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Mid Frequency Aperture Array Technology

Developments for the SKA

J. G. Bij de Vaate, P. Benthem, R. Witvers,

R. van den Brink

ASTRON

Dwingeloo, The Netherlands

vaate@astron.nl

Y. Zhang

Manchester University

Manchester, UK

David.zhang@manchester.ac.uk

S. A. Torchinsky

Station de radioastronomie

Observatoire de Paris, France

torchinsky@obs-nancay.fr

Abstract—This paper describes the technology developments

of antenna arrays for the mid frequency instrument of the

Square Kilometre Array radio telescope.

Keywords—radio astronommy; aperture arrays;

I. INTRODUCTION

The Square Kilometre Array, SKA [1], the next generation

radio telescope, is under development now. The SKA will be

100x more sensitive and will to survey the sky a million times

faster than any present radio telescope. The SKA will be

realized in Australia and in South Africa.

The SKA will be built in two phases: SKA1, which is

approximately 10% of the final system, followed by SKA2, the

complete system. Besides a ramp-up in size, SKA2 will also be

deployed with more advanced technology. One of the

advanced technologies which is being considered is Aperture

Array (AA) technology for the frequency band of 350-1450

MHz. An AA system in this frequency band would have

unprecedented performance advantages compared to a dish

based array: 1) A fully sampled unblocked aperture, 2) Large

field of view (~100 sq. deg @ 1GHz) and, 3) Multiple

independent fields of view. This creates an extremely fast

survey machine for HI at cosmological redshifts and will

allow a billion galaxy survey for Dark Energy science.

Note that low frequency aperture arrays are considered to

be more mature technology, extensively used in systems like

LOFAR [2] and therefore ready for SKA1.

II. SKA WORKPACKAGES

The SKA design effort is being executed by a number of

international consortia, all reporting to the SKA office at

Jordrell Bank (UK). The consortium for the AA-mid work

consists of ASTRON (lead), Observatoire de Paris, University

of Manchester, University of Cambridge, University of

Bordeaux, KLAASA (Hefei, China) and associate members

including African institutes. This Aperture Array MID

frequency (AAMID) consortium recently started the design

work which should lead to a Preliminary Design Review in

2016. Figure 1 gives an artist impression of a possible

realization of the AA-mid SKA system: hundreds of ~70m

diameter stations.

The work of the AAMID consortium builds on a long

international cooperation which started with the SKA Design

Study (SKADS), an EU FP-6 project, in 2005.

Fig. 1. Artist Impression of SKA2 AA-mid.

III. APERTURE ARRAY DEMONSTRATORS

The SKADS project realized two large AA demonstrators,

one in Westerbork, the Netherlands and one in Nançay,

France: the EMBRACE (Electronic Multi Beam Radio

Astronomy Concept) systems [3,4]. EMBRACE has 5000 to

10.000 Vivaldi antennas followed by a dual beam analogue

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beam former and a digital processing back-end. Figure 2 gives

a photograph of one the arrays. EMBRACE demonstrated

antenna array performance but also basic radio astronomy: HI

Milky Way observations, nearby galaxy and pulsar detections

[4].

Fig. 2 Photograph of the EMBRACE antenna array

Although successful, significant further development is

required in order to bring the SKADS technology to a

sufficient readiness level for SKA deployment. Cost reduction,

power consumption reduction and basic performance, e.g.

noise temperature, have to be improved.

IV. TECHNOLOGY DEVELOPEMENT

The following gives a snap shot of the technology

development work in AAMID:

A. Antenna design

Good results have been achieved with the Vivaldi antenna

in EMBRACE. EMBRACE is however a single polarization

instrument, therefore design effort will be required to realize a

dual pol. system [5]. Besides the Vivaldi a new planar antenna

concept is being considered, the Octagonal Ring Array (ORA)

[6]. The ORA concept has the potential of an improved

polarization performance and lower high volume production

cost.

B. Noise temperature

Fig. 3 Low Noise Tile (LNT) receiver noise measurements

The use of a recently released Skyworks transistor and an

improved Low Noise Amplifier (LNA) design demonstrated

good low noise temperatures, the Low Noise Tile (LNT) curve

in Figure 3. The frequency bandwidth in this test was limited

due to local RFI conditions. Besides off-the-shelf transistors

also custom IC are developed for the LNA.

C. Integration

EMBRACE uses a beam former IC (NXP process)

designed by the Obs. de Paris [7]. Development is required to

improve the current design, integrate more components on a

single chip and to reduce the complexity of the receiver by

means of direct sampling (>3Gsample/sec) for which an ADC

is developed.

D. Site environmental tests

The environmental conditions in the Karoo, the South

African SKA site, are very different from the conditions at

European sites. Therefore new tile development should

combine resistance to a hot desert climate with further cost

reduction. Figure 4 gives one the options which will be tested

at the SKA site.

Fig. 4. Design drawing of a new AA tile concept.

V. CONCLUSIONS AND FURTHER PLANS

High frequency aperture array technology is on the verge

to be mature enough for large scale deployment. After

completion of the AAMID technology development this will

be demonstrated with a science capable instrument (~2000m2

),

to be realized in the Karoo, as pathfinder for SKA2

REFERENCES

[1] The SKA telescope project, www.skatelescope.org

[2] LOFAR, www.lofar.org

[3] P. Benthem, et al., “EMBRACE: Results from an Aperture Array for

Radio Astronomy”, EuCAP 2012, Prague, Czech Republic.

[4] S. A. Torchinsky et al., “Characterization and Initial Results with

EMBRACE”, SF2A, Société Française d’Astronomie et

d’Astrophysique, June 2013, Montpellier, France

[5] M. J. Arts, B. Fiorelli, “Polarization studies of Vivaldi aperture arrays

for the Square Kilometre Array”, ICEAA 2013, Torino, Italy

[6] Y. Zhang, A. K. Brown, “Octagonal Ring Antenna for a Compact Dual- Polarized Aperture Array”, IEEE Transactions on Antennas and

Propagation, Volume 59, Issue 10, 2011

[7] S. Bosse, et al., “Beamformer ASIC in UHF-L band for the Square

Kilometre Array international project”, EuMIC 2010, Paris, France