Page 178 - Satellite Communications, Fourth Edition

158 Chapter Six

Figure 6.13 A scalar feed.

the hybrid-mode horn, that is, better main lobe symmetry, lower cross-
polarization, and a more efficient main beam with low sidelobes. Dual-
mode horns have been installed aboard various satellites (see Miya, 1981).
Horns which are required to provide earth coverage from geostation-
ary satellites must maintain low cross-polarization and high gain over
a cone angle of 9°. This is achieved more simply and economically
with dual-mode horns (Hwang, 1992).

6.12.2 Pyramidal horn antennas
The pyramidal horn antenna, illustrated in Fig. 6.14, is primarily
designed for linear polarization. In general, it has a rectangular cross
section a b and operates in the TE waveguide mode, which has the
10
electric field distribution shown in Fig. 6.14.
In general, the beamwidths for the pyramidal horn differ in the E and
H planes, but it is possible to choose the aperture dimensions to make
these equal. The pyramidal horn can be operated in horizontally and ver-
tically polarized modes simultaneously, giving rise to dual-linear polar-
ization. According to Chang (1989), the cross-polarization characteristics
of the pyramidal horn have not been studied to a great extent, and if
required, they should be measured.