ATA/SKA Issues

Jack Welch

ATA Log-periodic Feed 0.5-11 GHz

ATA Feed Gain vs. Frequency

G0 = 11.5 ± 1dB

Cross-Polarization Both Models

Expected Tsys for ATA

Tsys ˚K = 8 + 6.3√f(GHz) +7 + 7 +2.7 + 3 √f(GHz) (f(GHz))2.7

Diffractive spillover Ohmic losses CMBLNA

Atmosphere + geometric spillover

f(GHz) Tsys in K

0.5 51

1 34

4 33

10 40

Galaxy

Optimum Antenna Diameter SKA Memo #78

• Array of n antennas, each with diameter D total observing time T, point source sensitivity S for single pointing for survey

• Array cost

for single pointing

for survey

Optimum Antenna Diameter (more) SKA Memo #78

• Differentiate with respect to D to find the minimum cost for the array

for single pointing

for survey

ATA Antenna Constraints

• Feed Frequency Range: .5 GHz–11.2 GHz

• Feed Focus: s = 1.4 λ

• Feed focal ratio: .65

• Feed Gain: 11.5 db

• Antenna Diameter ~ 6m

Optical Design

• Key reference paper by Per-Simon Kildal PGAP, AP-31, #6 November 1983

• Aperture efficiency for two mirror system is

• ηIll is the illuminating efficiency; d is the secondary diameter; D is the primary; Cb is 1 for uniform illumination, ~1.5 for -10 dB taper. Cd Cb/π. The middle term is the aperture blockage. The third term is the edge diffraction loss. A0 is the amplitude edge illumination.

2

0

2

11

A

D

d

dC

D

dC dbIllA

Optical Design (continued)

• An optimum primary diameter for array survey sensitivity S∞ ND, with a modern wideband, single pixel feed system is ~ 6 m

• The secondary mirror should be no smaller than about 4 λ, 2.4 m for the longest wavelength of 60 cm.

• The blockage is large, and the diffractive sidelobes are high for symmetric Cassegrain with these reflector sizes

• An offset Gregorian is the better choice, which removes the middle term from the aperture efficiency expression and gives the low sidelobes of a clear aperture.

Paraxial PatternsO

ffse

t G

rego

rian

Off

set

Gre

gori

an

Pri

me

Foc

us

Cas

segr

ain

Beam Angle Beam Angle

ATA Optics: Offset Gregorian

2.4m secondary2.4m secondary

6m primary6m primary

Log-periodic FeedLog-periodic Feed with Actuatorwith Actuator

ShroudShroud

Radome

Gain Tables

Shroud Effects

• Echoes from the feed Pr = G2(90)

Pi 10 radius = -42 dB @ =500 MHz

• Primary Beam Waste: 4 bw 2• Secondary Beam Waste: 4 bw 3

Feed Patterns In And Out Of Focus

Antenna properties

• Surface RMS: 0.7mm (ok for 24 GHz)

• Aperture Efficiency: 0.6

• Low side lobes from clear aperture

• Feed at F/.65

• Good overall gain with feed at 6 GHz focus

1

10

19

28

37

46

55

64

73

82

91

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

Dish Position (arb.)

0.18-0.2

0.16-0.18

0.14-0.16

0.12-0.14

0.1-0.12

0.08-0.1

0.06-0.08

0.04-0.06

0.02-0.04

0-0.02

Ele

ctr

ic F

ield

Am

plit

ud

e (

arb

.)

Holographic Measurements at 4 GHz

Amplitude Phase

The Antenna Pattern

At 2.3 GHz

Spillover vs. Zenith Angle

Zenith Angle

Tsy

s

Antenna PropertiesOffset Gregorian ATA

• Surface accuracy 0.7 mm RMS (λ/20 @ 21GHz)

• Primary field of view 3.5º/f (GHz)

• Primary reflector blockage by secondary ≤ 2%

• Secondary diam = 2.4 m or 4 λ @ 500 MHz

• Optical pointing 10” RMS

• Assembly: 8 person days

• Tipping curve

20

10

0E

xces

s T

emp

erat

ure

(K

)

Elevation

*

* See slide show

Pointing Requirements

• Basic interferometry

• Single Antenna

• Multiple pointing interferometry (mosaicing)

Zero-spacing Problem

Single antenna transfer function

Multiple-pointing array transfer function

β= √u2+v2 D/λ

Pointing Errors Create Phase Errors

Effect Of Pointing Errors• With pointing errors δx and δy

• The effective transfer function is now

• At β = D/2λ, the expected value of

is 0.93 for σθ/Θ=0.1, a 0.1 beamwidth pointing error

• The same error results from the interferometry

• Altogether, 1/30th beamwidth pointing is required for map errors to be less than 10%

Filling In The Zero-spacing WithLarger Antennas

β= √u2+v2 D/λ

Single antenna transfer function

D = d array antenna diameter

2d3d

4d

Night Time Pointing

Long Term Tracking

GPS Pointing in the Daytime

ATA-42 from the Hat Creek hang glider launch

The ATA-42 Today

ATA Antenna Requirements