h. A radiation pattern is a diagram that shows you the directivity of
an antenna.
14. How energy is coupled to a radar antenna.
a. You know that a radar antenna consists of a feed and a reflector.
The feed is the radiating part of the antenna and is used to couple the RF
energy to the reflector. It may be a dipole, a horn, or a resonant cavity.
Figure 139 shows an example of each type of feed.
b. Part A of Figure 139 shows a dipole used to feed a parabolic
reflector.
You will learn about parabolic and other types of reflectors
later in this lesson.
Notice in Part A of Figure 139 that the dipole is
connected to a rigid coaxial transmission line.
One dipole element is
connected directly to the outer conductor of the coaxial line, and the other
element is connected to the inner conductor.
Each element is one-quarter
wavelength long, so that the two together form a half-wave dipole.
The
splash plate in front of the dipole is a small reflector that directs the
front radiation of the dipole back to the main dish reflector.
Figure 139.
Types of Antenna Feeds.
c. Horn-type feed in shown in Part B of Figure 139. It consists of a
waveguide with the open end flared to form a horn. The reason for flaring
the end of the waveguide is to match the impedance of the waveguide
transmission line to the impedance of free space. This provides a maximum
transfer of energy from feed to reflector. Horn-type feed is used primarily
in the higher radar frequency bands, such as the 3 cm band.
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