Tenth Summer Synthesis Imaging Workshop University of New Mexico, June 13-20, 2006 Antennas in Radio Astronomy Peter Napier

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

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

4 E.g., VLA observing at 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 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 General Antenna Types

7 Effective collecting area A(, ,  ) m 2 On-axis response A 0 =  A  = aperture efficiency Normalized pattern (primary beam) A(, ,  ) = A(, ,  )/A 0 Beam solid angle  A = ∫∫ A(, ,  ) d  all sky A 0  A = 2 = wavelength, = frequency Basic Antenna Formulas

8 f(u,v) = complex aperture field distribution u,v = aperture coordinates (wavelengths) F(l,m) = complex far-field voltage pattern l = sin  cos , m = sin  sin  F(l,m) = ∫∫ aperture f(u,v)exp(2  i(ul+vm)dudv f(u,v) = ∫∫ hemisphere F(l,m)exp(-2  i(ul+vm)dldm For VLA:  3dB = 1.02/D, First null = 1.22/D, D = reflector diameter in wavelengths Aperture-Beam Fourier Transform Relationship

9 Primary 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

14 VLA and EVLA Feed System Design

15 Aperture Efficiency A 0 =  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 dB dB = 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) ~ 0.97 BUT, 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 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

19 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 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 P in = k B T  (W), k B = Boltzman’s constant (1.38* J/ o K) When observing a radio source T total = T A + T sys Tsys = system noise when not looking at a discrete radio source T A = source antenna temperature T A =  AS/(2k B ) = KSS = source flux (Jy) Receiver Gain G B/W  Matched load Temp T ( o K) P out =G*P in P in Rayleigh-Jeans approximation Receivers

24 T A =  AS/(2k B ) = KSS = source flux (Jy) SEFD = system equivalent flux density SEFD = Tsys/K (Jy) Band (GHz)  T sys SEFD 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