Page 395 - Satellite Communications, Fourth Edition

The Space Link 375

the satellite’s transmit antenna. Referring back to Eq. (12.3), the power
output of the TWTA is given by

[P TWTA ] [EIRP] [G ] [TFL] D (12.56)
D
T D
] is found, the saturated power output rating of the TWTA
Once [P TWTA
is given by
[P TWTA S TWTA ] [BO] 0 (12.57)
] [P

Example 12.15 A satellite is operated at an EIRP of 56 dBW with an output BO
of 6 dB. The transmitter feeder losses amount to 2 dB, and the antenna gain is
50 dB. Calculate the power output of the TWTA required for full saturated EIRP.
Solution Equation (12.56):

[P TWTA ] [EIRP] D [G T ] D [TFL] D
56 50 2
8 dBW
Equation (12.57):

[P TWTA ] S 8 6
14 dBW (or 25 W)

12.9 Effects of Rain
Up to this point, calculations have been made for clear-sky conditions,
meaning the absence of weather-related phenomena which might affect
the signal strength. In the C band and, more especially, the Ku band,
rainfall is the most significant cause of signal fading. Rainfall results
in attenuation of radio waves by scattering and by absorption of energy
from the wave, as described in Sec. 4.4. Rain attenuation increases with
increasing frequency and is worse in the Ku band compared with the C
band. Studies have shown (CCIR Report 338-3, 1978) that the rain
attenuation for horizontal polarization is considerably greater than for
vertical polarization.
Rain attenuation data are usually available in the form of curves or
tables showing the fraction of time that a given attenuation is exceeded
or, equivalently, the probability that a given attenuation will be exceeded
(see Hogg et al., 1975; Lin et al., 1980; Webber et al., 1986). Some yearly
average Ku-band values are shown in Table 12.2.
The percentage figures at the head of the first three columns give the
percentage of time, averaged over any year, that the attenuation exceeds