Aeff/Tsys evaluation for
AAlo sensors
Cambridge 8th-10th December 2010
Federico Perini1, Jader Monari1, Giuseppe Virone2, Pietro Bolli3
1
INAF/IRA-Medicina radio telescopes
2 IEIIT/CNR-Politecnico di Torino
3 INAF/Osservatorio Astronomico di Cagliari
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Compare different
antennas?
White paper on the antenna sub-systems for the SKA project:
P.D. Patel, “Antenna Concepts Consideration for SKA”, part of the
SKA memo 91, “An SKA Engineering Overview” by P. Hall
G. Cortes Medellin, “Antenna Noise Temperature
Calculations,” SKA Technical Memo Series n° 95
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Antenna Temperature
General formulation:
Zenith
P  ; ,  T  ; ,   sin  d d

 
n
Tant  ; 0 ,  0 ,  0  
0
 0,
  0,2
P  ; ,  sin  d d

 
n
 0,
  0,2
0
Horizon
0 Direction in the azimuth plane
December 2010
b
where Pn(v;,) represents the antenna radiation
pattern and Tb(v;’,’), the brightness temperature
distribution (here Tb(v;’))
Pn computation is demanded to an external EM
Antenna CAD (GRASP, CST,…) whereas Tb is
evaluated by the Matlab code itself
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Coordinate reference systems
Two different coordinate reference systems: Pn is defined respect to the one
related to the antenna itself (z axes = main beam or pointing direction) whereas Tb
is defined respect to the Earth (z’ = local zenith)
Zenith
Circumference
at fixed  correspond to
several values of 
Ground
To compute the integral we need only one
coordinate system, so we need the
conversion from one to another:
      , , 0 ,  0 ,  0 
      , , 0 ,  0 ,  0 
   arccos sin 0 sin  sin   0   cos 0 cos 
 sin  0 sin  sin    0   cos  0 cos 0 sin  sin    0   cos  0 sin 0 cos 

  arctan 

cos

sin

cos




sin

cos

sin

sin




sin

sin

cos





0
0
0
0
0
0
0


December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Ground contribution
The brightness temperature can be split in 2 main contributions:
from sky (0° < ’ < 90°) and from ground (90° < ’ < 180°)
Tbsky  ;   Pn  ; , 
0     90
Pn  ; ,  Tb  ;    
Tb  ;   P  ; ,   Tb   ;   P  ; ,  90     180
Moreover the contribution from the ground (emission and scattering), is a
polarization dependent process:
Tb  ;    T sky  ;    T gnd      1  Tbsky  ;1   1   1   Tgnd
Tb   ;    Tsky  ;    Tgnd       1  Tbsky  ;1   1    1   Tgnd
2
   
1
cos    sin 
 cos    sin 
;    
cos    sin 
 cos    sin 
2
1
1

2
1
December 2010
2
2
2
2
1
2
1
2
2
1
2
1
1
2
Aeff/Tsys evaluation
TITLEfor AAlo sensors
1
AAVP Workshop
Brightness Temperature
The Tb model is the one proposed by [Cortes], which includes the effects of the
absorption coefficients of water vapour and oxygen, the CMB and the emission
from the galaxy
Atmosphere temperature
T
sky
b
 ;   T   e
  ,   0 , ha 
bo
 k

ha
0
H 2O
 ; z  k  ; z T  z  e
1   sin    1   z r   
O2
  ,   0 , z  
atm
2
e
T    T
bo
CMB
 T   
go
Zenithal opacity
dz
Effect of Earth curvature

0
Tgo and  are function of
the observed sky region
[Cortes] suggests
reasonable values as:
Tgo20K e 2.75.
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Tb calculator
ftp.ira.inaf.it/pub/outgoing/fperini/
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Ta calculator example 1:
BEST-1 @ 408MHz
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Ta calculator example 2:
32mt VLBI Dish @ 22GHz
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
AAlo Vivaldi antenna
Pattern at fmax=450MHz
Vivaldi Antenna, f=450MHz, 2=3.5
Pn resolution
,  (deg)
2.5
5
December 2010
Ta (K)
Tagnd (K)
Tasky (K)
69.4
69.1
52.4
51.6
17.0
17.5
Time comp.
(min)
13
3.5
For BEST-1 Tcomp=150 min,
using two different spacing for
Pn, fine (0.04°) for the main
beam and coarse (0.68° )
outside the main beam
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
AAlo Vivaldi antenna
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
TREC?
50Ohm
75Ohm
Trec = TLNA = 30K @ 300K
seems reasonable
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Aeff/Tsys?
Aeff (1  
Aeff
December 2010
Aeff
)
 ) T
Ta (1  
TSYS
K 2
K 2
N
*
Z

Z
K
LNA
Ant


_ elemZ  Z elem
LNA
Ant
Ta  TREC
REC
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Aeff/Tsys
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Double size antenna?
(Aeff/Tsys)2/(Aeff/Tsys)1
Freq [MHz]
theta=0°
(zenith)
theta=45°
phi=90°
theta=45°
phi=0°
70
120
260
450
0.84
1.59
1.74
0.93
1.12
0.82
2.97
1.83
0.87
1.79
0.47
1.11
Freq (MHz) Tant 1 [K] Tant 2 [K]
70
1785
1879
120
484
530
260
112
105
450
67
58
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop
Future works
• Tb(v;’) Tb(v;’,’)
• Considering the coupling effect
• Take in account the array factor at station level (antennas
positioning) and at full array level (stations positioning)
Stronger collaboration with all partners involved in this task
Thanks!
December 2010
Aeff/Tsys evaluation
TITLEfor AAlo sensors
AAVP Workshop