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Antennas in Radio Astronomy

Antennas in Radio Astronomy. Peter Napier. Outline. Interferometer block diagram Antenna fundamentals Types of antennas Antenna performance parameters Receivers. Radio Source. Radio Telescope Block Diagram. Receiver. Antenna. Frequency Conversion. Signal Processing. Signal

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Antennas in Radio Astronomy

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  1. Antennas in Radio Astronomy Peter Napier

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

  3. Radio Source Radio Telescope Block Diagram Receiver Antenna Frequency Conversion Signal Processing Signal Detection Computer Post-detection Processing

  4. Interferometer Block Diagram E.g., VLA observing at 4.8 GHz (C band) Antenna Front End IF Back End Correlator Key Amplifier Mixer Correlator X

  5. Importance of the Antenna Elements • 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.

  6. General Antenna Types Wavelength > 1 m (approx) Wire Antennas Dipole Yagi Helix or arrays of these Wavelength < 1 m (approx) Reflector antennas Wavelength = 1 m (approx) Hybrid antennas (wire reflectors or feeds) Feed

  7. Basic Antenna Formulas Effective collecting area A(n,q,f) m2 On-axis response A0 = hA h=aperture efficiency Normalized pattern (primary beam) A(n,q,f) = A(n,q,f)/A0 Beam solid angle WA= ∫∫ A(n,q,f) dW all sky A0 WA = l2l= wavelength, n = frequency

  8. Aperture-Beam Fourier Transform Relationship f(u,v) = complex aperture field distribution u,v = aperture coordinates (wavelengths) F(l,m) = complex far-field voltage pattern l = sinqcosf , m = sinqsinf F(l,m) = ∫∫aperturef(u,v)exp(2pi(ul+vm)dudv f(u,v) = ∫∫hemisphereF(l,m)exp(-2pi(ul+vm)dldm For VLA: q3dB = 1.02/D, First null = 1.22/D, D = reflector diameter in wavelengths

  9. Primary Antenna Key Features

  10. Types of Antenna Mount + 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

  11. Beam Rotation on the Sky Parallactic angle

  12. Reflector Types Prime focus Cassegrain focus (GMRT) (AT, ALMA) Offset Cassegrain Naysmith (VLA, VLBA) (OVRO) Beam Waveguide Dual Offset (NRO) (ATA, GBT)

  13. Reflector Types Prime focus Cassegrain focus (GMRT) (AT) Offset Cassegrain Naysmith (VLA) (OVRO) Beam Waveguide Dual Offset (NRO) (ATA)

  14. VLA and EVLA Feed System Design

  15. Antenna Performance Parameters Aperture Efficiency A0 = hA, h=hsf´hbl´hs´ht´hmisc hsf = reflector surface efficiency hbl = blockage efficiency hs = feed spillover efficiency ht = feed illumination efficiency hmisc= diffraction, phase, match, loss hsf = exp(-(4ps/l)2) e.g., s = l/16 , hsf = 0.5 rms error s

  16. Antenna Performance Parameters Primary Beam l=sin(q), D = antenna diameter in contours:-3,-6,-10,-15,-20,-25, wavelengths -30,-35,-40 dB dB = 10log(power ratio) = 20log(voltage ratio) For VLA: q3dB = 1.02/D, First null = 1.22/D pDl

  17. Antenna Performance Parameters Dq Pointing Accuracy Dq=rms pointing error Often Dq<q3dB/10 acceptable Because A(q3dB /10) ~ 0.97 BUT, at half power point in beam A(q3dB/2 ±q3dB/10)/A(q3dB/2) = ±0.3 For best VLA pointing use Reference Pointing. Dq=3 arcsec = q3dB/17 @ 50 GHz q3dB Primary beam A(q)

  18. Antenna Pointing Design Subreflector mount Reflector structure Quadrupod El encoder Alidade structure Rail flatness Foundation Az encoder

  19. ALMA 12m Antenna Design Surface: s = 25 mm Pointing: Dq = 0.6 arcsec Carbon fiber and invar reflector structure Pointing metrology structure inside alidade

  20. Antenna Performance Parameters 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

  21. Off-Axis Cross Polarization Cross polarized Cross polarized aperture distribution primary beam VLA 4.8 GHz cross polarized primary beam

  22. Antenna Holography VLA 4.8 GHz Far field pattern amplitude Phase not shown Aperture field distribution amplitude. Phase not shown

  23. Receivers Receiver Matched load Temp T (oK) Gain GB/W Pout=G*Pin Pin 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) Rayleigh-Jeans approximation

  24. Receivers (cont) TA = AS/(2kB) = KS S = source flux (Jy) SEFD = system equivalent flux density SEFD = Tsys/K (Jy) EVLA Sensitivities

  25. Corrections to Chapter 3 of Synthesis Imaging in Radio Astronomy II Equation 3-8: replace u,v with l,m Figure 3-7: abscissa title should be pDl

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