Page 234 - Satellite Communications, Fourth Edition

214 Chapter Seven

Figure 7.11 Satellite control system. (Courtesy of Telesat Canada, 1983.)

transponder. A typical transponder bandwidth is 36 MHz, and allowing
for a 4-MHz guardband between transponders, 12 such transponders can
be accommodated in the 500-MHz bandwidth. By making use of polar-
ization isolation, this number can be doubled. Polarization isolation
refers to the fact that carriers, which may be on the same frequency but
with opposite senses of polarization, can be isolated from one another
by receiving antennas matched to the incoming polarization. With linear
polarization, vertically and horizontally polarized carriers can be sep-
arated in this way, and with circular polarization, left-hand circular
and right-hand circular polarizations can be separated. Because the
carriers with opposite senses of polarization may overlap in frequency,
this technique is referred to as frequency reuse. Figure 7.12 shows part
of the frequency and polarization plan for a C-band communications
satellite.