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 -6.831068710-5 7.285044∙10-3 8.0577027∙10-1
1.6 -2.682889310-5 3.4836402∙10-3 8.869153∙10-1
2.3 -1.325603510-4 1.7229174∙10-2 4.4017117∙10-1
5 -4.6834953∙10-5 6.2403816∙10-3 7.9212981∙10-1
6 -5.819795910-5 9.4270958∙10-3 6.1824204∙10-1
8.3 -7.245727910-5 1.0623634∙10-2 6.1059261∙10-1
22 -4.20317910-5 5.90053310-3 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.

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