UNEQUALLY SPACED MICROSTRIP LINEAR ANTENNA ARRAYS FOR FIFTH-GENERATION BASE STATION

NOOR AINNIESAFINA BINTI ZAINAL

A thesis submitted in fulfilment of the requirements for the award of the degree of

Doctor of Philosophy (Electrical Engineering)

School of Electrical Engineering Faculty of Engineering

Universiti Teknologi Malaysia

SEPTEMBER 2018

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Specially dedicated to my lovely husband Abdul Halim Zaini, my lovely kidsMuhammad Adam and Muhammad Ammar, my mother Norsiah Abdul Rahman, my

dearest sibling, and in the memories of my father, Zainal Yahya.

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ACKNOWLEDGEMENT

Firstly, Praise to Allah, who has blessed me towards the completion of mythesis. I would like to thank my supervisor, Associate Professor Dr. Norhudah BintiSeman, for her excellent advice and expertise throughout my research work. Thisappreciation also goes to Professor Dr. Yoshihide Yamada and Dr. MuhammadRamlee Bin Kamarudin for their feedbacks and comments in numerous discussionsin producing successful research in UTM and also gave me guidance and supportnecessarily during the research and learning how to be a good researcher and betterperson in works. To the members of the Wireless Communication Centre (WCC) andresearch group, I owe sincere and earnest thankfulness for the unforgettable and uniqueexperience of my life. I would like to acknowledge my sponsors, the Ministry ofHigher Education Malaysia (MOHE), for sponsoring my studies.

Also, thanks to those in the School of Postgraduate Student, Faculty ofEngineering, Universiti Teknologi Malaysia, for providing excellent research andadministrative support. I would like to thank Associate Professor Dr. Tarmizi Aliand Mr. Ahmad Azlan Ab Aziz, from Universiti Teknologi Mara (UiTM) for theirexpertise in antenna measurement. I would like to take this opportunity to acknowledgeall staff and technician in WCC, Mr. Shaarul Shaari and Mr. Norhafizul Ismail for theirtechnical assistant.

My thanks go to my colleagues and other people that contributed to myresearch. I am hugely grateful to my mother Norsiah Abdul Rahman; my latefather; Zainal Yahya, my lovely husband; Abdul Halim Zaini, and my lovely kids;Muhammad Adam and Muhammad Ammar, for their endless love, care, supports andencouragements, especially throughout this period of my life and kept me going allthe way through my Ph.D. study. Most of all, Praise to Allah, who made all thingspossible. Alhamdulillah.

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ABSTRACT

Wireless technology communication has been continuously evolving towardsfuture fifth generation (5G), whereby multi-beam, multi-frequency, and low sidelobecharacteristics are required in the mobile base station. However, the low sidelobe levelof conventional mobile base station antenna led to more complex of feeding networkdesign in order to give an adequate excitation coefficients (amplitude and phase) toarray elements. Thus, the current base station antennas are difficult for wide frequencyuse due to frequency range is limited. Subsequently in this research, an unequallyspaced microstrip linear antenna arrays is proposed. The radiation pattern synthesis forlow sidelobe and grating lobes suppression over wide frequency use are investigated.In the first stage, a single antenna is designed at frequency 28 GHz followed by 16element linear arrays in order to achieve the gain requirement for mobile base stationantenna. Next, the design of antenna arrays with sidelobe reduction is proposed. Threeconfigurations of linear antenna arrays are designed, which are equally spaced array(ESA), unequally spaced array 1 (USA 1) and unequally spaced array 2 (USA 2) atfrequency f� = 28 GHz, f1 = 42 GHz and f2 = 56 GHz with a similar array aperture, inorder to investigate the antenna performance in wide frequency use characteristics.USA 1 and USA 2 are having different center spacing of array (dc), which aredc(USA1) = 0.6 mm and dc(USA2) = 0.5 mm, respectively. The simulation results areobtained by using High Frequency Structure Simulator (HFSS). The good results wereobserved, where the performance of sidelobe reduction are constant even though thefrequency changes. Due to the lack of measurement facilities at higher frequency than18 GHz, the antenna arrays are redesigned at lower frequency, which are 12 and 18GHz. In order to achieve a wide frequency operation, a wide frequency use of ESA⇤,USA 1⇤ and USA 2⇤ feeding network (which notation ⇤ indicates that the frequencyof 12 GHz is chosen as reference) are designed by using Advanced Design System(ADS). An equal line lengths (ln) with equal power ratio dividers were constructed.The sidelobe reduced from -13 dB for ESA⇤ to -19 dB for USA 2⇤. The measurementof S-parameter and radiation pattern are performed using a vector network analyzer(VNA) and anechoic chamber, respectively. The measured results were presented anda good correlation with simulations was observed. From the observation, the sidelobelevel and grating lobe suppression of USA 2⇤ is reduced rather well and recommendedfor wide frequency band for 5G mobile base station antenna.

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ABSTRAK

Teknologi komunikasi tanpa wayar terus berkembang untuk ke arah generasi kelima

(5G), di mana pelbagai alur, pelbagai frekuensi dan cuping sisi rendah diperlukan di stesen

pangkalan mudah alih. Walaubagaimanapun, stesen pangkalan mudah konvensional bercuping

sisi rendah rangkaian penyuapan menyebabkan rangkaian penyuapan yang lebih komplek untuk

menghasilkan pengujaan (amplitud dan fasa) yang cukup bagi setiap elemen. Maka, ketika ini,

antena stesen pangkalan mudah alih adalah sukar untuk mencapai penggunaan julat frekuensi

yang lebar kerana jalur frekuensi yang terhad. Seterusnya, tatasusunan antena mikrojalur linear

bersela tidak sama dicadangkan. Sintesis corak radiasi untuk pengurangan cuping sisi dan cuping

jeriji bagi penggunaan frekuensi yang lebar dikaji. Pada peringkat permulaan, antena tunggal

pada frekuensi 28 GHz direkabentuk diikuti dengan rekabentuk tatasusunan linear 16 elemen

bagi mencapai gandaan yang diperlukan oleh antena stesen pangkalan mudah alih. Seterusnya,

rekabentuk tatasusunan antena dengan pengurangan cuping sisi dicadangkan. Tiga konfigurasi

tatasusunan antena linear telah direkabentuk, iaitu tatasusunan sama jarak (ESA), tatasusunan

tidak sama jarak 1 (USA 1) dan tatasusunan tidak sama jarak 2 (USA 2) pada frekuensi 𝑓𝑜 = 28

GHz, 𝑓1 = 42 GHz dan 𝑓2 = 56 GHz dengan bukaan tatasusunan yang sama untuk kajian ke atas

prestasi antena dalam ciri frekuensi lebar. USA 1 dan USA 2 mempunyai jarak antara elemen di

tengah tatasusunan yang tidak sama, di mana 𝑑𝑐(𝑈𝑆𝐴 1) = 0.6 𝑚𝑚 dan 𝑑𝑐(𝑈𝑆𝐴 2) = 0.5 𝑚𝑚.

Simulasi dilakukan menggunakan High Frequency Structure Simulator (HFSS). Hasil keputusan

yang baik diperolehi, iaitu tahap pengurangan cuping sisi adalah tetap walaupun frekuensi

berubah. Disebabkan oleh kekurangan fasiliti pengukuran pada frekuensi tinggi melebihi 18

GHz, tatasusunan antena telah direkabentuk semula pada frekuensi 12 dan 18 GHz. Untuk

mencapai operasi frekuensi yang lebar, rangkaian penyuapan antena berfrekuensi yang lebar

untuk ESA*, USA* dan USA 2* (di mana tanda * menunjukkan frekuensi 12 GHz dipilih sebagai

rujukan) direkabentuk menggunakan Advanced Design System (ADS). Rangkaian pembahagi

kuasa dengan panjang laluan (ln) yang sama telah direkabentuk. Cuping sisi telah dikurangkan

daripada -13 dB bagi ESA* kepada -19 dB bagi USA 2*. Pengukuran ke atas parameter-S dan

corak radiasi masing-masing dibuat menggunakan Vector Network Analyzer (VNA) dan ruang

bebas gema. Keputusan yang baik ditunjukkan untuk simulasi dan pengukuran. Daripada

pemerhatian, pengurangan cuping sisi dan cuping jeriji yang baik bagi tatasusunan tidak sama

jarak 2* (USA 2*) telah diperolehi dan ia dicadangkan untuk jalur frekuensi yang lebar bagi

antena pangkalan mudah alih generasi kelima (5G).

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TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION iiDEDICATION iiiACKNOWLEDGEMENT ivABSTRACT vABSTRAK viTABLE OF CONTENTS viiLIST OF TABLES xLIST OF FIGURES xiiLIST OF ABBREVIATIONS xviiiLIST OF SYMBOLS xixLIST OF APPENDICES xx

1 INTRODUCTION 11.1 Research Background 11.2 Problem Statement 31.3 Objectives of the Research 41.4 Scope of the Research 41.5 Contributions of the Research 51.6 Thesis Outline 6

2 LITERATURE REVIEW 82.1 Introduction 82.2 Mobile Communication Systems 8

2.2.1 5G Technology 92.2.2 Mobile Base Station Antenna 122.2.3 Sidelobe Level for Base Station Antenna 13

2.3 Uniform and Nonuniform Antenna Arrays 142.3.1 Uniform Antenna Arrays 152.3.2 Nonuniform Antenna Arrays 17

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2.3.2.1 Density Taper of UnequallySpaced Arrays 18

2.3.2.2 Application of UnequallySpaced Antenna Arrays 19

2.4 Basic Design of Antenna Arrays 232.4.1 Linear Antenna Arrays with Sidelobe

Reduction 292.5 Feeding Network 34

2.5.1 Microstrip Line 342.5.2 Power Divider 362.5.3 Feeding Network Design Utilizing Power

Divider at 5G Band 382.6 Summary 41

3 RESEARCH METHODOLOGY 423.1 Introduction 423.2 Design Specification 423.3 Research Methodology and Flow Chart 443.4 Fabrication and Measurement 52

3.4.1 Fabrication 533.4.2 Measurement 55

3.5 Summary 58

4 DESIGN AND ANALYSIS OF MICROSTRIP PATCHANTENNA AT 28 GHZ 594.1 Introduction 594.2 Microstrip Patch Antenna Design 604.3 Microstrip Patch Antenna with Different Feeding

Technique at 28 GHz 624.3.1 Microstrip Patch Inset Feed 624.3.2 Microstrip Patch Coaxial Probe Feed 664.3.3 Microstrip Patch Proximity Coupled Feed 71

4.4 Summary 75

5 DESIGN OF UNEQUAL SPACED ANTENNA ARRAYS 765.1 Introduction 765.2 Antenna Arrays Configurations 775.3 Low Sidelobe Characteristics 81

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5.4 Implementation of 42 GHz and 56 GHz AntennaElement Using 28 GHz Array Configuration 855.4.1 Grating Lobe 91

5.4.1.1 Microstrip Linear Arrays Con-figurations with Parasitic Ele-ment 93

5.5 Summary 100

6 DEVELOPMENT AND INVESTIGATION OF UN-EQUALLY SPACED ANTENNA ARRAYS AT 12 GHZAND 18 GHZ 1036.1 Introduction 1036.2 Patch Element Configurations 104

6.2.1 12 GHz and 18 GHz Single AntennaDesign 104

6.2.2 Frequency Range for Wide FrequencyUse 107

6.2.3 12 GHz and 18 GHz Antenna Arrays 1086.2.4 Performance of Antenna Arrays Configu-

rations 1096.3 Design and Fabrication of Feeding Network 113

6.3.1 Design of Three Port Network (T-Junction) 115

6.3.2 Performance of Feeding Network 1276.4 Analysis of Low Sidelobe Characteristics 134

6.4.1 Configurations of Antenna Arrays withFeeding Network 134

6.4.2 Results and Discussions 1386.5 Summary 148

7 CONCLUSION AND FUTURE WORKS 1507.1 Introduction 1507.2 Conclusion 1507.3 Recommendation for Future Work 153

REFERENCES 155Appendices A – E 169 – 177

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LIST OF TABLES

TABLE NO. TITLE PAGE

2.1 Summary of literature for unequally spaced array for SLLreduction 21

2.2 Summary of microstrip antenna design at millimeter wavefrequency band 28

2.3 Summary of microstrip antenna arrays design 332.4 Summary of feeding network design utilizing power divider

for 5G antenna arrays 403.1 Parameters and specifications of proposed design 434.1 Theoretical and optimized parameters of design structure at

frequency 28 GHz 634.2 Dimension comparison of theoretical and proposed mi-

crostrip patch antenna (MPA) with coaxial probe feed atfrequency 28 GHz 68

4.3 Summary of simulated results 745.1 Element spacing of unequally spaced array (mm) 805.2 Simulated results of array at frequency 28 GHz 845.3 Summary of wide frequency characteristics simulated results 915.4 Differences of distance between element for 0.7�� (ESA) 925.5 Parameter dimension of patch antenna arrays and parasitic

element (mm) 945.6 Simulated summary of performances for three configurations

of array without and with parasitic elements 1006.1 Parameters of patch antenna 1076.2 Summary results of 16 elements with 16 input ports at

frequency 12 GHz 1126.3 Summary results of 16 elements with 16 input ports at

frequency 18 GHz 1126.4 Element spacing of array (mm) 1146.5 Calculation distance between the output ports of feeding

network 118

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6.6 Summary of impedances and widths of transmission lines 1256.7 Summary of transmission line lengths for feeding networks

for all array configurations at frequency 12 and 18 GHz 1266.8 Summary of the simulated and measured results of feeding

network 1316.9 Summarized transmission coefficient results (amplitudes) of

the feeding network 1336.10 The comparison of the simulation and measurement results of

the proposed antenna arrays 1396.11 Summary of radiation performance over wide frequency use

characteristics 1456.12 Summary of S21and Gt in both frequencies of 12 and 18 GHz 148

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LIST OF FIGURES

FIGURE NO. TITLE PAGE

2.1 Scenario of 5G’s services 92.2 Millimeter wave spectrum 112.3 Rain attenuation at microwave and millimeter wave

frequencies 112.4 Outdoor base station antenna 132.5 Radiation pattern of a mobile base station antenna 132.6 2D plot of radiation pattern 142.7 N element symmetric linear array placed along the x�axis 162.8 Unequally spaced linear array; (a) symmetrical and (b)

asymmetrical 172.9 Deterministic density taper of model current divided into

16 elements; (a) equally spaced array without densityillumination function and (b) unequally spaced array withillumination function (amplitude taper) 18

2.10 FR4 PCB grid array antenna for millimeter wave 5G mobilecommunications 25

2.11 Low profile unidirectional printed antenna for millimeterwave applications 26

2.12 Broadband printed sectorized coverage antennas for millime-ter wave wireless applications 26

2.13 stacked patch antenna arrays on LTCC substrate operated at28 GHz 27

2.14 Low sidelobe level series-fed microstrip antenna arrays ofunequal inter element spacing; (a) unequal inter elementspacing (IES) and nonuniform amplitude and (b) unequalinter element spacing (IES) and uniform amplitude 29

2.15 Stacked microstrip linear array with highly suppressedsidelobe levels and wide bandwidth 30

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2.16 Stacked series-fed linear antenna arrays with reducedsidelobe; (a) perspective view and (b) side view: T1 = 2.7mm, T2 = 1.2 mm, h = 0.8 mm and H = 5.5 mm 30

2.17 Microstrip antenna arrays feed by a series-parallel slot-coupled feeding network; (a) series feeding network and theplacement of coupling slots and (b) parallel feeding network 31

2.18 Systematic approach to sidelobe reduction in linear antennaarrays through corporate feed controlled excitation; (a) lineararray of twelve microstrip patch antenna (MPA) elements and(b) MPA as an array element 32

2.19 Microstrip line; (a) cross sectional view and (b) perspectiveview 34

2.20 Three-port network (T-junction) 372.21 Model of feeding network 382.22 Proposed integrated design 382.23 Configuration of a 20 × 20 element double layer waveguide

slot antenna with 2 × 2 sub-arrays due to partially corporatefeeding 39

2.24 Radiating section and parallel plate waveguide (PPW)corporate feed network (CFN); (a) cross section and (b) topview of the array 39

3.1 Flow chart of whole research 473.2 Flow chart of antenna design 503.3 Flow chart of feeding network design by using Advanced

Design System (ADS) 523.4 Fabrication process 543.5 Measurement process 573.6 The view of antenna array with metal plate; (a) front view and

(b) back view 574.1 Structure of microstrip patch antenna 604.2 Microstrip patch inset feed dimensional parameter; (a) 3D

view, (b) top view and (c) front view 624.3 The impedance variation with feed position of various notch

widths (g) for g = 0.1 mm, g = 0.2 mm and g = 0.3 mm 644.4 The impedance variation with feed position, for notch width,

g = 0.1 mm and the notch depth (Y�) for Y� = 2.0 mm, Y� =

2.1 mm and Y� = 2.2 mm 64

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4.5 Simulation performance of proposed microstrip antenna insetfeed line; (a) reflection coefficient (S11), (b) radiation patternand (c) gain 66

4.6 Structure of single patch antenna at 28 GHz 674.7 Simulated for proposed single antenna; (a) reflection

coefficient for different thickness of substrate and (b)reflection coefficient and SWR for substrate thickness h =0.508 mm 69

4.8 Simulated for single antenna at 28 GHz; (a) gain (h = 0.254mm), (b) gain (h = 0.508 mm) and (c) radiation pattern 70

4.9 Single patch 28 GHz proximity coupled feed; (a) 3D view and(b) layer view 71

4.10 Fabricated prototype of single patch at 28 GHz proximitycoupled feed; (a) prototype with SMA connector, (b) top layerand (c) bottom layer 72

4.11 Result of single patch antenna at 28 GHz; (a) simulation andmeasurement of reflection coefficient, S11, (b) simulation of3D pattern and (c) simulation of radiation pattern 73

5.1 Antenna arrays configuration 775.2 Antenna arrays’ spacing configurations; (a) equal spaced

array, ESA, (b) unequal spaced array 1, USA 1 and (c)unequal spaced array 2, USA 2 79

5.3 Equal spaced array; (a) simulated design of linear antennaarrays and (b) gain 81

5.4 Sidelobe characteristics at 28 GHz; (a) equally spaced array,ESA, (b) unequally spaced array 1, USA 1 and (c) unequallyspaced array 2, USA 2 82

5.5 E-plane radiation pattern at 28 GHz for ESA, USA 1 and USA2 83

5.6 First sidelobe [dB] vs. array spacing [mm] 845.7 Simulated current density distribution of the proposed array

configurations; (a) equally spaced array, ESA, (b) unequallyspaced array 1, USA 1 and (c) unequally spaced array 2, USA2 85

5.8 ESA antenna configuration with d = ��; (a) fundamentaldesign and (b) proposed design 87

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5.9 H-plane radiation pattern characteristics of f� = 28 GHz, f1= 42 GHz and f2 = 56 GHz; (a) equally spaced array, ESA,(b) unequally spaced array 1, USA 1 and (c) unequally spacedarray 2, USA 2 89

5.10 E-plane (y-z axis) of radiation pattern characteristics for ESA,USA 1 and USA 2 at frequencies of 28, 42 and 56 GHz. 90

5.11 Antenna gain vs. frequency 905.12 Simulated design; (a) ESA without parasitic, (b) ESA with

parasitic Design 1 and (c) ESA with parasitic Design 2 945.13 2D radiation patterns at 56 GHz with and without parasitic

elements; (a) ESA, (b) USA 1 and (c) USA 2 965.14 Gain performance at 56 GHz 975.15 USA 2 sidelobe performance in frequency 42 GHz; (a) 2D

rectangular plot and (b) polar plot 986.1 Patch element configurations; (a) 12 GHz and (b) 18 GHz 1056.2 Simulated and measured of reflection coefficient, S11 for

single feed antenna; (a) 12 GHz and (b) 18 GHz 1066.3 Simulated and measured of x�z radiation patterns (H-plane)

for single patch antenna 1076.4 Wide frequency on the same dimension size of feeding 1086.5 Linear array configurations for equally spaced array (dc = 0.7

��), unequally spaced array 1 (dc = 0.6 ��) and unequallyspaced array 2 (dc = 0.5��); (a) frequency 12 GHz and (b)frequency 18 GHz. 109

6.6 Simulated S11 and gain; (a) frequency 12 GHz and (b)frequency 18 GHz 110

6.7 2D pattern for ESA⇤, USA 1⇤ and USA 2⇤ configurations; (a)12 GHz and (b) 18 GHz 111

6.8 Array element spacing of unequally spaced array 1146.9 Concept of equal feed line design 1156.10 Schematic diagram of power divider design for antenna

arrays’ spacing configuration; (a) equally spaced array⇤

(ESA⇤) with dc = 0.7 ��, (b) unequally spaced array 1⇤ (USA1⇤) with dc = 0.6 �� and (c) unequally spaced array 2⇤ (USA2⇤) with dc = 0.5 �� 117

6.11 Length error (4l) of feeding network 1196.12 Simulation setup for ADS 1206.13 The important junction points of power dividers constituting

the feeding network 120

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6.14 Schematic of T-junction power divider at point #i 1216.15 Schematic of T-junction power divider at point #ii 1216.16 Schematic of T-junction power divider at point #iii 1226.17 Schematic of T-junction power divider at point #iv 1236.18 Schematic of T-junction power divider at point #iv circuit

after using quarterwave transformer (QWT) for Z3(ii) 1236.19 T-junction power divider circuit; (a) T-junction power divider

at point #iv and (b) calculation of quarterwave transformer 1246.20 Fabricated feeding networks; (a) ESA⇤, (b) USA 1⇤, (c)

USA 2⇤and (d) bottom view of ground plane for all feedingnetworks 128

6.21 Simulated reflection coefficient results of feeding network forall array configuration in frequency 12 GHz and 18 GHz 129

6.22 Simulated and measured reflection coefficients of feedingnetworks in frequency 12 GHz and 18 GHz; (a) ESA⇤, (b)USA 1⇤ and (c) USA 2⇤ 130

6.23 Transmission coefficients of 16 ports feeding network forboth frequencies; (a) ESA⇤, (b) USA 1⇤ and (c) USA 2⇤ 132

6.24 Amplitudes and phases of the feeding network at frequency12 and 18 GHz 133

6.25 Configuration of array (ESA⇤) and feeding network; (a)structure of array and feeding network and (b) feeding methodfor array element 135

6.26 Fabricated patch array at frequency 12 GHz; (a) ESA⇤, (b)USA 1⇤ and (c) USA 2⇤ 136

6.27 Fabricated patch array at frequency 18 GHz; (a) ESA⇤, (b)USA 1⇤ and (c) USA 2⇤ 136

6.28 Fabricated antenna at 12 GHz; (a) ESA⇤, (b) USA 1⇤ and (c)USA 2⇤ 137

6.29 Fabricated antenna at 18 GHz; (a) ESA⇤, (b) USA 1⇤ and (c)USA 2⇤ 137

6.30 Reflection coefficient, S11 results for all array configurationsversus frequency; (a) simulation and (b) measurement 138

6.31 Simulated and measured radiation patterns at 12 GHz; (a)ESA⇤ (b) USA 1⇤ and (c) USA 2⇤ 141

6.32 Simulated and measured radiation patterns in 18 GHz; (a)ESA⇤, (b) USA 1⇤ and (c) USA 2⇤ 142

6.33 Simulated 3D pattern for ESA⇤; (a) 12 GHz and (b) 18 GHz 1436.34 Simulated 3D pattern for USA 1⇤; (a) 12 GHz and (b) 18 GHz 143

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6.35 Simulated 3D pattern for USA 2⇤; (a) 12 GHz and (b) 18 GHz 1436.36 Simulated E-plane radiation pattern; (a) 12 GHz and (b) 18

GHz 1446.37 Simulated radiation patterns at 28 GHz for ESA⇤ , USA 1⇤

and USA 2⇤ 1466.38 Transmission coefficients, S21 for ESA⇤, USA 1⇤ and USA

2⇤; (a) 12 GHz and (b) 18 GHz 147B.1 Simulated reflection coefficients results of feeding network

for all array configurations in frequency 12 GHz and 18 GHz 171B.2 Simulated and measured reflection coefficient of feeding

network in frequency 12 GHz and 18 GHz (a) ESA⇤ (b) USA1⇤ (c) USA 2⇤ 172

C.1 GUI of the HFSS 173C.2 Far field radiation setup 174C.3 HFSS results 174D.1 Advance Design System (ADS) 175D.2 (a) Design the circuit (b) Simulation results 176D.3 (a) Convert circuit design to layout (b) Export feeding

network design to HFSS 176

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LIST OF ABBREVIATIONS

ADS - Advanced Design System

ASP - Aperture-stacked patch

AUT - Antenna under test

CDMA - Code Division Multiple Access

CFN - Corporate feed network

DEA - Differential evolution algorithm

DR - Dielectric Resonator

EM - Electromagnetic

ESA - Equally spaced array

5G - Fifth Generation

FBW - Fractional bandwidth

FCC - Federal Communications Commission

FDMA - Frequency Division Multiple Access

FM - Frequency modulation

FR4 - Flame Retardant 4

FSL - Free space loss

GL - Grating lobe

GSM - Global System for Mobile

HFSS - High Frequency Structure Simulator

HPBW - Half power beamwidth

IES - Inter element spacing

LMDS - Local Multipoint Distribution Services

LTCC - Low-temperature co-fired ceramic substrate

ITU - International Telecommunications Union

MIMO - Multiple input multiple output

MPA - Microstrip patch antenna

NLOS - Non-line-of-sight

PCA - Planar connected array

PCB - Printed circuit board

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PPW - Parallel plate waveguide

PTFE - Polytetrafluoroethylene

QWT - Quarterwave transformer

RET - Remote Electrical Tilt

RF - Radio frequency

SLL - Sidelobe level

SMA - Sub Miniature version A

TDMA - Time-Division Multiple Access

UV - Ultraviolet

US - United State

USA 1 - Unequally spaced array 1

USA 2 - Unequally spaced array 2

VNA - Vector network analyser

VSWR - Voltage standing wave ratio

WRC - World Radio Communications

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LIST OF SYMBOL

𝑓𝑜 - Operating frequency

𝑙𝑛 - Line length

H𝑜 - Permittivity of free space

H𝑟 - Relative permittivity / Dielectric constant

H𝑟𝑒𝑓𝑓 - Effective dielectric constant

P𝑜 - Permeability of free space

𝑣𝑜 - Space velocity of light

𝛿 - Tangent loss

𝑁 - A group of element

𝑑𝑛 - Element spacing

𝑤𝑛 - Complex weight

𝑘 - Free space wave number

E𝑛 - Phase excitation

O - Wavelength

R - Distance

h - Substrate thickness

𝑍𝑜 - Impedance of transmission line

𝐿𝑒𝑓𝑓 - Effective length

'L - Extension of length

𝐿𝑚𝑙 - Length of microstripline

𝑊𝑚𝑙 - Width of microstripline

W - Width of radiating patch

𝑆11 - Reflection coefficient

𝑆21 - Transmission coefficient

𝑇𝑥 - Transmit antenna

𝑅𝑥 - Receive antenna

𝐺𝑡 - Transmit gain

𝐺𝑟 - Receive gain

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LIST OF APPENDICES

APPENDIX TITLE PAGE

A List of Publication 169

B Simulation Result 171

C High Frequency Structure Simulator (HFSS) 173

D Advanced Design System (ADS) 175

E Substrate Choices and Datasheets for Rogers RT/Duroid 5880 177

CHAPTER 1

INTRODUCTION

1.1 Research Background

Mobile communications systems have continuously evolved and revolutionizedthe way people communicate, where the systems changed from fixed “point-to-point”to wireless that has more advantages. In future, the fifth generation (5G) mobile systemis the new technology that will drive the future communication evolution, throughincrease data capacity and lower latency [1, 2]. However, the spectrum below 6 GHz iscongested, therefore the spectrum above 6 GHz is being considered for the upcoming5G mobile technology [3].

The Federal Communications Commission (FCC) of the United States (US)allocated the frequency of spectrum bands above 24 GHz as operation band for mobileservices [4], where operation in a new millimeter frequency band is a way to avoid theovercrowded lower frequency spectrum. This frequency band provides large amountof spectrum to exploit large bandwidths in order to achieve very high data ratescommunication systems for high speed and efficient use [5].

In order to support 5G demands, wideband, low cost and low interference basestation antenna design has attracted the attention from both academia and industry.For this next 5G mobile communication system the distance between the antennas isgetting closer due to high frequency use. Thus base station antennas are requestedto achieve multi-band and low sidelobe level (SLL) characteristics [6, 7], where theSLL should be less than -15 dB to reduce the interference from the other signal [8, 9].Multi-band base station antenna design is one of the ways to avoid crowded installationspace due to limitation of the future antenna installation space such as on tower andthe roof of a building [10, 11]. One of the interests of designing the low sidelobe levelover wide frequency use antenna is on microstrip unequally spaced antenna arrays.

2

Unequally spaced antenna arrays is a nonuniform array or unequal spacingbetween adjacent element, which is able to control the radiation pattern [12]. Manytechniques have been proposed which has been presented and reported in [13, 14, 15]where a lot of effort has been done on the unequally spaced antenna arrays. Aconfiguration of unequally spaced arrays provides low sidelobe characteristics overwide frequency use, while this configuration is having uniform excitation coefficient(amplitude and phase) in all array elements.

An excitation coefficients to antenna arrays elements is determined by a feedingnetwork, where the changes of frequency operations will change an electrical lengthof transmission line and resulting a phase shift [16, 17]. Parallel feed has a wellcontrolled aperture distribution compared to series feed, which suffer from inherentphase difference caused by the differences in lengths of feed lines. Thus, the design ofa parallel feed network achieving equal excitation coefficients in the wide frequencyrange is seemed to be the solution [18] .

Besides, it is required to have equal magnitude and phase coefficients withnon-frequency dependence for wide frequency use implementation, which can beobtained by designing feeding network with uniform line length [18]. Thus, unequallyspaced arrays are deemed as the potential candidates due to their ability to achievelow sidelobe levels and suppress grating lobes in wide frequency use operations[12, 13, 14].

Therefore, in this thesis, an unequally spaced microstrip linear antenna arraysusing a wide frequency use of feeding network, that offers a low sidelobe level ispresented. The spectrum above 6 GHz, which is 28 GHz is chosen as the designatedfrequency band due to the availability of the band for mobile services [3, 4] andalso led to the increasing of bandwidth. However, the frequency will be scaleddown to 12 and 18 GHz for realization purpose, due to the limitation of radiationpattern measurement in higher frequency. Commercial electromagnetic simulatorswere employed in designing the feeding network and antenna array. For the feedingnetwork design, the Advanced Design System (ADS) is more easier to be employed.Then, the design of antenna arrays and their analysis performance are implemented inHigh Frequency Simulation Simulator (HFSS). Lastly, the feeding network from ADSwill be exported to HFSS simulator and the analysis of performances are performed inHFSS.

3

1.2 Problem Statement

Practically, the higher frequency use caused a shorter wavelength, which leadsthe future base station antennas to have closer distance to each other comparedto the present mobile base station antenna. Consequently, it will increase theinterference with another communication. Thus, in order to counter the effects ofattenuations, which reduce the strength of the signal, the mobile system shall deployantenna with higher gain [19, 20, 21]. The higher gain antenna has a signal that isconfined to a narrow solid angle [7] that can reduce the interference with anothercommunication system. However, Andrews in [22] stated that high gain in thenarrow beam communication is new to cellular communications. In addition, networkmodeling, analysis, design and optimization for preliminary status and spectrum 5Gstandardization are other challenges [22].

Besides, low sidelobes are required in order to reduce interference with anotherfrequency reuse cell [23, 24, 25, 26], where interference can be reduced by thereduction of the unwanted upper sidelobes, that is directed towards the neighboringcells. Due to that, near-in sidelobe reduction methods by designing unequal arrayspacing have been proposed in [27, 28, 29, 30, 31]. However, these previous workswere mainly focused on theoretical and numerical. Functional antenna configurationsand the acquired antenna characteristics should be clarified in the applications ofunequal spaced array for the upcoming 5G mobile. In addition, there has been noinvestigation made for the sidelobe level performance over wide frequency use.

For base station antenna, low sidelobe characteristics are achieved by givingadequate excitation coefficients (amplitude and phase) to array elements [32]. In thiscase, the feeding network is composed of many power dividers and feeder lines thathave different values. Here, phase values, which is determined by feeder line lengthshave severe frequency dependence. Therefore, the present base station antennas aredifficult for wide frequency use. In order to achieve an unequally spaced array over awide frequency use, a suitable feeding network circuit must be developed. Generally,excitation coefficients for unequally spaced array elements are uniform. Hence, T-junctions were proposed to be constructed with equal power division and phase.Therefore, the main design was subjected to the feeding network that must have equalline lengths from input point to the array elements which is placed in unequal spacing.The equality of line lengths ensures wide frequency use operation [13]. In addition, infuture 5G mobile system, wide frequency or multi-band antenna is requested [33] toprovide multifunctional operations for mobile communication [34].

4

Therefore, by considering these problems in designing the upcoming 5G basestation antenna, the unequally spaced linear antenna arrays will be proposed in order toachieve low sidelobe over wide frequency use operation, which is one of the featuresfor future 5G mobile base station antenna. Through this research, three configurationsof microstrip linear antenna arrays with the same array aperture have been proposedthat consist of equally spaced array (ESA), unequally spaced array 1 (USA 1) andunequally spaced array 2 (USA 2). The respective center spacing between element ofUSA 1 and USA 2 are 0.6�� and 0.5��, while the array apertures are similar for bothdesigns. The ESA design is chosen as a benchmark of the array, where the results ofUSA 1 and USA 2 are compared to ESA.

1.3 Objectives of the Research

The objectives of this research are stated as follows:

i. To design a microstrip single patch antenna and unequally spaced microstriplinear antenna arrays for base station with high gain and low sidelobe level in5G frequency band.

ii. To design the feeding network and integrate it with unequally spaced microstriplinear antenna arrays for high gain and low sidelobe level.

iii. To design the unequally microstrip linear antenna arrays at frequency 12 GHzand 18 GHz for realization due to the limitation of radiation pattern measurementin higher frequency in order to achieve high gain and low sidelobe over widefrequency use.

1.4 Scope of the Research

This research focuses on the design of an unequally spaced linear antennaarrays in the 5G frequency band. In order to achieve the research objectives, there areseveral steps to be completed. The designs consist of microstrip single patch antenna,microstrip antenna arrays, feeding network design and the combination of feedingnetwork and microstrip antenna arrays. Firstly the microstrip single patch antennaat 28 GHz with the various types of feeding is designed, simulated and optimized. Thecharacteristics of a feeding technique is studied based on antenna gain, return loss,

5

bandwidth and radiation pattern, and the suitable feeding technique is selected for theuse in the design of antenna arrays. Next, the characteristics and performance of theantenna arrays are investigated at frequency 28, 42 and 56 GHz.

In order to observe the microstrip linear antenna array’s sidelobe level,measurement of radiation pattern was taken in an anechoic chamber. In this work, thecharacteristics of the antenna such as gain, impedance bandwidth, radiation pattern,reflection coefficient, amplitude and phase differences between output ports areconsidered and discussed based on the requirements of 5G mobile system applications.However, due to limitation of measurement in higher frequency, realization are doneat 12 and 18 GHz. The designs are referred to the parameters and specifications listedin Table 3.1 in Chapter 3.

The simulation and optimization process of antenna design is performed usingHigh Frequency Structure Simulator (HFSS). While the feeding network is easier tobe designed by using Advanced Design System (ADS) due to the requirement of manyoutput ports. The RT/Duroid 5880 substrate (relative permittivity, "r = 2.2 and tangentloss, tan� = 0.0009) is chosen due to its low loss with thickness of 0.508 mm. Thefabrication and measurement are performed to ensure that comparable performancesbetween simulated and measured results. The measurement of S-parameter is carriedout by using a vector network analyzer (VNA) and radiation pattern measurement isperformed in an anechoic chamber.

1.5 Contributions of the Research

Two major contributions are presented in this research, which are as follows:

i. The designs of microstrip linear unequally spaced arrays over wide frequencyuse and its investigation on the effect of element spacing to the performance ofradiation pattern. The unequally spaced antenna arrays are designed at 28, 42and 56 GHz, which results in approximately 3 dB sidelobe reduction comparedto equally spaced arrays. Then, realization at 12 and 18 GHz, which resultsan approximately 6 dB sidelobe reduction compared to equally spaced antennaarray. All feeding ports having uniform amplitude and phase, then contribute tolow sidelobe level over wide frequency use.

6

ii. The design of equal power division and non-frequency dependent of feedingnetwork with uniform excitation coefficient for each output port. In previousworks [13, 14, 27, 28, 29, 30, 35] were mainly theoretical and numerical in naturewhereas practical examples are much needed in the applications of unequallyspaced antenna array for the upcoming fifth generation (5G). Therefore in thisresearch, the 16 parallel output ports for three configurations of arrays, whichare the equally spaced array⇤ (ESA⇤), unequally spaced array 1⇤(USA 1⇤) andunequally spaced array 2⇤ (USA 2⇤) are evaluated from the perspective of equalamplitude and phase which notation ⇤ indicates frequency 12 GHz that chosen isas reference. The large arrays are designed to have 16 elements due to the betteragreement between spaced tapered array (USA 1⇤ and USA 2⇤) and referencepatterns (ESA⇤) when optimum number of elements are used. The parallelfeeding network achieving equal excitation coefficients in the wide frequencyuse has been designed, which is suitable for this antenna arrays’ configuration.Thus, the wide frequency use antenna is achieved by employing this feedingnetwork, which results in constant sidelobe reduction even though the frequencyis changed. The sidelobe level is reduced from -13 dB, -16 dB and -19 dB forthe respective antenna array of equally spaced array ⇤ (ESA⇤ ), unequally spacedarray 2⇤ (USA 2⇤) and unequally spaced array 1⇤ (USA 1⇤).

1.6 Thesis Outline

This section discusses the thesis outline, which are organized into sevenchapters. In Chapter 1, the overview of the whole project is discussed, which includesthe research background, problem statement, objectives of the research, scope of theresearch, contributions to the research, and thesis outline.

While, Chapter 2 focuses on the literature reviews, which started from anoverview of a 5G mobile communication system, followed by mobile base station,microstrip patch antenna (MPA) and linear arrays. Then, the previous related worksare reviewed, which mainly focus on the design techniques, and characteristics of theunequally linear antenna arrays and feeding network design. Chapter 3 discussesthe methodology of this research. This chapter presents the research work flowsof the whole research. Also, the process of overall antenna design and feedingnetwork design are shown by the flow chart. The design parameters and specifications,optimization and simulation and measurement process, are introduced.

155

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