Antennas in Radio Astronomy
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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
5
• 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
6
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
7
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
8
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
10
+ 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
11
Parallactic angle
Beam Rotation on the Sky
12
Prime focus Cassegrain focus (GMRT) (AT, ALMA)
Offset Cassegrain Naysmith (VLA, VLBA) (OVRO)
Beam Waveguide Dual Offset (NRO) (ATA, GBT)
Reflector Types
13
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
15
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
16
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
17
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
18
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
20
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
21
Cross polarized Cross polarized
aperture distribution primary beam
VLA 4.8 GHz
cross polarized
primary beam
Off-Axis Cross Polarization
22
VLA 4.8 GHz
Far field pattern amplitude
Phase not shown
Aperture field distribution
amplitude.
Phase not shown
Antenna Holography
23
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
24
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)
25
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
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