STUDY OF THIN RESISTIVELY LOADED FSS

BASED MICROWAVE ABSORBERS

by

SITI NORMI ZABRI

B. Eng (Hons), MSc. (Eng)

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

DOCTOR OF PHILOSOPHY

In the Faculty of Engineering of

QUEEN‟S UNIVERSITY BELFAST

School of Electronics, Electrical Engineering and Computer Science

May 2015

i

Abstract

The purpose of this study was to develop new FSS based microwave absorber

designs to minimise the physical thickness, increase the bandwidth and provide radar

backscatter suppression that is independent of the wave polarization at large incident

angles. A new low cost, accurate and rapid printing technique is employed to pattern

the periodic arrays with the precise surface resistance required for each of the FSS

elements to optimize the performance of this class of absorber.

The electromagnetic behaviour of five new FSS based structures, two stand-alone

arrays, and three absorber arrangements, have been studied using CST Microwave

Studio software. The FSS structures consist of two closely spaced arrays of rings

with the conductor split at one or two locations to provide independent control of the

resonances. By careful design these are shown to exhibit coincident spectral

transmission responses in the TE and TM plane. Based on this design methodology, a

very thin 4-layer metal backed resistively loaded rectangular loop FSS absorber

which works from 0° - 22.5° is shown to give a wide band performance that is

independent of the orientation of the impinging signals. To reduce the manufacturing

complexity, a single layer FSS absorber which operates at 45° incidence has been

designed to give a polarisation independent performance by employing an array of

rectangular split loops with discrete pairs of resistive elements of unequal value

inserted at the midpoint of the four sides. A major increase in bandwidth is obtained

from a single layer FSS absorber which is composed of an array of nested hexagonal

loops. Moreover the use of the same surface resistance for all four elements in the

unit cell is shown to significantly simplify the construction of the structure which

was designed to provide radar cloaking from 0° to 45° incidence.

ii

A new manufacturing strategy is presented, where the required surface resistances

are obtained by employing an ink-jet printer to simultaneously pattern the FSS

elements on the substrate and digitally control the dot density of the nano silver ink

and aqueous vehicle mixture. Bi static measurements of the radar backscatter are

shown to be in good agreement with the numerical simulations for all three FSS

microwave absorber designs.

iii

Acknowledgements

First of all, I would like to thank my supervisor, Dr Robert Cahill for his guidance

and encouragement throughout the project. I owe him gratitude for his useful

suggestions and wide knowledge which helped me a lot. Without him, this project

would not have been completed within the time frame.

Special thanks to Ministry of Education Malaysia and Universiti Teknikal Malaysia

Melaka for providing financial support. Thanks also to Dr Alexander Schuchinsky

and Dr Gareth Conway for useful ideas and assistance on the project. Not forgotten,

Dr Robin Todd, Sarah Mohamad, Dr Efstratios Doumanis, Dr Robert Orr, Dr

Oleksandr Malyuskin, Dr Nurfarina Zainal, Dr Dmitry Zelenchuk, Dr Raymond

Dickie, Dr Steven Christie and Norfadzilah Ahmad for helping on software and

measurements and also Gerry Rafferty and Michael Major who put a lot of effort in

making the fabrication of absorber possible.

Lastly, I wish to thank my parents, Zabri Suhaimi Mat Khatib and Samsiah Mustaffa

for the constant encouragement and moral support provided and all member of staff

and PhD students of High Frequency Electronics Cluster for all the contributions

given.

iv

List of Publications

JOURNAL PUBLICATIONS

1. S. N. Zabri, R. Cahill, and A. Schuchinsky, “Polarisation independent split

ring frequency selective surface,” Electronics Letters, vol. 49, no. 4, pp. 245–

246, 2013.

2. N. Zabri, R. Cahill, and A. Schuchinsky, “Polarisation independent resistively

loaded frequency selective surface absorber with optimum oblique incidence

performance,” IET Microwaves, Antennas and Propagation, vol. 8, no. 14,

pp. 1198–1203, 2014.

3. S. N. Zabri, R. Cahill, and A. Schuchinsky, “Compact FSS absorber design

using resistively loaded quadruple hexagonal loops for bandwidth

enhancement,” Electronics Letters, vol. 51, no. 2, pp. 162–164, 2015.

4. S. N. Zabri, R. Cahill, and A. Schuchinsky, “Simpler, low-cost stealth,”

Electronics Letters, vol. 51, no. 2, p. 127, 2015.

5. S. N. Zabri, R. Cahill, G. Conway and A. Schuchinsky, “Inkjet printing of

resistively loaded FSS for microwave absorbers” Electronics Letters, to be

published.

CONFERENCE PUBLICATIONS

1. S. N. Zabri, R. Cahill, and A. Schuchinsky, “Ultra thin resistively loaded FSS

absorber for polarisation independent operation at large incident angles,” in

The 8th European Conference on Antennas and Propagation (EuCAP 2014),

The Hague, pp. 1363–1367, 2014.

v

List of Symbols and Abbreviations

Free Space Impedance

Surface Impedance

Free space permittivity

Relative permittivity

Free space permeability

Relative permeability

1D One Dimensional

2D Two Dimensional

3D Three Dimensional

CA Circuit Analog

CST Computer Simulation Tool

E Electric field

EBG Electromagnetic Band Gap

EM Electromagnetic

EOBR Edge of Band Ratio

FOM Figure of Merit

FSS Frequency Selective Surface

H Magnetic field

vi

HIS High Impedance Surface

LC Inductance (L), Capacitance (C)

PCB Printed Circuit Board

PNA Performance Network Analyzer

RAM Radar Absorbing Material

RCS Radar Cross Section

TE Transverse Electric

TM Transverse Magnetic

Reflection Coefficient

Transverse wavenumber

Propagation constant

Angular frequency

vii

Table of Contents

Abstract ........................................................................................................... i

Acknowledgements ........................................................................................ iii

List of Publications ........................................................................................ iv

List of Symbols and Abbreviations ............................................................... v

1 Introduction

1.1 Introduction to Microwave Absorbers ..................................................................... 1

1.2 Microwave Absorbers Types ................................................................................... 2

1.3 Application of Absorbers ......................................................................................... 3

1.4 Principle of Operation of an Absorber ..................................................................... 7

1.5 Past Research on FSS Based Absorbers ................................................................. 12

1.6 Objectives of the Research Project ........................................................................ 18

1.7 Structure of the Thesis ........................................................................................... 22

References ......................................................................................................................... 26

2 Polarisation Independent Frequency Selective Surfaces

Introduction ........................................................................................................... 33 2.1

Frequency Selective Surfaces ................................................................................ 34 2.2

2.2.1 Types of FSS Response .................................................................................... 34

2.2.2 FSS Element Types ........................................................................................... 36

2.2.3 Other FSS Design Considerations .................................................................... 38

2.2.4 Polarization Independent FSS Design .............................................................. 39

CST Microwave Studio ......................................................................................... 41 2.3

Single Layer Continuous Ring FSS ....................................................................... 43 2.4

Current Flow in A Continuous Ring FSS ............................................................... 49 2.5

viii

Modified Continuous Ring FSS Design ................................................................. 49 2.6

Single Split Ring FSS ............................................................................................ 51 2.7

2.7.1 Nested Single Split Ring FSS ........................................................................... 53

2.7.2 Double Layer Single Split Ring FSS ................................................................ 54

Double Layer Double Split Ring FSS Design ........................................................ 58 2.8

2.8.1 Single Layer Double Split Ring FSS ................................................................ 59

2.8.2 Double Layer Double Split Ring FSS .............................................................. 60

Experimental Validation ........................................................................................ 62 2.9

Conclusions ........................................................................................................... 67 2.10

References ......................................................................................................................... 68

3 Multi-Layer FSS Based Microwave Absorber

Introduction ........................................................................................................... 73 3.1

FSS Absorber Design Considerations .................................................................... 76 3.2

3.2.1 Optimum Value of Surface Resistance ............................................................. 79

3.2.2 Thickness to Bandwidth Ratio .......................................................................... 84

3.2.3 Figure of Merit (FOM) ...................................................................................... 86

3.2.4 Modified Square Loop Design for an Absorber Working At 45° Incidence ... 86

Split Square Loop FSS Absorber Design ............................................................... 88 3.3

Double Split Square Loop FSS Absorber for 45° Incidence Operation .................. 92 3.4

Square Loop FSS Absorber Design for 0° and 45° Operation ............................... 93 3.5

Conclusions ......................................................................................................... 100 3.6

References ....................................................................................................................... 103

4 Polarization Independent Resistively Loaded Single Layer FSS

Absorber with Optimum Oblique Incidence Performance

Introduction ......................................................................................................... 107 4.1

Angular Sensitivity of FSS Absorbers ................................................................. 108 4.2

ix

Numerical Optimization and Design .................................................................... 111 4.3

Sensitivity Analysis ............................................................................................. 118 4.4

Construction and Experimental Results ............................................................... 121 4.5

4.5.1 Thickness – Surface Resistance Relationship ................................................ 121

4.5.2 Measured Performance of Rectangular Loop FSS Absorber ......................... 128

Conclusions ......................................................................................................... 136 4.6

References ....................................................................................................................... 136

5 Compact Absorber Design Using Resistively Loaded Multi-Resonant

FSS for Bandwidth Enhancement

Introduction ......................................................................................................... 140 5.1

FSS Resistive Loading Methods .......................................................................... 142 5.2

Single Layer FSS Dipole Absorber Design .......................................................... 144 5.3

Two Layer FSS Dipole Absorber Design............................................................. 146 5.4

Single Layer Hexagonal Loop FSS Absorber Design .......................................... 155 5.5

5.5.1 Absorber Design and Simulated Performance ................................................ 156

5.5.2 Fabrication and Measured Results .................................................................. 161

Conclusions ......................................................................................................... 165 5.6

References ....................................................................................................................... 166

6 Inkjet Printing of Resistively Loaded FSS for Microwave Absorbers

Introduction ......................................................................................................... 169 6.1

Inkjet Printing for Printed Electronics ................................................................. 171 6.2

Parameter Settings ............................................................................................... 173 6.3

6.3.1 Colour Model .................................................................................................. 173

6.3.2 CMYK to RGB Conversion ............................................................................ 174

6.3.3 RGB and Dot Density ..................................................................................... 175

Preliminary Study ................................................................................................ 176 6.4

x

6.4.1 Mask Preparation and DipTrace Software Settings ........................................ 178

6.4.2 Printing Process and Settings .......................................................................... 179

6.4.3 Measured Performance of a Conductive Dipole............................................. 181

6.4.4 Nanosilver Ink and Aqueous Vehicle Mixture ............................................... 183

Experimental Validation ...................................................................................... 185 6.5

6.5.1 RGB and Surface Resistance Relationship ..................................................... 186

6.5.2 Construction and Measurement of Two Inkjet Printed FSS Absorbers ......... 195

Conclusions ......................................................................................................... 202 6.6

References ....................................................................................................................... 202

7 Conclusions and Future Work

Contribution of the Work Reported in This Thesis .............................................. 206 7.1

Future Work ......................................................................................................... 216 7.2

7.2.1 Resistively Loaded FSS Design for Optimum Absorber Performance .......... 217

7.2.2 Further Development of Nanosilver Ink ......................................................... 218

7.2.2.1 Curing Time and Surface Resistance Relationship .......................... 219

7.2.2.2 Multiple Layer Printing .................................................................... 220

7.2.2.3 Different Surface Resistances in a Unit Cell of a Nested FSS ......... 223

References ....................................................................................................................... 224

APPENDIX I: TACONIC Microwave Laminate Substrate

APPENDIX II: Y-Shield EMR Protection Conductive Paint

APPENDIX III: Novele™ IJ-220 Printed Electronics Substrate

APPENDIX IV: Metalon®

Conductive Inks and Aqueous Vehicle

APPENDIX V: Epson Stylus C88+ Inkjet Printer

1

Chapter 1

Introduction

1.1 Introduction to Microwave Absorbers

A microwave absorber is defined as a material or structure that attenuates the energy

in an electromagnetic wave. To be specific it can soak up the incident energy,

convert it into heat and therefore reduce the energy reflected back to the source [1].

The most common and well known electromagnetic absorber employed today is

pyramidal absorber which is used in an anechoic chamber to create a free space

environment for experimental purposes and also in microwave oven doors to prevent

the escape of radiation into the atmosphere. However, it was in 1952 that the first

microwave absorber was introduced, known as a Salisbury screen [2], it was

developed as a consequence of the use of radars during the World War 2 [3], [4].

Radar is a sensitive detection tool and since its growth, researchers have studied

various methods for reducing microwave reflections. The term radar cross section

(RCS) is a property of the target size, shape and the material from which it is

fabricated and is defined in terms of the ratio of the incident and reflected power [5].

The radar cross section has implications to survivability and mission capability. For

example, in the case of stealth aircraft, it is preferable to have a low RCS so that the

aircraft is less visible. Radar absorbers are one of the most effective methods used in

Chapter 1: Introduction 2

RCS reduction. The materials for reducing radar cross section rely on magnetic and

electric materials, while principles from physical optics are used to design absorber

structures. Since the introduction of the Salisbury screen, the design of

electromagnetic absorbers has been studied intensively to improve performance and

utility, and a large number of different approaches and designs have been reported in

the open literature.

1.2 Microwave Absorbers Types

Many different types of microwave absorbers have been developed to date. Vinot et.

al [3] classify the absorbers into two categories which are narrowband absorbers

and broadband absorbers. Salisbury screens [2], magnetic absorbers [6], Dallenbach

and Circuit Analog (CA) structures are examples of the narrowband type. Some

examples of broadband absorbers are the Jaumann [7], geometric transition [8], [9],

bulk and Chiral absorbers [10]. Examples of four different types of microwave

absorbers that have recently been reported in the literature are illustrated in Figure

1.1.

(a) (b) (c) (d)

Figure 1.1 Examples of different types of absorber structures, (a) Chiral metamaterial

absorber [11], (b) CA absorber [12], (c) geometric transition absorber [9], (d)

Jaumann absorber [13]

The Salisbury Screen is the earliest and simplest type of absorber which consists of a

continuous resistive sheet separated a quarter wavelength apart from a metal ground

Chapter 1: Introduction 3

plane. The Circuit Analog (CA) absorber, is similar in construction to the Salisbury

screen, but the continuous resistive sheet is replaced with a patterned resistive sheet

made of a lossy Frequency Selective Surface (FSS) separated a predetermined

distance from the ground plane. An FSS is a periodic surface which is the association

of identical elements placed in a one- or two-dimensional infinite array [14]. The

FSS screen has a periodic design described by a unit cell; it may contain either

metallic patches or apertures. In most FSS applications, the geometry of the FSS is

selected to obtain the type of response needed. By changing the geometry of the

structure, the electromagnetic properties of the FSS screen can be changed. In

addition, due to the frequency selective electromagnetic scattering properties of the

FSS, unlike Salisbury screen absorbers, the distance between the ground plane and

the FSS sheet is not necessarily required to be λ/4 and because of this the structure

can be designed to be thinner. Although CA absorbers are often classified as

narrowband absorbers, the exploitation of the FSS pattern in recent studies show that

a broadband performance can also be achieved [15], [16]. This type of structure is

studied in detail in this thesis.

1.3 Application of Absorbers

Traditionally the main application of microwave absorbers is in radar technology

because the exploitation of radar absorbing materials started shortly after the

introduction of radar. The term radar comes from the words radio detection and

ranging. As implied by the name, radar is capable of detecting the presence of a

target and also able to determine the range [4]. The ability of the radar in detecting

and tracking the target is due to echo signals, hence, it is important that the design

and operation of the radar should be capable of receiving these. This leads to the

Chapter 1: Introduction 4

term Radar Cross Section (RCS), which describes the target as an effective area or

the energy that reflects back towards the source. RCS is also known as

backscattering and the latter term is used throughout this thesis.

Radar backscatter (or RCS) can be explained by referring to the two scenarios

illustrated in Figure 1.2. For the conventional (or passenger) aircraft depicted in

Figure 1.2(a), the backscatter from the airframe is required to be as large as possible

so that it can be continuously tracked by the radar antenna. However, for the stealth

aircraft shown in Figure 1.2(b), it is required to reduce the backscatter to decrease the

visibility of the aircraft as „seen‟ by the radar antenna. There are four methods

available to reduce the radar backscatter from a target; shaping, passive cancellation,

active cancellation and use of absorbers [17]. Shaping is the primary method of

reducing the backscattered signal from stealthy aircraft and ships [17]. Passive

loading is a method in which the target is loaded at selected points with passive

impedances and is similar to the active loading method except that the latter uses

active elements [5]. The fourth method, absorbers, which is the topic of study in this

thesis, involves coating the target with a radar absorbing material (RAM). Although

shaping is very important and is the method used to electromagnetically cloak most

stealth aircraft, it redirects the radiation through specular reflection hence increasing

the probability of detection from bistatic radars [5]. Therefore, in addition to the

shaping method, microwave absorbing material can in principle be used to absorb the

remaining radar energy and ensure a low RCS in other sampling directions.

Chapter 1: Introduction 5

(a) (b)

Figure 1.2 Radar cross section (radar backscatter) of a (a) conventional aircraft, (b)

stealth aircraft [18]

Apart from aircraft, wind turbines also present a large RCS and the blade rotation can

cause further problems including disruption to the operation of air traffic control,

military and marine navigation in terms of inaccurate, misidentification and false

data [19]. Similar techniques that have been used for stealth aircraft can mitigate this

problem e.g. the blade shape can be modified to reduce the interaction with radar

signals. However due to aerodynamic constraints shaping alone may not be sufficient

[20] therefore an absorber is required to further supress the electrical noise caused by

the wind turbines and hence improve the radar performance [21]. The application of

microwave absorbers is also important in consumer electronics. For example as

illustrated in Figure 1.3, in electronics components, boards and circuits, it is used to

reduce the noise radiation from/ to adjacent components.

Chapter 1: Introduction 6

(a)

(b)

Figure 1.3 Various applications of microwave absorber sheets [22] (a) noise

reduction of electronic boards, (b) microwave interference reduction within a CS

converter

In recent literature [23], [24], work has been reported which shows that researchers

are now looking for more advanced features and improvements by placing CA

absorbers on radomes [23] to provide absorption and at the same time include a

transparent window for the operation of the antenna. The concept of an absorber with

a transparent window has been discussed in [25] for aircraft communication systems,

where a Jaumann absorber is combined with a low pass gangbuster-like FSS

combined with a polarizer. In [24] the authors introduced an absorptive frequency

selective radome to reduce the transmission losses, by using a resistive FSS to

Chapter 1: Introduction 7

replace the common resistive sheet. It was also reported recently [26], that a thin

metamaterial absorber was placed behind a reflectarray antenna to supress the back

lobe energy which is generated as a result of edge diffraction from the ground plane.

The concept which is illustrated in Figure 1.4 operates by absorbing the

backscattered electromagnetic waves and therefore this results in an increase in the

front to back ratio.

(a) (b)

Figure 1.4 Metamaterial absorber backed reflectarray antenna with improved front-

back ratio [26], (a) side view, (b) top view

1.4 Principle of Operation of an Absorber

The studies described in this thesis are mainly focused on developing absorbers in

which the design approach is based on the use of FSS. In this chapter the basic

principle of operation of the Salisbury screen is described because this serves to

illustrate how radar backscatter suppression is obtained for most classes of

microwave absorbers. Following this, the operating principle for FSS based

absorbers is briefly described but a more comprehensive explanation is presented in

Chapter 3.

Consider Figure 1.5(a) where a plane wave propagates in the forward z direction and

impinges at normal incidence on an absorber structure. The incoming wave consists

of the electric field, E, and magnetic field, H, which are mutually perpendicular and

Chapter 1: Introduction 8

are denoted by a blue and red line respectively. This field orientation determines the

type of polarization. When the induced E field is perpendicular to the incident plane,

it is known as Transverse Electric (TE) polarization (blue line) and when the induced

E field is parallel to the incident plane it is known as Transverse Magnetic (TM)

polarization (red line). When the incident wave propagates through free space and

impinges on the absorber surface, partial reflection occurs and this is characterized

by

(1.1)

where Zs is the surface impedance (which is different for TE and TM at oblique

incidence), is the free space impedance and is the reflection coefficient. The

absorber is designed to reduce the magnitude of the electromagnetic wave that

reflects back to the source. Based on Equation (1.1), zero reflection coefficient

( ) can be obtained based on two absorber theorems [27], that is: (1) matched

wave impedance and (2) matched characteristic impedance. The first theorem

requires that the surface impedance is equal to the free space impedance ( )

and the second theorem requires that the medium intrinsic impedance is equal to the

free space intrinsic impedance.

In general, the wave impedance Z is defined by the ratio of the E field phasor to the

H field phasor:

(1.2)

Equation (1.2) can be reduced to:

√

(1.3)

Chapter 1: Introduction 9

Ground plane

λ/4

Resistive sheet (Rs)

Z

λ/4 𝑍𝑠

In free space both and are equal to 1, therefore this expression gives the value

of the free space impedance Z (or normally for free space) which is 377 Ω.

The Salisbury screen structure is shown in Figure 1.5(a) and its equivalent circuit

which consists of a resistor Rs that represents a continuous resistive sheet is depicted

in Figure 1.5(b). Resonance occurs at the frequency where the spacing between the

resistive sheet and the ground plane is a quarter wavelength (with respect to the

centre resonance) because at this spacing the short circuit impedance of the metal

backing is transformed to an open circuit in parallel at the plane of the sheet.

Therefore only the resistor Rs is seen by the incident wave, and by selecting the

value of Rs to be 377 Ω, the structure will be matched to the free space impedance at

resonance. Figure 1.5(c) shows an example of the variation in the magnitude of the

reflected power for three different sheet resistances values.

(a)

(b) (c)

Figure 1.5 Electromagnetic wave absorber (a) TEM wave incident normally on the

Salisbury Screen, (b) equivalent circuit, (c) reflectivity for three different sheet

resistance values [3]

Chapter 1: Introduction 10

The operation of a CA absorber, which is also known as an FSS based absorber, is

similar to the Salisbury screen. But whereas a resistive sheet is used to construct the

former structure, the CA absorber employs a FSS screen which exhibits a frequency

dependent reactive component, except at resonance. A schematic of the FSS based

absorber is depicted in Figure 1.6(a) and the equivalent circuit of the arrangement is

illustrated in Figure 1.6(b). The operating principle of a FSS absorber is based on a

lossy High Impedance Surface (HIS). HIS terminology is used to describe a periodic

surface which is printed on a grounded dielectric slab [28]. Loss can be incorporated

into the absorber using different mechanisms: lumped resistors [29]–[33], resistive

patterns [12], [15], [16], [28], [34]–[44] or lossy dielectric [26], [45]–[48]. For the

work reported in this thesis, a resistive pattern (gaps or complete surface area of the

FSS elements) is used to create all of the absorbers studied. Consider Figure 1.6(a),

the equivalent circuit of the thin HIS consists of a parallel connection of the square

loop FSS impedance which can be represented by a series L (due to parallel strip), C

(due to inter-element gap), R (resistive loss of the loops) circuit, and the transformed

impedance, L‟ (assuming ), of the metal plate which is placed behind the

periodic array. Resonance occurs at the frequency where the imaginary parts of the

FSS impedance and the inductance presented by the ground plane cancel each other,

and by selecting the value of R which is used to represent the FSS loss, it is possible

to impedance match the structure to the free space impedance and thereby maximize

radar backscatter suppression.

Chapter 1: Introduction 11

(a) (b)

Figure 1.6 HIS based absorber (a) array of resistive FSS and, (b) equivalent circuit

In Figure 1.6, it is shown that the structure is represented by an equivalent circuit in

which the impedance at the surface of the structure is equal to:

(1.4)

is the impedance presented by the ground plane and is given by [15], [44]:

( ) (1.5)

where and are the impedances of the slab for TE and TM polarization and

is the propagation constant [44]:

( )

( )

(1.6)

√

(1.7)

is the transverse wavenumber which is a function of the incident wave angle, .

Therefore, based on the equivalent circuit shown in Figure 1.6, when designing

absorbers, the geometry of the FSS pattern should be optimized in such a way that

the imaginary part of the FSS impedance cancels out the ground plane impedance for

the required angle of incidence and electric vector orientation.

Ground plane

d Resistive

FSS 𝑍𝑠

𝑍𝐹𝑆𝑆 𝑍𝐺𝑃

Chapter 1: Introduction 12

1.5 Past Research on FSS Based Absorbers

Since its introduction the performance of the Salisbury screen has been improved by

adding multiple resistive layers, (known as a Jaumann absorber [7]), to widen the

reflectivity bandwidth. Moreover recent studies have focused on other methods to

increase their functionality including thickness reduction and performance stability at

different incidence angles and wave polarizations. In Section 1.2, it was mentioned

that the thickness of the absorber structure can be reduced while maintaining the

reflectivity bandwidth by exploiting the geometry of the periodic surfaces that are

used to construct CA absorbers. CA – RAM was introduced in 1956 [49] to improve

the performance of the Salisbury screen by creating sheets with geometric patterns

composed of lossy material, however, due to the lack of design rules and

methodology, practical implementation of the CA absorber arrangement (specifically

using resistive FSS) was largely forgotten about until design rules were introduced in

2010 [44].

In [15], the authors demonstrated that a simple metal backed lossy square loop FSS

based absorber, depicted in Figure 1.7, exhibits a similar performance as a Jaumann

absorber previously reported in [50]. This eliminates the need for a multilayer

structure hence this FSS based arrangement provides a significant reduction in the

thickness and fabrication complexity. Using a lossy FSS printed on 5 mm thick

dielectric substrate, the absorber produces a -10 dB reflectivity bandwidth of 114%

centred at 14.16 GHz. A similar performance is obtained for the Jaumann absorber

(Figure 1.7(b)) but for this arrangement the thickness is 15 mm.

Chapter 1: Introduction 13

(a) (b)

Figure 1.7 CA/HIS absorber based resistive FSS, (a) geometry of the structure, (b)

reflectivity of the (5 mm thick) HIS absorber and two-layer (15 mm thick) Jaumann

absorber at normal incidence [15]

The relationship between the FSS pattern geometry and the reflectivity bandwidth for

a 1 mm thick metal backed absorber (thin and narrowband absorber) is illustrated in

Figure 1.8 [44]. As shown in the plot, the square patch element gives the widest -10

dB reflectivity bandwidth (12%) because the capacitance is significantly greater than

the two other element shapes studied. The reason for this can be observed from the

following equation [51] which relates the reflectivity bandwidth to the equivalent

circuit components:

√ ( ) [√ ( ( ) √ ( ))

] (1.8)

where

( )

(1.9)

(

√

) (1.10)

Theoretically, square patch (and loop) FSS structures exhibit the same frequency

response for both TE and TM waves at normal incidence because of the pattern