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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
4.16 Radiation patterns for model 2, E plane . . . . . . . . . . . . . . . . . . . . . . . . 32
4.17 Radiation patterns for model 2, H plane . . . . . . . . . . . . . . . . . . . . . . . . 33
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
4.23 Radiation patterns for model 3, E plane . . . . . . . . . . . . . . . . . . . . . . . . 37
4.24 Radiation patterns for model 3, H plane . . . . . . . . . . . . . . . . . . . . . . . . 37
4.25 Radiation patterns for model 4, E plane . . . . . . . . . . . . . . . . . . . . . . . . 37
4.26 Radiation patterns for model 4, H plane . . . . . . . . . . . . . . . . . . . . . . . . 38
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
5.20 Measured and simulated radiation patterns for the aperture-coupled stripline
with stacked patch configuration (Model 3) . . . . . . . . . . . . . . . . . . . . . . 52
5.21 Measured and simulated radiation patterns for the aperture-coupled stripline
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
Chapter 2. Microstrip patch antennas
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
Chapter 2. Microstrip patch antennas
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
Chapter 2. Microstrip patch antennas
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
Chapter 3. Linearly polarized antennas
(a) Reflection coefficient (b) Smith Chart
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.
(a) Reflection coefficient (b) Smith Chart
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
Chapter 3. Linearly polarized antennas
(a) Reflection coefficient (b) Smith Chart
Figure 3.7: Microstrip line length sweep
(a) Reflection coefficient (b) Smith Chart
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].
(a) Reflection coefficient (b) Smith Chart
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
Chapter 3. Linearly polarized antennas
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
Chapter 4. Circularly polarized antennas
(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.
(a) Reflection coefficient (b) Smith Chart
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
Chapter 4. Circularly polarized antennas
(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
Chapter 4. Circularly polarized antennas
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
4.2. Aperture-coupled stripline patch with crossed slot
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
Chapter 4. Circularly polarized antennas
(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
(a) f=7.1 GHz, BW=60° (b) f=7.8 GHz, BW=54° (c) f=8.5 GHz, BW=60°
Figure 4.15: Radiation patterns for model 1, H plane
(a) f=7.1 GHz, BW=40° (b) f=7.8 GHz, BW=58° (c) f=8.5 GHz, BW=62°
Figure 4.16: Radiation patterns for model 2, E plane
32
4.2. Aperture-coupled stripline patch with crossed slot
(a) f=7.1 GHz, BW=60° (b) f=7.8 GHz, BW=54° (c) f=8.5 GHz, BW=60°
Figure 4.17: Radiation patterns for model 2, H plane
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
Chapter 4. Circularly polarized antennas
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
Chapter 4. Circularly polarized antennas
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
4.3. Aperture-coupled stacked patch with crossed slot
(a) f=7.1 GHz, BW=50° (b) f=7.8 GHz, BW=60° (c) f=8.5 GHz, BW=50°
Figure 4.23: Radiation patterns for model 3, E plane
(a) f=7.1 GHz, BW=64° (b) f=7.8 GHz, BW=50° (c) f=8.5 GHz, BW=66°
Figure 4.24: Radiation patterns for model 3, H plane
(a) f=7.1 GHz, BW=50° (b) f=7.8 GHz, BW=50° (c) f=8.5 GHz, BW=60°
Figure 4.25: Radiation patterns for model 4, E plane
37
Chapter 4. Circularly polarized antennas
(a) f=7.1 GHz, BW=60° (b) f=7.8 GHz, BW=58° (c) f=8.5 GHz, BW=62°
Figure 4.26: Radiation patterns for model 4, H plane
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
4.3. Aperture-coupled stacked patch with crossed slot
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
Chapter 5. Antenna prototype
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
Chapter 5. Antenna prototype
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
Chapter 5. Antenna prototype
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
Chapter 5. Antenna prototype
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
Chapter 5. Antenna prototype
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).
(a) f=7.1 GHz (b) f=7.8 GHz (c) f=8.5 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).
(a) f=7.1 GHz (b) f=7.8 GHz (c) f=8.5 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
Chapter 5. Antenna prototype
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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