array configurations studies and antenna element design...

Nima Razavi-Ghods3-5 May 2010, BYU, Provo, Utah USA

Array Configurations Studies and Antenna Element Design for Low

Frequency Band SKA Nima Razavi-Ghods*Eloy de Lera Acedo

University of Cambridge

International Workshop on Phased Array Antenna Systems for Radio AstronomyBrigham Young University, Provo, Utah, USA


Overview

• SKA AA-lo overview and specifications

• AA-lo antenna design and simulations

• AA-lo array configuration studies (regular vs sparse)

• Numerical simulation: sky noise contribution to Tsys

• Future efforts & collaborations


SKA AA-lo: Overview

• Wideband elements (dual-polarisation) covering a frequency range of 70 – 450 MHz.

• Direct digitisation of each signal path followed by polyphase filter and digital down conversion.

• Will involve multi-stage beamforming• Each AA-lo station providing an effective area of

~19,000 m2 (approx requiring 52 stations for full SKA)• The AAVP within PrepSKA is tasked to undertake

detailed design and costing work of AA-lo.


AA-lo Station Trial Specs.

Parameter Specification RemarksLow frequency MHz 70 Lowest frequency expected for the EoR

Nyquist frequency MHz 100 Frequency with element spacing is λ/2, defines max A-eff

High frequency MHz 450 Freq where sky noise is low, overlaps with AA-hi and/or dishes

Frequency coverage contiguous There are no gaps between low and high frequency

Bandwidth, max MHz 380 Individual beams can operate over the full frequency range

Polarisations 2 Orthogonal

Station diameter m 180 Determined from SKA2. Uses 250 arrays for expected SKA2 sensitivity

Geometric area m2 ~25,000

No. of element types 1 A single wide-band element type e.g. bow-tie or conical spiral

No. of elements 13,568 Each element is low gain, dual polarisation

Scan angle range deg ±45 Will operate at larger scan angles, but sensitivity not defined

Sensitivity @ 100 MHz m2/K 17Single array on boresight Sensitivity varies over the band.Assumes Tsys= Tsky= 1000K. 70% for appodisation.

Frequency channel kHz 250 Assumes 2048 channels splitting the full sample rate, further channelization will be required at correlator

Output data rate Tb/s 16Defines the survey performance of the array.

Can be used flexibly for frequency, bandwidth and number of beams


The BLU Antenna

Low profile Bow-tie antenna element covering 70 – 450 MHz.

GND @ λ/4 at max. freq.

No dielectric.


The BLU Antenna

• Infinite array simulations have been carried out to analyze sensitivity of a unit cell containing the BLU antenna versus the inter-element spacing, the antenna size and the tilt angle of the arms.


The BLU Antenna

• The sky will dominate a large part of the band. Furthermore, grating lobes will show up in the band.


The BLU Antenna

• In the infinite regular array, the sensitivity of a unit cell improves overall for more sparse configurations


The BLU Antenna

• Larger antennas bring a multi-lobulation issue at high frequencies.


The BLU Antenna

• An optimum tilt angle can be found for the antenna arms. The impedance match improves and so does the receiver noise. This is of special interest for the region in between the dense and sparse regions.


AA-lo Array Design Parameters• Array Size (fundamental limit on Aeff/Tsys) • Array Geometry (main beam and side-lobe response/profile)

– Fully filled grids (regular)– Sparse or thinned grids– Truly randomised grids

• Antenna Element (scan/polarisation response, mutual coupling)• Operating Frequency, Processing Bandwidth• Weighting Schemes (beam shape and side-lob profile)

– Spatial windows (e.g. Hamming, Hann, Kaiser)– Side-lobe profile (e.g. Dolph-Chebyshev/Taylor, Fourier design method)– Adaptive nulling

• Back-end processing– Fully digital core (any weighting in single or multiple stages)– First level analogue (some limitations in response)


Antenna Array Geometries


Randomised Array: AA-lo

d = λ/3 : 2λ


Technical Simulations• TA was analysed as the beam tracked three

cold patches on the sky over four and half hours.

• Array factor based simulations were carried computed using NFFT approach.

• AA-lo antenna configurations composed of 10,000 elements.

• Geometries included regular, triangular, sparse random, thinned, circular, and fully random.

• Four minimum inter-element separations of 0.5λ, 0.8 λ, 1λ, and 2λ.

• Weights: Uniform, Taylor and Dolph-Chebyshev (SLL = 35 dB)


The Observable Sky


SKA AA-lo Observable Sky

• Region 1: 09h07m12s 00°00’46’’, Region 2: 04h03m36s -34°48’00’’ Region 3: 04h45m00s -61°00’00’’

R1

R2

R3


Results for TA: Region 1


Results for TA: Region 2


Results for A/T: Region 1


Results for A/T: Region 2


Taylor Weighting


Future Work & Collaborations

• Faster and more accurate simulations of the station beam based on MBF approach (UCL collaboration).

• Computation framework for interferometric calculation (Oxford collaboration: OSCAR).

• Further analysis of beam synthesis techniques and weight calibration, Question: how often do the weights need to be updated.

• Work regarding optimal geometry, e.g. far out versus close in side-lobes.

• Scaled array design and simulation (some collaboration with Peter Hall)


Scaled BLU Antenna Array

• Scaled and small SKA AA-lo station

• 400 elements.• Scale: 30:1 of the BLU

antenna.• 1 m2 (the SMA)• UWB baluns.• Band: 2.1 GHz to 13.5

GHz.


System Temperature

η = Antenna radiation efficiency

TP = Physical temperature of antenna

Trec = Receiver noise temperature

(Antenna matching, LNA model, Friis formula)

(1 )sys recPAT T T Tη η= + − +

( )4

4

, , , , sin

, , sin

oboA

o

T P d dT

P d dπ

π

θ φ ν θ φ ν θ θ φν

θ φ ν θ θ φ

=∫∫

∫∫

rr

r

array configurations studies and antenna element design...

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