Page 386 - Satellite Communications, Fourth Edition

366 Chapter Twelve

Example 12.8 Repeat the calculation when the system of Fig. 12.6a is arranged
as shown in Fig. 12.6b.
Solution In this case the cable precedes the LNA, and therefore, the equivalent
noise temperature referred to the cable input is
3.16 (15.85 1) 290
T S 35 (3.16 1) 290 3.16 150
10 5
1136 K

Examples 12.7 and 12.8 illustrate the important point that the LNA
must be placed ahead of the cable, which is why one sees amplifiers
mounted right at the dish in satellite receive systems.

12.6 Carrier-to-Noise Ratio
Ameasure of the performance of a satellite link is the ratio of carrier power
to noise power at the receiver input, and link-budget calculations are often
concerned with determining this ratio. Conventionally, the ratio is denoted
by C/N (or CNR), which is equivalent to P /P N . In terms of decibels,
R
c C d [P ] [P ] (12.33)
R
N
N
Equations (12.17) and (12.18) may be used for [P ] and [P ], resulting in
R
N
c C d [EIRP] [G ] [LOSSES] [k] [T ] [B ] (12.34)
R
S
N
N
The G/T ratio is a key parameter in specifying the receiving system
performance. The antenna gain G and the system noise temperature
R
T can be combined in Eq. (12.34) as
S
1
[G/T] [G ] [T ] dBK (12.35)
S
R
Therefore, the link equation [Eq. (12.34)] becomes
c C d [EIRP] c G d [LOSSES] [k] [B ] (12.36)
N
N T
The ratio of carrier power to noise power density P /N may be the
0
R
quantity actually required. Since P kT B N B ,then
N
0
N
N
N
c C d c C d
N N B N
0
c C d [B ]
N
N 0