Concealment Materials and Techniques for mm-Wave Advanced Antenna Systems (AAS)

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STOCKHOLM, SWEDEN 2018

Concealment Materials and Techniques for mm-Wave

Advanced Antenna Systems (AAS)

ADHITYA BHARADWAJ SRIRAM

KTH ROYAL INSTITUTE OF TECHNOLOGY

SCHOOL OF ELECTRICAL ENGINEERING AND COMPUTER SCIENCE

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i

Concealment Materials and Techniques for mm-Wave Advanced Antenna Systems (AAS)

ADHITYA BHARADWAJ SRIRAM

Examiner:

Oscar Quevedo-Teruel

KTH Royal Institute of Technology, Sweden

Supervisors:

Mahsa Ebrahimpouri

KTH Royal Institute of Technology, Sweden

Ingolf Larsson

Ericsson AB, Gothenburg, Sweden

Livia Cerullo

Ericsson AB, Gothenburg, Sweden

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Abstract

With the advent of the 5G technology standard for communication, it is estimated that around 50 billion devices around the world will be interconnected under this standard [1]. This has prompted the need to investigate new materials, technologies and methods to conceal antennas for preserving the aesthetic value of urban centers, and preventing these antenna systems from being identified or damaged. The aim of this thesis work was to identify possible materials and investigate methods for concealing Ericsson’s 5G mm-Wave Ad- vanced Antenna System(AAS) Radio Base Stations. A study of economically viable, commercially available materials for concealment, and methods to determine their dielectric properties was done. A practical investigation of the effect some available concealment materials on the RF performance of the mm-Wave base station beams, was also performed. The results from the investigations and measurements performed led to some useful conclusions and understanding about the various concealment material solutions. Overall, the investigated concealment materials were found to have two prominent effects:

1. Reduction in EIRP (Equivalent Isotropic Radiated Power) of the base station beams (attenuation).

2. Distortion in the shape of the base station beams.

The boresight beams for all the investigated cases were found to be attenuated(effect 1) by the concealments, but the steered beams were found to be both attenuated in distorted(effect 2) by the concealments. In particular, concealments with a thin dielectric material mesh structure were found to have the least effect on the base station beams. All other thicker(> λ₀/2), composite concealments and even those with pure foam structures, were found to have a noticeable effect on the base station beams. In conclusion, thin dielectric mesh, films and paints could hence be of interest as concealment solutions for further investigations moving forward.

Keywords

AAS (Advanced Antenna Systems), EIRP(Equivalent Isotropic Radiated Power), mm-Wave(Millimeter Wave), Concealment Materials, Base Station.

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iii

Sammanfattning

I och med införandet av 5G-tekniken för kommunikation bedöms det att cirka 50 miljarder enheter runt om i världen kommer att vara uppkopplade via denna standard [1]. För att bevara det estetiska värdet av stadscentrum och förhindra att 5G-teknikens antenner identifieras eller skadas finns behov att undersöka nya material, tekniker och metoder för att dölja antenner. I detta examen- sarbete identifieras möjliga material samt undersöks metoder för att dölja Er- icssons 5G mm-vågs Advanced Antenna System (AAS) radiobasstationer. En studie av ekonomiskt genomförbara, kommersiellt tillgängliga material för döl- jande och metoder för bestämning av dess dielektriska egenskaper gjordes. En empirisk undersökning av effekten av några tillgängliga maskeringsmaterial på RF-prestanda hos loberna från mm-vågsbasstationer utfördes också. Resultaten från de undersökningar och mätningar som utfördes ledde till några användbara slutsatser och förståelse för egenskaperna hos de olika maskeringsmaterialen.

Sammantaget visade sig de undersökta maskeringsmaterialen ha två huvudsak- liga effekter:

1. Dämpning av EIRP(ekvivalent isotropisk utsrålad effekt) hos basstationsloberna.

2. Distorsion av basstationslobernas form.

De centrala basstationsloberna för alla undersökta fall visade sig dämpas (effekt 1) av maskeringsmaterialen, men de utstyrda loberna befanns vara både dämpade och förvrängda (effekt 2) av maskeringsmaterialen. Maskeringsma- terial med en tunn dielektrisk nätstruktur visade sig ha den minsta effekten på basstationsloberna. Alla andra tjockare, sammansatta material och även de med rena skumstrukturer visade sig ha en märkbar effekt på basstationsloberna.Sammanfattningsvis kan därför tunna dielektriska nät, filmer (plast- folier) och färger vara av intresse som lösningar för vidare undersökningar.

Keywords

AAS (Advanced Antenna Systems), EIRP(ekvivalent isotropisk utsrålad effekt), mm-Wave(Millimeter våg), Maskeringsmaterialen, Radiobasstationer.

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Acknowledgements

I am grateful to my managers at Ericsson AB, Magnus Holmén and Madelene Lundqvist, for providing me with this wonderful opportunity to do my thesis work in my area of passion. I would like to convey my profound gratitude to my supervisors at Ericsson, Livia Cerullo and Ingolf Larsson for all their invaluable guidance, support, feedback during this thesis work and for always being around to help me when I needed it. A big thanks to Ingolf and Bo Granstam, for helping me translate my thesis abstract to Swedish, proofreading and refin- ing it. I thank my supervisor at KTH, Mahsa Ebrahimpouri and my examiner, Professor Oscar Quevedo-Teruel for their support.

The first part of this thesis work would not have been possible without Bhushan Billade’s invaluable patience and guidance. I express my profound gratitude to Bhushan for teaching me so many things and helping me understand my own work as well as I have. Masoud Ramazani’s patience and support in helping me with the base station measurements were vital in ensuring my thesis work went smoothly. I express my gratitude to him for this.

A big thank you to all the wonderful friends I have made during this thesis work at Ericsson, for their wonderful company which kept me cheerful and enthusias- tic throughout this time. I would like to thank my friends who have supported me all these years, no matter where they are.

My family and most importantly my parents and grandparents, have been my pillars of support through all the highs and lows of life, helping me in any and every way they could, enabling me to achieve my pursuits. I am here today because of them and I would be eternally indebted to them. And finally, I thank Almighty God.

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v

List of Figures

2.1 Forward and backward propagating waves at a material interface 8 2.2 Electric and Magnetic Field Orientation at a material interface . 9

2.3 Single Layer Material Interface . . . 10

2.4 Transmission Line . . . 12

2.5 Different Types of Radome Wall Constructions . . . 16

2.6 Some Examples of Existing Concealment Solutions . . . 19

2.7 (a) Shroud Concealments[17] (b) Film Concealments[18] . . . . 20

2.8 An Open Resonator and A Split-Cylinder Resonator Setups . . 22

2.9 Free Space Measurement Setups . . . 22

2.10 Waveguide and Coaxial Transmission Line Measurement Setups 23 3.1 Concealment Sample 1 . . . 26

3.2 Concealment Sample 2 . . . 26

4.1 Free Space Measurement Setup . . . 34

4.2 Time-Domain Gating . . . 37

4.3 Model Fitting and Permittivity Estimation for Sample 1 . . . . 40

5.1 Base Station Concealment Measurement Setup . . . 50

5.2 28 GHz Base Station Front View without concealment material sample . . . 51

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5.3 28 GHz Base Station Top and Front Views with Concealment material sample . . . 52 5.4 Base Station Orientation in Starting and Ending Measurement

Positions . . . 53 5.5 Azimuth and Elevation Beam Steering with Base Station . . . . 54 5.6 Base Station V-Pol (Left) and H-Pol (Right) Beam Orientation 55 5.7 Solid, Pure Foam and Foam 1 Sandwich/Composite Samples . . 57 5.8 Foam 2 Composite Samples and Mesh Sample . . . 57 5.9 V-Pol Base Station Concealment Measurements for Foam 1 Sand-

wich/Composite Samples 2,3,4 and 10 . . . 59 5.10 H-Pol Base Station Concealment Measurements for Foam 1 Sand-

wich/Composite Samples 2,3,4 and 10 . . . 60 5.11 V-Pol Base Station Concealment Measurements for Pure Foam

Samples 5 and 6 . . . 61 5.12 H-Pol Base Station Concealment Measurements for Pure Foam

Samples 5 and 6 . . . 61 5.13 V-Pol Base Station Concealment Measurements for Solid Sam-

ples 1 and 9 . . . 62 5.14 H-Pol Base Station Concealment Measurements for Solid Sam-

ples 1 and 9 . . . 63 5.15 V-Pol Base Station Concealment Measurements for Foam 2 Sam-

ples 7 and 8 . . . 64 5.16 H-Pol Base Station Concealment Measurements for Foam 2 Com-

posite Samples 7 and 8 . . . 64 5.17 V-Pol Base Station Concealment Measurements for Mesh Sample

(Sample 11) . . . 65 5.18 H-Pol Base Station Concealment Measurements for Mesh Sample

(Sample 11) . . . 66

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Chapter 1

Introduction

1.1 Background

Modern times have brought with them a pervasive use of technology to make human lives increasingly convenient. Telecommunication technologies have seen a huge boom since the advent of the first telephones in the mid-19th century with the rapid growth of RF technologies during the era of the world wars and then onwards. What was once a distant dream, to be able to simply communi- cate over long distances, has now become an essential aspect of everyday life.

Some estimates predict that by 2020, around 50 billion devices worldwide would be interconnected under the 5G communication standard [1]. Antenna and RF technologies have been a vital driving force behind this relentless growth of telecommunication. Recent years have witnessed research into many new and innovative types of antenna solutions for telecommunication applications [2][3].

Antennas themselves have seen huge developments for a vast range of applications over the past century. The need to protect these antennas from extreme environments, and in some cases, to conceal them from visibility for aesthetic or security reasons, has also arisen, with the ubiquitous applications of antennas and antenna systems. Radomes perform the functionality of protecting the antenna systems from the harsh and perennially changing environmental conditions, across various geographic locations on earth.

Concealment of antenna systems, in particular, has been of great interest since the rise in the creation of defense and military radar systems in the latter half of the 20th century [4]. The development of radomes for concealing advanced radar and missile guidance systems from external visibility, continues to be ex- tensively investigated, as these systems move up in frequencies. The use of radomes in civilian applications arose majorly with the goal of shielding the antenna systems from the external environment and preserving the aesthetic value of modern buildings and structures in urban centers.

Concealment materials, here, refer to the use of certain materials over the radome-enclosed antenna systems, either as shrouds or as films and paint, with the prime objective of concealing, or hiding these antenna systems from visibility. Depending on the type and profile of the structures on which these antenna systems are to be installed, the concealment material can take various shapes

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2 Chapter 1. Introduction

and forms. From an RF perspective, the functionality of concealment materials is analogous to that of a typical radome. In this thesis work, some concealment materials are studied and analyzed for their effect on the RF performance of Ericsson’s 5G mm-Wave AAS (Advanced Antenna System) Radio base stations.

Their behavior for the steered base station antenna beams is investigated. This becomes particularly important for the AAS radio units, whose beams are elec- tronically steered over a service area of ±60 degrees along the azimuth direction, when installed in base station towers.

1.2 Purpose

This thesis work is expected to provide insight into the effects concealment materials have on the radio base stations, which will enable the development of better concealment solutions for 5G mm-Wave frequencies in the future. A part of this work also involves investigating and evaluating quick, low-cost and reasonably accurate methods for estimating dielectric properties of concealment materials. This is expected to be a benchmark test for determining dielectric properties of such materials in the future as well.

1.3 Outline

Evaluation of available methods for dielectric characterization of materials, and practical implementation of the optimum measurement method for dielectric characterization of some available material samples, form the first part of this thesis work. The design, implementation of suitable mechanical fixtures, and a practical investigation of the influence of some available material samples on the a mm-Wave radio base station beams forms the last part of this thesis work. Theoretical analysis, simulation work, and practical measurements in an anechoic chamber are within the scope of this master’s thesis project.

1.4 Goal

The goal of this master’s thesis work, as outlined previously, would be to investigate and identify the suitability of certain available materials in concealing Ericsson’s 5G mm-Wave AAS Radio Units, with minimal influence on its performance. Effort will also be dedicated to evaluating the best mechanical solutions for testing these material samples and their influence on the AAS radio unit’s RF performance, on the road to achieve the desired goals.

1.5 Work Flow Methodology

This section provides an overview of the work flow methodology followed during the thesis work.

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(a) Literature Study of Concealment Materials and Measurement Methods

A literature study was first performed with the aim of understanding the problem statement of the thesis, the materials that can be utilized for concealment, and the possible methods for the measurement and estimation of the complex permittivity of the available concealment material samples.

(b) S-Parameter Measurement for the Available Material Samples Based on the literature study and the resources available at hand, the Free Space Method was chosen to perform the insertion and reflection S-Parameter measurements, using two Standard Gain Horn antennas with the material samples. Further description of the Free Space measurement setup is presented in the next chapter.

(c) Post Processing and Optimization to Obtain Complex Permittivity Values for the Material Samples

Using the S-Parameter data obtained for all the concealment material samples using the free space measurements, the complex permittivity of the materials was estimated. The estimation was performed in MATLAB using the inverse ABCD matrix method described briefly in the previous chapter and in further detail in the next chapter. An Optimization was performed to estimate the best fit for the complex permittivity of the material samples, as will be described further in the next chapter.

(d) 28 GHz Antenna Base Station Concealment Measurements The next step in the thesis work was to study the performance of the concealment material samples by placing them in front of a mm-wave Antenna Base Station equipment, in an anechoic environment. This study is expected to provide an insight into how much the beams from the antenna base station are being degraded by these concealment materials. This step is expected to practically validate the suitability of these material samples in concealing the antenna base stations, with minimal effect on the base station beams.

(e) Post Processing and Analysis of Base Station Concealment Measurement Results

After the measurement of these concealment material samples in front of the antenna base station, some post processing is to be performed to visualize and analyze the measured data to derive conclusions on the performance of these concealment material samples. The results and findings of the thesis work would then be reported and presented.

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4 Chapter 1. Introduction

1.6 Sustainability

With concerns of rapid climate change in recent times due to greenhouse gas emissions, the impacts of technological growth on the planet have found major attention. With a six-fold increase in consumption of electronic devices over the past six decades, the ICT (Information and Communication Technology) industry, contributes to around 8 % of the total worldwide energy consumption [5]. This in-turn also contributes proportionally to the ICT industry’s carbon footprint. Sustainability and sustainable development have hence become important areas of focus for the ICT industry globally. The energy consumption and carbon footprint levels are only expected to rise further, with the advent of faster and high-capacity 5G mobile communication standard over the next few years.

Base stations are estimated to consume approximately 80 % of the total energy consumed by mobile networks [6]. With advanced beamforming and high data capacity features, the energy consumed by base stations becomes a critical aspect in 5G networks. The introduction of concealment solutions that scatter or absorb energy at 5G frequencies, would necessitate a higher supply of power to the base stations to compensate for these losses and provide a similar quality of coverage, compared to a case when no such concealments are present. Since concealment solutions are expected to be vital, moving forward, the direction of investigation performed in this thesis work becomes important, to identify concealment solutions that have a minimal impact on the 5G mobile base station beams. This could help eliminate the need to supply a larger amount of power to maintain the quality of coverage in the presence of concealments in such base stations. This would also be in line with target (12.A) of the United Nations Sustainable Development Goals (SDG), which aims at strengthening scientific and technological capacity to move towards more sustainable patterns of consumption and production [7].

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5

Chapter 2

Background Theory

The scope of this thesis work covers the theoretical areas defined by Maxwell’s equations, electromagnetic eave propagation in media and radome theory. The following subsections of this chapter briefly cover these topics in order to provide a theoretical foundation for the experimental work performed during the thesis.

2.1 Maxwell’s Equations and Dielectric Materi- als

James Clerk Maxwell in 1861-62 published his revolutionary Maxwell’s equations which, along with Lorentz’s Law form the foundation for the subject of Classical Electromagnetism. The Maxwell’s equations are stated below in their Differential form.

∇ × E = −∂B

∂t

∇ × B = J + ∂D

∂t

∇ · D = ρ

∇ · B = 0

(2.1)

Where E is the electric field intensity. D is the electric flux density and B is the magnetic flux density. ρ is the charge density and J is the current density.

D is sometimes also referred to as “Electric Induction”, indicating that it is an induced field density in response to the interaction of the incoming electric field strength with the surrounding material media. The current density J and the charge density ρ are also referred to as the “source quantities”, indicating they are the sources of the electromagnetic fields.

The following derivations are followed from [8], except wherever mentioned.

The constitutive relations between the Electric and Magnetic field intensities and the flux densities for an electromagnetic field propagating in material media are given as,

D = εE

B = µH (2.2)

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Where, ε and µ are the absolute values of permittivity and permeability, respectively, of the material media.

The absolute values of the permittivity and permeability of the media can be used to define relative permittivity ε_r and relative permeability µ_r as,

ε_r= ε ε₀ µ_r = µ

µ₀ (2.3)

Where ε₀ and µ₀, are the permittivity and permeability of vacuum. The refrac- tive index of the material media is then given as,

n =√

ε_rµ_r (2.4)

As a part of this thesis work, only the methods to find the complex permittivity of some concealment materials is explored. So, the focus from this point on shall only be on the permittivity of the material media, and not the permeability.

The absolute permittivity of a material media, in general, is a function of the frequency, f and is defined as a complex quantity, given as

ε(ω) = ε₀ε_r(ω) = ε⁰_r(ω) − jε⁰⁰_r(ω) (2.5) Where ω, is the angular frequency given by ω = 2πf .The complex nature of the permittivity of material media is representative of the energy storage and loss mechanisms inside the medium when an external electromagnetic wave interacts with it. For instance, let us consider an electric dipole inside a material media.

When an external, alternating electric field impinges upon the media, the dipole aligns itself with this field. But as the electric field alternates, the dipole tries to realign itself with the alternating electric field, by rotating. While rotating, the dipole loses energy through the mechanisms of heat generation from friction due to accelerating and decelerating motion of the dipole. The losses and phase difference between the electric field and the dipole determine how big the mag- nitude of the imaginary part of the permittivity is, as a function of frequency and material properties. The bigger the imaginary part of the complex permittivity, greater the energy being dissipated as heat. Thus, the imaginary part of the relative permittivity is a directly indicative of the losses in the material media [8].

The real part of the complex permittivity of the medium, ε⁰_r,is representative of the energy storage capability of the medium. A “dielectric” material is one that has an ability to store energy when an external electric field is applied across it.

If a DC voltage source is placed across a parallel plate capacitor, more charge is stored when a dielectric material is between the plates than if no material (a vacuum) is present between the plates. The dielectric material increases the energy storage capacity of the material(capacitor) by neutralizing the charges at its electrodes. The real part of the complex permittivity is, thus, a measure of how much energy from an external electric field is stored in a material [9].

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2.2. Electromagnetic Waves in Material Boundaries 7

Loss tangent (tan δ), is the ratio of the real and imaginary parts of the complex permittivity of the material.

tan δ = ε⁰⁰_r

ε⁰_r (2.6)

It can also be specified in terms of the quality factor or Q-factor, or the ratio of stored and dissipated energy as [9],

tan δ = ε⁰⁰_r ε⁰_r = 1

Q = Energy dissipated per cycle

Energy stored per cycle (2.7) In general, the imaginary part of complex permittivity (ε⁰⁰_r) is always greater than zero and is usually much smaller than real part of the complex permittivity (ε⁰_r).

2.2 Electromagnetic Waves in Material Bound- aries

2.2.1 Transmission and Reflection

The following derivations are followed from [10], unless mentioned otherwise.

Maxwell’s equations are simplified, by decoupling them and making assump- tions of a time harmonic, uniform, one dimensional plane wave propagation as described in [10]. The resulting one-dimensional differential equation has two possible solutions which describe a wave travelling in the forward (or, positive) and backward (or, negative) directions,

E(z, t) = E₊(z, t) and E(z, t) = E−(z, t)

H(z, t) = H₊(z, t) and H(z, t) = E−(z, t) (2.8)

They can be combined to form a general solution for a wave propagating in free space, along a single direction, at a single frequency, as described in [11]

E(z) = E+(z) + E−(z) = E0+(z)e^−jkz+ E0−(z)e^jkz H(z) = 1

η[E₊(z) + E−(z)] = 1

η[E₀₊(z)e^−jkz− E₀₋(z)e^jkz] (2.9)

Where η is the free space impedance having a value of 377 Ω.

Now we proceed to understand the behavior of these uniform plane waves nor- mally on a material interface or boundary. We can express the forward and

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Figure 2.1: Forward and backward propagating waves at a material interface [10].

backward propagating fields E₊ and E₋ as, E₊(z) = 1

2[E(z) + ηH(z)]

E₋(z) = 1

2[E(z) − ηH(z)] (2.10)

Two quantities that are useful in evaluating the behavior of the EM waves in a material interface are the wave impedance (Z), reflection coefficient (Γ), and transmission coefficient (T). They are defined as follows,

Z(z) = E(z) H(z) = η⁰ Γ(z) = E−(z)

E+(z) T (z) = E₊⁰ (z) E₊⁽z)

(2.11)

For a visual representation, we assume a planar interface (taken to be the xy- plane at some location z) separating free space having an impedance η and a dielectric media having a characteristic impedance, η⁰, as shown in figure 2.1. The quantity ρ here is the elementary reflection coefficient and τ is the elementary transmission coefficient for medium 1 (free space).

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2.2. Electromagnetic Waves in Material Boundaries 9

Figure 2.2: Electric and Magnetic Field Orientation at a material interface [8].

ρ⁰ and τ⁰ are the corresponding elementary reflection and transmission coefficients respectively, for medium 2 (dielectric/conducting material). Following the derivations in [11], the elementary reflection and transmission coefficients are defined as follows,

ρ = η⁰ − η

η⁰ + η = n − n⁰

n + n⁰, τ = 2η⁰

η⁰ + η = 2n n + n⁰ ρ⁰ = η − η⁰

η + η⁰ = n⁰− n

n⁰+ n, τ⁰ = 2η

η⁰ + η = 2n⁰ n + n⁰

(2.12)

It is important to remark here that the arrows in Fig. 2.1 indicate the direction of propagation of the Electromagnetic wave and not the direction of the Electric fields themselves. The electric and magnetic fields are oriented parallel to the boundary interface, along different planes as is depicted in figure 2.2.

The boundary conditions at the interface between the two medium demand the continuity of the total electric and magnetic fields across the interface boundary.

E = E⁰

H = H⁰ (2.13)

In terms of the forward and backward propagating waves, this condition can be expressed as,

E₊+ E− = E⁰₊+ E⁰₋ 1

η[E₊− E−] = 1

η[E⁰₊− E⁰₋] (2.14)

This relation can be written in a matching matrix form, relating the fields on either side of the interface,

E₊ E−

= 1 τ

1 ρ ρ 1

E⁰₊ E⁰₋

(2.15)

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Figure 2.3: Single Layer Material Interface [10].

From these two equations and recognizing from the above eqns. That, τ =1+ρ ,some useful relations can be derived, following the derivations in [10].

Γ = E−

E₊ =

1

τ[ρE₊⁰ + E₋⁰ ]

1

τ[E₊⁰ + ρE₋⁰ ] =

[ρ +^E

0

−

E⁰₊] [1 + ρ^E

0

−

E₊⁰ ] T = E₊⁰

E₊ = E₊⁰

1

τ[E₊⁰ + ρE₋⁰ ] = (1 + ρ)E₊⁰ [1 + ρ^E

0

−

E⁰₊]E₊⁰

(2.16)

Since there are assumed to be no incident waves from medium 2 towards medium 1 in our case, E₋⁰ (in Figure 2.1) and there is only one incident wave from medium 1 towards medium 2 (as in Figure 2.2). On simplifying eqns. (2.16), using this assumption, we get,

Γ = ρ

and T = 1 + ρ (2.17)

2.2.2 Single Layer Materials

We first briefly consider reflection and transmission for the case of a single layer material interface, following the description and derivation provided in [10].

Figure 2.3 provides a visual representation of a single layer material interface.

Some of the material samples in this thesis, to be tested as concealment materials for the base station antenna system, are single layer materials, and this analysis is expected to provide a brief background of the mathematical representation of this problem.

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2.3. Determination of Material Permittivity 11

The medium on the extreme left and extreme right (η_a and η_b) in figure 2.3 are assumed to be free space for our case, and the layer in the middle (η1), is assumed to be a dielectric material, for our case. Expressing this problem as a matching matrix, as described earlier in eqn. (2.15), we get the following relation,

E₁₊

E₁₋

= 1 τ₁

1 ρ₁ ρ₁ 1

E⁰₊ E⁰₋

= 1 τ₁

1 ρ₁ ρ₁ 1

e^jk¹^l¹ 0 0 e^−jk¹^l¹

E⁰₂₊

E⁰₂₋

= 1 τ₁

1 ρ₁ ρ1 1

e^jk¹^l¹ 1 1 e^−jk¹^l¹

1 τ₂

1 ρ₂ ρ2 1

E⁰₂₊

0

Assuming E₂₋⁰ = 0 , i.e. there is no wave returning to the material from free space at the interface between the material and free space (interface 2), multiplication of the matrices yields the following results,

E₁₊ = e^jk¹^l¹

τ₁τ₂ (1 + ρ₁ρ₂e^−2jk¹^l¹)E⁰₂₊

E₁₋ = e^jk¹^l¹

τ₁τ₂ (ρ₁+ ρ₂e^−2jk¹^l¹)E⁰₂₊ (2.19)

We can now derive the expressions for reflection and transmission coefficients as follows,

E₁₊ = e^jk¹^l¹

τ₁τ₂ (1 + ρ₁ρ₂e^−2jk¹^l¹)E⁰₂₊

E₁₋ = e^jk¹^l¹

τ₁τ₂ (ρ₁+ ρ₂e^−2jk¹^l¹)E⁰₂₊ (2.20)

The final expressions for reflection and transmission coefficients as follows, Γ = E₁₋

E₁₊ = ρ₁+ ρ₂e^−2jk¹^l¹ 1 + ρ₁ρ₂e^−2jk¹^l¹ T = E⁰₁₊

E₁₊ = τ₁τ₂e^−2jk¹^l¹ 1 + ρ₁ρ₂e^−2jk¹^l¹

(2.21)

The reflection and transmission coefficients are vital for the estimation of the complex permttivity of the concealment materials.

2.3 Determination of Material Permittivity

The permittivity of dielectric materials can be determined using an inverse ABCD transmission matrix method. This method follows from the relation

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Figure 2.4: Transmission Line

between ABCD parameters for a transmission line of length l, and Scattering Matrix parameters (S-Parameters), derived in [11].

For a given transmission line of length l, as shown in figure 2.4, the ABCD transmission matrix is defined as [11],

A B

C D

=

cos(βl) jZ₀sin(βl) j_Z¹

0sin(βl) cos(βl)

(2.22)

Where, Z0 is the characteristic impedance of the transmission line. Z0 is the same as η , mentioned in the previous section and the figures. β is the phase shift constant for a wave propagation through the transmission line. For the scope of this thesis work, we assume the transmission line to be a representation of a dielectric material layer of thickness l, in free space, for a free-space permittivity estimation setup. This method will be further described in chapter 4. For a multi-layered material, having say, n layers, we get a cascaded transmission matrix representation, containing a cascade of transmission lines of length l, corresponding to n material layers,

A B

C D

=

n

Y

j=1

A_n B_n C_n D_n

(2.23)

The phase shift constant β for a material is defined as, β = ω

√ε

c (2.24)

Where, as specified in the previous chapter, is the absolute permittivity of the material medium and c is the velocity of propagation of the mm-wave in the material medium. ω, is the angular frequency given by ω = 2πf . Z0 is the free space impedance is given as,

Z₀ = jωµ

β (2.25)

The two-port scattering parameters for the material medium can then be found from the ABCD parameter matrix using the following relation, described in

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2.4. Radomes and Concealment Materials 13

[11],

S₁₁= A + (B/Z₀) − CZ₀− D

A + (B/Z0) + CZ0+ D , S₁₂= 2(AD − BC) A + (B/Z0) + CZ0+ D

S₂₁= 2

A + (B/Z₀) + CZ₀+ D , S₂₂= −A + (B/Z0) − CZ0+ D A + (B/Z₀) + CZ₀+ D

(2.26)

Where the two-port scattering parameter matrix is represented as, S =S₁₁ S₁₂

S₂₁ S₂₂

(2.27) This scattering parameter matrix can be directly obtained from measurements, and the complex permittivity of the material medium, or, in our case, the concealment material sample under question, can be estimated. The exact methodology of measurement of the scattering matrix parameters and extrac- tion of complex permittivity for the samples is described further in detail in the next chapter.

2.4 Radomes and Concealment Materials

2.4.1 Radome

This section introduces the concept of "Radome". The term Radome, a port- manteau of the terms radar and dome, is a structure used to protect an antenna and its associated units from its physical environmental conditions [12]. The concealment material samples utilized in this thesis work, are intended to function like a radome from an RF(Radio Frequency) perspective. They must have a minimal impact on the RF performance of the base station antenna systems they conceal. A radome performs the same functionality from an RF perspective, although it differs from concealments in its physical functionality. The radome is intended to protect the antennas by shielding it from its physical environment, while the concealment is intended to hide/conceal base station antenna systems. Thus the RF parameters used to analyze radomes are valid for analyzing concealments as well.

The following section follows the definitions and discussions from [12], unless otherwise mentioned. It can be constructed in various shapes and sizes, depending on the physical shape, size and performance of the antenna it protects.

A radome is ideally expected to be completely transparent to the radio frequencies of interest and is expected not to degrade the electrical performance of the enclosed antenna in any way. A radome is expected to fulfill various structural (aeromechanical), electrical and environmental requirements, that in turn, constraint the selection of radome materials. Some of the most important properties analyzed for selecting candidate radome materials are [12]:

• Effect on the RF performance of the enclosed antenna;

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• Hardness and mechanical strength;

• Material density;

• Water absorption;.

• Rain erosion (particle impact) resistance;

• Effect of temperature variation on the electrical and mechanical parameters of the radome.

Radomes can be broadly classified into two categories based on the type of wall construction they use:

• Solid (monolithic) Radomes are ones that are made of foams or resins, sometimes consisting of reinforcements in the form of chopped glass fibers;

• Sandwich Radomes, consist of more than one layer of material, with alternating layers of high-density (high relative dielectric constant) and low- density (low relative dielectric constant) materials.

Depending on the ruggedness, mechanical strength and the other properties mentioned above, one of these two categories of radome wall constructions are utilized.

2.4.2 Monolithic Radomes

Many radomes that are required to be electrically-thin use a single-layer wall material and are termed monolithic wall radomes. Monolithic radomes are often reinforced with some form of fibers in order to enhance their mechanical properties. A simple formula for relative dielectric constant for reinforced radome materials is given as [12],

ε_m = V_Rlogε_R+ V_Flogε_F

V_R+ V_F (2.28)

Where,

ε_m = relative dielectric constant of mixture;

ε_R = relative dielectric constant of the resin;

εF = relative dielectric constant of reinforcement fibers;

V_R = volume of the dielectric resin;

V_F = volume of reinforcement fibers

Such monolithic radomes are quite common in millimeter-wave bands, both with and without reinforcements. This is because the millimeter-waves tend to attenuate rapidly in both material media and free space relative to lower frequencies [13].

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2.4.3 Classification of Radomes

Radomes can be classified based on the type of radome wall construction broadly as Monolithic and Sandwich radomes. While monolithic radome structures have a single material layer, Sandwich radomes usually more than one layers of two or more different materials. Sandwich radomes having low-density core materials and higher-density skin materials are common throughout the entire microwave spectrum because they function well over a broader range of frequencies than monolithic radomes and also have a higher strength-to-weight ratio. There are various different types of standard radome types that can be classified based on the type of wall construction [12]:

1. Style “a” Radomes: These are monolithic (single layer) structures, often having thickness equivalent to half their material wavelength. The wall thickness, in order to achieve lowest transmission loss, depends upon the angle of incidence, permittivity (or dielectric constant) of the wall material, and wavelength of the electromagnetic radiation.

t = mλ

2√

ε_r− sin²θ (2.29)

Here, λ is the wavelength of radiation and m is the wall order (an integer

>= 1). A style a radome is depicted in the figure 2.5.

2. Style “b” Radomes: These are monolithic radome structures having thin walls of the order of 0.1λ or lower in thickness, corresponding to the highest frequency at which the radome is to be used.

3. Style “c” Radomes: These are multi-layer radome structures consisting of three layers. Usually it is composed of two high-density skins outside and a low-density core material sandwiched in between the skin layers.

The permittivity of the skin layers is usually higher than that of the core layer.

4. Style “d” Radomes: These are multi-layer radome structures consisting of five or more layers of dielectric materials. There are an even number of low-density core layers sandwiched in between an odd number of high- density skin layers. The skin layers have a larger permittivity than the core layers. An increase in the number of layers is found to improve the broadband performance of the radomes, but as a tradeoff, the radome becomes more bulky and heavy.

5. Style “e” Radomes: Any other radome structure not covered in styles a to d can be categorized as style "e" radomes. This also includes a reversal of style "c" radomes i.e a three-layer structure having two low-density skin layers with a high-density core layer in between them.

Some of the most common types of radome wall constructions are shown in figure 2.5

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Figure 2.5: Different Types of Radome Wall Constructions [12].

2.4.4 Radome-Antenna Interaction

The interaction of radome with the enclosed antenna and the performance of the radome are usually evaluated using the following indicative parameters [12]:

1. Boresight Error (BSE) and Boresight Error Slope (BSES) Boresight is the angle or direction, relative to the radome enclosed transmitting antenna, where the signal from the antenna is incident perpen- dicular to the inner surface of the radome. Boresight Error (BSE) an indicates how much the angle of arrival of a received signal changes from the line of sight between the transmitting and receiving antenna. This error is introduced by the transmission of the signal through a radome.

Boresight Error Slope (BSES) is the rate at which the boresight error changes with the changing scan angle of the antenna inside the radome.

2. Registration Error

Registration error is difference in the boresight errors of the uplink and downlink frequencies of radome-enclosed antennas in satellite communication applications. The smaller the registration error, the better the alignment between the peaks of the transmitted and received beams.

3. Antenna Sidelobe Degradation

When an antenna is placed inside a radome, its sidelobe levels increase due to the distortion caused by the radome wall as the electromagnetic wave propagates through it. For well-designed, modern antennas, radomes alone can cause the sidelobe levels to be raised.

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4. Depolarization

The folding or transfer of energy from one antenna polarization to the other, due to the introduction of the radome in front of an antenna is known as radome Depolarization. An example can be a case when some energy from a right-hand circularly polarized (RHCP) signal is trans- ferred to the left-hand circularly polarized (LHCP) signal, on propagating through a radome wall. If this happens a RHCP receiving antenna will not receive all its intended energy. The radome wall curvature and a difference in complex transmission coefficient between orthogonal polarized vectors could cause depolarization. Polarization is important in modern telecommunication applications as it allows transmission of two signals on the same channel simultaneously, thus allowing us to maximize the band- width available for communication. Hence Depolarization is an important challenge to address.

5. Return Loss due to Presence of Radome

Antenna return loss for the radome-enclosed antenna can rise steeply due to reflections of the transmitted RF signal from the inner walls of radomes.

The rise in the return loss represents added gain/loss that can be critical in modern day 5G telecommunication systems and also in RADAR applications.

6. Insertion Loss due to Presence of Radome

Insertion loss is a reduction in signal strength as the electromagnetic wave propagates through the radome. Part of the power is lost due to reflections at the air/dielectric interface, as specified earlier. The layers of the radome also dissipate some part of the energy of the propagating electromagnetic wave. Loss tangent or tan δ of the material is a measure of these dissipative losses, caused by the radome.

2.4.5 Concealment Materials

The background provided on radomes in the previous sections is intended to provide a foundation for understanding this Master’s thesis work, which is con- cerned with Concealment Materials. Concealment Materials, in this case is the terminology used to refer to materials that are intended to be utilized to hide or conceal important antenna systems from being identified, avoiding any potential damage to these systems. Concealment materials, from an electromagnetic point of view can be visualized as a class of Radomes, that are intended to fulfil slightly different functionalities from that a conventional radome.

While the conventional radomes are utilized mainly to protect antennas from the environment conditions, Concealment materials are intended to hide the Antenna/Antenna systems in plain sight, so they cannot be easily identified visually. “Stealth Materials”, “Camouflaging Materials”, “RF transparent materials”, “low observability materials”, are other names commonly used to refer to

“Concealment Materials”.

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2.4.6 Background

As discussed in the introduction chapters, a global demand for faster mobile and telecommunication services has seen a vast growth in the mobile telecommunications networks in the past decade. The advent of high speed 5G cellular networks which are expected to be introduced in many global markets in the forthcoming years, has brought about a steep rise in the number of antenna and small cell base station deployments. The major reason for this has been the increasing need to maintain line-of-sight communication, due to increasing frequencies, which is inevitable, for larger bandwidths demanded by the 5G telecommunication standard [13]. This pervasive increase in the number of antennas deployed to deliver faster communication standards has become a contentious issue in large global mobile communication markets. For instance, many local communities in the United States of America have cited aesthetic, health, energy consumption and safety concerns to oppose base station antenna deployments, creating challenges, including base station relocation costs, for telecommunication service providers [14]. This issue is only expected to inten- sify with the deployment and rapid growth of 5G communication networks in the upcoming years.

Challenges from competitors and demands for high speed networks have forced telecommunications service providers to try and find low-cost and time-efficient solutions to this problem. Concealment of antennas and base stations has been identified as a promising, low-cost and time-efficient solution to address this challenge. Various manufacturers have come up with concealment materials to hide antennas and base stations in recent years, due to this reason. Figure 2.6 illustrates some commercially available antenna concealment/camouflaging solutions.

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Figure 2.6: Some Examples of Existing Concealment Solutions [15], [16].

These solutions, despite being quick and cheap, have not been backed by exhaus- tive scientific testing and evidence for their effect on the base station antenna beams. The testing and investigation of their effect on the RF performance of 5G Antenna base stations especially becomes critical, in order to understand if they degrade the quality of beams radiated and received from such base stations. Some important characteristics of concealments need to be analyzed in order to understand their effect on the base station antenna beams:

1. The materials used for concealing the base stations antennas, their RF properties and their material composition.

2. The shape of concealment, i.e. if they are shaped as solid shrouds around the antenna/base station figure 2.7(a) or if they are adhesive films that are uniformly applied on/wrapped around the base stations figure 2.7(b).

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Figure 2.7: (a) Shroud Concealments [17] (b) Film Conceal- ments [18]

3. The effect of the concealment materials on the base station antenna beams at the 5G mm-Wave carrier frequencies for antenna beams steered at different azimuth angles (boresight and oblique angles).

These characteristics will be studied for some samples provided by a commer- cial material manufacturer, along with their effect on a mm-Wave Base station antenna beams to determine if they are suitable for concealing these base stations with minimal effect on their RF performance. The material samples will be further discussed in the following chapter.

In summary, the concealment materials are utilized in order to hide antennabase stations from visibility and are manufactured usually as shrouds or as films, depending on the location of the base station. The Ericsson’s mm-Wave AAS Base Station is used for studying their effect on the base station beams steered both along boresight and along oblique angles.

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2.4.7 RF Material Characterization Methods

Response of materials to electromagnetic waves at millimeter wave frequencies is critical in order to evaluate their suitability as potential concealment materials for the base station antennas at these frequencies. In 5G telecommunication applications, the millimeter waves are expected to carry broadband information over long distances. Hence it is important to understand the RF properties of the materials concealing these 5G base station antennas to conclude if they degrade these millimeter waves beams. This becomes particularly important within the scope of this thesis work, since the material samples to be studied are custom manufactured using various dielectric material combinations and their RF properties are not known. The goal of dielectric measurements is to find the relative permittivity of the material under test for a specified orientation of electric field and frequency, and to analyze the insertion and reflection losses exhibited by these material samples in the frequency range.

When an electromagnetic field is incident on a material, the atoms, molecules, free charge, and defects in the material readjust their positions. This dynamic process of readjustment of the molecules in response to the field is termed relaxation. The response of the material depends strongly on material phase and lattice structure. In many solids, the molecules are not able to appreciably rotate in response to the applied fields, and therefore little relaxation response is observed, and the resulting permittivity can be low. The degree of crys- tallinity, existence of permanent dipoles, mobility of free charge, and defects all contribute to dielectric response [19]. There are various possible measurement techniques that have been studied for the characterization of dielectric materials over the past decades. Each of these methods has advantages and disadvantages depending on the the material sample shapes, sizes and the frequency of study [20]. The most common methods are discussed briefly.

(a) Resonator Methods

Resonator Methods consist of some form of resonant cavities. The resonant cavities are high Q-factor (Quality factor) structures that resonate at specific frequencies. A piece of sample material inserted into the cavity affects the resonant frequency (f) and Q-factor of the cavity. From these parameters, the complex permittivity of the material can be calculated at a single frequency.

Since the natural intrinsic resonances of dielectric materials do not occur at RF and microwave frequencies, the resonances measured here are geometric resonances due to the material and the cavity. A typical measurement system consists of a network analyzer, a resonant cavity fixture and software to make the calculations.

Typical Resonator Methods include, Cavity Resonators, Open-Resonators, Split- Cylinder Resonators, Microstrip Resonators, and Whispering Gallery Resonators.

Each has its own advantages depending on the sizes of the material under test and the frequency of operation. Some resonator setups are depicted in figure 2.9

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Figure 2.8: An Open Resonator(Left) [20] and A Split- Cylinder Resonator(Right) [21] Setups

Resonator Methods are the most accurate methods for permittivity and loss measurements owing to the sharp resonance peaks and their extremely high Q-factor values. Their major disadvantages are that the materials measured in these resonators must be extremely flat, and the permittivity measurements performed with these resonators are extremely narrow band measurements.

(b) Free Space Method

Free space methods use a pair of horn antennas to focus microwave energy at the material under test. This method is non-contacting and allows materials to be tested under high temperatures and hostile environments. This tech- nique measures reflection and transmission coefficients and from the measured data, dielectric constant, losses and complex permeability can be estimated as a function of frequency [21], [22], [23].

Figure 2.9: Free Space Measurement Setups [18]

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(c) Waveguide/Transmission Line Methods

Waveguide/Transmission line methods involve placing the material inside a por- tion of an enclosed transmission line. The line is usually a section of rectangular waveguide or coaxial airline. In case of a waveguide, a metallic waveguide is filled with the dielectric material under test. The transmission/reflection coefficient is measured using a VNA in order to derive the complex permittivity and loss tangent of the dielectric material.

Figure 2.10: Waveguide and Coaxial Transmission Line Mea- surement Setups [21]

Coaxial lines support EM wave transmission over a broad span of frequencies and can hence be utilized for broadband permittivity measurements for samples that can be machined to fit inside the coaxial line. Waveguides supporting different frequency bands can extend the possibility of permittivity estimation to mm-wave frequencies and thin samples are simpler to machine to fit in waveguides at sub-mm wave frequencies compared to coaxial lines. Thick dielectric samples can be difficult to machine to fit inside the waveguides and coaxial lines. Dispersive losses in the metal waveguide walls can affect also affect the permittivity measurements using a metallic waveguide.

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25

Chapter 3

Concealment Material Description

This chapter is intended to introduce the reader to the dielectric concealment material samples available and the work flow methodology adopted in this thesis work. As discussed earlier, these samples, provided by a concealment material manufacturer, are intended to hide antenna base stations. The motivation, as mentioned in the previous chapter, is to address and solve the challenges posed by communities in various countries to telecommunication service providers, citing aesthetic and health concerns against installing antenna base stations.

3.1 Concealment Material Sample Description

This section provides a brief overview of the available samples to be measured, with the intention of determining their complex permittivity, insertion and reflection losses and studying their effect on the mm-wave Base Station. All of the samples used in this thesis work are shroud concealments, which was earlier illustrated in section 2.4.4. There are in total 11 available samples:

(a) Sample 1

Sample 1, shown in figure 3.1, is a dielectric plastic material with an average thickness of approximately 1.4 mm. Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and breadth. This sample, as can be seen consists of an inconsistent back surface with ridges and many edges. These ridges and edges makes it quite challenging to measure the permittivity and loss tangent of this sample through the conventional measurement methods. Nonetheless, the insertion and return losses should give a reasonable picture of its behaviour as a concealment.

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Figure 3.1: Concealment Sample 1

(b) Sample 2

Sample 2, shown in figure 3.1, is a dielectric material with an average thickness of roughly 13.1 mm. Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and breadth. This sample, depicted in figure 3.2, consists of a foam-based core material with the exterior layer resembling a specific type of building façade with an uneven, corrugated surface. It can be categorized as a stlye "e" radome wall construction. Such a concealment could hide base stations placed on exterior building facades from view effectively by camouflaging/blending with the building facade.

(b) Sample 3

Sample 3 is a dielectric material with an average thickness of roughly 14.5 mm.

Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and

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3.1. Concealment Material Sample Description 27

breadth. This sample resembles a style "c" sandwich radome wall construction, with a foam-based core layer and corrugated skin material on both sides, as depicted in figure 3.3. The composition of the skin is not specified. It is also a solid shroud concealment like the two previous samples.

(b) Sample 4

Sample 4 is a dielectric material with an average thickness of approximately 14 mm. Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and breadth. This is also a style "c" sandwich radome wall construction.

This sample consists of a foam-type core layer and corrugated skin material on one side, as depicted in figure 3.4. The composition of the skin material, once again, is not specified. It is also a solid shroud concealment like the previous samples.

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(b) Sample 5

Sample 5 is a foam-type dielectric material with an average thickness of approximately 12.5 mm. Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and breadth. This is a monolithic radome wall construction, with just a core layer and no skin layers, as depicted in figure 3.5. It is also a solid shroud concealment like the previous samples.

(b) Sample 6

Sample 6 is also a foam-type dielectric material with an average thickness of roughly 19.4 mm. Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and breadth. This is also a monolithic radome wall construction, with just a core layer and no skin layers, as depicted in figure 3.6. It is also a solid shroud concealment like the previous samples.

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(b) Sample 7

Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and breadth. This sample resembles a style c sandwich radome wall construction, with a core layer and corrugated skin material on both sides, as depicted in figure 3.7. There is a noticeable irregularity (cut) at the centre of the sample parallel to its breadth. It is also a solid shroud concealment like the previous samples.

(b) Sample 8

Sample 8 is a dielectric material with an average thickness of approximately 6.5 mm. Its dimensions were measured to be roughly 30 cm x 30 cm, along its length and breadth. This sample consists of two layers, a core layer and corrugated skin material on only one side, as depicted in figure 3.8. It seemed to be constructed by adhesion of two sliced long pieces of the same sample. This resulted in a noticeable irregularity (cut) at the centre of the sample parallel to its length. It is also a solid shroud concealment like the previous samples. The exact material composition of the sample is not known.

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(b) Sample 9

Sample 9, shown in figure 3.1, is a dielectric plastic material with an average thickness of approximately 2.8 mm. Its dimensions were measured to be roughly 30 cm x 25 cm, along its length and breadth. This sample, as can be seen, has a curved profile and consists of an inconsistent back surface with ridges and many edges. These and the curved profile makes it quite challenging to measure the permittivity and loss tangent of this material sample through the conventional measurement methods. The insertion and return losses, however, should give a reasonable picture of its behaviour as a concealment material.

Figure 3.9: Concealment Sample 9 (b) Sample 10

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Sample 10 is a dielectric material with an average thickness of approximately 12.4 mm. Its dimensions were measured to be roughly 30 cm x 25 cm, along its length and breadth. This sample a foam-based core layer and a thin plastic film skin material on one side, as depicted in figure 3.10. It can hence be classified as a style "e" radome wall construction. It was one of the most uniform and flat of the available samples, making it more convenient for measurements. It is also a solid shroud concealment like the previous samples.

(b) Sample 11

Its dimensions were measured to be roughly 30 cm x 25 cm, along its length and breadth. This is a mesh sample, with lots of small holes in between, as depicted in figure 3.11. It can either be placed on a solid support structure to hold it in place.

Concealment Materials and Techniques for mm-Wave Advanced Antenna Systems (AAS)

Concealment Materials and Techniques for mm-Wave

Advanced Antenna Systems (AAS)

ADHITYA BHARADWAJ SRIRAM

Concealment Materials and Techniques for mm-Wave Advanced Antenna Systems (AAS)

ADHITYA BHARADWAJ SRIRAM

Examiner:

Supervisors:

Abstract

Sammanfattning

Acknowledgements

Contents

List of Figures

Chapter 1

Introduction

1.1 Background

1.2 Purpose

1.3 Outline

1.4 Goal

1.5 Work Flow Methodology

1.6 Sustainability

Chapter 2

Background Theory

2.1 Maxwell’s Equations and Dielectric Materi- als

2.2 Electromagnetic Waves in Material Bound- aries

2.2.1 Transmission and Reflection

2.2.2 Single Layer Materials

2.3 Determination of Material Permittivity

2.4 Radomes and Concealment Materials

2.4.1 Radome

2.4.2 Monolithic Radomes

2.4.3 Classification of Radomes

2.4.4 Radome-Antenna Interaction

2.4.5 Concealment Materials

2.4.6 Background

2.4.7 RF Material Characterization Methods

Chapter 3

Concealment Material Description

3.1 Concealment Material Sample Description