istic that would destroy the narrow pattern from the parabolic reflector. This destruction is prevented by the use of a hemispherical shield (not shown) that directs most of what would otherwise be spherical radiation toward the parabolic surface. Without the shield, some of the radiated field would leave the radiator directly, would not be reflected, and would serve no useful purpose. The shield makes the beam sharper, and concentrates the majority of the power in the beam. The same results can be obtained by using either a parasitic array to direct the radiated field back to the reflector, or a feed horn pointed at the paraboloid.
The radiation pattern of the paraboloid contains a major lobe, which is directed along the axis of the paraboloid, and several minor lobes, as shown in figure 2-41. Very narrow beams are possible with this type of reflector. View A of figure 2-42 illustrates the parabolic reflector.
Truncated Paraboloid
While the complete parabolic reflector produces a pencil-shaped beam, partial parabolic reflectors pro-
Figure 2-41.-Parabolic radiation pattern.
duce differently shaped beams. View B of figure 2-42 shows a horizontally truncated, or vertically shortened, paraboloid. This type of reflector is designed to produce a beam that is narrow horizontally but wide vertically. Since the beam is wide vertically, it will detect aircraft at different altitudes without changing the tilt of the antenna. It also works well for surface search radars to overcome the pitch and roll of the ship.
Figure 2-42.—Reflector shapes.
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