We present the design of a phased array system for future radio telescopes. This includes a system overview and recent results of the designed and implemented system, the Electronic Multi-Beam Radio Astronomy Concept (EMBRACE). Furthermore, simulations with a full-EM antenna simulator, combined with measurements on actual hardware, will provide information for the next design step, the Aperture Array Verification System (AAVS). With AAVS, we will prove design readiness of this novel array technology

Arts, M. J.

Benthem, P.

Kant, G. W.

Maaskant, Rob

Ruiter, M.

van der Wal, E.

Wijnholds, S. J.

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Aperture array development for future large radio telescopes
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Aperture Array Development for Future Large
Radio Telescopes
P. Benthem∗, G. W. Kant∗, S. J. Wijnholds∗, M. J. Arts∗, R. Maaskant∗, M. Ruiter∗, E. van der Wal∗
∗ASTRON
Oude Hoogeveensedijk 4, Dwingeloo, The Netherlands
benthem@astron.nl
Abstract—We present the design of a phased array system
for future radio telescopes. This includes a system overview
and recent results of the designed and implemented system, the
Electronic Multi-Beam Radio Astronomy Concept (EMBRACE).
Furthermore, simulations with a full-EM antenna simulator,
combined with measurements on actual hardware, will provide
information for the next design step, the Aperture Array Verifica-
tion System (AAVS). With AAVS, we will prove design readiness
of this novel array technology.
Index Terms—phased arrays, antenna arrays, array design,
mutual coupling, radio astronomy, array signal processing.
I. INTRODUCTION
Since the early 1990s, the global radio astronomy commu-
nity has been contemplating the design and development of
a radio telescope with nearly two orders of magnitude higher
sensitivity than current instruments. This radio telescope is
now known as the Square Kilometre Array (SKA) [1]. A key
figure of merit for the SKA is the survey speed, a measure how
fast a given fraction of the sky can be mapped out with a given
sensitivity. The survey speed is proportional to the product of
the square of the instantaneous telescope sensitivity and the
instantaneous field-of-view (FoV) [2].
The cost per unit survey speed for a given system concept
depends strongly on the operating frequency. For frequencies
below 1.5 GHz, the aperture array concept seems very at-
tractive [3]. A consortium of European research institutes led
by ASTRON in the Netherlands has therefore launched the
aperture array verification program (AAVP) to demonstrate
that astronomical aperture arrays are sufficiently mature to be
a viable concept for SKA over the frequency range between
500 and 1500 MHz. The aim is to show this point by building
the aperture array verification system (AAVS) and show that
it adheres to the SKA specifications.
The design of the AAVS will greatly benefit from the
experience gained from its precursor, the Electronic Multi-
Beam Radio Astronomy Concept (EMBRACE). EMBRACE
is a single polarization phased array demonstrator with two
stations, one in Westerbork, The Netherlands and the other in
Nanc¸ay, France. Figure 1 shows a photograph of one of these
stations. In the next section, we will describe the EMBRACE
antenna system in more detail, including recent results. The
antenna array simulations and measurements of the current
design will be presented next. In the last section, we will
outline the next steps towards the AAVS and SKA based
Fig. 1. Photograph of the Vivaldi elements of one of the EMBRACE stations.
TABLE I
EMBRACE DEMONSTRATOR MAIN REQUIREMENTS.
Requirement Remark Value
Number of stations In France and the
Netherlands
2
Total physical
collecting area
Both stations 300 m2
Aperture efficiency ≥ 0.8
Frequency range 0.5 – 1.5 GHz
System temperature @ 1 GHz ≤ 100 K
Instantaneous array
bandwidth
RF beams 100 MHz
Number of analog
beams
2
Polarization Single linear 1
Half power beam
width
RF beam @ 1 GHz > 15 deg
Scan range θ ≥ 45 deg
Side lobe levels w.r.t. main beam. No
grating lobes
≤ −13.2 dB
Signal dynamic range @ output A/D
converter
≥ 60 dB
design on the experience gained from EMBRACE.
II. EMBRACE
Table I summarizes the main requirements for the
EMBRACE antenna system. The system provides two inde-
pendent analog beams of approximately 16◦ beam width at
1 GHz.
A. Architecture
In the development phase towards SKA, it is imperative to
demonstrate the low cost potential of sufficiently large designs
like EMBRACE. Therefore, costs have been a key design
driver throughout the design process.
An EMBRACE station can be divided roughly into two
parts: a front-end and a back-end. The front-end consists
of the antenna array including the radome, the supporting
mechanics for the array and the signal distribution between
the tiles and the back-end. A large phased array system is
inherently distributed over a large area. Since active Low Noise
Amplifiers (LNAs) have to be integrated close to the elements,
the distribution of DC power already forms an important cost
driver. In EMBRACE the number of required cables is reduced
by multiplexing DC power, control signal and an analog RF
beam signal on one coaxial cable.
To achieve electrical continuity between tiles, the radiating
elements on the final tile design were oriented at 45 degree
angles with the tile edges. The final tile design contains 2×72
Vivaldi elements and has an area of 1.125 m2. The mechanical
design has evolved to a dual polarization arrangement of
Vivaldi radiators, in which each radiator is laser cut out of
a solid thin aluminum sheet and the individual radiators are
connected by extruded profiles.
The design of the phased array started with the scanning
requirement at the highest frequency of operation. This pro-
vided the element separation, producing a radiation pattern
free of grating lobes within the visible range. Each tile
produces two independent analog RF beam, which follow
from the combination of all the elements. Bot beams can be
electronically steered to the horizon using a combination of
phase shifters and time delay lines, ignoring the grating lobes
at the highest operating frequencies. The FoV of one of the
resulting RF beams is inversely proportional to the size of the
tile. It follows that with an element separation of 12.5 cm, the
array element array can meet the scanning requirement up to
45 degrees from zenith at 1.4 GHz in combination with the
FoV requirement.
More detailed information concerning the EMBRACE sys-
tem can be found in [4]
B. Recent Results
1) Pulsar First Light: EMBRACE has achieved the first
ever pulsar detection with an L-Band Aperture Array during
the night of 1-2 December 2010. This is an exciting and
important result, as the detection of such a weak astronomical
signal, especially considering the limited available collecting
area of EMBRACE, places high demands on the pointing
precision of the beam and the dynamical behavior of the
EMBRACE array at WSRT.
This first light measurement was carried out over 13 hours
at a central frequency of 1207 MHz, using a total bandwidth of
12 MHz. The detected pulsar, PSR B0329+54, has a rotational
period of 714 milliseconds, and a phase-averaged flux density
of 203 mJy. For this observation, the EMBRACE configuration
consisted of 24 tiles, resulting in a total effective collecting
area of 27 m2. The system temperature is approximately
100 K.
Figure 2 shows the pulsar signal as a function of time and
rotational phase, with the cumulative pulse profile, i.e. the sum
of all pulses, shown at the top (and repeated twice for clarity).
Only a 2-hour section of the data is shown because the signal
varies strongly in brightness due to scintillation.
Fig. 2. EMBRACE Pulsar First Light.
2) First HI Map: After the pulsar first light, EMBRACE
continued producing excellent results. On January 7, 2011 a
24 hours survey was held to detect HI. For this experiment
an 4x4 tile configuration was used, resulting in a collecting
area of 18 m2. An integration time of 14 seconds per pointing
was used. The available bandwidth of 12 MHz was processed,
resulting in sufficient frequency redundancy to correct for gain
variations and removal of sub-band selection filter effects,
located on the Remote Station Processing (RSP) boards. Figure
3 shows the measured HI map with EMBRACE in Equatorial
coordinates.
For this experiment we adapted the post-processing pipeline
to perform real-time FFT, correlation and integration off all
samples.
C. Antenna Array
The precursor of EMBRACE within ASTRON’s R&D pro-
gram on wide band aperture array technology, was the Thou-
sand Element Array (ThEA), which successfully demonstrated
a wide band phased array based on Vivaldi elements [5].
Therefore, the Vivaldi radiator was chosen as the appropriate
radiator to produce a scanning array covering a factor 3
in frequency. The dimensions of a single antenna element,
including feed geometry, are shown in table II, III, Fig. 4 and
Fig. 5.
Fig. 3. H1 detection with EMBRACE.
Fig. 4. Final feed geometry.
TABLE III
FINAL FEED BOARD DIMENSIONS.
Feed Dimensions Value
Stripline Width (Wl) 1 mm
Stub Radius (Rs) 15 mm
Stub Opening Angle (V) 96◦
Stub Outer Angle (Vo) 12◦
Board Width (p) 82 mm
Board Length (j) 70 mm
Fig. 5. Final antenna geometry.
TABLE II
FINAL ANTENNA DIMENSIONS.
Antenna dimensions Value
Element Separation (H) 124 mm
Aperture Height (A) 118 mm
Tapered Slot Length (Ls) 294 mm
Rate of Exponential Slotline (R) 0.01 mm−1
Cavity Length (D) 26 mm
Slotline Width (Ws) 2 mm
Constant Slotline Length (S) 15 mm
Cavity Ground Distance (G) 15 mm
Total Antenna Length (L) 350 mm
45 Degrees in E−plane
Broadside
45 Degrees in H−plane
0.2 0.4 0.6 0.8 1.2 1.6 1.8 2.01.0 1.4
1
2
3
4
5
6
7
8
f (GHz)
V
S
W
R
Fig. 6. Simulated active VSWR for an infinite array of EMBRACE-elements.
As the demonstrator stations are large phased arrays (20λ×
20λ), the array can be modeled using the infinite array
approach. We used a periodic boundary finite difference time
domain (PB-FDTD) code developed in [6]. The simulated
active VSWR for broadside scan, 45-degrees E-plane scan and
45-degrees H-plane scan is shown in figure 6.
For broadside scan the active VSWR is better than 2
between 0.3 and 1.5 GHz. Around 1.6 and 1.8 GHz two blind
scan angles occur. If the array is scanned 45 degrees in the
E-plane an additional scan blindness will occur around 1.4
GHz. Furthermore scanning the array 45 degrees in the E-
and H-plane degrades the performance between 0.7 and 1.1
GHz.
Verification measurements of the scattering parameters were
done with a single tile. Because a single tile can not be
considered as an infinite array, a (finite) tile is simulated using
an in-house developed full-EM antenna simulator (CAESAR
[7]).
To validate this simulator a small tile of 16 elements was
built, simulated and measured. A picture of this small tile is
shown in Fig. 7. Only one polarization of this tile (8 elements)
Fig. 7. Small test array.
0.5 1 1.5
1
2
3
4
5
6
7
Frequency (GHz)
VS
W
R
 
 
simulated
measured
Fig. 8. Active VSWR for center element of small array.
is used and equipped with feed boards.
The active VSWR of one of the two center elements is
shown in Fig. 8. The phase of the active reflection coefficient is
shown in Fig. 9. One can see that the correspondence between
measurement and simulation resembles very well. The active
VSWR of a single tile is shown in Fig. 10. The corresponding
phase active reflection coefficient is shown in Fig. 11.
III. NEXT STEPS
We are now verifying the results obtained from a full
system simulator (combination of full-EM antenna simulator
and circuit simulator) [7] with measurements on the actual
hardware. This is an important step, because one of the main
challenges for the AAVS will be to demonstrate that aperture
arrays can meet the SKA requirements within specified cost
envelop. This requires further improvement of the mechanical
design to allow efficient production and assembly of the
antenna system. Several routes have already been explored
0.5 1 1.5
−150
−100
−50
0
50
100
150
Frequency    [GHz]
de
gr
ee
s
 
 
simulation
measurement
Fig. 9. Phase of active reflection coefficient for center element of small
array.
0.4 0.6 0.8 1 1.2 1.4
1
2
3
4
5
6
7
Frequency (GHz)
VS
W
R
 
 
simulated
measured
Fig. 10. Active VSWR for center element of a single tile.
0.4 0.6 0.8 1 1.2 1.4
−150
−100
−50
0
50
100
150
Frequency    [GHz]
de
gr
ee
s
 
 
simulation
measurement
Fig. 11. Phase of active reflection coefficient for center element of a single
tile.
including copper Vivaldi profiles printed on a thin foil. We
need accurate performance simulations to assess new ideas
quickly.
Early experience with the EMBRACE stations in Wester-
bork and Nanc¸ay also showed, that to demonstrate aperture
arrays can meet the SKA requirements within power and cost
envelopes, a site is required with RFI levels similar to the
envisaged SKA sites. Therefore, AAVS will be built on the
SKA site, which will be known after the decision on the final
SKA site has been made around 2013.
The AAVS will consist of about 15 stations with a total
collecting area in the order of 2000 m2. The individual
components of the system are currently being designed and
reviewed. The array configuration of the AAVS stations is still
being discussed, but will be chosen such that it will not only be
a technological demonstrator, but will also fill the astronomical
niche of instruments with good short baseline coverage.
IV. CONCLUSIONS
In this paper we presented the design of a phased array
system for future radio telescopes. We also presented recent
results of EMBRACE, proving the system capabilities in an
early stage of evaluation. Full-EM simulation and measure-
ment results assure a well functioning next step design, ready
for scientific measurements, when placed on the correct site.
ACKNOWLEDGMENT
The authors would like to acknowledge the cooperation of
several European institutes within the project: The EMBRACE
team located at Astron, in The Netherlands, Observatoire de
Paris, France, INAF located in Bologna, Italy and Max Planck
Institute in Bonn, Germany. The authors have significantly
benefitted from numerous discussions with many colleagues
from Radio Astronomy institutes from all over the world. We
are grateful to all of them. This effort was supported by the
European Community Framework Programme 6, Square Kilo-
metre Array Design Studies (SKADS), contract no. 011938.
REFERENCES
[1] P. E. Dewdney, P. J. Hall, R. T. Schilizzi, and T. J. L. W. Lazio, “The
Square Kilometre Array,” Proceedings of the IEEE, , no. 8, pp. 1482–
1496, Aug. 2009.
[2] J. D. Bunton and S. G. Hay, “SKA cost model for wide field-of-view
options,” Experimental Astronomy, vol. 17, no. 1-3, pp. 381–405, 2004.
[3] A. van Ardenne, J. D. Bregman, W. A. van Cappellen, G. W. Kant, and
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[4] G.W Kant, M. Ruiter, E. van der Wal, and P. Benthem, “The building of
EMBRACE,” Widefields Science and Technologies for the SKA - SKADS
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[5] S. J. Wijnholds, A. G. de Bruyn, Bregman J. D., and J. G. bij de Vaate,
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