22-10-2012, 03:47 PM
Application Specificities of Array Antennas:
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Introduction
1.1 Array Antennas
With the ever increasing importance of wireless communication, antennas have occupied
a prominent position in everyday’s live. Though antennas are no more than
a transition between a guided wave on a transmission line system and a radiated
wave, many different ways of implementing this transition are in use. Indeed every
application has its specific demands regarding bandwidth, antenna size and radiation
directivity, resulting in a different antenna.
As a Fourier transform relates the currents on an antenna with the radiation pattern,
Sect. 3.2, it can be understood that in order to obtain a radiation pattern that
concentrates nearly all power in one spatial direction, the current carrying antenna
area should be large. One example of such antennas, are parabola dish antennas.
Another way of enlarging the antenna area, is (periodically) placing distinct antennas
into some array configuration, filling a much larger space. This however does not
allow current flowing over the entire area, as the current can only flow in the antenna
conductors, not in the space between the separate antennas. The disadvantage is
that in this way less spatial resolution can be obtained. The array of antennas, or
array antenna, has a wider main beam in its radiation pattern than an antenna of
the same size that allows current to flow over its entire area. Array antennas however
allow varying the direction of the main beam in an electronic way, without mechanical
movements and its associated problems such as wear and slow reconfiguration.
From an antenna point of view, it is natural to explain increased spatial resolution
of a structure of separate antennas based on the larger antenna area. From a signal
processing point of view, array antennas use multiple elements to sample a signal with
a spatial diversity in order to increase signal quality. Both approaches are equivalent.
Introduction
This work will only discuss the classical array antennas where each radiating element
is a translation of a base element, which is not the case for e.g. conformal arrays, and
where all array elements share the same signal, as opposed to the situation in true
Multiple Input Multiple Output (MIMO) systems.
Indeed, conformal arrays are generally spoken planar arrays that are bent and attached
to a surface with a certain curvature such as the body of an airplane, or the
hull of a ship. Consequently, the array elements are no longer translations of the
base element, but rotations are necessary to obtain all elements from the base element.
These rotations void the assumptions made in order to be able to split element
and location effects on the radiation properties of the array antenna, resulting in the
definition of the array factor. Hence classical array theory does not apply to conformal
array antennas, but fortunately literature, e.g. [1], is available on analysis and
synthesis of this specific array type.
Though literally spoken, any system using multiple antennas on both receiver and
transmitter can be regarded as a MIMO system, true MIMO supposes the use of
multiple channels. More specific, the information that is to be transmitted is divided
into several streams that are then divided in some way over (a subgroup of the)
multiple antennas [2, 3]. MIMO counts on the orthogonality of the channels, due
to the scattering and fading in the environment, and to the design of the sets of
receiving and transmitting antennas, to improve link capacity. Hence the antennas
in receiver and transmitter array do not share the same signal and moreover should
preferably not be translations of a base element, but have different radiation patterns
or polarizations and should preferably be spaced far apart [4].
Still of the classical phased arrays, an abundance of examples can be found in every
day life. Without having the intention of being exhaustive, a small enumeration of
systems that were studied, to a lesser degree or greater extent, during this work, is
given below:
• Communication: Global System for Mobile communication (GSM) base station
antennas (Gamma Nu EDBDP-900F/1800-17-65), satellite antennas for mobile
communication (IRIDIUM Sect. 4.4.2)
• RAdio Detection And Ranging (RADAR): missile detection radars (Thales
SMART-L), earth observation radars (RADARSAT Sect. 4.4.2), air traffic control
radars (Secondary RADAR in Bertem Sect. 3.3.2.3),
• Radio Astronomy: deep space probing (Square Kilometer Array (SKA) [5])
Outline of this Thesis
This thesis tackles array antennas used for yet two other selected, very different applications.
The first application, satellite communication, explained in detail in
Chapter 4, is a typical communication application. Bandwidths are generally spoken
small and all standard telecommunication engineering methods are valid. Designing
for space, however, requires special attention due to the hostile environment. Consequently,
in the design of a system for up link of in-situ collected data to an earth
observation satellite, much effort was spent on material and component selection.
Another interesting peculiarity of the design, was the application of an analog base
band implementation of a technique often used for digital beam forming.
Electromagnetic side channel analysis of cryptographic hardware, which is the second
application and is discussed in depth in Chapter 7, requires an approach sometimes
very different from standard telecommunication engineering methods. When
observing direct radiation of small currents performing cryptographic operations in
silicon hardware, the antennas are designed to be small and sensitive to magnetic
fields. Matching is not performed in order to assure power transfer, but to obtain a
high signal-to-noise ratio. The signal should be digitized with as less quantization error
as possible to allow calculation of correlation with a hypothesis in post-processing.
Array antennas should perform beam forming on very wide band signals and preferably
off-line to allow simultaneous monitoring of different active regions in the chip.
Covering two very distinct applications, allows to point out what aspects of array
theory and practice are application independent, and how applications will alter the
array design. Hence, besides the very technical treatments in this work, this leaves
some room for meta-discussions on the topic of array antennas in a more philosophical
way, which is an inevitable prerequisite for obtaining a Doctor of Philosophy (PhD)
degree.
In general, as explained at the beginning of Part I, the array antenna consists of:
• antenna elements,
• a signal shaping device for each antenna, to ensure constructive summing with
signals from other array elements,
• and the summing or combining network.
Obviously those three items will be discussed for both applications covered in this
work.
But firstly the general theory of array antennas is reviewed. At a sufficiently high
level of abstraction, some common possible ways of implementing beam forming will
be summed up. Then the mathematics of the array factor will demonstrate the
working principle of arrays and provide a basis for array topology design.
4 Chapter 1. Introduction
In a second part of this work, the application of array antennas for space communication
is discussed. After a chapter of introduction to this application, with some
definitions, its relevance, an overview of the current state of the art and reference to
the general design approach for space and its pitfalls, following chapters will zoom in
on the three parts of an array antenna, mentioned above: the antenna element, the
signal shaping device and the combining network.
The second application, also chronologically, is treated in a third part of this work.
Again, a first chapter introduces the reader to the world behind this application: side
channel analysis of cryptographic hardware. Here, again, the three parts, namely
antenna element, signal conditioning for constructive interference and signal combination
will make up the extensive treatment of the array for this application.
1.3 The Thesis at a Glance
Essentially, this work reviews much of the existing array theory and aims at indicating
what aspects of array theory and practice are application (in)dependent. This study is
performed by working out two applications in detail, namely satellite communication
and side channel analysis. Consequently, many problems and solutions at a smaller
scale, encountered in these two applications, are discussed throughout the work.
These smaller scale problems and solutions, in turn, are sometimes covered by existing
theory or common knowledge. When appropriate this existing material is reviewed.
Some new contributions, developed during the work reported here, are listed below,
to allow readers experienced in the field, to quickly browse to that parts that might
be of most interest to them.
• Sect. 4.1 describes a system design for in-situ data up link to an earth observation
satellite. This system would allow, for the first time in earth observation
history, to retrieve data from locations other than those that are under observation
with the imaging tool on board the satellite.
• Sect. 6.1 reports for the first time on an array that was designed and build by ir.
P. Delmotte to demonstrate an analog implementation of a technique commonly
used in digital beam forming.
• Sect. 6.2 adapts the design of Sect. 6.1 for usage on a satellite.
• Sect. 8.2 covers matching of several types of shielded loops.
• Sect. 8.3 generalizes the design of an RFID reader antenna, commonly described
in the literature for antennas that are small compared to the wavelength, to a
technique that is valid regardless of antenna size.
1.3. The Thesis at a Glance 5
• Sect. 8.4 calculates the maximal resolution of an inductive sensor based on
bandwidth and signal amplitude requirements.
• Appendix A elaborates on the possibility of compensating Doppler shift by
frequency scanning.
• Appendix D gives an improved signal processing technique to reconstruct an
image on a computer display after an interception of the radiation of the display
with an antenna.
The array content in Part III is very limited. While working on the side channel
analysis application, the lack of a good sensor element emerged. As the need for an
element, at the time of writing, was much stronger than the need for an array of
sensors, much effort was spent in studying and designing such element. As time is
however limited, this had its consequences on how much time could be invested in
studying arrays for this application.
invisible filling
overview
In this part, the general theory, applicable to any classical phased array antenna,
where the elements are translations of a base element, is reviewed.
Array antennas, when used for reception, essentially sample an incoming wave at
distinct spatial points. By combining the slightly different signals captured at all
those points, the original signal can be reconstructed, while interferers coming from
other sources, at other locations, can be suppressed, resulting in a higher Signal to
Noise Ratio (SNR).