Page 426 - Satellite Communications, Fourth Edition

406 Chapter Thirteen

Figure 13.4 Earth-station antenna gain pattern used in FCC/OST R83-2, revised
Nov. 30, 1984. (Courtesy of Sharp, 1984b.)

sidelobe gain function can be written as follows:
29 25 log
1
7
8 7
9.2
[G(
)] μ (13.6)
32 25 log
9.2
48
10 48
180
For the range of satellite orbital spacings presently in use, it is this
sidelobe gain function that determines the interference levels.

Example 13.4 Determine the degradation in the downlink [C/I] ratio when satel-
lite orbital spacing is reduced from 4° to 2°, all other factors remaining unchanged.
FCC antenna characteristics may be assumed.

Solution The decibel increase in interference will be
(29 25 log 2) (29 25 log 4) 25 log 2 7.5 dB

The [C/I] D will be degraded directly by this amount. Alternatively, from Fig. 13.4,

[G (2°)] [G (4°)] 21.4 13.9 7.5 dB