive transmitter module ……………….. 2 Figure 2.1 Two-arm Archimedean spiral antenna …………………...………....…… 9

Figure 2.2 Active region of the Archimedean spiral antenna …………………….... 10

Figure 2.3 Feeding of the slotted spiral antenna …………………………………… 15

Figure 3.1 Coupler ……………………………………………………………...….. 19

Figure 3.2 Marchand Balun ………………………………………………...……… 20

Figure 3.3 |S21| with change of coupling coefficient C …………………………..… 21

Figure 3.4 Simulation of the Marchand Balun in ADS ……………………...…….. 26

Figure 3.5 The magnitude of outputs of port2 and port3 of the Marchand Balun … 27

Figure 3.6 The phase difference of outputs of port2 and port3 of the Marchand Balun

(Ratio=S(2,1)/S(3,1)) ………………………………………………………………. 27

Figure 3.7 Electrical field of conductor backing slotline at 3GHz (HFSS) ……….. 30

Figure 3.8 Electrical field of conductor backing slotline at 5GHz ………………… 30

Figure 3.9 Electrical field of conductor backing slotline at 7GHz ………………… 31

Figure 3.10 The shunt resistance as a function of position for the slot spiral termination (reprinted from [40]) ……………………………………………….….. 32

Figure 3.11 The shunt resistance as a function of position for the slot spiral termination (reprinted from [40]) …………………………………………………... 33

Figure 3.12 Approach spiral with half circles (dark: spiral, gray: half circles) ……. 34

Figure 3.13 Masks to fabricate the slotted spiral antenna …………………………. 35

Figure 3.14 Simulation results of S11 for the slotted spiral antenna ……………….. 36

4 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 3.15 Simulation results of radiation pattern for the slotted spiral antenna at

3.3GHz, 4.7GHz and 6.4GHz.(unit: dB. blue:3.3GHz, red: 4.7GHz, green:

6.4GHz) ……………………………………………………………………………...36

Fig. 4.1 Quick Circuit Main Assemblies …………………………………………... 37

Figure 4.2 Interface of IsoPro …………………………………………………….... 39

Figure 4.3 Spin curve of Shipley 1818 photoresist ………………………………... 43

Figure 4.4 Laurell WS-400 spin coater ……………………………………………. 44

Figure 4.5 The slotted spiral antenna fabricated with photolithographic techniques. 46

Figure 5.1 Stepper motor circuits ………………………………………………….. 48

Figure 5.2 Interface of VEE program to test antennas ………………..…………… 49

Figure 5.3 Gain of the Log-Periodic antenna (unit: dBi) ………………………….. 51

Figure 5.4 S11 of the four identical slotted spiral antennas (unit: dB, Solid: P1, Dash:

P3, Dot: P5, Dash Dot: P6) ………………………………………...………………. 52

Figure 5.5 Gain of the #5 slotted spiral antenna (unit: dBi) ……………………….. 54

Figure 5.4 Radiation patterns of the four identical slotted spiral antennas at 5.15 GHz

(unit: dBm, Solid: P1, Dash: P3, Dot: P5, Dash Dot: P6) …………………………. 55

Figure 6.1 Array Factor for 4-element linear array with fixed distance between each

element (dash: 3GHz, line: 5GHz, dot: 7GHz) ………………………..………..…. 58

Fig. 6.2 Basic loaded-line phase shifter ……………………………………………. 61

Figure 6.3 450 loaded line phase shifter (HFSS) …………………………………... 62

Figure 6.4 Circuit layout of the loaded line phase shifter for a 4-element linear array

with phase shifter of 450 in ADS …………………………………………..……….. 63

5 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.5 Magnitudes of the four outputs of the loaded line phase shifter ……….. 64

Figure 6.6. Phase variations of the four outputs of the loaded line phase shifter…... 64

Figure 6.7 Layout of the 450 phase shifter at 5.15GHz ……………………………. 65

Figure 6.8 450 loaded line phase shifter …………………………………………… 66

Figure 6.9 Antenna array with 450 phase shifter …………………………………... 67

0 Figure 6.10 S11 of the 45 phase difference (unit: dB, Dash: P1, Dot: P3, Dash Dot:

P5, Dash Dot Dot: P6, Solid: Antenna Array) ……………………………..……….. 68

Figure 6.11 Antenna array with 450 phase difference at f = 5.15GHz (unit: dB, Dash:

P1, Dot: P3, Dash Dot: P5, Dash Dot Dot: P6, Solid: Antenna Array) ………..…… 69

Figure 6.12 Antenna array with 00 phase shifter …………………………………... 70

0 Figure 6.13 S11 of the 0 phase difference (unit: dB, Dash: P1, Dot: P3, Dash Dot: P5,

Dash Dot Dot: P6, Solid: Antenna Array) ………………………………………….. 71

Figure 6.14 Phased array with 0 phase difference at f = 5.15GHz (unit:dB, Dash: P1,

Dot: P3, Dash Dot: P5, Dash Dot Dot: P6, Solid: Antenna Array) ……..………….. 72

Figure 6.15 Gain of the 00 phase difference antenna array ………………………... 73

6 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Chapter 1 Introduction

The broadband wireless communication field has received special interest particularly in light of meeting the demands and needs of global connectivity. Antennas are an important part of any wireless system, and often determine their overall performance

[1]. The broadband antennas integrate several frequencies and functionalities under a single aperture. Spiral antennas are known for their ability to maintain nearly circular polarization and consistent gain and input impedance over a wide bandwidth [2].

2D monolithic integrated Transmitter/Receiver (T/R) modules are much smaller and lighter, more reliable, and much less expensive than the traditional waveguide-based receivers and transmitters. However, the conventional 2-dimensional monolithic microwave integrated circuits (2D-MMICs) technology has reached its limitation and cannot address the needs of the next generation of communication systems and microwave radars, so a 3D multilayer MMIC was developed at the University of

Cincinnati [1] to increase the functionality and compactness of microwave circuits.

Compact RF systems allow a greater degree of RF functionality per unit volume, while maintaining a high level of performance. Different circuit layers were isolated from each other by ground planes and monolithically interconnected by vertical via posts. This new technology considerably reduced the volume, weight and cost of the future microwave ICs, while still maintaining high level of performance and functionality, as shown in Figure 1.1. It has a very low profile, enabling the realization

1 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis of conformal phase array antennas. This configuration is also called vertically interconnected multilayer microwave monolithic circuit [3].

Figure 1.1 Vertically interconnected 3D passive transmitter module.

To make sure the T/R module is universally conformal to any arbitrarily curved surface, the depth must be negligibly small. For a conventional slotted spiral antenna with a ground plane, the substrate thickness is usually λg/4. A traditional slotted spiral antenna is basically short-circuited when built on a thin substrate, with a bottom ground plane less than 0.1λg away. A new Archimedean slotted spiral antenna is presented with a relatively thin dielectric substrate on a ground pane. With a narrow slot design, efficient broadband radiation has been reported from a spiral antenna even though it was monolithically fabricated on polyimide substrate with a thickness of

2 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

68um (0.0038λg) [4]. In this research, the slotted spiral antennas were slightly thicker with a thickness within 400um (0.005λg)

To properly excite the broadband slotted spiral antenna, a broadband balanced to

unbalanced line transition, balun is required. A high Q widebandwidth Marchand

balun was designed and integrated into the spiral antenna system in order to excite the

Mode-1 radiation. Performance of the processed slotted spiral antenna was then

measured in the anechoic chamber at the Microwave and Millimeter Wave Lab of the

University of Cincinnati.

The main obstacles in the design and fabrication of a wide-bandwidth printed antenna

are the use of a thin dielectric material and closely placed ground plane. The resonant

nature of slot antennas still limits the ultimate bandwidth that they can provide.

Linear or planar arrays do not have depth restrictions as long as the separations

between the modules do not exceed the free space half-wavelength, otherwise

unwanted side lobes are generated.

This thesis is organized into seven chapters. Chapter 1 introduces the novel 3D-

MMIC technology. A review of wideband antennas and the basic theory of spiral

antennas are presented in Chapter 2. Analysis and design of broadband slotted spiral

antennas including the Marchand balun are given in Chapter 3. The fabrication

3 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis procedure of the slotted Archimedean spiral antenna is described in Chapter 4.

Chapter 5 is focused on the antenna measurements, including the measurement setups for far field testing. Chapter 6 discusses the configuration and operation of the phased array antenna systems. A 4-element linear array using Archemidan slotted spiral antenna and PCB loaded-line phase shifter is presented in the same chapter. The thesis summary is discussed in Chapter 7.

4 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Chapter 2 Wideband Antenna

In many applications, an antenna must operate effectively over a wide range of frequencies. Let fU and fL be the upper and lower frequencies of operation for which satisfactory performance is obtained. The center frequency is denoted as fC. Then bandwidth as a percent of a center frequency Bp is

fU − f L B p = ×100% (2-1) f C

Bandwidth is also defined as a ratio Br by

fU Br = (2-2) f L

The bandwidth of narrow band antennas is usually expressed as a percent using (2-

1), whereas wideband antennas are quoted as a ratio using (2-2) [5].

2.1 Wideband Antennas

Resonant antennas, like dipole and microstrip antennas, accommodate standing waves only at certain frequencies and thus are inherently narrow-banded. On the contrary, antennas that are capable of bandwidths of 2:1 or more are classified as broadband antennas. In order to achieve a large bandwidth, those antennas usually have traveling waves rather than standing waves. They require structures that emphasize continuos angular changes in the direction and utilize materials with smooth boundaries.

Common practical broadband antennas include traveling wave antenna, helix, biconical antenna, bowtie antenna, sleeve antenna, spiral antenna, log-periodic

5 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis antenna and so on [5]. We know that resonant antennas have narrow bandwidths. For example, if we define the upper frequency fU and the lower frequency fL were determined by VSWR = 2.0 points, the half-wave dipoles have bandwidths of around

10%, and the patch microstrip antennas have bandwidths of around 5%.

The Characteristics that yield broadband behavior are:

1. Emphasis on angles rather than lengths.

2. Self-complementary structures.

3. Thick metal – “fatter is better.”

The wideband antennas usually require structures that do not emphasize abrupt changes in the physical dimensions involved, but instead utilize materials with smooth boundaries. Smooth physical structures tend to produce patterns and input impedances that also change smoothly with frequency. This simple concept is very prominent in broadband antennas. The traveling-wave antennas, helical antennas, biconical antennas, sleeve antennas, spiral antennas and log-periodical antennas are widely used as wideband antennas. Among these wideband antennas, spiral antennas are very suitable for the planar antenna techniques. Planar antennas are radiators constructed using planar circuit fabrication techniques, in which the top metallization layer is responsible for radiation. The planar antennas offer the advantages of low profiles, compatibility with integrated circuit technology, and conformability to a shaped surface. The main goal of this research was to design and fabricate an efficient

6 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis wideband planar antenna centered at 5GHz with broadside radiation compatible with the 3D-MMIC technologies. The antenna can easily be vertically interconnected with the rest of the circuit components of an RF T/R module [5]. The major problem is the design and fabrication of a wideband printed antenna with very thin dielectric thickness and closely placed ground [1].

2.2 Archimedean Spiral Antenna

Spiral antennas and their variations are usually constructed to be either exactly or nearly self-complementary. Two commonly used spiral antennas are equiangular spiral antenna [6] and Archimedean spiral antenna [7]. The equiangular spiral, also known as Log-spiral, possesses all the features of the frequency-independent antenna, and was invented first among all the spiral antennas. The arms of Archimedean spiral are linearly proportional to the polar angle rather than exponential as for the equiangular spiral, and flare out much more slowly [8].

The Archimedean spiral antenna is easily constructed using printed circuit techniques.

The equations for the two spirals in Figure 2-1 are

r = aψ + b and r = a(ψ + π ) + b (2.1)

Where r is radius of the spiral, ψ is the angle in radian, a is spiral growth rate, and b is the initial radius where the spiral begins [9].

7 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 2.1 Two-arm Archimedean spiral antenna

The simple geometry of the Archimedean spiral antenna affords an opportunity to explain an important operating principle in frequency-independent antennas. This is the “band” description of the radiation that is characterized by an active region responsible for radiation. Between the feed point of a frequency-independent antenna and the active region, currents exist in a transmission line mode and fields arising from them cancel in the far field. The active region occurs on that portion of the antenna that is one wavelength in circumference for curved structures. To demonstrate this, an equivalent rectangular spiral shown in Figure 2.2 is used. The horizontal and vertical scales represent the dimensions of the square microstrip spiral antenna with ground plane. From the figure, we can see the magnitude of surface current Js is higher at active region. Beyond the active region, currents are small, having lost

8 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis power to radiation in the active region. The antenna effectively behaves as if it is infinite in extent. Of course, the active region moves around the antenna with frequency. Since the geometry of a spiral is smooth, as frequency is reduced and the active region shifts to locations farther out on the spiral, the electrical performance remains unchanged. Hence, self-scaling occurs and frequency-independent behavior results.

Figure 2.2 Active region of the Archimedean spiral antenna [10]

We now give a physical explanation of how spiral antennas operate. At the active region where the circumference is one wavelength, it can be assumed that the current magnitudes over this region are nearly the same. The phase, however, shifts as the traveling waves progress along the arms. Since the circumference is electrically large in the active region, phase must be accounted for. In figure 2.1, the two arms are fed

0 180 out of phase at points F1 and F2. The current is inward from arm no.1 (-) and

9 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

outward for arm no.2 (+). The lengths of the arms out to A1 and A2, F1A1 and F2A2, are equal so the phase shifts from the feed to A1 and A2 are identical. The phase shifts

0 180 between A1 and A1’ and between A2 and A2’, because of the λ/2 differential path length. Adjacent points on different arms are now in phase because the 1800 phase shift counters the direction reversal introduced by half turn. In addition, the points opposite these pairs are in phase. This in phase condition leads to reinforcement of electric region.

A distinguishing feature of frequency-independent antennas is their self-scaling behavior. Most radiation takes place from that portion of the frequency-independent antenna where its width is half-wavelength or the circumference is one wavelength – the so-called active region. As frequency decreases, the active region moves to a larger portion of the antenna, where the width is a half-wavelength [5].

The final aspect that requires explanation is the circular polarization property. In the active region, points that are one-quarter turn around that spiral are 900 out of phase.

0 For example, the phase at point B1 lags that at point A1 by 90 . In addition, the currents are orthogonal in space. The current magnitudes are also nearly equal. Thus, all conditions are satisfied for circular polarized radiation: The radiated fields are orthogonal, equal in magnitude, and 900 out of phase. The left hand sense results from the left hand winding of the spiral [5].

10 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

2.3 Wire Spiral and Slot Spiral

Consider a metal antenna with input impedance Zmetal, and the dual structure, the complementary antenna where the metal is replaced by a slot. Assume it has input impedance Zair. Actually, complementary antennas are similar to a positive and negative in photography. The impedance relation is given by [5]

η 2 Z Z = (2.2) metal air 4

If the antenna is its own complement, which is called self-complementary, frequency- independent impedance behavior is achieved. A self-complementary property structure can be made to exactly overlay its complement through translation and/rotation. We also have

η Z = Z = = 188.5Ω (2.3) metal air 2

In this research, we suppose the impedance of the slotted spiral antenna is 120Ω.

2.4 Cavity-backed Slot Antenna

Based on the above discussion, it is apparent that the spiral produces a broad main beam perpendicular to the plane of the spiral. Most applications require a unidirectional beam. This is created by backing the spiral with a ground plane to eliminate the back radiation of the slot antenna. The most common construction approach is to use a metallic cavity behind the spiral, forming a cavity-backed

Archimedean spiral antenna. The directivity of the slot antenna is increased and

11 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis mutual effects are reduced. The cavity of CBS is essentially a shorted rectangular waveguide. Single mode operation, usually TE10 mode, is preferred at the design frequency, and the dimension of cavity can be chosen accordingly. The electrical field distribution in the slot is sinusoidal and in the direction of the slot width. Since the whole cavity was very thin, the depth of the cavity was much less than on half of the guide wavelength (λg) at the interested frequency [1]. This introduces a fixed physical length, the distance to the ground plane, thereby altering the true frequency independent behavior. This is corrected in most commercial units by loading the cavity with absorbing material to reduce resonance effects. Typical performance parameter values for the cavity backed Archimedean spiral are HP=750, |Axial Ratio|

= 1dB, Gain = 5dBi over a 10:1 bandwidth for more. The input impedance is about

100Ω, and is nearly real [5].

2.5 Slotted Archimedean spiral antenna

The slotted spiral antennas have been widely studied for a long time [11-18]. The slotted spirals support the propagation of a slotline traveling wave, i.e. the magnetic currents, and are traditionally configured in a planar form. For the slot line spirals, there are difficulties in the feed design and integration of the multi-mode excitation.

Furthermore, the necessity of terminating the slot arms to achieve a commonly required broadband performance, and problems associated with the excitation of cavity modes coupled with the relatively complex integration of the infinite coaxial

12 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis balun feed all contribute to additional challenges in design of slot spirals. On the other hand, slot spirals are smaller, shallower and have better gain. The printed microstrip spirals require an absorber filled cavity for broadband performance.

Most microstrip antennas have narrow-bandwidth, poor radiation efficiency and hence low gain, which are highly dependent on the dielectric layer thickness. Increasing the substrate thickness can also increase the bandwidth and efficiency. However, surface waves are excited as the height increases. The surface waves travel on the substrate and they are scattered at circuit discontinuities. This degrades the antenna radiation characteristics and polarization characteristics. The problem becomes worse in array applications because the surface waves enhance mutual coupling.

The performance of patch antennas is extremely poor when incorporated into thin film dielectric substrates. The electromagnetic dual of the dipole antennas, the slot antenna, can provide wider bandwidth with smaller size due to less dependence on the substrate thickness. The current of a slot antenna is distributed on the metal sheet around the slot, and less energy is stored between the antenna and ground plane.

A slot antenna may be conveniently energized with a coaxial transmission line or a waveguide, and then fed with two vias, as shown in Figure 2.3. The power radiated back into the substrate excites surface waves if a CPW or microstrip line is used as a feed line on a thick substrate. Parallel plate mode is excited if a stripline is used.

13 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Shorting pins were used in stripline-fed slot antennas to suppress the parallel plate mode [1].

Figure 2.3 Feeding of the slotted spiral antenna

The respective upper and lower frequencies of the bandwidth are usually determined by the smallest and largest circumferences of the spiral structure. Circular spiral shape provides a smooth change when the current adjusts with the frequency. Broader arms and more tightly wound spirals exhibit smoother and more uniform patterns with smaller variations in beam-width with frequency.

14 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

2.6 Shorting Pins

Although a conductor backing behind a standard slotline or coplanar waveguide is attractive to achieve enhanced mechanical strength. The conductor backing would also allow additional circuit integration on its other side, providing the necessary electrical isolation in between. However, such a conductor backed configuration, unlike a standard slotline or coplanar waveguide, can leak significant power to the parallel plate mode. This leakage can be particularly prohibitive for thin substrates and/or higher frequencies, making the conductor-backed geometries potentially dangerous to use [19].

Several possible methods include:

1) The use of conducting shorting pins to suppress excitation of the parallel plate mode;

2) The use of a “hybrid configuration”, where a small dielectric-guide structure with a significantly high dielectric constant compared to that of the surrounding substrate is coupled underneath the slotline or coplanar wave guide;

3) The use of a two-layered conductor backing configuration, instead of the standard configurations with only a single uniform substrate between the parallel plates

4) Loading the conductor-backed geometry on top using a thin, high dielectric- constant substrate.

In this research, the shorting pins were used to suppress the parallel plate mode. On

15 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis the other hand, the shorting pins can not be too close to spiral arms, or the performance of the spiral antenna will be affected [19].

16 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Chapter 3 Slotted Spiral Antenna

This chapter begins with a discussion of balun circuit of the slotted spiral antenna.

Then the design of slotted spiral antenna will be discussed.

3.1 3D Broadband Balun for 2-arm Slotted Spiral Antenna

In balanced mixers, frequency doublers and push-pull amplifiers, baluns convert unbalanced signals into balanced signals and vice versa. Many analog circuits require balanced inputs and outputs in order to reduce noise and high order harmonics as well as improve the dynamic range of the circuits. Various types of baluns have been reported for applications in microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs) [20].

Planar baluns have been implemented using various types of transmission lines, including coupled microstrip lines, coupled coplanar waveguide (CPW) lines, and multilayered broadside-coupled microstrip lines. The single-layer edge-coupled microstrip and CPW baluns are easier to fabricate, but have smaller bandwidth ratios

[21]. Multilayered planar baluns, on the other hand, are more complicated to fabricate, but they provide larger bandwidth ratios [22]. Among them, the planar version of the

Marchand balun is perhaps one of the most popular because of its ease of implementation and wide bandwidth. The planar Marchand balun consists of two sections of quarter-wave coupled lines, which may be realized using microstrip

17 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis coupled lines, Lange coupler, or multilayer coupler structures [23].

The Marchand balun is a popular RF component used in broadband communication systems designs. The Marchand balun compensated balun was implemented originally in coaxial cables, and later converted into planar structures suitable for miniaturized system integration [24]. For use in MIC’s and MMIC’s, wide bandwidth and compactness of baluns are of high interest [25].

3.1.1 Marchand Balun

For a coupler as in Figure 3.1, each arm is one quarter-wavelength long at the center frequency of operation.

λg/4

P2, Through

P3, Coupled P4, Isolated

Figure 3.1 Coupler

The scattering matrix for ideal couplers with infinite directivity and coupling factor C is given by [26]

18 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

⎡ 0 C − j 1− C 2 0 ⎤ ⎢ ⎥ ⎢ C 0 0 − j 1− C 2 ⎥ [S]coupler = (3-1) ⎢− j 1− C 2 0 0 C ⎥ ⎢ ⎥ 2 ⎣⎢ 0 − j 1− C C 0 ⎦⎥ The planar Marchand balun consists of two coupled sections, a block diagram of the

Marchand balun is shown in Figure 3.2. In general, the impedances Z1 and Z0 are different. So in addition to providing balanced outputs, the balun also needs to perform impedance transformation between the source and load impedances [26]. It provides balanced outputs to load terminations Z1 from an unbalanced input with source impedance Z0.

P2 P3

Figure 3.2 Marchand Balun

We first consider the case where the source and load impedances are equal to Z0. For symmetrical baluns, the scattering matrix of the balun can be derived from the scattering matrix of two identical couplers.

The S-parameters of the balun can then be obtained by using the voltage wave’s

19 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis relationships [27],

⎡ 1− 3C 2 2C 1− C 2 2C 1− C 2 ⎤ ⎢ 2 j 2 − j 2 ⎥ ⎢ 1+ C 1+ C 1+ C ⎥ ⎢ 2C 1− C 2 1− C 2 2C 2 ⎥ [S]balun = ⎢ j 2 2 2 ⎥ (3-2) ⎢ 1+ C 1+ C 1+ C ⎥ ⎢ 2C 1− C 2 2C 2 1− C 2 ⎥ − j ⎢ 2 2 2 ⎥ ⎣ 1+ C 1+ C 1+ C ⎦

2C 1− C 2 For | S |= , we can plot |S12| in Matlab as a function of the coupling 12 1+ C 2 factor C as shown in Figure 3.3.

Figure 3.3 |S21| with change of coupling coefficient C

-1/2 -1/2 From the Fig. 3.3, |S12| reaches its maximum value of 2 when C = -4.8dB = 3 .

20 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

When the balun load terminations are changed from Z0 to Z1, S matrix of the balun

’ has to be modified from [S]balun to [S] balun.

' −1 + −1 + [S] balun = [A] ([S]balun − [Γ] )([I] − [Γ][S]balun ) [A] (3-4) where [I] is the identity matrix, while [Г] and [A] are given by

⎡0 0 0 ⎤ ⎡0 0 0 ⎤ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ ⎢ ⎥ Z − Z Z − Z [Γ] = ⎢0 1 0 0 ⎥, [A] = ⎢0 2 1 0 0 ⎥ (3-5) ⎢ ⎥ ⎢ ⎥ Z1 + Z 0 Z1 + Z 0 ⎢ Z − Z ⎥ ⎢ Z − Z ⎥ ⎢0 0 1 0 ⎥ ⎢0 0 2 1 0 ⎥ ⎣⎢ Z1 + Z 0 ⎦⎥ ⎣⎢ Z1 + Z 0 ⎦⎥

The S-parameters of the balun are then given by

⎡ 2Z 2 Z 2 Z ⎤ ⎢ 1− C 2 ( 1 +1) 2C 1− C 1 2C 1− C 1 ⎥ Z Z Z ⎢ 0 j 0 − j 0 ⎥ ⎢ 2 2Z 2 2Z 2 2Z ⎥ ⎢ 1+ C ( 1 +1) 1+ C ( 1 +1) 1+ C ( 1 +1) ⎥ ⎢ Z 0 Z 0 Z 0 ⎥

⎢ 2 Z1 2 Z1 ⎥ ⎢ 2C 1− C 2C 1− C ⎥ Z 2 Z ' ⎢ 0 1− C 0 ⎥ [S] balun = j j (3-6) ⎢ 2 2Z1 2 2Z1 2 2Z1 ⎥ ⎢ 1+ C ( +1) 1+ C ( +1) 1+ C ( +1) ⎥ Z Z Z ⎢ 0 0 0 ⎥

⎢ 2 Z1 Z1 ⎥ ⎢ 2C 1− C 2C( ) ⎥ Z Z 1− C 2 ⎢− j 0 j 0 ⎥ ⎢ 2Z 2Z 2Z ⎥ 1+ C 2 ( 1 +1) 1+ C 2 ( 1 +1) 1+ C 2 ( 1 +1) ⎢ Z Z Z ⎥ ⎣⎢ 0 0 0 ⎦⎥

Equation 3-6 Shows that the use of identical coupled sections results in balun outputs of equal amplitude and opposite phase, regardless of the coupling factor and port terminations [25]. To achieve optimum power transfer of -3dB to each port, we require

21 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

' ' 1 | S b,21 |=| S b,31 |= 2

We can get

1 C = (3-7) 2Z 1 +1 Z 0

Z1 is Zout. When all the ports are terminated with the same impedance, such as 50Ω, where the impedance transforming ratio is unity, the required coupling factor C is

1/ 3 , which is -4.8dB and not -3dB. The misuse of commonly assumed -3dB couplers will result in an insertion loss and output isolation of

IL = 20log | T |= 20log | S 21 |= −3.5dB (3.8) and return loss of

RL = 20log | Γ |= 20log | S11 |= −9.5dB (3.9)

When C = -4.8dB, we obtain

⎡ j j ⎤ ⎢ 0 − ⎥ ⎢ 2 2 ⎥ ⎢ j 1 1 ⎥ [S]balun = (3.10) ⎢ 2 2 2 ⎥ ⎢ j 1 1 ⎥ ⎢− ⎥ ⎣ 2 2 2 ⎦

This is the best attainable S matrix of a lossless balun. It is matched at the input and has transmission coefficients of -3dB with opposite phase [26].

A strong coupling C = -4.8dB, (neglecting the impedance transformation) is required for both of the two coupled line sections to obtain a well designed Marchand balun.

Also, high even-mode impedances to reject even-mode excitations are required in

22 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Marchand baluns for broadband performance. Additional methods were studied to achieve the strong coupling [28-31]. For the edge-coupled structure, the coupling factor is largely dependant on the gap between two coupled lines. For MMICs design, the gap between two coupled lines can be a few micrometers, thus such tight coupling is still achievable in the edge-coupled way. However, for MIC designs using the printed circuit board (PCB) fabrication technology, Marchand Baluns usually tend to be broadside coupling lines, or multilayer structures to achieve tight coupling, which are high in cost or complex in structure [23].

Because balun plane uses strip conductor, and only a small area is covered with conductor, the ground plane of the balun can be considered as that of the antenna too.

On the other hand, the antenna plane uses slot conductor, and since only a small area is covered by the slot, the antenna plane can be considered as the ground plane too.

That means the two balun layers could be approximately considered as the striplines.

3.1.2 Simulation Results of Marchand Balun

The Marchand balun was performed with edge-coupled stripline (SCLIN) first, but because a high coupling (C_dB=-4.8dB) is required, the gap between two striplines is less than 10 microns. This small distance is not only difficult to fabricate for PCB technology, but also can not give correct simulation results in ADS. Later the

Marchand balun was built with broadside-coupled stripline (SBCLIN).

23 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Assuming an input impedance of 50Ω, and the odd-mode impedance of the slotted spiral antenna to be 100Ω, we obtain using equation 3.7

1 1 1 C = = = = −7dB (3.11) 2Z 2*100 5 out +1 +1 Z in 50

Then the distance between two layers of SBCLINis calculated by LineCalc in ADS.

Three layers of RT6002 were bound together to build the Marchand Balun, and the parameters of RT6002 from datasheet were shown in Table 3.1 [32].

PCB Dielectic Constant εr Thickness of substrate Thickness of metal Tanδ

RT6002 2.94 127um 8um 0.002

Table 3.1 The parameters of the PCB RT6002

To keep the output impedance of Marchand balun to be 100Ω, the input impedance could be adjusted to meet the parameters of RT6002. Using LineCalc in ADS, we can get the coupling coefficient C to be -8.27dB. Corresponding input impedance becomes 35Ω. The Marchand balun was designed with center frequency of 5GHz with the parameters were shown in Table 3.2.

LCoupler WCoupler SCoupler LConnector DVia Zin Zout

8.74mm 0.31mm 127um 279um 457um 35Ω 100Ω

Table 3.2 The parameters of the Marchand Balun

24 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

The Marchand balun with the parameters shown in Table 3.2 was simulated in ADS, shown in Figure 3.4.

S-PARAMETERS Meas MeasEqn Var VAR Var Var Eqn Eqn Eqn VAR Eqn VAR Meas1 VAR1 VAR2 VAR3 S_Param Ratio=S(2,1)/S(3,1) W_Coup=308.5 S_Coup=126.7 L_Coup=8742 SP1 Start=3 GHz Stop=7 GHz Step=0.1 GHz

SSub SBCLIN SBCLIN SSUB CLin1 CLin2 SSub1 Subst="SSub1" Subst="SSub1" Er=2.94 W=W_Coup um W=W_Coup um Mur=1 S=S_Coup um S=S_Coup um B=381 um Term L=L_Coup um Term Term L=L_Coup um T=8 um Term1 Term2 Term3 Cond=5.8E+7 Num=1 Num=2 Num=3 TanD=0.002 Z=35 Ohm Z=100 Ohm Z=100 Ohm

Figure 3.4 Simulation of the Marchand Balun in ADS

From Figure 3.5, we can see that the simulated magnitudes of the two outputs are both nearly -3dB from 3GHz to 7GHz.

From Figure 3.6, we can see that the phase difference of the two outputs is perfectly

1800 from 3GHz to 7GHz.

25 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

-2 ) )) )

3 2 -4 , , 1 (1 ( S (S ( -6 B B d d -8

-10 37456 freq, GHz

Figure 3.5 The magnitude of outputs of port2 and port3 of the Marchand Balun

200

100 o) i t a

R 0 e(

phas -100

-200 37456 freq, GHz

Figure 3.6 The phase difference of outputs of port2 and port3 of the Marchand

Balun (Ratio=S(2,1)/S(3,1))

26 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

3.2 Conductor Backing Slotline

For a traditional slotline, the closed form approximate expressions for slot wavelength and characteristic impedance are given as follows [33, 34].

For 2.22 ≤ ε r ≤ 3.8,0.006 ≤ h / λ0 ≤ 0.06.

and 0.0015 ≤ W/λ0 < 0.075

0.945 λs 6.3(W / h) ε r 8.81(ε r + 0.95) =1.045 − 0.365lnε r + − [0.148 − ]⋅ ln(h / λ0 ) (3.15) λ0 238.64 +100W / h 100ε r

(ε − 2.22)π Z = 60+ 3.69sin[ r ]+133.5ln(10ε ) W / λ 0s 2.36 r 0

+ 2.81[1− 0.011ε r (4.48+ lnε r )](W / h)ln(100h/λ0 ) +131.1(1.028− lnε r ) h/λ0 (3.16) W / h +12.48(1+ 0.18lnε r ) 2 ε r − 2.06+ 0.85(W / h)

For 0.075 ≤ W/λ0 < 1

0.835 0.48 λs 0.621ε r (W / λ0 ) (ε r + 2) =1.194 − 0.24 ln ε r − − 0.0617[1.91 − ] ⋅ ln(h / λ0 ) (3.17) λ0 (1.344 + W / h) ε r

Z =133+10.34(ε −1.8)2 +2.87[2.96+(ε −1.582)2]⋅{(W/h+2.32ε −0.56) 0s r r r (3.18) 2 1/2 2 2 ⋅[(32.5−6.67εr )(100h/λ0) −1]} −(684.45h/λ0 )(εr +1.35) +13.23[(εr −1.722)W/λ0}

Conductor backed slotline has many advantages: lower Z0, less dispersion, improved mechanical strength and so on [35]. However, there are no closed form expressions

27 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis for effective wavelength and characteristic impedance for conductor backed slotline.

Conductor backing slotline could be simulated in HFSS, and effective wavelength can be measured from the electrical field distribution. From Figures 3.6-3.8, we can measure the effective wavelengths of conductor backed slotline at different frequencies. These are summarized in Table 3.2.

Frequency Effective Wavelength λg rRadiation Ring

3GHz 76.0mm 12.1mm

5GHz 41.5mm 6.6mm

7GHz 27.3mm 4.4mm

Table 3.3 Effective wavelengths of conductor backing slotline from HFSS

Finally, we chose the radius of radiation ring for the highest frequency to be 1mm, and that for the lowest frequency to be 13mm. That means, we designed the slotted spiral antenna with b = 1mm and a = 0.438mm/rad.

28 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 3.7 Electrical field of conductor backing slotline at 3GHz (HFSS)

Figure 3.8 Electrical field of conductor backing slotline at 5GHz

29 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 3.9 Electrical field of conductor backing slotline at 7GHz

3.3 Terminations of the arms

The outer portion of each arm could be occupied by a termination of some type, to minimize reflections and axial ratio. The initial impedance for the synthesis was quite large, at 1500Ω, to ensure minimal refection from the beginning of the termination.

The final impedance in the synthesis was set to 150Ω to ensure sufficient overall loss.

The standard synthesis yielded the impedance as a function of position and the overall length of the transformer [36-39].

The termination of a cavity backed slotted spiral antenna can be implemented using

30 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

60 resistors, distributed equally along the overall transformer length obtained from the synthesis [39]. The terminations were realized using 1%, 0603-package chip resistors, as shown in Figure 3.9. A plot showing the resulting shunt resistance across the slot as a function of position in the slot, in guide wavelengths, is shown in Figure 3.10. The values of the individual chip resistors were chosen to most accurately approximate the theoretical impedance curve of the Klopfenstein taper within the limits of availability

[40].

Figure 3.10 The shunt resistance as a function of position for the slot spiral

termination (reprinted from [41])

31 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 3.11 The shunt resistance as a function of position for the slot spiral

termination (reprinted from [41])

However, because of the restricted surface area our design, the terminations were not prcatical in this research.

3.4 Final Design

Since the simulation of the real spiral arms took too long time in HFSS, and the real spirals are not available in CoralDRAW, the spiral arms with half circles were substituted with increasing radii to approach the real spiral arms (Figure 3.11). Both the appearance and the simulations results are very close to the real spiral arms. So in

32 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis this thesis, the half circle approximation spirals were used.

Figure 3.12 Approach spiral with half circles (dark: spiral, gray: half circles)

From Bosui’s research, the slot width should be less than or equal to the substrate thickness [3]. Based on the discussions of the previous section, the parameters of the slotted spiral antennas used in this work are summarized in Table 3.2.

a N LSlot Rmin Rmax DVia

478um/rad 4 2mm 1mm 13mm 457um

Table 3.4 The parameters of the slotted spiral antenna

Figure 3.12 shows the masks to fabricate the slotted spiral antenna. The Marchand balun was bent so as to make sure the two end vias of Marchand balun lie on the

33 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis metal part of the top slotted spiral antenna layer instead of the slot area.

Figure 3.13 Masks to fabricate the slotted spiral antenna

The slotted spiral antenna with the above parameters was simulated in Ansoft HFSS.

The S11 simulation results are shown at Figure 4.13. From the figure, we see S11 could be as low as -15dB at the region close to 3.3GHz, 4.7GHz, 6.4GHz, although S11 could not always keep as low as -10dB between 3GHz and 7GHz. One of the possible reasons is the lack of terminations at the two ends of arms as well as the presence of the Marchand balun conductors between the antenn and ground planes. The radiation patterns at 3.3GHz, 4.7GHz and 6.4GHz are shown at Figure 4.14. From the figure,

34 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

we can see at the frequencies with low S11, the antenna has good forward radiation patterns, and the gains are as high as 0dB.

Figure 3.14 Simulation results of S11 for the slotted spiral antenna

Figure 3.15 Simulation results of radiation pattern for the slotted spiral antenna

at 3.3GHz, 4.7GHz and 6.4GHz.(unit: dB. blue:3.3GHz, red: 4.7GHz, green:

6.4GHz)

35 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Chapter 4. Fabrication Procedure

The fabrication procedures for the 3D slotted spiral antenna are presented in this chapter. The slotted spiral antennas were fabricated both with the milling machine and the photolithographic techniques separately.

4.1 Milling Method

4.1.1 Introduction

The preliminary slotted spiral antennas were fabricated by the milling machine Quick

Circuit 3000 provided by the T-tech Inc. The Quick Circuit Model 3000 system [Fig.

4.1] is composed of two main assemblies: the milling table with the standard spindle assembly, and the controller. The controller supplies the power to the milling table and acts as the communication link between the table and the computer.

Fig. 4.1 Quick Circuit Main Assemblies

Some kind of vacuum system is also required for the proper operation of the Quick

Circuit system. The vacuum system is used to hold the board as well as to remove

36 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis particles generated by the operation of the Quick Circuit. Proper use of the vacuum will help ensure accurate milling of board features as well as increase tool life. A solenoid or an air cylinder controls the spindle head assembly. A pneumatic controller regulates the air cylinder on the Quick Circuit. The controller houses an air valve, regulator and pressure gage. The stepper motors turn the lead screws in precise amounts to position the spindle head assembly above the table. The solenoid raises and lowers the spindle above the table. The drill motor rotates the chuck.

IsoPro programs the Quick Circuit system to drill, mill and route following the circuit board design. In this research, IsoPro 2.5 was used as shown in Figure 4.2. The prototype was generated in dxf format from HFSS. Finally, the dxf format files were imported into IsoPro. [42]

The endmill tools with diameter 11mil (279um) were used to mill the unneeded metal.

The locating holes were put as close to the edge of PCB to increase the efficient area.

Furthermore, the design should not be put too close to the locating holes so that the interference of the holes could be small.

37 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 4.2 Interface of IsoPro

When the fabricated antennas were observed under a microscope, we could see that the edges were not very smooth. Furthermore, besides the metal, some of the substrate of PCBs was also removed together. That means, the thickness of the PCB near the slots is not 5mil anymore. Actually, using the micrometer, the thickness of the PCBs near slots was measured to be approximately 4 mils.

4.1.2 Bonding

Rogers 3001 bonding film was chosen to bond the three layers of PCBs. Rogers 3001 bonding film is a thermoplastic chloro-fluorocopolymer with a thickness of 1.5mil

38 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

(38.1um). 3001 bonding film features a low dielectric constant and low loss tangent at microwave frequencies, ensuring minimum interference with the electrical function of bonded stripline and other multilayer constructions [43].

The procedures are as following:

1. Clean all the surfaces to be bonded with acetone and DI water.

2. Put the bonding films between the PCBs and use the big holes to do the alignment.

3. Put the thin metal threads across the eight small alignment holes to fix the PCBs

and the bonding films.

4. Put the aligned PCBs together with the bonding films between the clamps, and

apply moderate pressure on it.

5. Put the clamped PCBs in the oven and set the oven to be 2200C.

6. After the temperature of the oven reaches 2200C, leave the clamped PCBs in the

oven for another 30 minutes.

7. Let it cool to room temperature.

8. Take the clamped PCBs out of the oven, and keep the PCBs clamped until it cools

down to room temperature.

Because of the slot roughness, the results of the preliminary slotted spiral antennas were not so good.

39 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

4.1.3 Soldering

Four vias between the upper layer of the Marchand balun and the spiral antenna plane are made by solder wires. The bonding film through the via holes were removed by a drill bit. After putting a small piece of solder wire in the via hole, solder was melted using a soldering iron and then the via holes were filled with solder. In this way, the via connections were made between the outputs of the Marchand balun and the inputs of slotted spiral antenna.

Furthermore, the metal threads were soldered on the copper surface on the upper spiral antenna plane and the bottom ground plane to act as the shorting pins. In this way, the parallel transmission mode was depressed.

4.2 Photolithographic Techniques

Finally, the slotted spiral antennas were fabricated with the photolithographic techniques.

4.2.1 Introduction

Photolithography is a process used in microfabrication to selectively remove parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitive chemical, photoresist, on the substrate. A series of chemical treatments then engraves the exposure pattern into the material underneath

40 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis the photoresist. Photolithography resembles the conventional lithography used in printing, and shares some fundamental principles with photography. It is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously [44].

4.2.2 Spin Coating

Spin coating is the most common technique for depositing thin polyimide films. The thickness of the spin-coated polyimide film depends strongly on the viscosity and concentration of the solution and can be controlled over a wide range by varying the angular speed of the spinning, spin time and dispense volume. The most sensitive parameter here is the spin speed according to the power law relationship: t=kω3, where t is the film thickness and ω is the angular spin frequency. On general, high viscosity solutions spun at low angular speeds for a short period of time resulted in thicker films.

In this research, Shipley1818 (S1818) was used as the photoresist. S1818 is positive photoresist. Figure 4.3 shows the spin curve of S1818 photoresist. Normally, S1818 was spun with 500rpm for 10 seconds for spreading, 4000rpm 30 seconds for spin coating. [45]

41 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Spin Curve of Shipley 1818

1.6

1.4

1.2 ess(um) 1

Thickn 0.8

0.6 0 1000 2000 3000 4000 5000 6000 7000 RPM

Figure 4.3 Spin curve of Shipley 1818 photoresist

Adhesion promoters are frequently used to improve the initial adhesion of photoresist to various surfaces to reduce the degradation in adhesion due to environmental effects.

Hexamethyldisilazane (HMDS) is a commonly used adhesion promoter. Normally

HMDS is spun with 500rpm for 10 seconds for spreading, 4000rpm 30 seconds for spin coating.

Laurell WS-400 spin processor was used to do the spin coating (Figure 4.4) [46].

42 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 4.4 Laurell WS-400 spin coater

The following are the steps used to fabricate the spiral antennas with photolithographic techniques.

1. Cleaning the PCBs with Actone, Methenol, DI water.

2. Put HMDS on PCB and spin with 500rpm 10 seconds for spreading, 4000rpm 30

seconds for spin coating.

3. Put S1818 on PCB and spin with 500rpm 10 seconds for spreading, 4000rpm 30

seconds for spin coating.

4. Put in the oven set at 90-950C for 20 minutes for soft baking.

5. Expose under UV lights for 15 seconds.

6. Dip in the diluted Microposit 351 (DI water:Microposit 351 = 5:1) for 90 seconds

for development.

7. Rinse in DI water.

43 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

8. Put in the oven set at 1200C for 20-30 minutes for hard baking.

9. After the PCBs cool down, put in FeCl3 for 20 minutes.

10. Rinse the PCBs in DI water, Acetone and then DI water.

For the bottom layer, between step 6 and step 7 above, step 2 and 3 should be repeated on the back side to protect from FeCl3.

A drill tool with diameter 18mil is used to drill the via holes. The three layers were bonded together using the bonding film 3001 mentioned in section 4.1.2. Then the soldering procedure mentioned in section 4.1.3 was done to connect the two outputs of the Marchand balun and the two inputs of the slotted spiral antenna, as shown in

Figure 4.5.

44 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 4.5 The slotted spiral antenna fabricated with photolithographic techniques

45 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Chapter 5 Results of Antenna Measurements

The characterization of an antenna includes its input impedance, directivity, gain, efficiency, radiation pattern and axial ratio. The slotted spiral antennas were tested with antenna characterization system in the Microwave and Millimeter Electronics

Lab at the University of Cincinnati.

5.1 Antenna Measurement

The far field condition is

r > 2D 2 / λ (5.1)

Where D is the diameter of the antenna and λ is the wavelength at a given frequency.

For example, for an antenna with diameter 5cm, r = 0.12m at f = 7GHz. In order to provide a controlled environment and to minimize electromagnetic interference, the antennas are normally tested in an anechoic chamber. The chamber is a 1.2x1.2x1.2m anechoic chamber. The inner walls of the chamber were covered with pyramidal absorbing material to eliminate the reflection of the waves from the wall. The pyramidal absorbers were 4” thick which could provide 30dB absorption as low as at

3GHz. At 10GHz, the absorption of the normal incident wave is better than 40dB. The distance between the transmitter and the receiver was 85cm. The receiving antenna was also connected to a stepping motor which could rotate the transmitter 1.80 per step. The stepping motor used for antenna testing was a 2-phase 5.1V bipolar motor.

46 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

The maximum current for each phase was 1 amp. The stepping motor was driven by a

L298 dual full-bridge driver. The drive circuit for the L298 is attached as Figure 5.1

[47].

Figure 5.1 Stepper motor circuits

5.1.1 Far Field Measurements

An HP VEE program was used to synchronize the rotation of the stepping motor and to acquire data from the signal source and the signal detector. The program steps the motor rotation, pauses while data is being taken from the sweep oscillator, power meter and the spectrum analyzer. It then steps the motor again. After the required number of steps to achieve a full rotation, 3600, the program generates the radiation pattern of the antenna under test at the horizontal plane and stores the data. One can obtain the radiation pattern of the Antenna under Test (AUT) in other planes by

47 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis adjusting the orientation of the antenna. The interface of the VEE program is shown in

Figure 5.2.

Figure 5.2 Interface of VEE program to test antennas

The gain of the antennas can be measured according to the Friis transmission formula given below [5]:

2 2 Pr ⎛ λ ⎞ Pr ⎛ c ⎞ = ⎜ ⎟ Gr Gt or = ⎜ ⎟ Gr Gt (5.2) Pt ⎝ 4πR ⎠ Pt ⎝ 4πRf ⎠

Then we can derive the equation to measure the gain:

2 1 Pr ⎛ 4πRf ⎞ Gr = ⎜ ⎟ (5.3) Gt Pt ⎝ c ⎠

If two identical antennas are used, that is Gt = Gr the gain can be derived from

48 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

1/ 2 ⎛ P ⎞ 4πR ⎜ r ⎟ Gr = Gt = ⎜ ⎟ (5.4) ⎝ Pt ⎠ λ

Gt is the gain of the transmitting antenna, Gr is the gain of receiving antenna, Pr is the received power, Pt is the transmitted power, R is the separation distance between the antennas.

We used the commercial wideband log-periodic antenna as the transmitter antenna, and the spiral antenna as the receiver antenna. If the spiral antenna was used as the transmitter antenna, when it was turned toward the anechoic chamber wall, some power would be reflected by the pyramidal absorbing wall, thus increasing the measurement error.

Based on equation (5.4), we can get the gain of the log-periodic antennas using two identical log-periodic antennas, as in Figure 5.3 expressed in dBi.

49 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 5.3 Gain of the Log-Periodic antenna (unit: dBi)

5.1.1 Polarization Measurements

Theoretical radiated fields of a spiral antenna are circularly polarized (CP). The quality of the CP is represented by the axial ratio (AR). The polarization measurement method requires that a linearly polarized antenna, usually a dipole or a small horn is rotated in the plane of polarization, which is taken to be perpendicular to the direction of the incident field, and the output voltage of the probe is recorded to determine the

AR of the antenna under test (AUT) [48]. However, such measurement setup for the proper AR measurement is not available in the Microwave and Millimeter Wave

Electronics Lab. Therefore, no detailed polarization measurements were performed on any of the antennas fabricated in this thesis.

50 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

5.2 Experimental Results

A number of slotted spiral antennas were fabricated using the processing methods summarized above. For the first step, the S11 of the slotted spiral antennas were measured with the network analyzer HP8510C. From Figure 5.4, we can see only narrow discrete bandwidths where S11 < -10dB (VSWR < 2.0) appear in the measured results.

Figure 5.4 S11 of the four identical slotted spiral antennas (unit: dB, Solid: P1,

Dash: P3, Dot: P5, Dash Dot: P6)

Figure 5.4 is consistent with the simulated data shown in Fig. 4.13. Main reason for the discrete frequency response may be due to the slot width being 279um and the

Marchand balun upper strip layer being 125 um below the antenna layer. That means that for each half a slot turn, the condition that the slot width should be equal to or

51 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis less than the substrate thickness is being violated. Therefore it is possible that the radiation at these half-slot-turns may be preventing the effective radiation condition of the slot line over a continuous band.

Other possible reasons why antenna S11 does not show wideband response are:

1. Because of the limitation of the experimental conditions, the slotted spiral

antennas were bound with 3 layers of PCBs using the bonding film, instead of

depositing polyimide using the spin coater and depositing metal layer using

sputtering deposition. There were some gaps between the three layers of PCBs.

2. The widths of the Marchand balun strips are very narrow; therefore there is need

for a very precise strip alignment. While the alignment in this research was very

approximate, the performance of the Marchand balun would not be as good as

simulation results.

3. The soldering was realized by inserting a small piece of solder into the vias, and

then melted it using an iron. However, we can’t guarantee that the top layer had

good contact with the output of the Marchand Balun.

4. The terminations were not performed at the two ends of the slotted arms. The

reflected back waves may influence the distribution of the currents in the slots.

The far field performance of the slotted spiral antennas was measured in the anechoic chamber at the Microwave and Millimeter Wave Electronics Lab.

The gain of #5 slotted spiral antenna was measured based on (5.3), as shown in Figure

52 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

5.5. We can see that the peak gain is nearly -12dBi and occurs at f = 5.15GHz. Note also that these results are based on using a linearly polarized transmitting antenna for a circularly polarized slotted spiral antenna.

Figure 5.5 Gain of the #5 slotted spiral antenna (unit: dBi)

The radiation patterns of the four slotted spiral antennas were measured using the measurement system described above. The input power is 10dBm at f = 5.15GHz.

From Figure 5.6, we can see, at f = 5.15GHz, the radiation patterns of the four slotted spiral antenna are very close. The four almost identical slotted spiral antennas are later used for building an antenna array at f = 5.15GHz.

53 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 5.4 Radiation patterns of the four identical slotted spiral antennas at 5.15

GHz (unit: dBm, Solid: P1, Dash: P3, Dot: P5, Dash Dot: P6)

54 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Chapter 6 Phased Array Antenna

Phased antenna arrays offer the unique capability of electronic scanning of the main beam. By changing the phase of the exciting currents in each element antenna of the array, the radiation pattern can be scanned through space [5]. In this chapter, several common used antenna arrays are discussed. A linear phased antenna array was built with four identical slotted spiral antennas fabricated in chapter 5 and using a loaded line phase shifter.

6.1 Array Theory

Arrays are found in many geometrical configurations. The most elementary is that of a linear array in which the array element centers lie along a straight line. The elements in an array can also form a planar array. A popular planar array is the rectangular array in which the element centers are contained within a rectangular area. A class of arrays that is still emerging is that of conformal arrays, where the array element locations conform to a nonplanar surface. This is a great advantage for arrays on the skin of a vehicle such as an airplane. The slotted spiral antennas in this research are very suitable to build conformal arrays because of their small thickness.

The radiation pattern of an array is determined by the type of individual elements used, their orientations, their positions in space, and the amplitude and phase of the currents

55 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis feeding them. To simplify the discussion of array, we will begin by letting each element of the array be an isotropic point source. The resulting radiation pattern is called the array factor (AF) [5].

The array factor, AF, is a function of the number of elements, their geometrical arrangement, their relative magnitudes, relative phases, and their spacing.

6.1.1 Linear Array

Let’s consider a linear array of N equally spaced elements.

N −1 jβd cos θ jβ 2 d cos θ jβ 3d cos θ jn βd cos θ AF = I 0 + I 1e + I 2 e + I 3 e + L = ∑ I n e (6.1) n = 0

Where d is the distance between each element, β is the phase constant and θ is the phase of each elements. If the current has a linear phase progression, we can separate the phase explicitly as

jnα I n = An e (6.2)

Defineψ = βd cosθ +α , then

N −1 jnψ AF = ∑ An e (6.3) n=0

The normalized array factor for N elements which are uniformly excited (UE), equally spaced (ESLA) and centered about the coordinate origin is:

sin(Nψ / 2) f (ψ ) = (6.4) N sin(ψ / 2)

As N increases, the main lobe narrows and there may be more side lobes in one period.

Since wavelength decreases when frequency increases, the distance between the

56 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis elements in term of wavelength becomes longer, so the grating side lobes may appear at higher frequencies.

Figure 6.1 Array Factor for 4-element linear array with fixed distance between

each element (dash: 3GHz, line: 5GHz, dot: 7GHz)

Figure 6.1 shows the calculated results for a 4 element linear array. For this case, the phase difference was 450, and the distance between the spiral antennas was 35mm, corresponding to 0.35λ0 at f = 3.0GHz, 0.6λ0 at f = 5.15GHz, and 0.82λ0 at f =

7.0GHz. Grating lobes that may appear depends on the distance between element spacing d and phase angle α between elements.

1. d<λ0/2, no grating lobes.

57 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

2. λ0/2

3. d>λ0, at lease one grating lobe.

6.1.2 Feed Network for Phased Array Antenna

Feed networks can take one of three basic geometric forms: parallel, series, or space.

Parallel and serial feed are most commonly used in microstrip arrays or slot antenna arrays. The parallel feed is also called a corporate feed because of its similarity to an organization diagram of a corporation. The path length to each element from the feed point is equal; thus, the phase of the excitations will also be equal. The three- dimensional multilayer integrated circuit technique is to place the feed network and radiator on multiple layers that are isolated with ground planes.

6.2 Linear Slotted Spiral Antenna Array

A simpler approach was adopted to make a four-element linear array, instead of planar array. Loaded line type phase shifter fabricated on PCB is used instead of RF MEMS switch phase shifters. The loaded line phase shifter is connected to four identical monolithic broadband slotted spiral antennas to form a 4-element linear array.

The loaded line phase shifter uses “shunt stubs” which are added to the microstrip feeding network to provide phase shifting. A tilted radiation beam can be detected first with the phase shift in the feeding network. Then the shunt stubs can be trimmed off

58 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis manually or by chemical etching in order to remove the phase shift in the feeding network.

6.2.1 Loaded line phase shifter design

In order to electronically steer the main beam of an antenna array, a phase shifter is needed to be incorporated into the system. A phase shifter is a two-port network that provides phase difference between the output and input signals. Various phase shifters can be classified into two categories: digital and analog phase shifters.

A digital phase shifter can provide phase shift at only a few predetermined discrete values while an analog phase shifter tunes the phase shift in a continuous manner with a corresponding continuous variation of control signals. Digital phase shifter has gained in popularity since it is compatible with computer control in beam steering.

Analog phase shifter tunes the phase shift in a continuous manner with a corresponding continuous variation of control signals [1].

Loaded line phase shifter is a digital phase shifter design that is useful for small amount of phase shift. The mechanism of phase shift in this circuit is based on the loading of a uniform transmission line by a small reactance. This is illustrated in

Figure 6.2

59 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Fig. 6.2 Basic loaded-line phase shifter

The transmitted wave in this 2-port network undergoes a phase shift ∆φ that depends on the normalized susceptance b=B*Z0. The reflection and transmission coefficient T caused by b can be written as

1− (1+ jb) − jb Γ = = 1+ (1+ jb) 2 + jb 1/ 2 (6.1) 2 ⎛ 4 ⎞ −1 T = 1+ Γ = = ⎜ ⎟ e− j tan (b / 2) 2 + jb ⎝ 4 + b 2 ⎠

Thus, for a basic loaded line phase shifter, the phase shift in the transmitted wave introduced by the susceptance is

b ∆φ = tan −1 ( ) 2

∆φ will be positive for capacitive (i.e. positive b) loading, and negative for inductive loading (i.e. negative b). That is, the transmitted wave advances in phase compared to the transmitted wave for b=0.

In this research, the loaded line phase shifter was designed on ULTRA2000, and the parameters of the PCB are shown in Table 6.1.

60 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

PCB Dielectric Constant εr Thickness of substrate Thickness of metal

ULTRA 2000 2.45 506um (20mil) 35um (1oz )

Table 6.1 The parameters of the PCB ULTRA 2000

At f = 5.15GHz, λ0 = 58.25 mm. We chose the distance between each elements d0 =

0 0.6*λ0 = 34.95mm. Figure 6.3 illustrates the parameters of the 45 loaded line phase shifter. shifter at Frequency = 5.15GHz.

L50Ω1

L25Ω1 L50OC The simulation was done with ADS with the distance between each elements fixed and leave L25Ω2, L25Ω3, L25ΩOC, L50Ω2, L50Ω3, and L50ΩOC flexible, as shown in Figure

6.2. We set the goals that the magnitudes of the four outputs are all close to -6dB and the phases of the four outputs increase by 450, and then did the optimization. L50Ω2 L50Ω3

L25ΩOC

λg/4(25Ω) L25Ω2 L25Ω3

Figure 6.3 450 loaded line phase shifter (HFSS)

61 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.4 Circuit layout of the loaded line phase shifter for a 4-element linear

array with phase shifter of 450 in ADS.

62 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

By optimization in ADS shown in Figure 6.4, we can derive the results of the

loaded line phase shifter. These are shown in Figures 6.5 and 6.6. ) ) ) ) )

5) 4) 3) 2 -4 , 1, 1, 1, 1 ( ( ( ( S S S S . . . . -5 P P P P S S S S . . . . 1 1 1 1

P P P P -6 S S S S . . . . 1 1 1 1 s s s s i i i i -7 s s s s y l a

naly naly naly n -8 A l a n inalA inalA inalA i

F F F F -9 ( ( ( (

B 5.10 5.12 5.14 5.16 5.18 5.20 dB dB dB d freq, GHz

Figure 6.5 Magnitudes of the four outputs of the loaded line phase shifter. ) ) ) ) ) ) ) ) 5 4 3 2 , , , , 100 1 1 1 1

.SP.S( .SP.S( .SP.S( .SP.S( 50 P1 P1 P1 P1 .S .S .S .S 1 1 1 1

s s s s 0 i i i i s s s s y y y y l l l l a a a a n n n n -50 A A A A l l l l a a a a n n n n i i i i F F F F ( ( ( ( -100 e e e e s s s s

a a a a 5.10 5.12 5.14 5.16 5.18 5.20 h h h h p p p p freq, GHz

Figure 6.6. Phase variations of the four outputs of the loaded line phase shifter.

63 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

W25Ω λg/4(25Ω) W50Ω L25ΩOC L50ΩOC

3.71 mm 9.84 mm 1.417 mm 13.93 mm 16.9 mm

L25Ω1 L25Ω2 L25Ω3 L50Ω1 L50Ω2 L50Ω3

31.25 13.4 15.8 14.91 14.3 5.0

Table 6.2 The parameters of the 450 loaded line phase shifter (unit: mm)

For the input, we used CPWG, Z0 = 50Ω, CPWG: W = 1.53 mm, G = 1.575mm.

The design was realized with the parameters listed in Table 6.2. The mask is shown in

Figure 6.7. The 450 loaded line phase shifter was fabricated using the milling machine at the Department of Electrical and Computer Engineering, as shown in Figure 6.8.

Figure 6.7 Layout of the 450 phase shifter at 5.15GHz

64 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.8 450 loaded line phase shifter

6.2.2 Phased Array with Loaded line phase shifter

The four identical slotted spiral antennas fabricated in Chapter 5 were soldered on the loaded line phase shifter fabricated in section 6.2.1, as shown in Figure 6.9.

65 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.9 Antenna array with 450 phase shifter

S11 of the antenna array was measured with the network analyzer and the results were compared with those of single slotted spiral antenna as shown in Figure 6.10. From

Figure 6.10, we can see at 5.15GHz, S11 of antenna array is nearly -5dB.

66 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

0 Figure 6.10 S11 of the 45 phase difference (unit: dB, Dash: P1, Dot: P3, Dash

Dot: P5, Dash Dot Dot: P6, Solid: Antenna Array)

Radiation patterns of the antenna array were measured with the antenna test system and the results were compared with those of a single slotted spiral antenna, shown in

Figure 6.11. Since the input power for the single spiral antenna is 10dBm, so that of the antenna array should be four times of the single antenna, which is 16dBm. From

Figure 6.11, we can see that magnitude of the radiation from antenna array is almost equal to that of the best single slotted spiral antenna. One of the possible reasons is that loss was introduced by the soldering points between the slotted spiral antennas and the loaded line phase shifter. Furthermore, the main beam deviation was not observed. One of the possible reasons is the radiation patterns of the four single slotted spiral antennas are not the same.

67 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.11 Antenna array with 450 phase difference at f = 5.15GHz (unit: dB,

Dash: P1, Dot: P3, Dash Dot: P5, Dash Dot Dot: P6, Solid: Antenna Array)

Then the two loaded lines (L25ΩOC and L50ΩOC) were removed and then phase difference with the four outputs disappeared also, as shown in Figure 6.12. Now it becomes the phased array without phase difference.

68 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.12 Antenna array with 00 phase shifter

S11 and radiation patterns of the antenna array were measured with the network analyzer and the results were compared with those of single slotted spiral antenna as shown in Figure 6.13. From figure 6.13, we can see at 5.15GHz, S11 of antenna array is nearly -10dB.

69 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

0 Figure 6.13 S11 of the 0 phase difference (unit: dB, Dash: P1, Dot: P3, Dash Dot:

P5, Dash Dot Dot: P6, Solid: Antenna Array)

Radiation patterns of the antenna array with 00 phase difference was measured with the antenna test system and the results were compared with those of single slotted spiral antennas as shown in Figure 6.14. From figure 6.14, we can see that the magnitude of the antenna array was improved comparing with the single slotted spiral antenna.

70 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.14 Phased array with 0 phase difference at f = 5.15GHz (unit:dB, Dash:

P1, Dot: P3, Dash Dot: P5, Dash Dot Dot: P6, Solid: Antenna Array)

The log-periodic antenna mentioned in chapter 5 was used as transmit antenna to measure gain of the antenna array without phase difference. This is shown in Figure

6.15. From Figure 6.15, we can see the gain of the slotted spiral antenna is flatter than that of single slotted spiral antenna in Figure 5.5.

71 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Figure 6.15 Gain of the 00 phase difference antenna array.

72 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

Chapter 7 Conclusion

7.1 Thesis Summary

The slotted spiral antenna with Marchand balun and array systems were studied in this thesis. The slotted spiral antennas were simulated with HFSS. The slotted spiral antennas were fabricated using the milling machine and the photolithographic techniques. The slotted spiral antennas were measured in the anechoic chamber in the

Microwave and Millimeter Electronics Lab. The slotted spiral antennas were designed to work from 3-7GHz. The Marchand balun exhibits good performance from 3-7GHz.

The magnitudes of two Marchand balun outputs both decrease nearly -3dB and phase difference between the two outputs is exact 1800. However, the performance of the slotted spiral antennas did not meet our expectation. The bandwidth with S11 lower than -10dB (VSWR<2) is only around 5%.

7.2 Future Work

Three layers of PCBs were bound together using bonding film to build the slotted spiral antennas. The possible gaps between the layers would cause the error of design.

The bad connect of vias were another reason to cause the error.

One of the possible reasons why the results were not good was the coupling between the slotted spiral antenna arms and Marchand Balun. To solve this problem, we can move the Marchand balun outside the cavity. That means, we can isolate slotted spiral

73 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis antenna and Marchand balun with a ground plane, and use vertically interconnected via to connect the two outputs of Marchand balun and the two inputs of slotted spiral antenna.

74 Slotted Spiral Antennas and Widebandwidth Array Systems MS Thesis

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