Antennas. Greg Taylor. University of New Mexico Spring Astronomy 423 at UNM Radio Astronomy

Transcription

1 Antennas Greg Taylor University of New Mexico Spring 2011 Astronomy 423 at UNM Radio Astronomy

2 Radio Window 2 spans a wide range of λ and ν from λ ~ 0.33 mm to ~ 20 m! (ν = 1300 GHz to 15 MHz )

3 Outline 3 Fourier Transforms Interferometer block diagram Antenna fundamentals Types of antennas Antenna performance parameters Receivers Dipole Antennas

4 Fourier Transforms 4

5 Fourier Transforms 5

6 6 Mixer Bandpass filter, IF amplifier Square law detector Software

7 E.g., VLA observing at 4.8 GHz (C band) Interferometer Block Diagram 7 Antenna Front End IF Back End Correlator

8 Importance of the Antenna Elements 8 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.

9 General Antenna Types 9 Wavelength > 1 m (approx) A e = Gλ 2 /4π Wire Antennas Dipole Yagi Helix or arrays of these Wavelength < 1 m (approx) Reflector antennas Feed Wavelength = 1 m (approx) Hybrid antennas (wire reflectors or feeds)

10 Basic Antenna Formulas 10 Effective collecting area A(ν,θ,φ) m 2 On-axis response A e = ηa η = aperture efficiency Normalized pattern (primary beam) A(ν,θ,φ) = A(ν,θ,φ)/A e Beam solid angle Ω A = A(ν,θ,φ) dω all sky A e Ω A = λ 2

11 Aperture-Beam Fourier Transform Relationship 11 f(u,v) = complex aperture field distribution u,v = aperture coordinates (wavelengths) F(l,m) = complex far-field voltage pattern l = sinθcosφ, m = sinθsinφ F(l,m) = aperture f(u,v)exp(2πi(ul+vm)dudv f(u,v) = hemisphere F(l,m)exp(-2πi(ul+vm)dldm For VLA: θ 3dB = 1.02/D, First null = 1.22/D, D = reflector diameter in wavelengths

12 The Standard Parabolic Antenna Response 12

13 Primary Antenna Key Features 13

14 Types of Antenna Mount 14 + 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

15 Beam Rotation on the Sky 15 Parallactic angle

16 Reflector Types 16 Prime focus (GMRT) Cassegrain focus (AT) Offset Cassegrain (VLA) Naysmith (OVRO) Beam Waveguide (NRO) Dual Offset (ATA)

17 Reflector Types 17 Prime focus (GMRT) Cassegrain focus (AT) Offset Cassegrain (VLA) Naysmith (OVRO) Beam Waveguide (NRO) Dual Offset (ATA)

18 Effelsberg 100-m telescope near Bonn, Germany 18

19 Reflector Types 19 Dual Offset Unblocked Aperture (GBT)

20 VLA and EVLA Feed System Design 20

21 Example Feed Horn 21

22 Focal Plane Arrays 22 8 x 9 Array for 2-7 GHz Ivashina Et al.

23 Antenna Performance Parameters 23 Aperture Efficiency A 0 = ηa, η = η sf η bl η s η t η misc η sf = reflector surface efficiency η bl = blockage efficiency η s = feed spillover efficiency η t = feed illumination efficiency η misc = diffraction, phase, match, loss η sf = exp( (4πσ/λ) 2 ) e.g., σ = λ/16, η sf = 0.5 rms error σ

24 Antenna Performance Parameters 24 Primary Beam πdl l=sin(θ), D = antenna diameter in wavelengths db = 10log(power ratio) = 20log(voltage ratio) For VLA: θ 3dB = 1.02/D, First null = 1.22/D contours: 3, 6, 10, 15, 20, 25, 30, 35, 40 db

25 Antenna Performance Parameters 25 Pointing Accuracy Δθ = rms pointing error Δθ Often Δθ < θ 3dB /10 acceptable Because A(θ 3dB /10) ~ 0.97 BUT, at half power point in beam A(θ 3dB /2 ± θ 3dB /10)/A(θ 3dB /2) = ±0.3 θ 3dB Primary beam A(θ) For best VLA pointing use Reference Pointing. Δθ = 3 arcsec = θ 3dB 50 GHz

26 26 Antenna Pointing Design Subreflector mount Reflector structure Quadrupod El encoder Alidade structure Rail flatness Foundation Az encoder

27 ALMA 12m Antenna 27 Surface: σ = 25 µm Pointing: Δθ = 0.6 arcsec Carbon fiber and invar reflector structure Pointing metrology structure inside alidade

28 Antenna Performance Parameters 28 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

29 Off-Axis Cross Polarization 29 Cross polarized aperture distribution Cross polarized primary beam VLA 4.8 GHz cross polarized primary beam

30 Antenna Holography 30 VLA 4.8 GHz Far field pattern amplitude Phase not shown Aperture field distribution amplitude. Phase not shown

31 Other Concerns 31 Pointing errors, especially at high frequencies Gain curves Atmospheric opacity corrections

32 Practical concerns continued Opacity corrections and tipping scans Can measure the total power detected as a function of elevation, which has contributions T sys = T 0 + T atm (1 e τ 0a ) + T spill (a) and solve for τ 0. Or, make use of the fact that there is a good correlation between the surface weather and τ 0 measured at the VLA (Butler 2002): 32 and apply this opacity correction using FILLM in AIPS

33 Noise Temperature Rayleigh-Jeans approximation P in = k B T Δν, k B = Boltzman s constant 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 = ηas/(2k B ) S = source flux (Jy) SEFD = system equivalent flux density SEFD = Tsys/K (Jy) Receivers Matched load Temp T ( o K) P in Receiver Gain G B/W Δν P out =G*P in Planned EVLA Sensitivities Band (GHz) η T sys SEFD

34 Hertz Dipole 34 A e = Gλ 2 /4π G=1.5 for Hertz Dipole G = 2.5 at 20 MHz G = 4.0 at 60 MHz for LWA

35 LWA Antenna 35

36 20 MHz 3D 36

37 E and H-Plane Antenna Pattern 37

38 Further Reading 38