Sizing the Communications Payload

We now have determined the satellite transmitter power and antenna aperture size required to support our links. These parameters have the greatest impact on satellite mass, and thus on the cost of the system. In this section we will describe these components and estimate their mass. This process is summarized in Table 13-9 in Sec. 13 3.

Up to now we have considered only the parabolic reflector antenna, which is best suited for applications where the peak gain is above 20 dB and beamwidth is less than 15 deg. For lower-gain, wider-beam applications, we may prefer other types of antennas with lighter mass and simpler design, especially at frequencies below 1 GHz (see Table 13-14). For example, an Earth-coverage satellite antenna has a beamwidth just big enough to illuminate the Earth. At geosynchronous altitude, this beamwidth is

TABLE 13-14. Antenna Types tor Satellite Systems. (Formulas from Jasik [1961]) In these equations, C, D, L, and A are in m and f is in GHz.

Antenna

J-Å’BDGOOD

hoi T

vfCi-

Coaxial Lira

Beam Shape

Conical

Conical

Conical

Toroidal

Typical Max Gain (dBI)

0 0

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