X-BAND ANTENNA FOR CUBESAT SATELLITE

Laboratory of Electromagnetics and Antennas

Semester Project:

X-BAND ANTENNA FOR CUBESAT SATELLITE

September 2016 - January 2017

Author:Joana Maria Llull Coll

Advisors:Professor Anja Skrivervik, EPFL

Professor Juan Manuel Rius Casals, UPCSupervisor Miroslav Veljovic, EPFL

Acknowledgements

First of all, I would like to thank Prof. Juan Manuel Rius Casals, at UPC, for his discussions

regarding my final project and his recommendation to the Laboratory of Electromagnetics and

Antennas (LEMA) at EPFL. I would also like to thank Prof. Anja Skrivervik for her willingness to

host me and for giving me the opportunity to work on this project. Moreover, I would also like

to thank her for her guidance and mentoring during my stay at LEMA. In this aspect, I would

also like to express my gratitude towards Miroslav Veljovic, who acted as my co-supervisor

while working on his PhD; his dedication and help allowed me to quickly introduce me to the

topic and the tools used for antenna design. I would also like to mention Dr. Marcos Alvarez

Folgueiras, who helped me during the prototyping stage and their measurements.

Some other people that I would like to mention are Dr. Santiago Capdevila Cascante for his

assistance, support and help along the work; Ismael Vico Triviño, who arrived to LEMA at

the same time, for his companionship during the realization of the project; and all LEMA

members for their reception and good moments.

Finally, this project would not have been possible without the support of the Laboratory of

Electromagnetics and Antennas (LEMA) and the workshops of EPFL for the realization and

measurement of the antenna prototypes.

i

AbstractThis project describes the process of design and manufacturing of an X-Band (7.1-8.5 GHz)

antenna for a CubeSat satellite, facing many of the design criteria and challenges of pico-

satellites. A CubeSat is a type of miniaturized satellite for space research that is made up of

multiples of 10×10×10 cm3 units and has a mass of no more than 1.33 kilograms per unit. It

can be manufactured using commercial off-the-shelf (COTS) components for its electronics

and structure. Limited size, low mass, circular polarization, high gain and wide bandwidth are

the main challenges of the antenna design. A design of an aperture-coupled patch antenna

has been performed to overcome the difficulties and fulfill all the requirements for CubeSats

antennas. The antenna consists of one or two rectangular patches, fed by a microstrip line

through a crossed slot in the ground plane. The two patches are separated from the ground

plane using a layer of hard, low-permittivity foam. After the study of several antenna models,

a stacked patch antenna (two patches) accomplished most of the requirements. The antenna

models have been simulated using a 3-D electromagnetic simulation software, HFSS. Simula-

tion results show a gain of 7.2-9.4 dB within the entire frequency range, attaining the higher

gain at the central frequency of 7.8 GHz. An impedance bandwidth of 27.8 % has been ob-

tained for this design. Radiation pattern shows a beamwidth of 50°-66° in the whole frequency

range. Good circular polarization is achieved with an axial ratio bandwidth of 33.3 %. The best

models have been manufactured and measured on the network analyzer and in an anechoic

chamber. Measured results show an impedance bandwidth of 22 % and a gain within 8.2-9.5

dB, which seems to be higher than in the simulation results. Good circular polarization is also

achieved in the measured results.

Key words: CubeSat, X-Band, circular polarization, gain, bandwidth, beamwidth, radiation

pattern, aperture-coupled patch antenna, stacked patch, microstrip line, permittivity, network

analyzer, anechoic chamber.

iii

ResumenEste proyecto describe el proceso de diseño y fabricación de una antena X-Band (7.1-8.5

GHz) para un satélite CubeSat, frente a muchos de los criterios de diseño y desafíos de los

picosatélites. CubeSat se trata de un satélite miniaturizado, dedicado a la investigación espacial

que está compuesto por múltiples unidades cúbicas de dimensiones 10 × 10 × 10 cm3 y con una

masa no superior a 1.33 kilogramos la unidad. Este puede ser fabricado usando componentes

comerciales (COTS) para su composición electrónica y estructural. En el tamaño reducido,

su baja masa, su polarización circular, la alta ganancia del dispositivo y en el amplio ancho

de banda se encuentran los principales desafíos del diseño de la antena. Se ha realizado un

diseño de una patch antena para superar las dificultades y satisfacer todos los requisitos del

diseño. La antena consta de dos patch rectangulares, alimentadas por una línea microstrip a

través de una ranura cruzada en el respectivo plano de masa. Ambas patch están separadas de

la alimentación y el plano de masa mediante una capa de espuma dura de baja permitividad

dieléctrica. Después del estudio de varios modelos de antena la compuesta por dos patch ha

satisfecho los requisitos necesarios. Los modelos de antena han sido diseñados utilizando un

software de simulación electromagnético 3-D, HFSS. Los resultados obtenidos muestran una

ganancia de 7.2 - 9.4 dB dentro de la gama de frecuencias de interés, alcanzando la ganancia

más alta en la frecuencia central de 7.8 GHz. Para este diseño se ha obtenido un ancho de

banda de impedancia del 27.8%. El diagrama de radiación muestra una ancho de haz de 50 ° a

66 °. Se consigue una buena polarización circular mostrando un ancho de banda del axial ratio

de 33.3%. Los mejores modelos han sido medidos en el analizador de redes y en la cámara

anecoica. En los resultados medidos se observa un ancho de banda de impedancia del 22%

y la ganancia adquiere valores entre 8.2 - 9.5 dB. Dichos resultados también muestran una

buena polarización circular.

Palabras clave: CubeSat, banda X, polarización circular, ganancia, ancho de banda, ancho de

haz, diagrama de radiación, antena patch, antena con doble patch, axial ratio, alimentación

por aperura, línea microstrip, permitividad, analizador de red, cámara anecoica.

v

ResumAquest projecte descriu el procés de disseny i fabricació d’una antena X-Band (7.1-8.5 GHz)

per un satèl·lit CubeSat, davant molts dels criteris de disseny i desafiaments dels picosatèl·lits.

CubeSat es tracta d’un satèl·lit miniaturitzat dedicat principalment a la investigació espacial

que està compost per múltiples unitats cúbiques de dimensions 10 × 10 × 10 cm3 i amb una

massa no superior a 1,33 quilograms la unitat. Aquest pot ser fabricat utilitzant components

comercials (COTS) per a la seva composició electrònica i estructural. La mida reduïda, la seva

baixa massa, la seva polarització circular, l’alt guany del dispositiu i l’ampli ample de banda

composen els principals reptes del disseny de l’antena. S’ha realitzat un disseny d’una patch

antena per superar les dificultats i complir tots els requisits del disseny. L’antena consta de

dues patch rectangulars, alimentades per una línia microstrip a través d’una ranura creuada

en el respectiu pla de massa. Les dues patch estan separades de l’alimentació i el pla de massa

mitjançant una capa d’espuma dura de baixa permitivitat dielèctrica. Després de l’estudi de

diversos models d’antena la composta per dos patch ha proporcionat els millors resultats

tenint en compte els requisits de l’antena. Els models d’antena han estat dissenyats utilitzant

un programa de simulació electromagnètica en 3-D, HFSS. Els resultats obtinguts mostren un

guany de 7,2-9,4 dB dintre de la gamma de freqüències d’interès, aconseguint el guany més

alt a la freqüència central de 7,8 GHz. Per a aquest disseny s’ha obtingut un ample de banda

de impedància del 27,8 %. El diagrama de radiació mostra una amplada de feix de 50 ° a 66 °.

S’aconsegueix una bona polarització circular mostrant un ample de banda de l’ axial ratio

de 33,3 %. Els millors models han estat mesurats en l’analitzador de xarxes i en la cambra

anecoica. En els resultats mesurats s’observa un ample de banda de impedància del 22 % i el

guany ocupa valors ente 8.2 - 9.5 dB. Aquests resultats també mostren una bona polarització

circular.

Paraules clau: CubeSat, banda X, polarització circular, guany, ample de banda, ample de feix,

diagrama de radiació, antena patch, axial ratio, alimentació per aperura, línia microstrip,

permitivitat, analitzador de xarxa, càmera anecoica.

vii

ContentsAcknowledgements i

Abstract (English/Spanish/Catalan) iii

List of figures xi

1 Introduction 1

1.1 Cubesat satellite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 CubeSat challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Project requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Microstrip patch antennas 5

2.1 Microstrip radiators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1.1 Microstrip patch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Feeding techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 Transmision line feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.2 Slot feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.2.3 Coaxial feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.3 Antenna polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Linearly polarized antennas 15

3.1 Aperture-coupled microstrip patch with rectangular slot . . . . . . . . . . . . . . 16

3.2 Aperture-coupled microstrip patch with H slot . . . . . . . . . . . . . . . . . . . 21

4 Circularly polarized antennas 23

4.1 Aperture-coupled microstrip patch with crossed slot . . . . . . . . . . . . . . . . 23

4.1.1 Study of circular polarization quality . . . . . . . . . . . . . . . . . . . . . 25

4.2 Aperture-coupled stripline patch with crossed slot . . . . . . . . . . . . . . . . . 29

4.3 Aperture-coupled stacked patch with crossed slot . . . . . . . . . . . . . . . . . . 34

5 Antenna prototype 41

5.1 Manufacturing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.2 Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.2.1 Set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

5.2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

ix

Contents

6 Project planning 55

6.1 Gantt diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.2 Cost plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

7 Conclusion and future developments 59

Bibliography 64

x

List of Figures1.1 CubeSat 1U in space [1] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Possible structures for a CubeSat (Edited from [2]) . . . . . . . . . . . . . . . . . 2

2.1 Microstrip resonator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Different resonant modes [3] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Different patch shapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Transmission line feed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

2.5 Aperture-coupled antenna with rectangular slot . . . . . . . . . . . . . . . . . . . 8

2.6 Different slot shapes [4] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

2.7 Strip-Slot-Foam-Inverted –Patch (SSFIP) . . . . . . . . . . . . . . . . . . . . . . . 10

2.8 Patch antenna with coaxial line feed . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.9 (a)Left-hand circular polarization (b)Right-hand circular polarization (c) Polar-

ization ellipse [5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 HFSS mesh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

3.2 Aperture-coupled patch with rectangular slot model . . . . . . . . . . . . . . . . 16

3.3 Patch size sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

3.4 Slot width sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.5 Slot length sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

3.6 Microstrip line width sweep and its influence on the input impedance . . . . . 19

3.7 Microstrip line length sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.8 Foam height sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.9 Best result achieved for aperture-coupled patch with a microstrip line and fed

by a rectangular slot. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.10 Aperture-coupled patch with ’H’ slot . . . . . . . . . . . . . . . . . . . . . . . . . 22

4.1 Different ways to circularly polarize a patch . . . . . . . . . . . . . . . . . . . . . 23

4.2 Aperture-coupled antenna with crossed slot structure . . . . . . . . . . . . . . . 24

4.3 Feeding line length sketch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.4 Sweep of the larger slot (S1) and its length influence on the reflection coefficient

and axial ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.5 Sweep of the shorter slot (S2) and its length influence on the reflection coefficient

and axial ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

4.6 Microstrip patch simulation results – S11 and smith chart . . . . . . . . . . . . . 27

xi

List of Figures

4.7 Microstrip patch simulation results – Gain and radiation pattern . . . . . . . . . 28

4.8 Microstrip patch simulation results – Axial Ratio . . . . . . . . . . . . . . . . . . . 28

4.9 Aperture-coupled stripline structure with a single patch . . . . . . . . . . . . . . 29

4.10 Stripline patch simulation results – Reflection coefficient . . . . . . . . . . . . . 30

4.11 Stripline patch simulation results – Smith Charts . . . . . . . . . . . . . . . . . . 30

4.12 Stripline patch simulation results – Gain . . . . . . . . . . . . . . . . . . . . . . . 31

4.13 E plane and H plane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.14 Radiation patterns for model 1, E plane . . . . . . . . . . . . . . . . . . . . . . . . 32

4.15 Radiation patterns for model 1, H plane . . . . . . . . . . . . . . . . . . . . . . . . 32



4.18 Stripline patch simulation results – Axial ratio . . . . . . . . . . . . . . . . . . . . 33

4.19 Aperture-coupled stacked patch model . . . . . . . . . . . . . . . . . . . . . . . . 34

4.20 Stacked patch simulation results – Reflection coefficient . . . . . . . . . . . . . . 35

4.21 Stacked patch simulation results – Smith Charts . . . . . . . . . . . . . . . . . . . 35

4.22 Stacked patch simulation results – Gain . . . . . . . . . . . . . . . . . . . . . . . . 36





4.27 Stacked patch simulation results – Axial ratio . . . . . . . . . . . . . . . . . . . . . 38

5.1 Board inside the oven . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

5.2 Photoresist covering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5.3 Masks for UV exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

5.4 UV exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.5 Revealing process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.6 Acid etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

5.7 Final drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.8 Different layers of the final antenna . . . . . . . . . . . . . . . . . . . . . . . . . . 44

5.9 Antenna for stacked patch configuration . . . . . . . . . . . . . . . . . . . . . . . 45

5.10 Network analyzer block diagram [6] . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.11 VNA measures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

5.12 Antenna inside the anechoic chamber, LEMA . . . . . . . . . . . . . . . . . . . . 47

5.13 Aperture-coupled stripline with a single patch (Model 1) – Reflection coefficient 48

5.14 Aperture-coupled stripline with stacked patch configuration (Model 3) – Reflec-

tion coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

5.15 Aperture-coupled stripline with stacked patch configuration (Model 4) – Reflec-

tion coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5.16 Aperture-coupled stripline with a single patch (Model 1) – Gain . . . . . . . . . 50

5.17 Aperture-coupled stripline with stacked patch configuration (Model 3) – Gain . 50

5.18 Aperture-coupled stripline with stacked patch configuration (Model 4) – Gain . 51

xii

List of Figures

5.19 Measured and simulated radiation patterns for the aperture-coupled stripline

with a single patch (Model 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51


with stacked patch configuration (Model 3) . . . . . . . . . . . . . . . . . . . . . . 52


with stacked patch configuration (Model 4) . . . . . . . . . . . . . . . . . . . . . . 52

5.22 Aperture-coupled stripline with a single patch (Model 1) – Axial ratio . . . . . . 53

5.23 Aperture-coupled stripline with stacked patch configuration (Model 3) – Axial ratio 53

5.24 Aperture-coupled stripline with stacked patch configuration (Model 4) – Axial ratio 54

xiii

1 Introduction

1.1 Cubesat satellite

A CubeSat is a type of miniaturized satellite for space research that is made up of multiples of

10×10×10 cm3 units and has a mass of no more than 1.33 kilograms per unit [7], its structure

can be seen in Figures ( 1.1, 1.2) and table 1.1. Multiple CubeSats can be joined together to

form a satellite with increased volume and mass constraints as shown in the table beneath. It

can be manufactured using commercial off-the -shelf (COTS) components for their electronics

and structure.

Cubesat program started in 1999 at California Polytechnic State University (Cal Poly) and

SSDL (Space Systems Development Laboratory) of Stanford University under the leadership

of Robert J. Twiggs [7]. The idea was to cover an educational need to define a meaningful

satellite mission that could be developed within a time frame of one or a couple of years, have

a low cost and therefore a low mass to reduce launching costs. Accordingly, CubeSats are

most commonly put in orbit by deployers on the International Space Station, or launched as

secondary payloads on a launch vehicle. CubeSats are satellites intended for low Earth orbit

(LEO) and most commonly involve experiments which can be miniaturized or serve purposes

such as Earth observation or amateur radio. Many of them are used to demonstrate spacecraft

technologies that are targeted for use in small satellites or that present questionable feasibility

and are unlikely to justify the cost of a larger satellite. A list of some of the CubeSats in space is

available [8], defining its type, mission, status and organization in charge.

CubeSat designation Maximum size Maximum massCube (1U) 10cm×10cm×10cm 1 kgDouble cube (2U) 10cm×10cm×20cm 2 kgTriple cube (3U) 10cm×10cm×30cm 3 kg6 pack (6U) 10cm×20cm×30cm 6 kg

Table 1.1: CubeSat structures

1

Chapter 1. Introduction

Figure 1.1: CubeSat 1U in space [1]

Figure 1.2: Possible structures for a CubeSat (Edited from [2])

Different classifications can be used to distinguish the different types of miniaturized satellites.

1. Mini-satellite (100 – 500 kg)

2. Micro-satellite (10 – 100 kg)

3. Nano-satellite (1 – 10 kg)

4. Pico-satellite (0.1 – 1 kg)

5. Femto-satellite (0.01 – 0.1 kg)

CubeSat satellites belong to the genre of pico-satellites [9] or micro-satellites depending on its

structure.

1.2 CubeSat challenges

CubeSat satellites are not short of technical challenges; they usually require innovative propul-

sion, attitude control, communication and computation systems. For instance, micro-/nano-

satellites have to use electric propulsion, compressed gas, vaporization liquids, such as butane

or carbon dioxide, or other innovative propulsion systems that are simple, cheap and scalable

2

1.3. Project requirements

[10]. Despite this challenges this paper will focus on antenna design challenges and its proper-

ties; in within these hurdles we can outline the following:

Small size and low mass: this includes the need of low power consumption, inexpensive

materials and low area but with capacity to mount the solar cells [11].

Circular polarization: polarization mismatch losses need to be eliminated with only 3 dB loss

regardless of antenna orientation [12].

Impedance matching: there is a need to minimize signal reflection and maximize the power

transferred (minimize power losses) [13].

High gain and large bandwidth, are needed for long distance communications so the contact

period with ground stations is increased. This characteristics are also needed to enable inter-

satellite communications [14, 15].

These are the main characteristics that will be faced in this antenna design but for some

CubeSat designs frequency re-configurability or beam steerability is also a requirement. These

characteristics solve the need for more patterns to be radiated at different frequencies, po-

larizations to enhance systems performance and steerability helps power to be saved by

providing the antenna beam to focus on the desired direction of radiation [10].

Some works have examined the suitability of existing planar antenna designs for the use on

pico-satellites [10]. After reading about different comparison between planar antenna designs

in terms of mass, size gain, type of polarization and operating frequency band this project is

focused on the design and measurements of an aperture-coupled patch antenna.

1.3 Project requirements

Inside the remarked characteristics and challenges that a CubeSat antenna involves, this

project will focus on the following requirements specified on table 1.2.

Antenna requirementsX-band frequency 7.1 – 8.5 (GHz)Gain 7 – 10 (dB)Beamwidth 40 – 50 (°)Dimensions 100x100x10 (mm3)Polarization Circular polarization(CP)

Table 1.2: Project requirements

The main demanding task is to achieve all the requirements due to the dependency between

each one of them; therefore prioritization within these requirements must be made depending

on further applications. For this antenna design, the most important factors are to achieve a

good circular polarization in the whole X-band and to accomplish the stated dimensions of the

antenna. The purpose of this work is focused on the antenna design, but not the integration

of the antenna to the CubeSat. HFSS will be used which is a software for 3-D, full-wave,

electromagnetic modeling.

3

2 Microstrip patch antennas

2.1 Microstrip radiators

Printed microstrip radiators were originally proposed in early 1953 which lead to the first

microstrip antenna later on in 1974, [16]. The structure of a microstrip resonator consists of

an upper conductor which can acquire different shapes like the ones shown in Figure 2.3. This

conductor is located on the top of a dielectric substrate and a ground plane at the bottom as

can be seen in Figure 2.1.

Figure 2.1: Microstrip resonator

When the signal frequency approximates to the resonant frequency, the amplitude of the

currents flowing through the conductor increases, the patterns produced by this currents

at the resonant frequency are called the resonant modes of the structure (Figure 2.2). This

resonances arise when the conductor’s size is half of the wavelength. This modes only depend

on the shape and size of the structure and are independent of the excitation feed, we can

obtain them by solving the following eigenvalue equation.

[X ]~Jn =λn[R]~Jn (2.1)

5

Chapter 2. Microstrip patch antennas

Figure 2.2: Different resonant modes [3]

Where ~Jn are the characteristic currents, λn are the eigenvalues, R and X are the real and

imaginary parts of the impedance matrix Z of the structure, which is obtained from the Method

of Moments solutions [17]. The lightweight, small volume, low-profile planar configuration

and ease of mass production using printed-circuit technology leading to a low fabrication

cost of microstrip antennas, has made it suitable for them to be used in many applications

such as aircraft, missiles, satellites, ships and many others [18, 19]. They are also much

easier to be integrated with other microstrip systems on the same substrate, allow triple

frequency operations and dual use for linear and circular polarization. To counterbalance

all this advantages, a narrow bandwidth, lower gain and lower power-handling capability is

achieved compared to usual microwave antennas [20].

2.1.1 Microstrip patch

We can distinguish two main types of resonators that are differentiated by their wide-length

ratio, from one hand we can find a resonator made with a narrow conductor strip called a

dipole [21], whereas a wider resonator is known as a microstrip patch [22]. Radiation patterns

and gains of both types of resonators are not so different but the effect on other characteristics

of the antenna like input impedance, side lobes or polarization can deviate from one type

to another. A broader beam is radiated when the signal frequency is close to the resonant

frequency making the input signal contribute to radiation and making the resonator behave

as an antenna.

From both types of microstrip resonators a broader bandwidth is produced by microstrip

patches. One of the dimensions of the resonant radiator must be half of the guided wavelength,

6

2.2. Feeding techniques

and the resonant dimension depends on the shape of the patch conductor, this is why printed

patch antennas use radiating elements of a wide variety of shapes: squares, rectangular, circle,

ring, triangle, ellipse and many others; see examples in Figure 2.3.

Figure 2.3: Different patch shapes

The selection of the shape depends on the parameter (bandwidth, crossed polarization,

antenna size etc) that needs to be optimized. It is important to remark that the properties of

the substrate, mostly its dielectric constant εr and its heigh h, play a fundamental role in the

performance of the printed antenna. The radiation efficiency of the antenna will also depend

upon the material used, these conclusions will later be shown in Chapter 3.

2.2 Feeding techniques

Feeding Technique is one of the most important design factor in antennas, a selection of

feeding technique requires efficient power transfer between radiating and feed structure.

Several ways of feeding a microstrip patch exist, in between them we can find the transmission

line feed, which happens to be the simplest way to feed a microstrip patch antenna. A coaxial

feed, proximity coupled feeding, buried Feed and Slot feed are different ways, but for the first

model designed, a slot feeding technique has been used.

2.2.1 Transmision line feed

Connecting a microstrip line directly to the edge of the patch is the simplest way to feed a

microstrip patch (Figure 2.4). Simultaneous optimization cannot be achieved if the patch

and the microstrip line are on the same level, this brings a compromise between the two to

avoid the feed line radiating too much at the discontinuities, as the antenna performance can

be degraded by this effect. From the other side, there is a considerable amount of reactive

power located under the patch that degrades the radiation caracteristics and decreases the

bandwidth [23]. In addition, the surface-wave excitation conditions must also be satisfied

[24].

7


Figure 2.4: Transmission line feed

2.2.2 Slot feed

The aperture-coupled microstrip patch antenna feed technique was introduced in 1985 [25]

that includes electrically isolated microstrip transmission lines and patch conductors. These

structures are electromagnetically coupled through a small aperture in the isolating ground

plane (Figure 2.5). Source electromagnetic fields are concentrated between the microstrip

Figure 2.5: Aperture-coupled antenna with rectangular slot

line and ground plane to excite primarily guided waves as opposed to radiated or surface

waves. Guided waves are dominant if the dielectric is electrically thin (<λ/50) and has a large

permittivity relative to free space (εr > 5). At the radiating patch, it is desirable to decrease

guided waves under the patch and increase radiated waves at the patch edges. This requires

8

2.2. Feeding techniques

an electrically thick dielectric (>λ/10) substrate with a relatively low permittivity (εr < 3).

Compromising between the two conflicting criteria results in surface waves, reduced radiation

efficiency due to guided waves below the patch, and increased side lobes levels and cross-

polarization levels from spurious feed line radiation [26]. Coupling between the two sides of

the ground plane is provided by the slot on the ground plane but radiation has to be avoided as

it would produce spurious radiation toward the back of the antenna; therefore slot dimensions

are a critical parameter and should be chosen to avoid resonances withing the working band.

This technique will be used along this project antenna models. One of its main advantages is

the feed as it is a microstrip line but from the other hand a slot on the ground plane must be

cut which makes the structure more complicated to be fabricated as it involves two dielectric

layers that are bonded on both sides of the conductor [23].

Smaller aperture areas result in lower back radiation levels, increasing efficiency and improving

radiation in the back region. A thin rectangular aperture produces better coupling, and

this coupling can also be increased by playing with the length and width of the rectangular

apertures. There are many slot shapes that can be used to achieve this (Figure 2.6). By adding

Figure 2.6: Different slot shapes [4]

slots at the end of the rectangular aperture, the field becomes nearly uniform along the

aperture and hence coupling increases. Coupling also increases by changing the aperture

shape to ’U’ and ’L’ slots. If a higher resonant impedance also has to be achieved a ’bow tie’ is

a good option [4].

The SSFIP principle

One of the main limitations of the Microstrip antennas is their narrow bandwidth. Therefore,

a lot of efforts have been done to improve the bandwidth of these popular antennas. Strip-

Slot-Foam-Inverted –Patch (SSFIP) have been shown to be capable of wider bandwidth up to

more than 30 %.

9


The main structure of this model consists on an inverted patch, sometimes covered by an

epoxy sheet to be protected from rusting. Ideally the dielectric substrate of a microstrip

antenna would be air, with εr = 1, but the patches should be supported in some way this is

why hard foam is used as its dielectric permittivity is quite similar to that of air, with εr = 1.07.

This rigid foams help to achieve a lightweight but resistant structure which makes it a perfect

candidate to be used in aerospace systems. Then the ground plane is placed with its aperture

separated by a substrate from the feeding line [23]. The structure is shown in Figure 2.7.

Figure 2.7: Strip-Slot-Foam-Inverted –Patch (SSFIP)

As it has been pointed out before a thick substrate is needed for the radiation of the patch not

to cancel out as, patches on thicker substrates have wider bandwidth. This thickness leads to

spurious coupling that is compensated by selecting a suitable low permittivity substrate.

On the design of the first model we will see the effects of different parameters separately and

its effect on the overall results.

2.2.3 Coaxial feed

The use of a coaxial line that is perpendicular to the ground plane is another way to feed a

patch. This type of feeding was one of the first considered in the development of microstrip

antennas. The outer conductor of the coaxial cable is connected to the ground plane, and

the center conductor is extended up to the patch antenna as we can see in Figure 2.8. By

positioning the feed properly the patch can be matched to the line, so the input impedance

depends on the feed position. In this structure the radiator and the feeding system are located

on the two sides of the ground plane, this two sides can be designed independently to reach

10

2.3. Antenna polarization

the desired radiation from the patch from one hand, and the feed structure from the other

hand [23]. Most of the coaxial feed theoretical developments and models were developed to

characterize the injection of current into the patch accurately. The intrinsic radiation from the

coaxial feed is small and can be neglected for thin substrates becoming more significant with

thicker substrates [27].

Figure 2.8: Patch antenna with coaxial line feed

Coaxial feed are cheap to instal and easy to modify, however, its physical design is hard to make

because holes have to be be drilled through the substrate and then conductors are introduced

through this holes and soldered into the patch.

2.3 Antenna polarization

Polarization is a parameter applying to waves that specifies the geometrical orientation of the

oscillation. In an electromagnetic wave, both the electric field and magnetic field are oscillating

but in different directions. When it is talked about the polarization of light it normally refers

to the polarization of the electric field. Light which can be approximated as a plane wave in

free space or in an isotropic medium, propagates as a transverse [28] wave where both electric

and magnetic fields are perpendicular to the propagation direction. When the electric field

vector of an electromagnetic wave at a fixed point in space is constantly pointing in a fixed

direction, although its magnitude is not constant, linear polarization is defined. There are

two forms of linear polarization (LP); vertical polarization occurs when the electric field is

perpendicular to the surface of the Earth and when the field is parallel to the surface it is called

horizontal polarization. Both directions can be used simultaneously on the same frequency

[29]. However, circular polarization (CP) mode is preferred for many systems, this type of

polarization takes place when the trajectory of the tip of the electrical vector rotates about the

11


propagation axis as a function of time. The sense of a CP wave is determined by the rotation

direction of the E vector as it describes a circle, this rotation is always defined following the

propagation direction of the wave. When this rotation is generated clockwise it is called a

right-hand CP (RHCP), and a left-hand CP (LHCP) when it is generated counterclockwise. An

imperfect circularly polarized field instead of a circle is sometimes generated as we can see in

Figure 2.9, therefore elliptical polarization is obtained.

Figure 2.9: (a)Left-hand circular polarization (b)Right-hand circular polarization (c) Polariza-tion ellipse [5]

The ratio between major and minor axis seen in Figure 2.9 (c), is defined as the Axial Ratio

(AR); its value will be 1 when perfect wave propagation is given meaning only one hand of

propagation is generated. It can coincide that both RHCP and LHCP magnitudes components

are the same and so the circle formed by the tip of the E vector degenerates into a line, the

polarization becomes linear and AR infinite. In any two orthogonal cuts like the horizontal

and vertical terms Eh and Ev have the same amplitude and shifted in phase (±90°) RHCP or

12

2.3. Antenna polarization

LHCP wave components may be expressed as:

ERHC P = 1p2

(EH + j EV )

ELHC P = 1p2

(EH − j EV )

Hence, combining the amplitude and phase response of two orthogonal waves, the radiation

patterns generated by a CP antenna can be plotted [5]. Specific equipment costs are marginally

less for lineal polarization but there are several key advantages that make circular polarization

more appealing. Lineal polarization is not affected by the Faraday effect which causes a

rotation of the plane of polarization which is linearly proportional to the component of the

magnetic field in the direction of propagation; this becomes a significant problem when exact

signal alignment is needed. CP is also more resistant to signal degradation due to atmospheric

conditions, has easier installation and higher link reliability [2].

13

3 Linearly polarized antennas

The aperture-coupled microstrip patch antenna (ACMPA) is of great interest since it allows

the separation of the radiating element (the microstrip patch) and the feed network (50 ohm

microstrip transmission line) with a conductive layer (ground) between them. The radiating

microstrip patch element is etched on the top of the antenna substrate, and the microstrip feed

line is etched on the bottom of the feed substrate. The thickness and dielectric constants of

these two substrates may be chosen independently to optimize the distinct electrical functions

of radiation and circuitry. In order to achieve the objectives of this project, an aperture patch

antenna has been chosen [23].

The first step has been to study the behavior of every component in a basic model to then be

able to modify the correct parameters to reach the desired results. Aperture coupled patch

antennas involve performance critical parameters including substrate thickness, substrate

dielectric constant, microstrip feed line, ground plane slot, and patch dimensions and relative

locations. A parametric study on a basic model has been completed to determine performance

effects of critical parameters to develop a design procedure.

All the results shown below have been simulated using HFSS which is a software for 3-D,

full-wave, electromagnetic modeling. HFSS uses what is known as the Finite Element Method

(FEM) which is a numerical technique for finding approximate solutions to boundary value

problems for partial differential equations, such are electromagnetic problems described by

Maxwell equations. It subdivides a large problem into smaller, simpler parts that are called

finite elements, and then approximates the physical quantities along these elements using

simple functions. The simple equations that model these finite elements are then assembled

into a larger system of equations. Solving the large system gives the solution for the entire

problem. An example of such subdivision is shown in Figure 3.1.

15

Chapter 3. Linearly polarized antennas

Figure 3.1: HFSS mesh

3.1 Aperture-coupled microstrip patch with rectangular slot

The study of the most basic model of an aperture-coupled patch was made to be able to then

tune the parameters properly and completely understand the behavior of our antenna. To

proceed with this, the dielectric sheet that covers the patch to protect it was removed and the

foam was ideally replaced by air to simulate the following results, shown in Figure 3.2. This

simplified the structure and accelerated the simulations run by HFSS but kept the results close

enough to the real influence each component of the antenna has on the results. Gain

(8-10dB), circular polarization, good return loss and beamwidth(around 50 degrees) were the

main objectives to be accomplished on the desired working bandwidth 7.1-8.4 GHz (X-band).

(a) Model: Top View (b) Model: Side view

Figure 3.2: Aperture-coupled patch with rectangular slot model

The first step was to achieve a good matching on the working bandwidth so the effect each

parameter has on the matching of our antenna can be seen.

16

3.1. Aperture-coupled microstrip patch with rectangular slot

Patch Size

It has to be taken into account that in practice, the sides of the patch are not perfect magnetic

walls since these walls radiate energy and the fields in the patch extend beyond the physical

dimensions of the patch. When the size of the patch is increased, the resonant frequency

decreases so we will choose the size patch for our antenna to work on the desired frequency,

see Figure 3.3. A square patch has been used in this simulation results. The width W of the

patch must be less than the wavelength in the dielectric substrate material so that higher order

modes are not excited, this excitation can also be avoided by using multiple feeds [30].

(a) Reflection coefficient (b) Smith Chart

Figure 3.3: Patch size sweep

For maximum coupling, the patch should be centered over the slot. Moving the patch relative

to the slot in the H plane direction has little effect, while moving the patch relative to the slot

in the E-plane (resonant) direction will decrease the coupling level [31].

Slot Dimensions

In the aperture coupled microstrip antenna, the most common method of controlling the

coupling to the microstrip feed line is to vary the size of the aperture. Small coupling aperture

also limits the antenna substrate thickness that may be used. As we can see in Figure 3.4, as

the slot width is increased, the resonant frequency becomes slightly smaller, but it is not a

critical parameter to match the antenna. The width of the slot affects the coupling level, but to

a much less degree than the slot length. The ratio of slot length to width is typically 1/10.

17



Figure 3.4: Slot width sweep

On the other hand, the length of the aperture becomes more crucial for antenna matching, see

Figure 3.5. A variation in the slot length introduces a change in the inductive input reactance

of the line, canceling out its resonant behavior [32]. The coupling level is primarily determined

by the length of the coupling slot, as well as the back radiation level. The slot should therefore

be made no larger than is required for impedance matching.


Figure 3.5: Slot length sweep

Feeding line dimensions

Dimensions and location of the feed line also have a critical effect on the impedance band-

width of the antenna. The use of a radiating aperture results in a high level of coupling, which

18

3.1. Aperture-coupled microstrip patch with rectangular slot

must be reduced to properly impedance match the antenna. This coupling decreases with

feeding line width. The results can be seen in Figures 3.6 and 3.7.

Besides controlling the characteristic impedance of the feed line, the width of the feed line

affects the coupling to the slot. To a certain degree, thinner feed lines couple more strongly to

the slot. For maximum coupling, the feed line should be positioned at right angles to the center

of the slot. Deviating the feed line from the slot will reduce the coupling, as will positioning

the feed line towards the edge of the slot [31].

Figure 3.6: Microstrip line width sweep and its influence on the input impedance

Foam layer thickness

By changing the substrate material, the dielectric constant of the substrate changes εr . A wide

variety of substrate materials have been found to exist suitable for microstrip patch antenna

design with mechanical, thermal, and electrical properties which are attractive for use in both

planar and conformal antenna configurations [33]. Here, we calculate the antenna parameters

by varying substrate thickness (air in this case). When the thickness is increased, the coupling

from the slot to the patch is reduced. Then, the slot area must be increased to restore the

coupling level but not too large because it will then resonate (Figure 3.8). From the analysis, we

can conclude that the use of substrate material with higher dielectric constant in microstrip

patch antenna design, results degradation of antenna performance.

19



Figure 3.7: Microstrip line length sweep


Figure 3.8: Foam height sweep

Best result achieved with a rectangular slot

Since the various parts of the antenna interact, it is not trivial to optimize and determine the

best design procedure.

For this model, the microstrip feed line of the ’optimal’ design is placed on a 0.51 mm thick

substrate of relative permittivity εr = 2.33 using RogersRT/duroid 5870 as the substrate material.

The slot dimensions for this result are 13 mm×0.49 mm (0.338 λ0×0.013 λ0 ), the feeding line

length is 28.1 mm (0.73 λ0)with a width of 1.39 mm (0.036 λ0) that produces a characteristic

impedance Z0 of 50 Ω.The radiating square patch is 13 mm×13 mm (0.338 λ0×0.338 λ0 ),

20

3.2. Aperture-coupled microstrip patch with H slot

on a 0.1 mm epoxy sheet. For this final result, the air has been substitute by hard foam due

to its similar permittivity εr Foam and εr Ai r = 1. The substrate size used for the simulation

has been 40 mm×40 mm, but an analysis of the substrate size has been performed to study

its influence and it does not make a significant difference to the results shown in Figure 3.9.

Using this model, a bandiwdth of 8.33 % is achieved which is not enough for the objectives of

this antenna. Several changes have been made to the model in order to increase the desired

bandwidth [34].


Figure 3.9: Best result achieved for aperture-coupled patch with a microstrip line and fed by arectangular slot.

3.2 Aperture-coupled microstrip patch with H slot

The Slot length affects the coupling level and the back radiation level. The slot should be made

no larger than is required for impedance matching. The patch is normally centered over the

aperture making the slot shape and size be the dominant mechanism for coupling. This is

how the best coupling depends on the aperture shape.

By adding slots at the end of the rectangular aperture (H-shaped aperture) as shown in Figure

3.10, the field becomes nearly uniform along the aperture and hence the coupling increases

[35]. The aim of changing the model is to increased the bandwidth to reach at least 18 % and

the optimization of this design has not shown bandwidth improvement but better coupling.

21


Figure 3.10: Aperture-coupled patch with ’H’ slot

In conclusion, this study has shown that the most important parameters to be taken into

account when designing an aperture-coupled antenna are: feeding line length and width,

patch dimensions, aperture dimensions specially the length of the slot, overall height of the

antenna and the materials used on the design as a change in the permittivity is also critical.

From the other hand the width of the slots and the size of the ground plane do not have a big

influence on the matching.

22

4 Circularly polarized antennas

Circular polarization was explained on chapter two ’Microstrip Antennas’, in order to see its

real influence on an antenna the aperture-coupled patch antenna model shown in the previous

chapter will be circularly polarized. Dual polarization capability in microstrip antennas can

be obtained either by using two orthogonal feeds [36], or with a diagonal coupling slot and

a slightly rectangular patch [37] similar to circularly polarized patches with a single probe

feed, but the resulting axial ratio bandwidth is very narrow. Somewhat improved axial ratio

bandwidth can be obtained by using a crossed slot with a single microstrip feed line through

the diagonal of the cross, which is the chosen design for this project. Circular polarization is

generated by exciting two orthogonal patch modes in phase quadrature with the sign of the

relative phase determining polarization hand, this can also be achieved by orthogonal feeds

or a single feed degenerated mode patch [35](Figure 4.1).

Figure 4.1: Different ways to circularly polarize a patch

4.1 Aperture-coupled microstrip patch with crossed slot

In order to achieve a wideband circularly polarized aperture-coupled microstrip antenna,

several changes have been made to the previous design. A crossed slot shifted 45 has been

used (Figure 4.2) instead of a rectangular slot like shown in Figure 3.2. This procedure has

leaded to bandwidth enhancement and achieving a good circular polarization. The model

23

Chapter 4. Circularly polarized antennas

was first simulated with both slots S1 and S2 with the same length, but good matching in the

whole desired bandwidth was not performed.

(a) Model: Top view (b) Slots

(c) Model: Side View

Figure 4.2: Aperture-coupled antenna with crossed slot structure

A study of each parameter of this model was proceeded before executing the best result with

this model and the following conclusions were settled.

This model showed two resonant frequencies on our working band and they were matched

taking into account the following conclusions:

Patch size is the main parameter to determine the working bandwidth, the bigger its size the

lower the bandwidth. It was also simulated what the results would be if the patch was thicker

(not simulated as a sheet of pec) and the result was that adding copper to the patch affected

the second resonance bringing it to a smaller frequency.

Slot lengths have a big impact on the first resonant frequency, larger apertures make the first

resonant frequency smaller.

Feeding line length is also a critical parameter. Up to length in case [1], see Figure 4.3, as

the microstrip feeding line is larger, the first and second resonant frequencies are smaller,

24

4.1. Aperture-coupled microstrip patch with crossed slot

from case [2] making the line larger only decreases the first resonance making the second one

remain more or less the same.

Figure 4.3: Feeding line length sketch

By optimizing each of the parameters mention before a good matching on the whole desired

bandwidth was achieved, increasing it from 8.33 % to 20.2%.

4.1.1 Study of circular polarization quality

Circular polarization was also a requirement for the antenna. The aperture-coupled configu-

ration exhibits very low cross-polarization levels, making it well suited to circularly polarized

antenna designs. A common technique for producing circular polarization is to excite two

orthogonal linearly polarized elements with a 90 phase. This method can be utilized by

using either two off center coupling apertures [38] or a crossed slot. The crossed slot retains

symmetry and therefore is capable of producing circularly polarized radiation with very good

polarization purity. It also permits the use of slot lengths greater than half the patch width,

which is critical to achieving adequate coupling when thick antenna substrates are used for

wide bandwidths. The ratio between S1 and S2 is a decisive parameter to achieve good circular

polarization.

The following Figure 4.4 shows how the length of S1 influences the axial ratio and matching.

25


(a) Reflection coefficient (b) Axial ratio

Figure 4.4: Sweep of the larger slot (S1) and its length influence on the reflection coefficientand axial ratio

The following Figure 4.5 shows how the length of S2 influences the axial ratio and matching.

(a) Reflection coefficient (b) Axial ratio

Figure 4.5: Sweep of the shorter slot (S2) and its length influence on the reflection coefficientand axial ratio

Result achieved with microstrip aperture-coupled patch model fed by a crossed slot that best

accomplishes the requirements

On simulation results shown from now on, air is placed by hard foam (Rohacell HF) of 2.75 mm

height which has a dielectric loss tangent of 0.003 at the desired working frequency. PEC was

26

4.1. Aperture-coupled microstrip patch with crossed slot

also replaced by copper to make the results the most realistic possible, this changes introduced

some losses and readjustments had to be made for the results to fulfill all the requirements.

Good matching was achieved in the desired bandwidth by substituting the rectangular slot by a

crossed slot shifted 45 . On this case, the microstrip feed line of the ’optimal’ design is etched

on a 0.51 mm thick substrate of relative permittivity εr = 2.33 using RogersRT/duroid 5870 as

the substrate material. The slot dimensions for this result are 17 mm×0.49 mm (0.44 λ0×0.013

λ0 ) fort the long slot S1, and 9.4 mm×0.49 mm (0.24 λ0×0.013 λ0 ) for the short slot S2. The

feeding line length is 23.4 mm (0.61 λ0) with a width of 1.39 mm (0.036 λ0) that produces a

characteristic impedance Z0 of 50Ω. The radiating square patch is 12.5 mm×12.5 mm (0.33

λ0×0.33 λ0 ) that is attached to the hard foam with a conductive pasta. The substrate size used

for the simulation has been 40 mm×40 mm. Using this model, a much wider bandiwdth (22.7

%) is also achieved which now accomplishes the matching requirements of the antenna.


Figure 4.6: Microstrip patch simulation results – S11 and smith chart

Gain and radiation pattern

Gain is represented on Figure 4.7a, as we can observe it is higher than 7 GHz at all the frequency

range being lower at lower frequencies. Radiation pattern on Figure 4.7b shows a beamwidth

of 60° at the frequency of 7.8 GHz, the central frequency where RHCP gain is represented in

purple and LHCP gain is the red curve.

27


(a) Gain (b) Radiation pattern at f=7.8GHz

Figure 4.7: Microstrip patch simulation results – Gain and radiation pattern

Axial ratio

Axial ratio for circular polarization is ideally 1, which is when the ratio between the minor

axis and the major axis is 1. This means that the rotation direction of the ~E following the

propagation direction of the wave describes a perfect circle. This is very hard to achieve in

practice in the all frequency range, so we will consider a good circular polarization when AR is

equal or lower than 3, being 4 or lower also an acceptable value.

Figure 4.8: Microstrip patch simulation results – Axial Ratio

28

4.2. Aperture-coupled stripline patch with crossed slot

Even though this model accomplishes all the parameter needed, some changes need to be

done for it to be improved. The main problem of this model is that the feeding line is not

isolated. This isolation is needed to avoid the feeding line of the antenna to interact with other

systems on the satellite or even other possible devices in space. Imperfections or changes

cannot be made to the antenna once it is launched so all preventions must be though of and

carried out previously.

4.2 Aperture-coupled stripline patch with crossed slot

Isolation problem leads us to this model which consists on adding a second ground plane

to segregate the feeding network from any other devices. This ground plane is placed on

top of the feeding line using a hard foam (Rohacell HF) of 3.21 mm thickness in between the

feeding network and the additional ground plane. Two models have been pursued using the

structure shown in Figure 4.9. This models parameters are specified the table 4.1, the rest of

the parameters and the structure are preserved the same as in the previous model on section

4.1.

Figure 4.9: Aperture-coupled stripline structure with a single patch

Parameters(mm) Model 1 Model 2Feeding line length 23.4(0.608 λ0) 23.82(0.619 λ0)

Slot 1 length 16.4(0.426 λ0) 15.9(0.413 λ0)Patch size 12.4(0.322 λ0) 12.62(0.328 λ0)

Table 4.1: Stripline models dimensions

Return loss

As we can see in the simulation when measuring S11 paramenters, see Figure 4.10, the antenna

is also properly matched in the whole X band frequency, being the one from model 2 more

accurate as shown on the reflection coefficient figure 4.10 and the smith charts on Figure

4.11. It was explained on Chapter 2, that the overall thickness of the antenna provokes a

wider bandwidth; adding the second ground plane and another hard foam has increased the

29


thickness so the bandwidth achieved with this model is 25 % for model 1 and 25.4% for model

2, being wider than the microstrip bandwidth which was 22.7 %.

Figure 4.10: Stripline patch simulation results – Reflection coefficient

(a) Model 1 (b) Model 2

Figure 4.11: Stripline patch simulation results – Smith Charts

Gain

We can observe that the gain shown in Figure 4.12 is below 7dB between 7.1-7.4 GHz and it is

30


lower than the gain achieved on the previous model.

Figure 4.12: Stripline patch simulation results – Gain

Radiation patterns

Radiation patterns at different frequencies and planes and beamwidth(BW) for each frequency

can be seen in Figures (4.14,4.15) for model 1, and Figures(4.16, 4.17) for model 2. Plane E is

refered to Phi=0°, and plane H to Phi=90° as Figure 4.13 shows.

Figure 4.13: E plane and H plane

31


(a) f=7.1 GHz, BW=50° (b) f=7.8 GHz, BW=60° (c) f=8.5 GHz, BW=60°

Figure 4.14: Radiation patterns for model 1, E plane


Figure 4.15: Radiation patterns for model 1, H plane



32




It can be concluded that the bad crossed polarization may be the main disadvantage when

using this models.

Axial ratio

Circular polarization is good for both models as the axial ratio is lower than 2 in most of the

frequency range, see Figure 4.18

Figure 4.18: Stripline patch simulation results – Axial ratio

Comparing this stripline model with the previous model with a single ground plane and a

micro stripline we can conclude that circular polarization improves in the stripline model but

33


gain decreases being lower than the minimum value. This is the main reason why a stacked

patch model was created, to increase the gain of the antenna.

4.3 Aperture-coupled stacked patch with crossed slot

Electromagnetically coupled microstrip patches with stacked configurations have recently

gone through a great deal of development due to their performance features. Mainly this

include large bandwidth, higher gain and/or dual frequency operation [39]. High gain and

large bandwidth are main parameters that need to be accomplished by our antenna.

On this case, the microstrip feed line of the ’optimal’ design is etched on a 0.51 mm thick

substrate of relative permittivity εr = 2.33 using RogersRT/duroid 5870 as the substrate material.

The slot dimensions, feeding line length and patch sizes are shown in table 4.2 for each of

the models. The substrate size is 40 mm×40 mm. As Figure 4.19 displays, the patch area of

patch 1 (driven square patch) is smaller than patch 2 (parasitic patch) to achieve improved

performance. The center of both driven and stacked patches lay one over other. The driven

patch is positioned 1.3 mm away from the ground plane with the slots separated by hard foam.

The parasitic patch is also placed on hard foam of 1.3 mm thickness.

Figure 4.19: Aperture-coupled stacked patch model

Parameters(mm) Model 3 Model 4Feeding line length 23.4(0.608 λ0) 23.82(0.619 λ0)

Slot 1 length 16.4(0.426 λ0) 15.33(0.398 λ0)Patch 1 size 11(0.286 λ0) 11.39(0.296 λ0)Patch 2 size 13.4(0.348 λ0) 12.64(0.328 λ0)

Table 4.2: Stacked patch models dimensions

Return loss

One of the main improvements by using a stacked patch configuration is the achievement

of a wider bandwidth. S11 parameters can be shown on Figures 4.20 ,4.21. Model 3 has a

34

4.3. Aperture-coupled stacked patch with crossed slot

bandwidth of 27.2% while model 4 achieves a bandwidth of 27.8%. These are slightly wider

than the results achieved until now. Stacked patch is characterized to achieved much wider

bandwidth but this is due to the increment of the overall antenna thickness, and in this case

the stacked patch configuration has a similar thickness than the other models so the results

are mostly improved by increasing the gain.

Figure 4.20: Stacked patch simulation results – Reflection coefficient

(a) Model 3 (b) Model 4

Figure 4.21: Stacked patch simulation results – Smith Charts

35


Gain

Simulated gain results are above 7 GHz for both models being model’s 3 gain superior than 8

GHz in all X band. This gain improvement from model 3 has leaded to a less well matched

S11 and an aggravation of the quality of circular polarization. These deterioration are not

that strong on model 4 as the gain has not increased sharply on this model, but an overall

improvement of the results can be seen with the stacked patch configuration.

Figure 4.22: Stacked patch simulation results – Gain

Radiation patterns

Radiation patterns at different frequencies and planes can be seen in Figures (4.23,4.24) for

model 3, and Figures(4.25, 4.26) for model 4.

36








37




Axial ratio

Circular polarization is better with model 4 as the axial ratio results are lower than 2 and flatter

for that model, as for model 3 a degression can be seen on axial ratio results with respect to

previous models, but it still within the desired range. This results are reflected on Figure 4.27.

Figure 4.27: Stacked patch simulation results – Axial ratio

38


Previous models shown above, have been optimized by using the optimization function of

HFSS. The optimization carried out has been a Quasi Newton optimization. In optimization,

quasi-Newton methods (a special case of variable metric methods) are algorithms for finding

local maxims and minims of functions. Quasi-Newton methods are based on Newton’s method

to find the stationary point of a function, where the gradient is 0. Newton’s method assumes

that the function can be locally approximated as a quadratic in the region around the optimum,

and uses the first and second derivatives to find the stationary point. In higher dimensions,

Newton’s method uses the gradient and the Hessian matrix (B) of second derivatives of the

function to be minimized. Quasi-Newton methods usually generate an estimate of (B−1) [40].

The overall conclusion of this chapter is that a microstrip aperture-coupled patch antenna can

accomplish all the parameter characteristics but it is not the most efficient model to be used

in space, so in order to isolate the feeding network a stripline model has to be used instead.

This stripline model achieves all requirements but has a low gain at the desired frequency

so a stacked patch configuration is proceeded in order to fulfill all the requirements. On

the following chapter, this results will be compared to the real results when measuring the

antenna.

39

5 Antenna prototype

5.1 Manufacturing

Manufacturing of the four models of the stripline aperture-coupled patch antenna and the

stacked patch prototypes was carried out by ourselves using the equipment and machinery

provided by LEMA and EPFL. The etching procedure for the patches and specially the ground

plane with the slots and feeding line on the RogersRT/duroid 5870 sheet can be classified as

the most complex step when it comes to manufacturing.

ETCHING PROCEDURE

1. Board Cleaning

Board cleaning is an essential part to avoid possible oils, fats or other substances to interfere

with the etching process. The board has to be placed inside a sink with approximately 1 cm

of water and needs to be cleaned on both sides. Water should wet all the surface as copper

should not ’refuse’ it, which would indicate the presence of fat. Once it is covered by water, the

two sides of the board have to be rinsed with alcohol. To end this step, the board has to be

placed in a drying oven as shown in Figure 5.1. It is very important to avoid contact with the

cleaned sides and hold it by the edges.

Figure 5.1: Board inside the oven

41

Chapter 5. Antenna prototype

2. Photoresist covering

The process consists of covering the entire substrate with a photosensitive coating material

in order to completely coat the unit, this is called the immersion technique [41], see Figure

5.2. Once the board is coated it has to be placed inside the baking oven for 8 minutes and 30

seconds at 80° C. This time can vary with the oven used.

Figure 5.2: Photoresist covering

3. UV exposure

The first thing to do is printing the layout on a transparent film. In our case print the patches,

feeding lines and ground plane with the crossed slots are printed. The feeding line and the

slot in the ground plane are on the two different sides of the dielectric slab. They need to be

located at the right position with respect to each other, which requires a careful alignment of

the two masks on each side of the board. This is done using alignment signs on the masks.

The masks that were used are shown in Figure 5.3. These masks are located on both sides of

the board and exposed to UV, see Figure 5.4, this way the light ’eats’ all the surface leaving

copper in the shadowed parts of the template. The exposure time is critical, it should not be

larger than 2 minutes. Once the UV exposition is over, the board must be placed in a basin

with some revelat and the basin needs to be constantly moved from left to right and back and

forth. The photoresist is removed where it was exposed to the UV light as shown in Figure 5.9.

Then the board is dried with paper.

(a) Patch template (b) Microstrip line and ground planemasks

Figure 5.3: Masks for UV exposure

42

5.1. Manufacturing

Figure 5.4: UV exposure

(a) Microstriplines (b) Ground planes

Figure 5.5: Revealing process

4. Acid etching

Etching is a critically important process step. Etching must entirely remove the top layer

of a multilayer structure, without damaging the underlying or masking layers. The etching

system’s ability to do this depends on the ratio of etch rates in the two materials. The board is

introduced inside the etching device fixing the speed, see Figure 5.6. Then the substrates are

ready to be used, Figure 5.7.

Figure 5.6: Acid etching

43


Figure 5.7: Final drying

5. Cut and drill

Once the different layers of the antenna are manufactured, they need to be accurately cut,

drilled and stacked together. An SMA connector has been used and soldered to the antenna.

The different antenna layers for the stacked patch model are shown in Figure 5.8. For the

single patch model the second patch layers 6 and 7 are not used. Figure 6.2 shows one of the

final antennas for the stacked patch configuration once the layers are stacked together and

the connector is soldered.

Figure 5.8: Different layers of the final antenna

During this process, when cutting the ground substrate, there were some manufacturing

44

5.2. Measurements

problems and the feeding substrate of the model 2 got damaged, making it impossible to

proceed with the manufacturing. Model 1, 3 and 4 shown in chapter 4 were manufactured and

measured with no inconvenience.

Figure 5.9: Antenna for stacked patch configuration

5.2 Measurements

5.2.1 Set up

Two main equipments will be used to measure the different parameters of the antenna de-

signed. On one hand S11 parameters were be measured using the network analyzer in the lab

environment and on the other hand gain, polarization and radiation pattern were measured

in the anechoic chamber environment, also using a network analyzer.

A network analyzer is an instrument that measures the network parameters of electrical

networks. Today, network analyzers commonly measure S–parameters because reflection and

transmission of electrical networks are easy to measure at high frequencies, and these are often

used to characterize two-port networks such as amplifiers and filters, but they can be used

on networks with an arbitrary number of ports [42]. For the measurements a vector network

analyzer (VNA) will be used, this type of network analyzer measures both amplitude and phase

properties. Figure 5.10 shows a generalized block diagram of the major signal-processing

sections that a Network Analyzer requires in order to measure the incident, reflected and

transmitted signals.

The signal processing-sections are the following [6]:

1. Source for stimulus, the signal source supplies the stimulus for the stimulus-response test

system either sweeping the frequency of the source or its power level.

2. Signal-separation devices, it measures a portion of the incident signal to provide a reference

for ratioing. It also separates the incident and reflected traveling waves at the input of the

device under test (DUT).

3. Signal down conversion and detection. Directivity is one of the most important parameters

for couplers, is a measure of how well a coupler can separate signals moving in opposite

directions.

45


Figure 5.10: Network analyzer block diagram [6]

4. Processor to calculate and review the results, now reflection and transmission data is format-

ted in ways that make it easy to interpret the measurement results.

Figure 5.11 pictures the measurements of the antenna using the VNA (HP 8720C).

Figure 5.11: VNA measures

Gain, circular polarization and radiation patterns were measured using the anechoic chamber,

Figure 5.12 shows the integration of the antenna to the chamber to proceed the tests of the

measurements.

46

5.2. Measurements

Figure 5.12: Antenna inside the anechoic chamber, LEMA

If the radiation pattern of an antenna was measured in a laboratory, reflections from walls,

ceiling, floor or equipments would modify its shape adding dips or forcing the signal in

certain radiations [43]. A good but expensive solution to avoid these reflections is to make

the measurements in an anechoic chamber. An anechoic chamber (meaning non-reflective

and non-echoing) is a room designed to absorb reflections of either sound or electromagnetic

waves. They are also insulated from exterior sources of noise. The combination of both aspects

means they simulate a quiet open-space of infinite dimension, which is useful when exterior

influences would otherwise give false results [44].

5.2.2 Results

Reflection coefficient

First results have been measured with the VNA. Reflection coefficient Figures for each of

the manufactured models can be seen in Figures (5.13,5.14,5.15). The black trace shows the

measured results which is compared to the respective simulated results for each model shown

in the colored traces. Prototypes were measured both after screwing and gluing.

47


Figure 5.13: Aperture-coupled stripline with a single patch (Model 1) – Reflection coefficient

Figure 5.14: Aperture-coupled stripline with stacked patch configuration (Model 3) – Reflectioncoefficient

48

5.2. Measurements

Figure 5.15: Aperture-coupled stripline with stacked patch configuration (Model 4) – Reflectioncoefficient

Measured results show worse matching than simulation results, and more losses also appear

when the different layers of the antenna are not sticked together. This can be due to possible

gaps of air in between the layers that can be avoided when they are glued together. Model 1

which corresponds to a single patch with a stripline, does not fulfill the matching requirements

but both models with two patches, comply with the demanded specifications. It can be seen

that the measured results response is shifted towards lower frequencies. The permittivity

of the foam and the substrates is higher in real materials than the simulated ones. A higher

permittivity (εr ) leads to a lower wavelength, therefore the electrical length is decreased so the

resonant is lowered.

λ= Co

fpεr

(5.1)

Measured bandwidths for models 1, 3 and 4 respectively are 17.2%, 22% and 21.6%. Possible

causes for these lower values can be a misalignment of the different layers. Manufacturing

process can also cause some undesired deterioration of the result because of the losses

introduced by the connector, the adapter and the screws. Another factor that could have

influenced the results is the surface flatness of the foam or the board.

49


Gain

The anechoic chamber has been utilized for gain measurement and results can bee seen on

Figures (5.16,5.17,5.18). Again, measured results are shown in black and are compared to their

respective simulation results.

Figure 5.16: Aperture-coupled stripline with a single patch (Model 1) – Gain

Figure 5.17: Aperture-coupled stripline with stacked patch configuration (Model 3) – Gain

50

5.2. Measurements

Figure 5.18: Aperture-coupled stripline with stacked patch configuration (Model 4) – Gain

In order to obtain the gain a reference antenna was used, this antenna was a horn ’Narda 642’

(5.4 – 8.2 GHz). For this gain measurement, the probe antenna inside the chamber is aligned

with the standard gain (reference) antenna and the gain is measured, then this one is replaced

by the antenna under test (AUT), which is the aperture-coupled patch antenna in our case.

This way, we can reference both gains. Gain measurements show results very close to the

simulated ones having smaller values at lower frequencies and being the gain even higher

than the simulations results at higher frequencies.

Radiation patterns

Radiation patterns measured are shown in the diagrams below in comparison to the simulation

results. Black traces show the measurements in the anechoic chamber while the colored traces

red, blue and green are for models 1, 3 and 4 respectively. Far fields were measured from θ=-85°

to θ=85°.

(a) f=7.1 GHz (b) f=7.8 GHz (c) f=8.5 GHz

Figure 5.19: Measured and simulated radiation patterns for the aperture-coupled striplinewith a single patch (Model 1)

51


Model 1 measures show a beamwidth of 58° for the lowest frequency (f = 7.1 GHz), 84° for the

central frequency (f = 7.8 GHz) and 40° for the higher frequency (f = 7.1 GHz).


Figure 5.20: Measured and simulated radiation patterns for the aperture-coupled striplinewith stacked patch configuration (Model 3)

Model 3 measures show a beamwidth of 54° for the lowest frequency (f = 7.1 GHz), 26° for the

central frequency (f = 7.8 GHz) and 44° for the higher frequency (f = 7.1 GHz).


Figure 5.21: Measured and simulated radiation patterns for the aperture-coupled striplinewith stacked patch configuration (Model 4)

Model 4 measures show a beamwidth of 63° for the lowest frequency (f = 7.1 GHz), 85° for

the central frequency (f = 7.8 GHz) and 44° for the higher frequency (f = 7.1 GHz). Radiation

pattern is out of the desired range at low frequencies. Model 3 also shows the best results in

comparison with the requirements in this case. We can deduce from the radiation patterns

that poor cross polarization is one of the main limitations of our antenna.

Axial ratio

Axial ratio diagrams show good results for circular polarization as it can be seen in Figures

(5.22, 5.23, 5.24). Measurements show very good results as an axial ratio lower than 3 dB is

achieved in all the frequency band and for the stacked patch models is lower than 2 dB in most

of the frequency range.

52

5.2. Measurements

Figure 5.22: Aperture-coupled stripline with a single patch (Model 1) – Axial ratio

Figure 5.23: Aperture-coupled stripline with stacked patch configuration (Model 3) – Axialratio

53


Figure 5.24: Aperture-coupled stripline with stacked patch configuration (Model 4) – Axialratio

54

6 Project planning

6.1 Gantt diagram

Four main parts of the project can be distinguished.

Research is one of the most important parts of the project and is when time is invested in

learning, revising concepts of the topic that is going to be worked in, reading papers and

documents about the antenna design and understanding the challenges of the project in order

to be able to provide the best solution possible. During this period I dedicated myself on

research about CubeSat antennas and possible antennas that could be used understanding its

advantages and disadvantages. HFSS tutorials are essential to be able to manage the software

and understand the results given by the software, so this is included on the research part of

the project.

Once the main objectives were clear I started designing the basic model of the antenna, as

it was the first model, it took longer to design it and evaluate the results, it was important to

understand the behavior of the basic model of the antenna to be able to make the correct

modifications and achieve the desired results afterwords. As it is reflected on the diagram, the

deep study of the lineal polarized model took longer whereas the evolution of this model was

faster and more efficient once the basic behavior of the antenna was understood and after

having acquired a greater HFSS knowledge. This simulation part also involves the optimization

of the final designs before starting the physical design and measures. To achieve the most

realistic results possible, all the real materials that were going to be used for the antenna were

included in the simulation before starting the optimization process.

The last part of the project was to proceed with the physical design of the antenna. The

manufacture and measures were carried through as explained previously on chapter 5.

Document plan for the project and a critical review were delivered in order to document the

main objectives of the project and any change that could have occurred, in my case there

were no changes from the main objectives. Project final memory was done in parallel with the

design. Finally, the oral defense will be prepared once the memory is delivered.

55

Chapter 6. Project planning

September October November December January

Task 1.1Task 1.2Task 2.1Task 2.2Task 2.3Task 2.4

Task 3Task 4.1Task 4.2Task 4.3Task 4.4

TasksResearch Manufacturing Measurements DocumentsTask1.1-Research Task2.1-LP antenna Task3-Physical design Task4.1-Project planTask1.2-HFSS Tutorials Task2.2-CP antenna Task4.2-Critical review

Task2.3-Stacked Patch Task4.3-MemoryTask2.4-Optimization Task4.4-Oral defense

Table 6.1: Gantt sections

6.2 Cost plan

An estimated cost plan of the project has been made. The cost of the engineers has been con-

sidered to be 40 CHF per hours if considering the client/enterprise requires the service from

the engineer. This cost has been estimated taking into account that according to MyScience.ch

[45], the average gross wage of an engineer per year in Switzerland is just over 100.000 CHF.

This price will vary depending on the country the project is made. In my case, this project has

been done as a Master Project, so engineer working hours should not be taken into account as

it is part of my bachelor program. The number of hours dedicated for the design section will

also change depending on the engineer, an experienced engineer would need less hours for

the design but will also have a bigger cost.

HFSS license has been considered to be 2500 CHF, this will be if considering that the project is

being made at a university as they have 98% of discount on software licenses, if the project

was accomplished on a private enterprise the license would grow up to 100.000CHF, so the

proportional cost for the project would also increase.

Material prices have been approximated to the quantity required for the antenna considering

that the project is done somewhere where the materials are available, otherwise if the materials

had to be ordered, a minimum quantity is required so the prices would rise up. This prices are

approximate prices where LEMA (Laboratory of electromagnetics and antennas, EPFL) buys

the material, but they would not vary from one country to another as most of this materials

are ordered abroad.

56

6.2. Cost plan

Design procedure Cost (CHF)Antenna design (engineer-600h) 24.000

HFSS licence 180

Total cost for Design 24.180

ManufacturingFoam (1 Slide) 30FR4 (3 Slides) 7

Conductive paste 2RogersRT/duroid 5870 (1 Slide) 72

Manufacturing(equipment) 200Manufacturing (engineer-24h) 960

Total cost for manufacture 1.271

MeasuringAnechoic Chamber 500

Measuring (engineer-16h) 640

Total cost for measuring 1.140

TOTAL COST OF THE PROJECT 26.591 CHF

Table 6.2: Description of the costs of the project

Prices for all the measures of the parameters of the antenna take place on the anechoic cham-

ber which cost has been considered around 500 CHF without including the engineer who is in

charge of the measurements.

In my case this project has been carried out at LEMA where most of the equipment and

machinery is available to carry out the experimental part and the manufacturing.

57

7 Conclusion and future developments

The main purpose of this project was to design an X-band antenna for a CubeSat satellite that

satisfied some of the main challenges that this process involves. The principal requirements

to be accomplished were high gain within 7 – 10 dB, small dimensions of a maximum of

100x100x10 (mm3), a beamwidth of 40 – 50 in all the frequency range and circular polariza-

tion of the antenna.

To proceed with a design that satisfied all the requirements, an aperture-coupled patch an-

tenna using the slot feeding technique was chosen. After the study of the main model to

understand the effect of each parameter and its influence on the results, a crossed slot with

one or two patches seemed to give the desired results. Four main models accomplished most

of the design criteria, two of them with a single patch and the other two with a stacked patch.

The results and comparison of each of the models are summarized in table 7.1.

Single patch Stacked patchParameter Model 1 Model 2 Model 3 Model 4Impedance bandwidth(%) 25.0 25.4 27.2 27.8AR bandwidth (%) 20.2 20.1 33.3 32.0Beamwidth () 50 – 60 40 – 62 50 – 66 50 – 62Gain (dB) 6.2 – 9.1 6.4 – 9.3 8.3 – 9.2 7.0 – 9.4

Table 7.1: Simulation results for each model

As we can conclude from the results, models 1 and 2 with a single patch and a stripline do not

follow gain and beamwidth requirements. The stacked patch model (two patches) was then

designed to increase the gain which also leaded to a narrower beamwidth. Patch, feeding line

and slots dimensions of model 3 achieve a higher and more constant gain but from the other

hand the matching in the frequency range is worsened. Both designs using stacked patch

configuration satisfy the requirements and objectives of the antenna.

After the optimization of the design in HFSS, it was proceeded with the manufacturing of all

59

Chapter 7. Conclusion and future developments

the models. During this process, model 2 was damaged so its measurements results could not

be completed. Measured results with the VNA and anechoic chamber are shown on table 7.2.

Single patch Stacked patchParameter Model 1 Model 2 Model 3 Model 4Impedance bandwidth(%) 17.2 – 22.0 21.6Axial ratio < 3 dB – < 3 dB < 3 dBBeamwidth () 58 – 84 – 26 – 54 44 – 85Gain (dB) 5.8 – 9.4 – 8.2 – 9.5 6.6 – 9.9

Table 7.2: Measured results for each model

Measurement results for the rest of the models show more losses, specially matching losses,

than simulated results. Possible causes for this losses could be a misalignment of the different

layers. Manufacturing process can also cause some undesired deterioration on the result

because of the losses introduced by the connector, the adapter and the screws. Another

factor that could have influenced the results is the surface flatness of the foam or the board.

Measured results response is shifted towards lower frequencies.

In conclusion, after the study of possible antennas for a CubeSat satellite [10], an aperture-

coupled stacked patch antenna has given the desired results according to the requirements.

Two models with two patches have been simulated and measured. Model 3 dimensions show

the most adequate results for this antenna design characteristics.

One of the future developments that could be done to continue this project is the integration

of the antenna to the CubeSat. In this case, the CubeSat will work in two frequency bands

(X-band and S-band), so this X-band antenna will need to be integrated with another S-band

antenna which works at lower frequencies. This integration could lead to undesired coupling

and cross-polarization losses and worsen the results. During the design of this antenna, we

have seen that the ground plane size does not have a big influence on the results, so a possible

solution for this integration could be to use the CubeSat metallic structure as the ground

plane of the antenna. An aperture-coupled patch antenna using slot feeding technique has

shown to be a good option for this project but this does not make it the unique solution.

Depending on the further applications of the satellite, other antennas may be more suitable.

Aperture-coupled antenna with a coaxial feed could be a good option but some techniques

would need to be done to enhance its bandwidth and increase the gain. An antenna array may

also be a good solution to cover all the requirements [10].

60

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