Page 187 - Satellite Communications, Fourth Edition

Antennas 167

6.15 Double-Reflector Antennas
With reflector-type antennas, the feeder connecting the feed horn to the
transmit/receive equipment must be kept as short as possible to mini-
mize losses. This is particularly important with large earth stations where
the transmit power is large and where very low receiver noise is required.
The single-reflector system described in Sec. 6.14 does not lend itself very
well to achieving this, and more satisfactory, but more costly, arrangements
are possible with a double-reflector system. The feed horn is mounted at
the rear of the main reflector through an opening at the vertex, as illus-
trated in Fig. 6.22. The rear mount makes for a compact feed, which is an
advantage where steerable antennas must be used, and access for servic-
ing is easier. The subreflector, which is mounted at the front of the main
reflector, is generally smaller than the feed horn and causes less blockage.
Two main types are in use, the Cassegrain antenna and the Gregorian
antenna, named after the astronomers who first developed them.

6.15.1 Cassegrain antenna
The basic Cassegrain form consists of a main paraboloid and a subre-
flector, which is a hyperboloid (see App. B). The subreflector has two

Figure 6.22 A 19-m Cassegrain antenna. (Courtesy of TIW
Systems, Inc.)