Investigation of Wide Band mm-Wave Radome1092557/FULLTEXT01.pdf · Investigation of Wide Band mm-Wave Radome ... from76-77GHz ... The radome cover of a radar sensor is the backbone

IN DEGREE PROJECT ELECTRICAL ENGINEERING,SECOND CYCLE, 30 CREDITS

, STOCKHOLM SWEDEN 2017

Investigation of Wide Band mm-Wave Radome

SHREYASI BHOWMIK

KTH ROYAL INSTITUTE OF TECHNOLOGYSCHOOL OF ELECTRICAL ENGINEERING

Investigation of Wide band mm-Wave RadomeTechnology

SHREYASI BHOWMIK

Master’s Thesis at EESupervisor: Mahsa E.Hamklar

Examiner: Oscar Q.Teruel

TRITA-EE 2017:008

Abstract“Radome” is a necessary shield for any type of antenna. Itsshape, thickness, mechanical and electrical properties de-fine its application. This thesis mainly revolves around thetechniques, used for radome analysis in W-band with thehelp of electromagnetic solvers of High Frequency Struc-ture Simulator (HFSS) and CST Microwave Studio. Mea-surement of permittivity of many materials is not made atsuch high frequencies and hence one is not sure about itsmaterial characteristics. This thesis includes a survey onmaterials which are applicable in W-band regime and givea sturdy performance with single type of material or com-monly known as ‘Style A’ radomes. The thesis further de-scribes, the shape and thickness modification of the radome.It contemplates on the the choice of solver involved and thedifferent advantages and disadvantages of the electromag-netic solvers with respect to this particular case. Severaltrials have been done on each of these platforms and onlythe relevant results have been presented. The antenna stud-ied is a “Travelling Wave Microstrip Patch Antenna” whichis subjected to different sizes of ground plane in order toobtain the realistic environment for it to be simulated in.It was observed that as the thickness of radome increased,the gain through the radome enclosed antennas decreasedbut the side lobe level performances improved. The simu-lations done in HFSS have a closer resemblance to theoryof radome performance.

From the perspective of measurement, the return lossand insertion loss has been recorded for the radome in theW-band. A radome piece having different thicknesses hasalso been measured using the Rhode & Shwarz VNA. Asa step further, to study the radome pattern characteristic,Fabry perot periodic resonating dipoles are simulated ona radome of desired thickness. The height of the radomefrom the ground plane is optimized in order to get a 3-dB gain at the region of interest. After implementationit was observed that, around 76.5 GHz the setup which isdescribed later on acquired a gain of 20.33 dB. Althoughit is a high gain and low profile solution, it comes witha cost of narrow beam-width. Thus one has to optimizethe number of periodic elements along with phase taperingin order to achieve the desirable beam characteristics. Inshort, the thesis outlines the theory and thinking behinddesigning a radome, its effect on the antenna and how onecan resolve the issues.

Referat

“Radom” är ett nödvändigt sköld för varje typ av an-tenn. Dess form, tjocklek, mekaniska och elektriska egen-skaper definiera dess tillämpning. Denna avhandling beskri-ver främst kring de tekniker som används för radom ana-lys i W-band med hjälp av elektromagnetiska simulering iHFSS och CST Microwave Studio. Mätningar av dielekt-ricitetskonstant av många material görs inte vid så högafrekvenser och därmed en inte säker på dess materialegen-skaper. Denna avhandling ingår i en undersökning om ma-terial som är tillämpliga i W-band regim och som ger enrobust prestanda med en enda typ av material eller all-mänt känd som Style Aradomer. Avhandlingen beskrivervidare, formen och tjockleken modifiering av radomen. Denöverväger om valet av solver inblandade och de olika för-delar och nackdelar med de elektromagnetiska lösare medavseende på det aktuella fallet. Flera försök har gjorts påvar och en av dessa plattformar och endast relevanta resul-tat har presenterats. Antennen studeras är en “TravellingWave Micro Patch Antenna” när simuleras med ett jord-plan storlek 18.6λ X 25λ. Denna antenn har utsatts för oli-ka storlekar av jordplanet i syfte att erhålla den realistiskmiljö som den skall simuleras i. Det observerades att närtjockleken av material ökade, förstärkningen genom rado-men slutna antennerna minskade men nivå föreställningarSido lob förbättras. Dessutom är en bättre matchning erhål-lits med tjockare radomer. Simuleringarna görs i HFSS haren närmare likhet med teoretiska uttalanden av radomenprestanda. Ur mätningen, har avkastningen förlust och in-länkningsdämpningen registrerats för radomen i W-bandet.Radomen har olika tjocklekar har mätts med hjälp av Rho-de & Shwarz VNA. Som ett ytterligare steg för att förbättrai radomen mönster egenskap, Perot Fabry periodiska reso-nans dipoler simuleras på en radom med önskad tjocklek.Höjden på radomen från jordplanet är optimerad för att fåen 3dB förstärkning vid området av intresse. Efter genom-förandet konstaterades att omkring 76,5 vid installationenGHz vid som beskrivs senare förvärvat en vinst på 20,33dB. Även om det är en hög förstärkning lösning och lågprofil lösning, kommer det med en kostnad av smal smala-re bredd av strålning. Sålunda behövs en optimering av detantalet periodiska element tillsammans med fas avsmalnan-de i syfte att uppnå de önskvärda balkegenskaper. I korthetbeskriver avhandlingen teori och tänkande bakom utformaen radom, dess effekt på antennen och hur man kan lösaproblemen för att få en mer realistisk lösning.

Preface

This report is the final thesis in the master study in Investigation of mm-waveRadomes at Acreo Swedish ICT in association with the department of Electro-physics, Faculty of Electromagnetic Engineering, at the KTH-Royal Institute ofTechnology, Sweden.

The report has been written in the period February 2016 - October 2016. Thework has combined doing robust simulation in commercially available electromag-netic solvers, mechanical and electrical design as well as laboratory measurements.The nature of the topic, has been very beneficial for me as it helped me gain knowl-edge in one of the most challenging field in the automotive industry with regard toantennas. It has given me a deep insight on the electromagnetic solvers and theirfunctioning, the intuition of understanding the electromagnetic theory involved be-hind the design of radomes, the hands on experience of doing lab measurements.

I would like to thank my examiner Oscar Q. Terual and supervisor Mahsa E.Hamlkar at the KTH Royal Institute of Technology for their constant guidanceand advice which helped me achieve the goals of the thesis. I would also like toextend my heart felt gratitude to Michael Salter, Duncan Platt and Lars Petterssonat Acreo Swedish ICT for providing me with this opportunity and helping me outthroughout the thesis, to overcome the hurdles. I would also like to extend mygratitude to Markus Laudien of Ansys who was of immense help and guidanceduring the thesis. Last but not the least I would like to thank my family for theirfaith in me. I would like to thank my classmates Amir Torki and Veit Langrock fortheir constant encouragement throughout Masters. I would like to thank my friendsSoumik Dagupta and Deepa Krishnamurthy for their constant feedback during thedraft of the thesis.

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Contents

Contents v

List of Figures vii

List of Tables ix

1 Introduction 11.1 Overview of Radomes . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Antennas employed with Radomes for Automotive Radars . . . . . . 21.3 Problem Description . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Fundamentals of Radome 52.1 Radome Characteristics and Types . . . . . . . . . . . . . . . . . . . 52.2 Radome Interaction with the Antenna . . . . . . . . . . . . . . . . . 72.3 Basic Radome Requirements . . . . . . . . . . . . . . . . . . . . . . . 82.4 Advances in Radome Material . . . . . . . . . . . . . . . . . . . . . . 92.5 Basic Radome Calculations . . . . . . . . . . . . . . . . . . . . . . . 10

3 Implementation of the Radome and Travelling Wave Antenna(TWA) 133.1 Characteristics of TWA . . . . . . . . . . . . . . . . . . . . . . . . . 133.2 Radome structure and modification . . . . . . . . . . . . . . . . . . . 143.3 Size of Ground plane and its impact . . . . . . . . . . . . . . . . . . 153.4 Simulations performed in HFSS and CST Microwave Studio environ-

ment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.4.1 Choice of Solver . . . . . . . . . . . . . . . . . . . . . . . . . 203.4.2 Implementation of the set up and simulation in CST and HFSS 23

4 Measurement Techniques 304.1 Measuring Insertion Loss (IL) and Return loss (RL) of Material 1 . . 30

5 Implementation of Fabry-Perot Resonator 345.1 Fabry Perot Resonators . . . . . . . . . . . . . . . . . . . . . . . . . 345.2 Optimizing the Fabry Perot Resonator . . . . . . . . . . . . . . . . . 36

v

6 Future Work 41

Bibliography 42

List of Figures

1.1 Evolution of radar technology over time has reached the miniaturiza-tion of 79 GHz using 28nm silicon CMOS technology. Imec is now alsoworking on 140 GHz radar chips. (Source: imec) . . . . . . . . . . . . . 3

2.1 Solid wall . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.2 (a) Low dielectric constant with honey comb core (b) Low dielectric

constant with foam core . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 (a) Double sandwich (b) More than five layers . . . . . . . . . . . . . . . 72.4 High dielectric constant core . . . . . . . . . . . . . . . . . . . . . . . . 72.5 Reflection-transmission through a dielectric slab . . . . . . . . . . . . . 102.6 Reflection through half wave radome . . . . . . . . . . . . . . . . . . . . 11

3.1 Travelling wave series fed microstrip patch array antenna operating at76.5 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.2 Modified radome shape . . . . . . . . . . . . . . . . . . . . . . . . . . . 153.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

(a) The Return Loss (S11) of the TWA patch array antenna for allthe three cases in CST Microwave Studio . . . . . . . . . . . . 16

3.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17(b) The Return Loss (S11) of the TWA patch array antenna for all

the three cases in HFSS . . . . . . . . . . . . . . . . . . . . . . 173.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

(a) E-plane pattern of the TWA Patch array in all the three casessimulated in CST Microwave Studio . . . . . . . . . . . . . . . 17

3.4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18(b) H-plane pattern of the TWA Patch array in all the three cases

simulated in CST Microwave Studio . . . . . . . . . . . . . . . 183.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

(a) E-plane pattern of the TWA Patch array in all the three casessimulated in HFSS . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19(b) H-plane pattern of the TWA Patch array in all the three cases

simulated in HFSS . . . . . . . . . . . . . . . . . . . . . . . . . 19

vii

3.6 The TWA patch array antenna enclosed in the radome . . . . . . . . . . 203.7 The comparison of various E-M solvers . . . . . . . . . . . . . . . . . . . 213.8 Lit and Shadowed regions for a PO assigned dielectric region . . . . . . 223.9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

(a) E-plane radiation pattern of Material 1 at 76.5 GHz of thickness1.18 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

(b) H-plane radiation pattern of Material 1 at 76.5 GHz of thickness1.18 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26(a) E-plane radiation pattern of Dyneema at 76.5 GHz of thickness

1.3 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26(b) H-plane radiation pattern of Dyneema at 76.5 GHz of thickness

1.3 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.11 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

(a) E-plane radiation pattern of Rexolite at 76.5 GHz of thickness1.23 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

(b) H-plane radiation pattern of Rexolite at 76.5 GHz of thickness1.23 mm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.12 Return loss of Material 1 radome enclosed antenna compared to Antennawith large finite GND plane . . . . . . . . . . . . . . . . . . . . . . . . . 28

4.1 The vertically aligned Material 1 Radome between the W-band horns . 304.2 Return loss and insertion loss when there is no radome . . . . . . . . . . 314.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

(a) The insertion loss through the radome of Material 1 in horizon-tal and vertical alignments at 0 degree. . . . . . . . . . . . . . 32

(b) The return loss through the radome of Material 1 in horizontaland vertical alignments at 0 degree. . . . . . . . . . . . . . . . 32

4.4 The return loss and insertion loss recorded through pieces of 6.27mm ,4.72mm and 2.49mm of Material 1 . . . . . . . . . . . . . . . . . . . . . 33

5.1 Arrangement of the simulation set up of the PRS . . . . . . . . . . . . . 365.2 Cell design of Fabry Perot resonator . . . . . . . . . . . . . . . . . . . . 375.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

(a) Reflection and transmission band of the unit cell . . . . . . . . 375.3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

(b) Reflection and Transmission phase coefficient of the unit cell atnormal incidence . . . . . . . . . . . . . . . . . . . . . . . . . . 38

5.4 Implementation of the Fabry Perot resonator using the dipole elementsfrom 76-77 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

5.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39(a) Gain realized in the E-plane with the dipole elements . . . . . 39

5.5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40(b) Gain realized in the H-plane with the dipole elements . . . . . 40

List of Tables

2.1 Some common radome materials at 10 GHz . . . . . . . . . . . . . . . . 10

3.1 Parameters of the TWA antenna. . . . . . . . . . . . . . . . . . . . . . 143.2 summarizes the performance of all the three cases implemented in both

the solvers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.3 The list of materials of the radome and their dielectric properties . . . . 243.4 Comparison of the solved radome materials between CST and HFSS at

76.5 GHz . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

5.1 Optimization parameter for Fabry Perot Unit Cell . . . . . . . . . . . . 36

ix

Chapter 1

Introduction

1.1 Overview of Radomes“Radome” is a portmanteau of the words, “Radar and Dome”. It functions as astructural and weatherproof enclosure to the antenna in such a manner that it hasleast interference with the transmission from it. Their history and developmentcomes a long way as they have withstood the test of blowing winds and thermalinstabilities . In order to balance aerodynamic forces as well have good thermalproperties throughout, the shape and material of the radomes have been a topic ofinterest since ages.It is interestingly a component which both the RF engineer and a Mechanical engi-neer can describe but each would have their own jargon. In an industrially viableproduct it is imperative to satisfy the quench of both the engineers and thus, thisis a pure optimization problem. Thus depending upon the type of application, theradomes have either a nose cone type shape, flat slab with rounded edge struc-tures or spherical structures. In this manner a simple plastic cover which is just aprotective cover for antennas against environmental damage became a RF simula-tion problem. Moreover, just designing the shape and choosing the material doesnot help to solve the problem efficiently, but also one has to take care about thethickness of the slab, the dielectric properties as well as the distance of it from theradiating structure. These parameters are explained further in the chapters below.Many applications such as base station antennas , maritime satellites, aircraft, mis-sile guiding systems as well as radars employ radomes [14]. This thesis, deals withradomes in radar applications and emphasizes on the point that when employing itwith a radar, how the radome should not change the antenna radiation character-istics and maintain a similar gain characteristics. It is intuitive that enclosing theradome on the antenna, will incur losses and that there will be a degradation inthe performance of the antenna. But, the challenge lies in the fact of compensatingthese losses or atleast getting the antenna upto the desired performance level. Forthis purpose, simulating the antenna along with the radome becomes very crucial.In today’s times, we have very powerful simulators, [1, 2] ,which can handle elec-

1

CHAPTER 1. INTRODUCTION

trically large structures and go upto very high frequencies. One can model themapproximately and try to achieve as much as a realistic environment as possible.With these commercially available electromagnetic simulators it is possible to varyeach parameter of interest and study its influence on the antenna. These parametersand formulations will also be explained later on in the proceeding chapters.

1.2 Antennas employed with Radomes for AutomotiveRadars

The automotive industry is witnessing a paradigm shift in its radar frequency rangeand is slowly moving towards frequencies of 76-81GHz [13]. Popularly termed as themm-wave radars, they now employ sensors and other devices, which are operatingat these frequencies. Thus, the radome market from its conventional choice ofmaterial is also forced to progress parallelly. The fundamental reason behind thisfrequency shift is that to achieve a wider bandwidth, which will permit a highresolution of multiple objects when the radar is scanning. An indispensable partof the radar system is the antenna which has evolved with time. For the abovementioned frequency range, planar array antennas are a popular choice due to theirlow profile, low cost as well as adaptability to integrate in existing systems. Sincethe frequency range gets higher, the devices also shrink causing a space constraintin the mounting region. Thus for obtaining the effective antenna aperture efficiency,one has to carefully levy the choice. The popular argument can also be to place alens antenna, but the difficultly to construct in the given area and the cost makes ita less competitive choice to planar array antennas for automotive radars inspite oftheir incurring high losses. Moreover, the element widths can be tapered to achievethe required performance in Side Lobe Levels. From an industrial perspective, theycan be reliably reproduced with existing standard common circuit board processes.Along with the Rx-Tx antennas, many other devices are also integrated on thesame circuit board, which is also enclosed by the radome. Thus, the challenge inthis thesis is to solve an electrically large W-band radome-antenna interaction.

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Figure 1.1: Evolution of radar technology over time has reached the miniaturizationof 79 GHz using 28nm silicon CMOS technology. Imec is now also working on 140GHz radar chips. (Source: imec)

1.3 Problem DescriptionAs mentioned previously, there is a unanimous momentum towards a worldwide-harmonized frequency allocation for automotive radars from 76GHz-81 GHz band-width. Multiple objects cannot be identified if they appear in the same congestedfrequency range, as they will have low resolution. With the allocation of new fre-quency bandwidth of around 5 GHz, it will be possible to distinguish each object.Also the size of devices corresponding to these wavelengths have miniaturized dras-tically. In other words, the size of high frequency circuits and radar componentshave reduced as the frequency has escalated. ‘Radomes’ are electromagnetic win-dows which covers the antenna and yet makes it transparent to the environment.They are much-needed electromagnetic as well as mechanical device, which providethe necessary support, robustness, weather resistance as well as good radiation per-formance for the antenna. Under all operational circumstances, the radome shouldnot influence or disturb the performance of the antenna and hence is a very cru-cial component from design perspective. For analyzing the reflection, transmissionand losses through the radome, the dielectric constant, loss tangent as well as thematerial thickness are significant parameters in the regime of operation. For anaccurate and reliable idea about the analysis of the radome the following tasks wereinvestigated further:

1. Performing a literature survey of the materials available -Since the antenna operates in the frequencies from 73 GHz to 81 GHz, thematerials to be used for the radome need to be measured in this frequencyrange and a study needs to be performed about how sensitive they are. Its

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mechanical properties too need to be investigated, as it has to withstandextreme temperatures.

2. Analyzing the given radome structure in HFSS simulation environment andstudying its shape and material properties coupled with the travelling wavepatch antenna array -This case study, which is to be elucidated further has taken into account anarray of travelling wave patch antenna array which has a resonance around76.5 GHz within an enclosed radome. The reflection coefficient, the radiationpattern that is the E and H-plane patterns has to be analyzed with optimumthickness, optimum shape as well as correct permittivity. For these types ofsimulations, HFSS internally has many solvers. One has to judge which solverwill take into account all the problems and solve each component involved ina realistic manner.

3. Comparing the same design and recreating the environment in CST for arriv-ing at the same results -This particular step always adds value from an academic perspective since,different solvers employ different algorithms and to compare their solutions isalways a way to identify errors or get an assurance that the problem has beenpicturized and modelled in a correct manner.

4. Measuring the Radome performance -This step was performed to check the insertion loss, reflection coefficient andto get an insight of the permittivity of the material at the frequency of interest.

5. Overview of the concept of Fabry Perot Resonators -Based upon the conclusion of the results obtained, a Fabry Perot model hasbeen suggested in order to verify that a 3-dB gain as per theory is obtainedin E and H-plane pattern of the radome for future models.

The Thesis is organized as follows:

• Chapter 2 is dedicated to Literature Review of radomes, the errors they in-troduce in the antenna and their important parameters to be taken into con-sideration. It also explains, basic radome calculations.

• Chapter 3, outlines the basic theory and concept visualization of single layerradome.It also comprises of the comparison of results of HFSS versus CSTMicrowave Studio.

• Chapter 4 consists of the Measurement Set up and its results about the per-formance of the radome.

• Chapter 5 introduces the concept of Fabry Perot resonators and their appli-cation at such high frequencies.

4

Chapter 2

Fundamentals of Radome

2.1 Radome Characteristics and TypesThe radome cover of a radar sensor is the backbone of the design as it can haveinfluence on sensitivity, radiation pattern and immunity to vibrations. The motiveof an efficient radome design is to reduce reflections at the surface of the cover andtransmit waves “losslessly” [10]. Thus, the cover material can be thought of as a lenswhich either focuses or disperses waves uniformly. For this purpose a constant anduniform thickness of the material is required and it must also have a good surfacesmoothness. The radome surface should preferably be covered with a hydrophobicsubstance like a variant of silicon or Teflon.

It was from the time of World War II, 1940’s that radome become such a boilingtopic in the microwave community. Materials such as plywood, polyseterene fiber,plexi glass etc. were employed and it became the then state of the art. Signifi-cant contributions were made during that period by Cady [4] [9] who described theelectrical design of normal and streamlined radomes and their installation, togetherwith the theory of reflection and transmission of electromagnetic waves throughdielectric materials. Hansen, [9] focused on large ground radomes, their environ-mental, structural and design problems, as well as on methods of construction,including metal spaced frame radomes covered with thin dielectric materials, etc.Nowadays Frequency Selective Surfaces, Transformation optics, Rasorbers, etc areused to improve the radome performance.

As antennas, climbed up in frequencies, the radome materials also changed andso did their construction, installation, tooling and transportation. Classically theradome constructions can be classified as follows:

a. Air pressure supported dielectric structure

b. Self-supporting dielectric structure

c. Widely spaced metallic or non-metallic frames supporting dielectric windows

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CHAPTER 2. FUNDAMENTALS OF RADOME

d. Metal surfaces covered with dielectric covered slotted or other shaped win-dows.

One can also distinguish them based on their dielectric wall construction: [14]

a. Style A: Radomes are half wave wall solid (monolithic) radomes.

b. Style B: Radomes are thin-wall monolithic structures with a wall thicknessequal to or less than 0.1λ wavelengths at the highest operating frequency.

Figure 2.1: Solid wall

c. Style C: Radomes are also called A-sandwich multilayer walls. These consistof three layers: two high-density skins and a low-density core material. Thedielectric constant of the skins is greater than the dielectric constant of thecore material.

Figure 2.2: (a) Low dielectric constant with honey comb core (b) Low dielectricconstant with foam core

d. Style D: Radomes (C-Sandwich) are multilayer walled structures having fiveor more dielectric layers. These have an odd number of high-density skinlayers and an even number of low-density core layers. The dielectric constantsof the skins are greater than the dielectric constants of the core materials.As the number of layers is increased, the broadband frequency performance isimproved.

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Figure 2.3: (a) Double sandwich (b) More than five layers

e. Style E: Radomes are defined here in as all possible radome wall constructionnot fitting into styles A through D. These include the B-sandwich and others.The B-sandwich is similar to the A-sandwich except it consists of two low-density skins and a high-density core material. The dielectric constant of theskins is less than the dielectric constant of the core material.

Figure 2.4: High dielectric constant core

2.2 Radome Interaction with the AntennaThe antennas employed in the radar application have very strict requirements andhence it needs to be thoroughly checked as the radome can alter their performances[14] [9] in the following ways:

a. Transmission loss: This results from the reflection against the radome walls,reflection and diffraction around the corners and edges as well as polarizationshift. The remainder of the loss occurs from dissipation within the dielectriclayers. The material loss tangent or tanδ is a measure of these dissipativelosses.

b. Pattern and polarization distortions: This results in the change of mainbeam direction and shape, increase in side lobe level and cross polarizationcomponents also increases.

c. Boresight Error (BSE): BSE is a bending of the angle of arrival of a receivedsignal relative to its actual angle of arrival or line of sight. This phenomenonoccurs due to distortions of the electromagnetic wave-front as it propagatesthrough a dielectric radome wall. BSE depends on frequency, polarization,and antenna orientation. In production, the radome manufacturing toleranceaffects the BSES characteristics of the radome.

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d. Registration error: Registration error is the difference in uplink frequencyand downlink frequency BSE in the case of radome-enclosed SATCOM anten-nas. It is important that this parameter be sufficiently small to assure thatboth the transmit beam and receive beam peaks are coaligned on the samesatellite.

e. Antenna Voltage Standing Wave Ratio (VSWR): The antenna VSWRcan greatly increase from RF power reflected from the inner radome wallsurface.

Inspite of these disadvantages, on the other hand one gets a large number ofoperational, structural as well as economic advantages when the radome is mounted.To mention a few, the antenna now is protected from the harsh environment anddoes not need any special maintenance. Although the radome, will wither with time.The over-all system is much more accurate and reliable as it is not susceptible toany changes.

2.3 Basic Radome RequirementsRadomes have to satisfy aerodynamic, structural and electromagnetic requirements,as well as environmental and installation problems, such as occur when they areinstalled on ship, submarine, ground, aircraft, missile, vehicle etc. The electro-magnetic requirements could call for certain limits to transmission loss, aberration,polarization, pattern distortion, reflection levels etc., while the structural designmust satisfy the shape, size and load requirements under all conditions with anadequate safety factor.

Environmental requirements [14]Typical environmental conditions that may be called for are:

a. Temperature: Ground radomes −56°C to +80°C.

b. Relative humidity: 100%

c. Pressure: Ground radomes may have to stand pressures resulting from windspeeds of up to 240km/h.

d. Loads: Ice, hail and snow loads have to be allowed for, together with possibleimpact loads from birds, stones etc.

e. Rain: On ground radomes, rain adhesion should be minimized.

f. Vibration, shock and acceleration: Ground radomes should withstand seismicloads, e.g. up to 1g, depending upon site.

g. Fire, lightning and radiation hazards: Materials should be fire retardant, andthe radome should withstand salt, dust, insects, fungi, fuels, etc.

8


In conclusion, this thesis focuses primarily on mechanical parameters like strength,hardness, material density, water absorption, rain erosion and extreme climate tem-perature withstanding capacity, etc.

2.4 Advances in Radome MaterialThe key to obtain an optimized radome performance is evaluating the electricalproperties of the material at possible wavelengths. The influential parameters arethe material dielectric constant and the dissipation loss factor at the frequency ofinterest. [4]

The radome wall construction are typically of either the monolithic type orthe sandwich design type. Since the focus in this thesis is mostly on monolithicradomes, a detailed outline and some commonly used materials shall be presented.The monolithic radomes often incorporate fiber reinforcement in order to enhancethe mechanical properties. Thus the relative dielectric constant that we obtain fromthe mixture [4] is as follows:

εm = VR log εR + VF log εFVR + VF

(2.1)

Where,

εm = relative dielectric constant of mixtureεR = relative dielectric constant of the resinVr = Volume of dielectric resinVf = Volume of reinforcement fibers(2.1) Assumes that the mixture is uniform and isotropic

Table 2.1 lists down few materials which are commonly used in the X band.But as we are dealing in the mm-wave band, monolithic radomes are manufacturedwithout reinforcement. These are mostly termed as the organic materials. There isalso another type of material called the inorganic or ceramic material. The advan-tage that inorganic materials have is that they can last at very high temperaturessuch as 500C. These materials are suitable for high velocity applications such asmissile systems. To name a few, aluminium oxide, Rayceram, Pyroceram, etc.

After 1999, when Rodger M. Wasler formally defined metamaterials, there wasan advancement to study antenna radome metamaterials. Research was done usingmultilayered structures composed of two materials which together had a refractiveindex as close to air or below (mention reference number Liu et al). They can beused to compensate BSE, improve gain, directivity, etc. but yes they come at theprice of a narrower beam-width.

Another type of inclusions on the radome walls are commonly known as Fre-quency Selective Surfaces (FSS) [18]. This layer can be arranged inside, outside

9


or even embedded within the radome wall. They are usually a periodic array ofetched antenna elements that can be used to either reflect or transmit electromag-netic waves with a frequency discretion. A FSS can take many forms and strips,slots, square loops, omega units and Jerusalem crosses have all been used in arrayconfiguration on the radome surfaces.

Material Permittivity (ε) Loss tangent (δ)Polycarbonate 2.9 0.005

E-glass 6.06 0.004Epoxy 4.4 0.016Nylon 3.03 - 3.21 0.014 - 0.02Teflon 2.1 0.0003

Polystyrene 2.55 0.0004

Table 2.1: Some common radome materials at 10 GHz

2.5 Basic Radome CalculationsUnder, ideal condition a radome should be invisible to the electromagnetic waves,and that can be achieved by matching the width of the material slab. Materialswith low dielectric constants and tanδ losses is preferred as it will lead to lessreflections at the boundary. However, one cannot ignore mechanical, economic andenvironmental factors while ultimately making the choice.

Since, monolithic radomes are studied in detail in this thesis, the radome calcu-lations [10, 11] show cased will be pertaining to monolithic dielectrics. A dielectricis characterized by its characteristic impedance given by:

ZD = 377Ω√ε

(2.2)

Where εr is relative pemittivity of the material.

In other words, when the incident wave strikes on the surface of the dielectric apart of the wave is transmitted and a part of it is reflected back (as shown in Figure2.5).

Figure 2.5: Reflection-transmission through a dielectric slab

10


Figure 2.6: Reflection through half wave radome

A radome that is electrically thin (less than 0.1λ) will generally deliver good RFperformance. This is because signal reflections at the freespace-dielectric boundaryare cancelled out by out-of-phase reflections from the dielectric-free space boundaryon the other side of the dielectric material. Unfortunately, electrically thin radomesprovide very little thermal insulation and are not suitable for locations with widetemperature extremes and a requirement for controlled temperatures.

Another radome approach that works well is,the half-wavelength-thick solid lam-inate, as in Figure 2.6. In this case, the wave travels 180° through the laminate, itis then reflected with a phase shift of −180°, and travels another 180° on the returntrip to achieve the net 180° phase shift required for cancellation. [10,15]

λ0 = c0fc

(2.3)

λm = λ0√εr

(2.4)

Tm = nλm2 (2.5)

fc: resonating frequencyεr: dielectric constantc: speed of lightλ0: free space wavelengthλm: wavelength in dielectric

Optimal distance between antenna and radome [10] allows minimizing the effectsof reflections caused by the radome. These effects become minimal if the wavesreturned at the antenna are in phase with the transmitted waves. The radomes arevery sensitive to this distance as it affects the radiation pattern drastically. Theoptimum distance, dm is given by:

11


dm = λ02 (2.6)

Distances < λ0/2 should be avoided while distances greater than that are notas critical and can be optimized through various algorithms.

12

Chapter 3

Implementation of the Radome andTravelling Wave Antenna(TWA)

3.1 Characteristics of TWAAs mentioned previously, planar antennas are the popular choice at automotivefrequencies due to their low cost, low profile and easy to integrate technology. Inthis thesis, the operating regime is 73 GHz to 81 GHz and the patch array antennahas a resonating frequency of 76.5 GHz. Figure 3.1 and table 3.1 summarize thecharacteristics of the antenna. A patch antenna mainly propagates surface waveson the radiating edges of the E–plane and it travels along the column as indicatedby the arrow towards E-plane [13]. Comparatively, the H-plane pattern is not in-fluenced by the surface waves as the elements do not excite them in that directionof propagation.

Figure 3.1: Travelling wave series fed microstrip patch array antenna operating at76.5 GHz

In order to suppress the grating lobes, care is taken that the mutual distancebetween each element in the column is lesser than half the free space wavelength.According to theory, since the column contains few antenna elements, series feeding

13

CHAPTER 3. IMPLEMENTATION OF THE RADOME AND TRAVELLING WAVEANTENNA(TWA)

Parameter SpecificationType Series fed TWA

Solution Frequency 76.5 GHzSubstrate Roger 3003

εr of Roger 3003 3.045Thickness 0.13 mm

Small GND size 4.6λ X 1.53λLarge GND size 18.6λ X 25λ

Table 3.1: Parameters of the TWA antenna.

is the feasible solution which will also support beam forming in the elevation plane.

The critical task, which affects the matching of an array is the length of the highimpedance line which ensure the transformation at the edge of the patch. It is thislength, that dictates the phase distribution across the array, which in turn influencesthe radiation pattern. The main beam should point in the broadside direction, sothe interconnecting lines’ lengths are adjusted for 360° phase shift between theinput edges of consecutive patches. If we observe carefully, the impedance line isterminated in a fashion that after the last patch element there is an inward inset.This inset length controls the global input reactance and thus improves the matchingwithout affecting the center frequency. Keeping these rules in mind, this kind ofantenna can be designed to obtain a complete set up of Rx-Tx arrays with differentfeeding network. As the number of columns are increased, care has to be taken toadjust the space between them and tuning of the feeding network, which can bedone with a differential single element patch, etc. [13] But, in this thesis for now,we will deal with one such column to study the influence of radome on it, usingdifferent electromagnetic simulators. The radiation pattern and return loss of theantenna is described in the sections below.

3.2 Radome structure and modificationAlong with the automotive market, the radomes employed for them also are under-going a revolutionary change . Analysis techniques too , have been modified andbetter and powerful simulators have been introduced in order to obtain realistic andstable results. But, as the complexity of solutions and approaches have improveda price we pay for that is computation time which has become the most vied forfactor today.

As more and more sensors are incorporated and the Rx-Tx antennas need tobe mounted, an electrically large radome is needed. With electrically large struc-tures, comes the baggage of constructing them properly so that the simulator candivide them into their adaptive meshes. Moreover , one has to keep in mind the

14


hardware specification of the device also which can afford to store such complexmatrix calculations. In conclusion, the structure needs to be made as simple aspossible for the purpose of simulation. As the interest is to study the shape of theradome and realize its effect on the radiation pattern, one must keep in mind thesymmetry, aperture and bends involved in the shape. As the number of radius ofcurvature increases to realize the shape, it is true that the internal reflections willindeed be minimized but simulating such a structure with different aperture radiiwill consume a lot of memory power.

The original radome structure studied in this thesis has a size of approximately56λ X 37.5λ (Where λ is calculated at 76.5 GHz). Thus, the structure is quite largeand consumes a lot of computation power. Meshing such a structure accurately andsimplifying it can be quite a challenge at these high frequencies. Hence, the ideawas to create a similar shaped model but of a smaller size. This would give an edgeof a faster computation time as well as give a perspective about the behavior of thevarious radome materials tested. Figure 3.2 illustrates the modified radome shapeused for the study further. The electrical size of this radome is 22.5λ X 12.53λ.Thus , a smoothly rounded and symmetrical shape was designed . This reducedthe number of meshing cells involved with the simulators. In this shape, just thematerial and thickness parameters, were varied to study the radome performances.

Figure 3.2: Modified radome shape

3.3 Size of Ground plane and its impactThe electrical size of the ground plane determines the field radiated from the an-tenna. Thus, the termination of the ground plane is of grave importance. Properedge shaping and termination reduces the effect of leaky waves.

In the case of TWA patch array, an infinite ground plane was modelled in bothCST and HFSS. Since , practically one has to have a finite size of the ground plane,termination of the surface waves is desired [15]. This would help to prevent spuriousradiation patterns from the antenna radiation. Thus, a study has been performedbased on three different cases in CST and HFSS. A comparison of those results ispresented along with their reported return loss and gain values. The following three

15


cases considered are:

(i) Case 1: Antenna placed on infinite ground plane

(ii) Case 2: Antenna placed on finite small ground plane

(iii) Case 3: Antenna placed on finite large ground plane

The electrical sizes of the small and large electrical ground plane is mentionedin Table 3.1. The concept behind including the word infinite and finite is that, fromthe simulator point of view ,an infinite ground plane will be multiple of wavelengthscompared to the antenna structure whereas in the finite structure it would exactlybe of the size as specified to be. From Figure 3.4 and Figure 3.5, it is deduced that,in the infinite ground plane condition ,the side lobe levels and other spurious radi-ations from the edges is stopped and when one deals with terminating the groundplane these radiations show up. But, their level is so low and their effect on thegain is minimal and hence it can be lived with.

(a) The Return Loss (S11) of the TWA patch array antenna for all the three cases inCST Microwave Studio

Figure 3.3

16


(b) The Return Loss (S11) of the TWA patch array antenna for all the threecases in HFSS

Figure 3.3

(a) E-plane pattern of the TWA Patch array in all the three cases simulated inCST Microwave Studio

Figure 3.4

17


(b) H-plane pattern of the TWA Patch array in all the three cases simulated inCST Microwave Studio

Figure 3.4

(a) E-plane pattern of the TWA Patch array in all the three cases simulated inHFSS

Figure 3.5

18


(b) H-plane pattern of the TWA Patch array in all the three cases simulated inHFSS

Figure 3.5

Antenna Parameters CST HFSS

Frequency=76.5 GHz Gain(dB)(E/Hplane)

Gain(dB)(E/Hplane)

Antenna with infiniteGND 13.9/13.9 13.7/13.7

Antenna with finitesmall GND 13.06/13.06 13.7/13.7

Antenna with finitelarge GND (wave-port/lumpedport)

12.92/12.92 13.42/13.42

Table 3.2: summarizes the performance of all the three cases implemented in boththe solvers

Thus we see that both the simulations in CST and HFSS with the same envi-ronment as seen in Table 3.2 yields similar results.

19


3.4 Simulations performed in HFSS and CST MicrowaveStudio environment

3.4.1 Choice of SolverThe radome model as shown in Figure 3.2 was implemented on top of the TWApatch array antenna in both the HFSS and CST environment. The set up lookedlike as shown in Figure 3.6.

Figure 3.6: The TWA patch array antenna enclosed in the radome

The main quest that remained to be solved was still the type of electromagneticsolver to be considered for obtaining the accurate results . The choice of optimumsolver depending upon the 3-D geometry and material of the structure is to be made.

In this thesis, HFSS and CST have been considered for doing the simulation. InHFSS, there are the following types of solvers:

• Integral equation (IE) solver which is based on Method of Moments (MoM)

• Finite Element Method (FEM) which is a frequency domain solver based ondifferential form of Maxwell’s equation

• Physical Optics (PO) solver which is based on the principle of high frequencyasymptotic solver.

• Hybrid Solver, namely as Finite Element-Boundary Integral (FE-BI) and Hy-brid PO solvers.

In short, it is a choice between the complexity of materials involved versus theelectrical size of the problem. There is an interesting depiction of this chart shown inFigure 3.7 which is extracted from [6]. “Meshing” though a very important triggerword in the discussions ahead either is used to reduce the problem size or increaseit in critical areas. In CST, the Finite Difference Time Domain (FDTD) solverwhich is based on full wave solving technique, generates partial differential forms ofthe Maxwell equations and adjusts them to central difference equations and thendiscretizes .

To begin with firstly with the FEM solver which is the most popular solverin HFSS. In this particular solver, there is a fixed volume of “box” defined [17].This box encloses the boundary conditions and geometry to be solved. The mesh

20


is discretized into tetrahedrons and it is much denser near the geometry. The mainunknown that the solver solves for is the field quantity. Thus, this type of mesh-ing is called the volume based meshing. Though this type of solver is very goodwith complex geometry or materials, the slack that one faces that it needs a lot ofcomputation space to store the coefficients of the matrix and moreveover for thisproblem one would prefer an open scattering environment.

Next, moving on with the Physical Optics solver [3]. It employs the technique ofilluminating a smooth and locally flat region. It is not a full wave solution and hencemust be thought twice before implementing it. The structure which is declared asa PO region needs to be atleast 10λ times large and also 10 wavelengths away fromthe radiating source, which is not the case in our problem statement [5]. Moreover,the radome structure is highly rounded and has curved surfaces which will lead tomulti path reflections. Thus, while solving for the currents on the surface, it solvesfor the currents on the ‘lit’ region and declares the rest as a ‘shadowed’ region asshown in Figure 3.8. Hence, the entire surface of the radome is ‘NOT’ taken intoaccount while analyzing the solution.

Figure 3.7: The comparison of various E-M solvers

21


Figure 3.8: Lit and Shadowed regions for a PO assigned dielectric region

Lit Region:j = (1−R)n×H inc (3.1a)

e = (1 +R)n× Einc × n (3.1b)

Shadowed Region:j = −Tn×H inc (3.2a)

e = −Tn× Einc × n (3.2b)

Where,

R =k2z − k2

zs_µ − (k2z − k2

zs_µ)e−2jkzsdd

(kz + kzs_µ)2 − (kz − kzs_µ)2e−2jkzsdd

T = 4(kzkzs_µ)e−jkzsdd

(kz + kzs_µ)2 − (kz − kzs_µ)2e−2jkzsdd

kz = k0, kzs = kz√εrµr, kzs_µ = kzs/µr

Thus, moving onto the IE solver [2] used in HFSS. It is based on method of mo-ments technique, to solve for the currents on the surfaces of dielectric and metallicobjects in an open scattering environment. This solver utilizes adaptive meshingtechnique which designs the optimum mesh needed around the object to consider inorder to reduce the complexity and time involved in the iterative solving technique.This particular solver was closely related to a possible solution for the thesis, sinceit models open scattering problems reliably.

The FE-BI approach [16] involves a box around the antenna could be solvedwith FEM inside it and a conformal boundary around the radome where it is solvedas IE region within and interaction can be recorded between the two boundarieswith this hybrid FE-BI technique. The slack in this method is that the, dielectricregion is a IE region and the ground plane which is a metallic IE region cannot bein contact with each other in a hybrid solving technique.

22


Thus, the simulations in HFSS were performed with the antenna solved in aFEM region and the large ground plane declared as IE region as the ground planeshould be exposed to open scattering and the radiation pattern obtained was linkedto the input field in the radome model which was simulated in the IE environment.The results of this simulation are discussed in section 3.4.2.

Finally, boiling down to the FDTD solver which was employed in CST Mi-crowave studio for simulation is a full wave 3D simulator but based on partialdifferential equations of the Maxwellian equations. Boundary conditions can be setin the domain and a box is formed. The entire volume of the simulation domain isdiscretized, usually using hexahedral mesh cells. FDTD uses a time stepping algo-rithm which updates the field values across the mesh cell time-step by time-step,thereby explicitly following the electromagnetic waves as they propagate throughthe structure. One of the significant benefits over the FEM method is that FDTDtechnique does not require a matrix solve and thus very large problems can oftenbe addressed using surprisingly small amounts of computer memory.

On the other hand, the FDTD faces a problem known as “Stair-stepping edges”[12]. The orthogonal grid structure of the FDTD method implies that edges ofstructures within the simulation have edges that follow the grid structure. This canbecome a problem for curved surfaces, for which greater accuracy is sought .

3.4.2 Implementation of the set up and simulation in CST and HFSSAs shown in Fig 3.6, the set up was implemented in FDTD solver of CST and the IEcoupled with FEM solver in HFSS. The thickness of the materials tested were λ/2and 3λ/2. Only for Material 1 which was the initially specified material, a nominalmanufactured thickness has been implemented. Table 3.3 summarizes the materialstested along with their thickness and dielectric properties.

23


Radome MaterialProperties

Loss tangent(tanδ) Thickness (mm)

Material 1(=2.74)Measured: 20 GHz 0.0009 (@20 GHz)

1.183.55Nominal= 3.2

Dyneema ST-17( =2.28)Measured: 60 GHz

0.00018(@60 GHz)

1.33.89

Rexolite 1422( =2.53)Measured: 500 GHz

0.00066(@10 GHz)

1.233.69

Table 3.3: The list of materials of the radome and their dielectric properties

Fig 3.9 illustrates the radiation pattern of the E and H- planes when Material1 was used as the radome material having a thickness of half-wavelength as shownin Table 3.3. Similarly, Figure 3.10 reflect on the performance of Dyneema as amaterial and Figure3.11 project the results with Rexolite as radome,also at thick-nesses of half-wavelength. Each of these thicknesses have been executed in CSTand HFSS environments. The figures illustrate the comparison between the samecase being implemented in different solvers. Table 3.4 documents the performanceof these antenna enclosed radomes in both CST and HFSS with regard to the gainand side lobe levels.

Material Thickness (mm) Type of Solver Gain (dB) SLL (dB)

Material 1 1.18 CST 13.00 -16.4HFSS 15.04 -18.47



Dyneema 1.3 CST 13.00 -11.40HFSS 13.45 -19.23

Dyneema 3.89 CST 13.30 -12.60HFSS 13.45 -19.90

Rexolite 1.23 CST 12.90 -11.30HFSS 14.92 -18.56

Rexolite 3.69 CST 13.40 -13.00HFSS 13.06 -20.50

Table 3.4: Comparison of the solved radome materials between CST and HFSS at76.5 GHz

24


(a) E-plane radiation pattern of Material 1 at 76.5 GHz of thickness 1.18 mm

(b) H-plane radiation pattern of Material 1 at 76.5 GHz of thickness 1.18 mm

Figure 3.9

25


(a) E-plane radiation pattern of Dyneema at 76.5 GHz of thickness 1.3 mm

(b) H-plane radiation pattern of Dyneema at 76.5 GHz of thickness 1.3 mm

Figure 3.10

26


(a) E-plane radiation pattern of Rexolite at 76.5 GHz of thickness 1.23 mm

(b) H-plane radiation pattern of Rexolite at 76.5 GHz of thickness 1.23 mm

Figure 3.11

27


From Figure 3.9-3.11 and Table 3.4, we deduce the following conclusions:

• From Figure 3.9-3.11 and Table 3.4 the recordings of Material 1, Dyneema andRexolite has agreed with the theory. It illustrated the fact, that apart fromthe calculated thicknesses, the side lobe levels and the gain from the radome-antenna system will be inferior, as in the case of Material 1 with thickness of3.2mm. Both, HFSS and CST solutions verify that, Material 1 of thickness3.2mm is clearly not a viable solution.

• Generally, it was observed that as the thickness of the materials increasefrom λ/2 to 3λ/2 there is a decrease in the gain but improvement in the sidelobe levels. From Table 3.4, for the antenna-radome system solved in CST,an anomaly was observed in Material 1 of thickness 3.55mm. The side lobelevels had abruptly increased. This can be attributed to requirement of densermeshing cells.

• From Table 3.4, it is also seen that for all the cases, the set up in CST hasrecorded higher side lobe levels than HFSS. This can be attributed to the factthat, the entire radome-antenna set up was in one complete simulation filewhile that in HFSS was solved with linked radiation fields. This, could be areason for CST to record more accurate side lobe levels.

• It is observed that the radome structures having a thickness of half-wavelengthsolutions in all the three materials have not lost in gain, bore sight error andside lobe level. This behavior is verified by the radomes solved in HFSS andis in accordance with theory.

Figure 3.12: Return loss of Material 1 radome enclosed antenna compared to An-tenna with large finite GND plane

28


The fast solution method and short time required in CST helps to visualizethe matching parameter S11 of the entire set up. The Figure 3.12 showcases thematching of the radome-antenna set up for Material 1 at the frequency of operation.The figures compare the basic antenna with large ground plane to the radomeenclosed set up of different thicknesses. Similar characteristics is also observed forthe radomes composed of Dyneema and Rexolite.

29

Chapter 4

Measurement Techniques

4.1 Measuring Insertion Loss (IL) and Return loss (RL) ofMaterial 1

Rhode and Schwarz VNA ZVA 24 was the tool used to record the insertion andreturn loss through the material of the radome. The main features of this kit are,it has W-band extenders and WR-10 calibration kit. It has two mm-wave horns ofW-band aligned to each other. The distance between them is 13.8 cm. The radomeis placed between them at 6.9 cm. It was vertically aligned between them as shownin Figure 4.1.

Figure 4.1: The vertically aligned Material 1 Radome between the W-band horns

The S-parameters were measured between the two horn antennas when no

30

CHAPTER 4. MEASUREMENT TECHNIQUES

radome was kept.

Figure 4.2: Return loss and insertion loss when there is no radome

Next, the insertion loss as well as return through the radome was recordedkeeping the radome between them.It was obtained by S21open − S21radome. Theradome was kept in horizontal and vertical alignment (as shown in Fig 4.1 whichcorresponds to vertical alignment). The following results were obtained.

31


(a) The insertion loss through the radome of Material 1 in horizontal and verticalalignments at 0 degree.

(b) The return loss through the radome of Material 1 in horizontal and verticalalignments at 0 degree.

Figure 4.3

The next step was to cut a rectangular piece of Material 1 and de-embed itwithin the waveguides so that an accurate return loss and insertion loss of variousthicknesses can be seen. It was done just to get an idea about the matching ofradomes at various thicknesses. Figure 4.4 shows the return loss and insertion loss

32


through the thicknesses of 6.28 mm, 4.72 mm and 2.49 mm. These pieces were cutin a manner so that it perfectly fits the face end of WR-10 waveguide.

The thinnest radome thickness has broader matching in this frequency regionand it has the lowest insertion loss as well. Depending upon the region of operationwe have observed that simulations must be done and thickness must be optimized.Thus these measurements, gave an overall idea about the performance of Material1.

Figure 4.4: The return loss and insertion loss recorded through pieces of 6.27mm ,4.72mm and 2.49mm of Material 1

33

Chapter 5

Implementation of Fabry-PerotResonator

5.1 Fabry Perot ResonatorsAs seen from Chapter 3, the gain of the antenna and antenna enclosed radomewas comparable. Sometimes, it is desirable to add features such as high gain ina particular bandwidth, etc. in both the E an H-plane in few applications. Forthat purpose, there are many new concepts of ‘rasorbers’ and even transformationoptics for that matter which changes the permittivity profile of the object as thewaves traverse through it. In [8] we have also seen the implementation of matchinglayers for achieving the desirable gain. One such long researched and comparativelycheaper solution is “Fabry Perot Resonators”.

This technique delves into the concept of Partially Reflecting Surfaces (PRS)which produces a leaky wave and beam forming affect when excited by a groundplane wave guide aperture which in our case is a TWA Microstrip Patch Antenna.This kind of solution is well suited for our problem where the radome can be con-verted to a Partially Reflecting surface. The gain and amplitude depend on thereflection from the PRS and also the distance of it from the ground plane. Onemust optimize the height involved in order to get the maximum gain from thestructure. The partial reflecting surfaces can be obtained by designing a periodicarray of patches, apertures, dipoles, etc. which are also termed as Frequency Selec-tive Surfaces. These structures either support complete reflection or transmissionwithin the frequency range of interest. They characteristically show case a 3-dBgain both in the E and H-plane around the resonating frequencies.

The basic design criteria for obtaining them are:

The reflecting surfaces in this scenario are the ground plane and the PRS. Thereflection coefficient of the ground plane Rgr = 1 and the corresponding reflection

34

CHAPTER 5. IMPLEMENTATION OF FABRY-PEROT RESONATOR

phase φgr = −π. The reflection coefficient and the phase of the PRS vary withfrequency [7]. The schematic diagram of the antenna, using a partially reflectivearray, is shown in Fig 5.1. An open-ended rectangular waveguide operating in theW-band , placed within the ground plane, is used as the primary antenna since it issimple and has adequate bandwidth. The antenna function can then be describedas follows. Waves emerging from the primary antenna travel long paths as a resultof in multiple reflections between the ground plane and the PRS. A phase shift isintroduced by the path length, the total reflection on the ground plane and also bythe phase of the reflection coefficient of the PRS. The transmitted power can becalculated by the interference of the waves partially transmitted through the PRS.The sum of the transmitted rays yields an analytic formula for the power pattern,given by:

P (θ) = |1−R2(θ)|1 +R2(θ)− 2R(θ)cos

[φ(θ)− π − 4πLr

λ0

]F 2(θ) (5.1)

where R(θ) is the complex reflection coefficient of the PRS as a function of θ, λ isthe free-space wavelength, and F (θ) is the radiation pattern of the primary antenna.The resonant distance, Lr, is the distance between the PRS and the ground plane.Maximum power at boresight is obtained when,

φ(0)− π − 4πLrλ0

= 2Nπ (5.2)

or,

Lr =(φ(0)π− 1

)λ04 +N

λ02 , N = 0, 1, 2... (5.3)

When equation 5.2 is inserted in equation 5.1, and θ is considered at 0 degrees,then gain at boresigt is obtained to be:

G = 1 +R

1−R (5.4)

Thus, to obtain a linearly increasing (with frequency) phase response will resultin a maximum gain within a certain frequency range. The gain maximum willbe determined from the magnitude of the reflection coefficient. Under these twoconditions high gain and wide bandwidth can be obtained.

35


Figure 5.1: Arrangement of the simulation set up of the PRS

5.2 Optimizing the Fabry Perot ResonatorIt has been seen that for a conducting element array the phase of the reflection co-efficient is negative and decreases with frequency. At resonance (total reflection) itsvalue becomes −π. In the case of an aperture array, the reflection phase is positiveand, too, decreases with frequency until resonance (total transmission) where it be-comes zero. Thus, a unit cell was designed firstly to get an idea about the reflectionand phase parameter. Thus, a metallic element of a dipole was designed havingwidth, length and thickness and periodicity. These parameters were optimized ona dielectric block of Rexolite having a thickness of 1.23mm. Table 5.1 comprises ofthe parameters used to design the cell. Figs 5.2, 5.3 (a) and (b) illustrate the unitcell design, the reflection and phase coefficients of the unit cell respectively.

Parameters ValuesLength of dipole 1.31 mmWidth of dipole 0.4 mm

Height of superstrate 1.96 mmThickness of superstrate 1.23Material of superstrate RexoliteThickness of copper 20 µm

Table 5.1: Optimization parameter for Fabry Perot Unit Cell

36


Figure 5.2: Cell design of Fabry Perot resonator

(a) Reflection and transmission band of the unit cell

Figure 5.3

37


(b) Reflection and Transmission phase coefficient of the unit cell at normal incidence

Figure 5.3

After optimizing the unit cell, an array of these structures was implemented(asshown in Figure 5.4) . The array size is of 11 X 11 such elements on a largeground plane (size mentioned in table 5.1). A WR-10 waveguide was used to excitethe structure. The superstrate layer was placed first at Lr : 1.96mm(λ/2) abovethe ground plane and then 5.88 mm (3λ/2) respectively. As the multiple of halfwavelengths increase, gain increase but the side lobe level performance deteriorates.Fig 5.5 (a) and (b) show the 3-dB gain increase in both the E and H-plane at theabove-mentioned heights.

38


Figure 5.4: Implementation of the Fabry Perot resonator using the dipole elementsfrom 76-77 GHz

(a) Gain realized in the E-plane with the dipole elements

Figure 5.5

39


(b) Gain realized in the H-plane with the dipole elements

Figure 5.5

Thus concluding, it is observed that on one hand these structures have a verydirective beam gain of 20.33 dB at 76.5 GHz. The directivity from about 13 dB leapsby 7dB at the frequency of interest. This low profile solution has the transmissionband in control and if the elements are tapered correctly they can exhibit goodphase control as well. On the other hand, it is a narrow band solution, making itapplicable only for pencil beam solutions.

40

Chapter 6

Future Work

After obtaining the results, as seen in Chapter 3, there is a slight disagreement inthe nature of the patterns when simulated in CST and HFSS. It would be interestingto bridge the gap by either taking real time radiation pattern measurement of theradome materials or by creating a denser mesh near the critical areas of the radome.For the purpose of getting a much more real feel of the problem, it would be highlybeneficial to take radiation pattern measurement of the radome which was notpossible due to faults in the range. Depending on that, one could firstly view thedeviation between simulation and measurements. The different types of materialsinvolved and their calculated thicknesses should be cut into pieces so that they canbe de-embedded within the measurement horns as done previously. In that manner,insertion loss and return loss can also be checked and extraction of permittivity inthe right manner can also be done. For the purpose of getting closer to the realsize of the constructed radome, after judging the measurement results, the closestanalyzing technique can be used to simulate the actual size of large radome withsuitable hardware. Besides, the array formed by Fabry Perot resonating elementsdesigned to obtain the 3dB can be further optimized and also tapering can be done inorder to obtain the desirable phase and amplitude. Materials such as “Rasorbers”also can be investigated or the radome can be transformed into a lens antennalike structure having different permittivities and thicknesses [8]. In conclusion, theresults obtained presently give a firm grounding for performing further investigationsand improvement.

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Bibliography

[1] CST Microwave Studio - Time Domain Solver. https://www.cst.com/products/cstmws.

[2] Ansys. HFSS Integral Equation Solver Option. Technical report.

[3] Ansys. HFSS PO hybrid region. Technical report, 2016.

[4] R.H.J Cary. Handbook of Antenna Design, volume 2. Cambridge UniversityPress, 1983.

[5] Ugo d’Elia, Giuseppe Pelosi, Christian Pichot, Stefano Selleri, and MassimoZoppi. A physical optics approach to the analysis of large frequency selectiveradomes. Progress in Electromagnetics Research, 138:537–553, 2013.

[6] J. J. DeLisle. What are the differences between various em-simulation numericalmethods? 2010.

[7] Az P Feresidis and JC Vardaxoglou. High gain planar antenna using opti-mised partially reflective surfaces. IEEE Proceedings-Microwaves, Antennasand Propagation, 148(6):345–350, 2001.

[8] Biebl Fitzek, Rasshofer. Comparison of matching layers for automotive radomedesign. 8:49–54, 2010.

[9] E. G. Geterud. Design and Optimization of Wideband Hat-Fed Reflector An-tenna with Radome for Satellite Earth Station. PhD thesis, Chalmers Univer-sity of Technology, 2012.

[10] RFbeam Microwave GmbH. Radome(radar enclosure). 2013.

[11] L. Griffiths. A fundamental and technical review of radomes. 2008.

[12] Marshall Innan. Numerical Electromagnetics. Cambridge University Press,2011.

[13] Pablo Joerg. Planar antenna technology for mm-wave automotive radar, sens-ing, and communications. 2010.

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https://www.cst.com/products/cstmws

https://www.cst.com/products/cstmws

BIBLIOGRAPHY

[14] D. J. Kozakoff. Analysis of Radome-Enclosed Antennas. Boston: Artech House,2 edition, 1997.

[15] Shu-Kun Lin. Gps/gnss antennas. by b. rama rao, w. kunysz, r. fante and k.mcdonald, artech house, 2012; 420 pages. Remote Sensing, 5(2):808, 2013.

[16] Hong Fu Meng and Wen-Bin Dou. A hybrid method for the analysis of radome-enclosed horn antenna. Progress In Electromagnetics Research, 90:219–233,2009.

[17] D. Morris. Which electromagnetic simulator should i use? 2014.

[18] Dapeng. Wu. 76-81 GHz planar antenna development and utilization for auto-motive radar applications, 2016.

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