Page 246 - Satellite Communications, Fourth Edition

226 Chapter Seven

where l is the wavelength of the signal, D is the reflector diameter, and
is the aperture efficiency. A typical value for is 0.55. The 3-dB
I
I
beamwidth is given approximately by Eq. (6.33) as
l
> 70 degrees

3dB
D
The ratio D/l is seen to be the key factor in these equations, the gain
2
being directly proportional to (D/l) and the beamwidth inversely pro-
portional to D/l. Hence the gain can be increased and the beamwidth
made narrower by increasing the reflector size or decreasing the wave-
length. In comparing C-band and Ku-band, the largest reflectors are
those for the 6/4-GHz band. Comparable performance can be obtained
with considerably smaller reflectors in the 14/12-GHz band. Satellites
used for mobile services in the L-band employ much larger antennas
2
2
(with reflector areas in the order of 100 m to 200 m ) as described in
Chap. 17.
Figure 7.22 shows the antenna subsystem of the INTELSAT VI satel-
lite (Johnston and Thompson, 1982). This provides a good illustration
of the level of complexity which has been reached in large communica-
tions satellites. The largest reflectors are for the 6/4-GHz hemisphere
and zone coverages, as illustrated in Fig. 7.23. These are fed from horn
arrays, and various groups of horns can be excited to produce the beam
shape required. As can be seen, separate arrays are used for transmit and
receive. Each array has 146 dual-polarization horns. In the 14/11-GHz
band, circular reflectors are used to provide spot beams, one for east and
one for west, also shown in Fig. 7.23. These beams are fully steerable.
Each spot is fed by a single horn which is used for both transmit and
receive.
Wide beams for global coverage are produced by simple horn anten-
nas at 6/4 GHz. These horns beam the signal directly to the earth with-
out the use of reflectors. Also as shown in Fig. 7.22, a simple biconical
dipole antenna is used for the tracking and control signals. The complete
antenna platform and the communications payload are despun as
described in Sec. 7.3 to keep the antennas pointing to their correct loca-
tions on earth.
The same feed horn may be used to transmit and receive carriers
with the same polarization. The transmit and receive signals are sepa-
rated in a device known as a diplexer, and the separation is further
aided by means of frequency filtering. Polarization discrimination also
may be used to separate the transmit and receive signals using the
same feed horn. For example, the horn may be used to transmit hori-
zontally polarized waves in the downlink frequency band, while simul-
taneously receiving vertically polarized waves in the uplink frequency