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Phased Array Feed (PAF) Design for

the LOVELL Antenna based on the

Octagonal Ring Antenna (ORA) Array

M. Yang, D. Zhang, L. Danoon and A. K. Brown,

School of Electrical and Electronic Engineering

The University of Manchester, United kingdom

Anthony.brown@manchester.ac.uk

LOVELL Dish

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Parabolic reflector

D=76.2m (diameter of the dish)

F=22.9m (focal length)

f=1-4GHz (target)

theta_0:angle at the rim of the reflector dish is about 79.5 degree

Gain of LOVELL Dish

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𝐺 =4𝜋

𝜆2𝜀𝐴𝑃 =

𝜋2𝐷2

𝜆2𝜀

ε : aperture efficiency

𝐴𝑃: the physical area of the aperture

ORA Unit Cell Design for 1-4GHz

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Element separation: 35mm

Height of the structure: 42mm

H1 (distance between the ground plane and active layer): 29mm

H2 ( distance between the active layer and passive layer):13mm

Capacitor:0.35pF

Dual polarisation: Vertical/Horizontal polarisation

ORA Unit Cell Design for 1-4GHz contd.

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Return Loss > 10dB from 1-GHz

0.5 1 1.5 2 2.5 3 3.5 4 4.5-80

-70

-60

-50

-40

-30

-20

-10

0

Frequency(GHz)

dB

S11

S21

Size of ORA for LOVELL

With the scanning angle of 0.3 degree, there is an offset of 0.2m from the focus of the LOVELL dish, therefore the radius of the finite ORA (in circular shape) needs to be 0.2m at least.

Beam Deviation Factor (BDF), ‘Antenna theory and design’ by Stutzman and Thiele, pp.341

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

0.2

0.4

0.6

0.8

1

1.2

1.4

Offset(m)

Main

beam

direction t

heta

1GHz

4GHz

2.5GHz

BDF formula

Circular ORA for LOVELL

The finite ORA has 17 elements across its diameter with two edge elements loaded for the purpose of impedance match

In total there are 117 elements used (white squares)and yellow squares indicate the loaded elements

Size of the ORA:0.595 (m) x 0.595 (m)

Radiation Pattern of a Horn Antenna (reference)

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A horn antenna with a taper of -12dB at the rim of the reflector is used in GRASP across the whole frequency range

Good illumination efficiency about 75%

-100 -80 -60 -40 -20 0 20 40 60 80 100-12

-10

-8

-6

-4

-2

0

Angle of incidence (degree)N

orm

aliz

ed r

adia

tion p

att

ern

(dB

)

Proposed Excitation of ORA

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The central single element excited from 2.2GHz-4.0GHz

A good illumination efficiency (horn antenna as a reference) when used to illuminate LOVELL

Multiple elements excitation has lower illumination efficiency, therefore results in lower gain for the LOVELL

Normalized Radiation Pattern of the ORA (CST)

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Frequency (GHz) Aperture taper efficiency (t) Spillover efficiency (s) Illumination efficiency of LOVELL (t*s)

2.2 0.7587 0.9694 0.7355

3.0 0.7969 0.9769 0.7785

4.0 0.8174 0.9763 0.7980

Horn 0.8395 0.8997 0.7553

Radiation Pattern of LOVELL @2.2GHz (GRASP)

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Phi=0 Phi=90

Radiation Pattern of LOVELL @3GHz (GRASP)

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Phi=0 Phi=90

Radiation Pattern of LOVELL @4GHz (GRASP)

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Phi=0 Phi=90

Radiation Pattern of LOVELL @2.2GHz (Co-polar)

Phi=0 and Phi=90(GRASP)

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Radiation Pattern of LOVELL @3GHz (Co-polar)

Phi=0 and Phi=90(GRASP)

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Radiation Pattern of LOVELL @4GHz (Co-polar)

Phi=0 and Phi=90(GRASP)

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Overlapping of the Beamwidth

at 2.2GHz, 3GHz and 4GHz

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For PAF, it is desirable that the HPBW of one beam is overlapping with the HPBW of the next beam

This will be able to offer a larger field of view therefore more space of sky can be observed at once, which is one of the advantages of using PAF to feed the reflector dish instead of using a traditional horn antenna

Commercial LNA(ZX60-P33ULN+, MMIC)

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LNA based on ATF-35143

(Avago discreet)

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Steady State Heat Flow (Whole PAF)30 oC ambient, 400 W/m2 solar flux

Solar Heat, +1256 W

Convection

-303 WRadiation

-713 W

LNA Heat, +29 W

Convection, +16 WRadiation, +17 W

Co-axial cables, +6 W

Heat from sides, and

support structure, +67 W

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Cooling System Installation

Cryogenerator and

Water Chiller

Vacuum Jacket Piping

Cryogenic PAF Lovell Radio Telescope

Future integration work

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Future Work

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Illumination of the LOVELL antenna by single ORA element excitation is not beingoptimal and can be further optimized. The whole point is we don’t just uniformlyexcite the array, we will allow the beamformer to use non uniform weights thusoptimising the Array feed pattern. This work was not done yet so it is important tosay this is preliminary results and not yet optimised . We expect significantlyimproved results with weight optimisation.

For ORA PAF design, the optimal excitation scheme needs further investigation.The goal of the optimization will be to shape the ORA array pattern to get close toa Gaussian beam of an idealized horn antenna. Because the pattern of theGaussian beam is independent of the frequency, this poses a challenge for thedesign of ORA PAF. Generally the pattern of finite array will change as frequency:the gain increases with increasing frequency and it results in a narrower HPBW ifthe number of elements being excited remains the same across the wholefrequency range

What might be a possible solution to this challenge is to excite different number ofelements for the ORA PAF at different frequencies. A general rule is to excitesmaller number of elements at higher frequencies than that at lower frequencies.The optimization of the excitation of the ORA PAF could be something which can betaken forward for future work