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5. Efficiency and Temperatures |
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5. Efficiency and Temperatures |
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The antenna gain is defined as :
m = 0.5 (non polarized radiation)
Ag = geometric area
kB = Boltzmann constant
ηA = antenna efficiency
For the Medicina antenna, the constants are :
ηA is the overall efficiency, estimated assembling all the signal degradation factors.
The antenna gain varies according to the elevation and it reaches a maximum at 45°. A good interpolation is obtained with a second degree curve, such as :
The coefficients of the normalized polynomials, at each frequency, are listed in the following :
|
Frequency (GHz) |
a | b | c |
| 1.4 | -6.8310687∙10-5 | 7.285044∙10-3 | 8.0577027∙10-1 |
| 1.6 | -2.6828893∙10-5 | 3.4836402∙10-3 | 8.869153∙10-1 |
| 2.3 | -1.3256035∙10-4 | 1.7229174∙10-2 | 4.4017117∙10-1 |
| 5 | -4.6834953∙10-5 | 6.2403816∙10-3 | 7.9212981∙10-1 |
| 6 | -5.8197959∙10-5 | 9.4270958∙10-3 | 6.1824204∙10-1 |
| 8.3 | -7.2457279∙10-5 | 1.0623634∙10-2 | 6.1059261∙10-1 |
| 22 | -4.203179∙10-5 | 5.900533∙10-3 | 7.929185∙10-1 |
Tab. 5.1 : Normalized gain curves, coefficients
The sensitivity can be estimated as follows :
= receiver constant (=1)
Tsys = system temperature
G = gain (K/Jy)
Δν = bandwidth
τ = integration time
n = integration number
NIF = available channels (= 1,2)
The system temperatures and the sensitivities of the Medicina antenna are listed in Table 5.2:
|
ν0 (GHz) |
T receiver (K) |
Tsys (K) |
ηA (%) |
G (K/Jy) |
SEFD (Jy) |
Band (MHz) |
|
| 1.4 | 50 | 58 | 41 | 0.120 | 483 | 2x80* | 38.2 |
| 1.6 | 60 | 64 | 36 | 0.106 | 604 | 2x80* | 47.8 |
| 2.3 | 40 | 58 | 43 | 0.125 | 464 | 2x160* | 26.0 |
| 5 | 14 | 26 | 58 | 0.160 | 163 |
2x400 2x800 |
5.8 4.1 |
| 6 | 57 | 65 | 50 | 0.145 | 676 | 2x400 | 23.9 |
| 8.3 | 25 | 40 | 48 | 0.141 | 284 | 2x800 | 7.1 |
| 22 | 20-30 | 55 | 38 | 0.110 | 500 | 2x2000 | 8.0 |
Tab. 5.2 : Sensitivity of the antenna at 45°, assuming τ = 1 sec, n=1, NIF = 2
| Primary Focus |
| Cassegrain Focus |
* Usually at these frequencies the available bandwidth is less than the maximum allowed by the receivers, beacuse of interferences.
Note that the gain values listed in Table 5.2 are related to a single channel of the antenna (according to the above definition of gain). To obtain the gain of the antenna in its complete configuration (two channel LHC, RHC) the value must be doubled.
