Page 385 - Satellite Communications, Fourth Edition

The Space Link 365

temperature of a lossy network referred to the input at the antenna
when the antenna is used in receiving mode.
If the lossy network should happen to be at room temperature, that
is, T T , then a comparison of Eqs. (12.26) and (12.30) shows that
0
x
F L (12.31)
This shows that at room temperature the noise factor of a lossy network
is equal to its power loss.

12.5.6 Overall system noise temperature
Figure 12.6a shows a typical receiving system. Applying the results of
the previous sections yields, for the system noise temperature referred
to the input,

(L 1)T 0 L(F 1)T 0
T T ant T (12.32)
S
e1
G 1 G 1
The significance of the individual terms is illustrated in the following
examples.

Example 12.7 For the system shown in Fig. 12.6a, the receiver noise figure is
12 dB, the cable loss is 5 dB, the LNA gain is 50 dB, and its noise temperature
150 K. The antenna noise temperature is 35 K. Calculate the noise temperature
referred to the input.
Solution For the main receiver, F 10 1.2 15.85. For the cable, L 10 0.5 3.16.
5
For the LNA, G 10 . Hence,
(3.16 1) 290 3.16 (15.85 1) 290
T S 35 150
10 5 10 5
> 185 K

Figure 12.6 Connections used in examples illustrating overall noise
temperature of system, Sec. 12.5.6.