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Abstract
Perhaps there
is no other antenna project in the
world has elicited more interest
on the part of scientists and engineers
as has the Square Kilometer Array
(SKA). SKA is a
radio telescope,
which is being developed as a joint
project - truly international
in nature - with participation from
19 countries, including the UK,
Netherlands, South Africa, Canada,
the US, Australia, New Zealand,
China and India. When fully developed,
it will have a total collecting
area of approximately one
square kilometer,
will operate over a wide range of
frequencies, and its size will make
it 50 times more sensitive than
any other radio instrument built
to-date. Unlike conventional phased
arrays, SKA will rely upon digital
signal processing for beamforming,
and will be able to survey the
sky more than ten
thousand times faster than has been
possible hitherto. As a consequence,
it will require extremely high-performance
central-computing engines, as well
as long-haul optical fiber links,
with a capacity that would exceed
the existing
Internet traffic
of Europe! The array of receiving
stations, which will comprise the
telescope, will extend out to a
distance of 3,000 km from a concentrated
central core, enabling the system
to push - almost to its ultimate
limit - the tradition of
radio astronomy,
namely providing the highest
resolution
images. The SKA
will combine the signals received
from thousands of small antennas
spread over a distance of more than
3000 kms to simulate a giant radio
telescope capable of extremely high
sensitivity and angular resolution.
The SKA will also have a very large
field-of-view (FOV) with a goal
at
frequencies below
1 GHz of 200
square degrees
and of more than 1 square degree
(about 5 full Moons) at higher frequencies.
One innovative development is the
use of
phased-array technology
to provide multiple FOVs. This will
greatly increase the survey speed
of the SKA and enable multiple users
to observe different pieces of the
sky simultaneously. The unique combination
of a very large FOV and high sensitivity
would enable the SKA to explore
far deeper into the Universe than
has ever been done before.
The SKA will
be built in the
southern hemisphere,
either in
South Africa or
Australia &
New Zealand, where
the view of our own
galaxy, namely
the
Milky Way, is best
and
radio interference
least. With a budget of over €1.5
billion, construction of the SKA
is scheduled to begin in
2013, with initial
observations projected by 2017,
and full operation expected to begin
by 2022. The goal of the project
is to probe into the mysteries of
our universe, its very origin, as
well as its evolution starting all
the way from the big bang.
As mentioned
earlier, the SKA will scan and map
the sky with a sensitivity of two
orders of magnitude higher than
the present-day radio interferometers.
It is projected to ultimately operate
over a large frequency band, ranging
from approximately 300 MHz up to
20 GHz, and the projected goal of
the receiver sensitivity metric,
Aeff/Tsys,
to be on the order of 20000 m2/K,
where
Aeff
is the effective collecting area
and
Tsys
is the system equivalent noise temperature.
A back of the envelope calculation
quickly shows that for a canonical
50K system noise temperature, the
total collecting area would have
to be equal to 1 km2,
which is obviously extremely large.
The challenge, therefore, is to
minimize the system-noise temperature,
which is a very challenging task
indeed for non-cooled antenna systems.
Five projects
have been identified by the radio
astronomy community as being the
key science drivers for the SKA.
They are:
• Cradle of
Life
• Probing the
Dark Ages
• The origin
and evolution of Cosmic Magnetism
• Strong field
tests of gravity using pulsars and
black holes
• Galaxy evolution,
cosmology and dark energy
The SKA project
is a long-term endeavor with a truly
international flavor. A number of
different antenna technologies are
being considered each of which operates
in a certain frequency sub-band.
Currently, one of these designs,
is being implemented by ASTRON of
Netherlands, and an artist’s impression
of the Square Kilometer Array being
fabricated by the above organization
is shown in Fig. 1. The Australian
version of the SKA is shown in Fig.2,
together with a sketch of the image
captured by the telescope (anticipated).
Both aperture
(Fig.3) and focal plane array (Fig.4)
concepts are being developed at
ASTRON as SKA pathfinders. These
instruments will cover a substantial
part of the frequency spectrum,
namely (0.01
< f
<
10) GHz. Below 300 MHz, electrically
small dipole antennas are positioned
over a non-uniform grid whose sparsity
level increases with the distance
to the center of the array. Above
300 MHz, contiguous arrays of Tapered
Slot Antennas (TSAs) are better
suited for both the aperture and
focal plane array concepts. Because
the number of antenna elements is
relatively large, the manufacturing
cost needs to be minimized. This
requires a high level of integration
with the attached electronics.
The design and
analysis of large TSA arrays constitutes
a challenging task for the reasons
explained below. To increase the
operational frequency bandwidth,
the outer edges of the TSA fins
are (entirely) connected to the
adjacent elements to preserve the
continuity of the surface current
across TSA boundaries. Discontinuities
introduced by slots and gaps of
sufficient size tend to radiate
and, consequently, disrupt the impedance
and radiation characteristics. One
consequence of this type of connection
is that the numerical analysis of
the entire array problem can no
longer be reduced to that of analyzing
a single isolated TSA element, as
is done conventionally when analyzing
a phased array. As a result, few
existing commercial software tools
can handle large finite antenna
arrays because of the excessive
burdens on the cpu time and
memory placed by the simulation
problem at hand. The usual approach
to circumventing this difficulty,
namely imposing the periodic boundary
conditions to reduce to the idealized
(doubly-periodic infinite) array
simulation problem to that of a
unit cell, is also not practical
in this particular case, because
the excitation scheme can be non-uniform,
and the edge truncation effects
can be significant.
As mentioned
previously, several different antenna
configurations are being considered
by different groups around the world,
for different frequency sub-bands.
A focal plane array (FPA) is being
designed by CSIRO in Australia for
the ASKAP project. The reflector
and feed combination is illustrated
in Fig.5. The FPA for this project
is a checkerboard type of array,
also shown in this figure, which
has its own unique design and simulation
challenges.
The presentation
will be divided into three parts.
The first of these will describe
the overall SKA project in some
detail, and will include the motivation,
long-range plans and the present
status. Next, it will discuss some
newly developed simulation techniques
for both the TSA (ASTRON) and the
FPA/reflector systems for designing
these highly complex antenna configurations.
(For some representative results,
see Figs.6 and 7). Finally, we will
look at the Crystal Ball and venture
some predictions on the future of
this gargantuan project, which is
not only very unique but also an
unusually exciting development from
the perspective of the antenna engineer.
Before closing
we mention that an excellent source
of reference for the SKA project
is the SKA website (www.skatelescope.org),
and the reader is encouraged to
visit the website for further information.
Finally, we would
like to have knowledge his colleagues
Rob Maaskant of ASTRON and Stuart
Hay of CSIRO, who have been the
principal players in designing the
antennas and systems for the SKA
at their respective organizations,
and have been very generous very
generous in providing wealth of
information on their antenna projects
to this author.
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