antennas and receivers in radio astronomy2 eleventh synthesis imaging workshop, june 10-17, 2008...

Eleventh Synthesis Imaging WorkshopSocorro, June 10-17, 2008

Antennas and Receivers in

Radio Astronomy

Mark McKinnon

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Eleventh Synthesis Imaging Workshop, June 10-17, 2008

Outline

• Context• Types of antennas• Antenna fundamentals• Reflector antennas

– Mounts– Optics

• Antenna performance– Aperture efficiency– Pointing – Polarization

• Receivers

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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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VLA @ 4.8 GHz (C-band) Interferometer Block Diagram

Antenna

Front End

IF

Back End

Correlator

KeyAmplifier

Mixer

X Correlator

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Types of Antennas

• Wire antennas– Dipole– Yagi– Helix– Small arrays of the above

• Reflector antennas• Hybrid antennas

– Wire reflectors– Reflectors with dipole

feeds

m)1( >λYagi

Helix

m)1( <λm)1( ≈λ

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Basic Antenna FormulasEffective collecting area A(ν,θ,φ) m2

On-axis response A0 = ηAη = aperture efficiency

Normalized pattern(primary beam)A(ν,θ,φ) = A(ν,θ,φ)/A0

Beam solid angle ΩA= ∫∫ A(ν,θ,φ) dΩ

all sky

A0 ΩA = λ2

λ = wavelength, ν = frequency

ΔΩΔ= ννφθνφθνφθ ),,(),,(),,( IAP

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What determines the beam shape?

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

F(l,m) = complex far-field voltage patternl = sinθcosφ , m = sinθsinφ

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

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

For VLA: θ3dB = 1.02/D, First null = 1.22/D, D = reflector diameter in wavelengths

Aperture-Beam Fourier Transform Relationship

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Antenna Mounts: Altitude over Azimuth

• Advantages– Cost – Gravity performance

• Disadvantages– Zone of avoidance – Beam rotates on sky

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Parallactic angle

Beam Rotation on the Sky

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Antenna Mounts: Equatorial

• Advantages– Tracking accuracy– Beam doesn’t rotate

• Disadvantages– Cost– Gravity performance– Sources on horizon at

pole

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Prime focus Cassegrain focus

Offset Cassegrain Naysmith

Beam Waveguide Dual Offset

Reflector Optics

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Reflector Optics: Limitations

• Prime focus– Over-illumination (spillover) can increase system temperature

due to ground pick-up– Number of receivers, and access to them, is limited

• Subreflector systems– Can limit low frequency capability. Feed horn too large.– Over-illumination by feed horn can exceed gain of reflector’s

diffraction limited sidelobes• Strong sources a few degrees away may limit image dynamic range

• Offset optics– Support structure of offset feed is complex and expensive

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Prime focus Cassegrain focus(GMRT) (AT)

Offset Cassegrain Naysmith(VLA) (OVRO)

Beam Waveguide Dual Offset(NRO) (GBT)

Reflector Optics: Examples

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Feed Systems

VLA

EVLA

GBT

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Antenna Performance: Aperture Efficiency

On axis response: A0 = ηAEfficiency: η = ηsf × ηbl × ηs × ηt × ηmisc

ηsf = Reflector surface efficiencyDue to imperfections in reflector surfaceηsf = exp(−(4πσ/λ)2) e.g., σ = λ/16 , ηsf = 0.5

ηbl = Blockage efficiencyCaused by subreflector and its support structure

ηs = Feed spillover efficiencyFraction of power radiated by feed intercepted by subreflector

ηt = Feed illumination efficiencyOuter parts of reflector illuminated at lower level than inner part

ηmisc= Reflector diffraction, feed position phase errors, feed match and loss

rms error σ

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Primary Beam

l=sin(θ), D = antenna diameter in contours:−3,−6,−10,−15,−20,−25,wavelengths −30,−35,−40 dB

dB = 10log(power ratio) = 20log(voltage ratio)VLA: θ3dB = 1.02/D, First null = 1.22/D Voltage radiation pattern, |F(l,m)|

πDl

Antenna Performance: Aperture Efficiency

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Subreflectormount

Quadrupod

El encoder

Reflector structure

Alidade structure

Rail flatness

Az encoder

Foundation

Antenna Pointing: Practical Considerations

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Pointing AccuracyΔθ = rms pointing error

Often Δθ < θ3dB /10 acceptable,because A(θ3dB /10) ~ 0.97

BUT, at half power point in beamA(θ3dB /2 ± θ3dB /10)/A(θ3dB /2) = ±0.3

For best VLA pointing use Reference Pointing. Δθ = 3 arcsec = θ3dB /17 @ 50 GHz

Δθ

θ3dB

Primary beam A(θ)

Antenna Performance: Pointing

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Antenna can modify the apparent polarization properties of the source:

• Antenna structure– Symmetry of the optics – Reflections in the optics– Curvature of the reflectors

• Quality of feed polarization splitter– Constant across the beam

• Circularity of feed radiation patterns– No instrumental polarization on-axis,– But cross-polarization varies across the beam …

Antenna Performance: Polarization

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VLA 4.8 GHz cross-polarizedprimary beam

Off-Axis Cross Polarization

Cross-polarized aperture distribution

Cross-polarizedprimary beam

Field distribution in aperture of paraboloid fed by electric dipole

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• Reference received power to the equivalent temperature of a matched load at the input to the receiver

• Rayleigh-Jeans approximation to Planck radiation law for a blackbody

Pin = kBT Δν (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

Receiver

Gain GB/W Δν

Matched load @ temp T (oK) Pout=G*Pin

Pin

Receivers: Noise Temperature

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TA = ηAS/(2kB) = KS

S = source flux (Jy)

SEFD = system equivalent flux density

SEFD = Tsys/K (Jy)

129078.3440-5083658.3926-4060655.5118-2638537.5412-1831131.568-1226228.604-824527.622-423621.501-2 SEFDTsysηBand (GHz)

EVLA Sensitivities

Receivers: SEFD

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EVLA Q-Band (40-50 GHz) Receiver

LO

Ref

35dB

RC

P IF

Out

8-18

GH

z

40-50GHz

NoiseDiodePol

LNA

Old

New

x3

Dewar

NRAOCDL

35dB

LNA

NRAOCDL

Dorado4IWC45-1

PamtechKYG2121-K2

(w/g)

Noise/COMNC 5222

ENR > 20 dB

LC

P IF

Out

8-18

GH

z16

-19.

5 G

Hz

IsolatorMica

T-610S1010-20 GHz

PA8207-2F16.0-19.3 GHz

Magic-TMDL

22TH12B

RCP

LCP

TCal

0∀3

dBmVariable

AttenuatorNRAO

DC-BlockInmet8055H

0.01-18 GHz

IsolatorMICA

T-708S408-18 GHz

24dB

AtlanticMicrowaveAMC 1233

Septum Polarizer& Cal Coupler

IsolatorDorado

4IWN45-1A(UG38 → UG599)

Tripler/Mixer Assembly Spacek

3XM45-8.4-0.1L/RRF=40-50 GHz

Post-AmpModuleCaltech

3XM45-8.4-0.1L/RRF=40-50 GHz

Limiting LO AmplifierNorden

N03-4010

16.0-19.5 GHzPOut = 21.0 ± 0.5 dBm

for ±6 dBm input

x3

DC-BlockInmet8055H

0.01-18 GHz

IsolatorMICA

T-708S408-18 GHz

24dB

IsolatorDorado

4IWN45-1A(UG38 → UG599)

Tripler/Mixer Assembly Spacek

3XM45-8.4-0.1L/RRF=40-50 GHz

Post-AmpModuleCaltech

3XM45-8.4-0.1L/RRF=40-50 GHz

IsolatorDitom

D3I75107.5-10 GHz

Integrated

18 dBm

LO SplitterMAC Tech

40-50 GHz

Remove

Some New

TCal

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Equation 3-8: replace u,v with l,mFigure 3-7: abscissa title should be πDl

Corrections to Chapter 3 of Synthesis Imaging in Radio Astronomy II

antennas and receivers in radio astronomy2 eleventh synthesis imaging workshop, june 10-17, 2008...

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