Tenth Summer Synthesis Imaging Workshop

University of New Mexico, June 13-20, 2006

Antennas in Radio Astronomy

Peter Napier

2Outline

• Interferometer block diagram• Antenna fundamentals• Types of antennas• Antenna performance parameters• Receivers

3Radio Telescope Block Diagram

Radio Source

Receiver

FrequencyConversion

Signal Processing

SignalDetection

ComputerPost-detection

Processing

Antenna

4E.g., VLA observingat 4.8 GHz (C band)

Interferometer Block Diagram

Antenna

Front End

IF

Back End

Correlator

Key

Amplifier

Mixer

X Correlator

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• Antenna amplitude pattern causes amplitude to vary across the source.• Antenna phase pattern causes phase to vary across the source.• Polarization properties of the antenna modify the apparent polarization of the source.• Antenna pointing errors can cause time varying amplitude and phase errors.• Variation in noise pickup from the ground can cause time variable amplitude errors.• Deformations of the antenna surface can cause amplitude and phase errors, especially at short wavelengths.

Importance of the Antenna Elements

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Wavelength > 1 m (approx) Wire AntennasDipole

Yagi

Helixor arrays of these

Wavelength < 1 m (approx) Reflector antennas

Wavelength = 1 m (approx) Hybrid antennas (wire reflectors or feeds)

Feed

General Antenna Types

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Effective collecting area A(,,) m2

On-axis response A0 = A = aperture efficiency

Normalized pattern(primary beam)A(,,) = A(,,)/A0

Beam solid angle A= ∫∫ A(,,) d

all sky

A0 A = 2 = wavelength, = frequency

Basic Antenna Formulas

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f(u,v) = complex aperture field distributionu,v = aperture coordinates (wavelengths)

F(l,m) = complex far-field voltage pattern

l = sincos , m = sinsin

F(l,m) = ∫∫aperturef(u,v)exp(2i(ul+vm)dudv

f(u,v) = ∫∫hemisphereF(l,m)exp(-2i(ul+vm)dldm

For VLA: 3dB = 1.02/D, First null = 1.22/D, D = reflector diameter in wavelengths

Aperture-Beam Fourier Transform Relationship

9Primary Antenna Key Features

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+ Beam does not rotate + Lower cost

+ Better tracking accuracy + Better gravity performance

Higher cost Beam rotates on the sky

Poorer gravity performance

Non-intersecting axis

Types of Antenna Mount

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Parallactic angle

Beam Rotation on the Sky

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Prime focus Cassegrain focus (GMRT) (AT, ALMA)

Offset Cassegrain Naysmith (VLA, VLBA) (OVRO)

Beam Waveguide Dual Offset (NRO) (ATA, GBT)

Reflector Types

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Prime focus Cassegrain focus (GMRT) (AT)

Offset Cassegrain Naysmith (VLA) (OVRO)

Beam Waveguide Dual Offset (NRO) (ATA)

Reflector Types

14VLA and EVLA Feed System Design

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Aperture Efficiency

A0 = A, = sf bl s t misc

sf = reflector surface efficiency

bl = blockage efficiency

s = feed spillover efficiency

t = feed illumination efficiency

misc= diffraction, phase, match, loss

sf = exp((4/)2)

e.g., = /16 , sf = 0.5

rms error

Antenna Performance Parameters

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Primary Beam

l=sin(), D = antenna diameter in contours:3,6,10,15,20,25,wavelengths 30,35,40 dBdB = 10log(power ratio) = 20log(voltage ratio)

For VLA: 3dB = 1.02/D, First null = 1.22/D

Dl

Antenna Performance Parameters

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Pointing Accuracy = rms pointing error

Often < 3dB /10 acceptable

Because A(3dB /10) ~ 0.97BUT, at half power point in beam

A(3dB /2 3dB /10)/A(3dB /2) = 0.3

For best VLA pointing use Reference Pointing.

= 3 arcsec = 3dB /17 @ 50 GHz

3dB

Primary beam A()

Antenna Performance Parameters

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Subreflector mount

Quadrupod

El encoder

Reflector structure

Alidade structure

Rail flatness

Az encoder

Foundation

Antenna Pointing Design

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Surface: = 25 m

Pointing: = 0.6 arcsec

Carbon fiber and invar

reflector structure

Pointing metrology structure

inside alidade

ALMA 12m Antenna Design

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Polarization

Antenna can modify the apparent

polarization properties of the source:• Symmetry of the optics• Quality of feed polarization splitter• Circularity of feed radiation patterns• Reflections in the optics• Curvature of the reflectors

Antenna Performance Parameters

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Cross polarized Cross polarized

aperture distribution primary beam

VLA 4.8 GHz

cross polarized

primary beam

Off-Axis Cross Polarization

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VLA 4.8 GHz

Far field pattern amplitude

Phase not shown

Aperture field distribution

amplitude.

Phase not shown

Antenna Holography

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Noise Temperature

Pin = kBT (W),

kB = Boltzman’s constant (1.38*10-23 J/oK)

When observing a radio source Ttotal = TA + Tsys

Tsys = system noise when not looking

at a discrete radio source

TA = source antenna temperature

TA = AS/(2kB) = KS S = source flux (Jy)

Receiver

Gain G B/W

Matched load Temp T (oK)

Pout=G*PinPin

Rayleigh-Jeans approximation

Receivers

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TA = AS/(2kB) = KS S = source flux (Jy)

SEFD = system equivalent flux density

SEFD = Tsys/K (Jy)

Band (GHz) Tsys SEFD

1-2 .50 21 236

2-4 .62 27 245

4-8 .60 28 262

8-12 .56 31 311

12-18 .54 37 385

18-26 .51 55 606

26-40 .39 58 836

40-50 .34 78 1290

EVLA Sensitivities

Receivers (cont)

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Equation 3-8: replace u,v with l,m

Figure 3-7: abscissa title should be Dl

Corrections to Chapter 3 of Synthesis Imaging in Radio Astronomy II