b. The impedance of the dipole is determined by the depth to which the web is inserted and the position
of the dipole with respect to the opening in the waveguide. Usually, the waveguide has a tapered section on the
end and the web is inserted in this section. This provides a very good impedance match between the waveguide
and space.
c. To obtain the desired radiation pattern, several dipole elements may be mounted on the web. The
most common arrangement though, is a single dipole with a reflecting element. The reflecting element can be
either another dipole or some type of reflecting material shaped according to the beam pattern desired.
2-26.
TAPERED HORN
a. The gain of a waveguide radiator may be increased by enlarging the aperture. This is done by
attaching a flare or horn to the waveguide, as shown in figure 97. The waveguide termination is commonly
known as a tapered horn antenna. The tapered horn antenna is designed to transform a transverse wave at the end
of the waveguide to a similar transverse wave at the end of the tapered horn without causing attenuation. The
throat of the tapered horn (the junction between the tapered horn and the waveguide) serves as a filter device and
allows only a single mode to be propagated freely to the aperture. The tapered horn will not support propagation
of a particular mode unless the transverse dimensions of the tapered horn are greater than the dimensions of the
waveguide.
Figure 97. Tapered horn antenna.
b. The dimensions of the open end of the tapered horn are chosen to obtain the desired radiation pattern
and to prevent spherical distortion of the propagated wave. The taper of the horn serves to match the impedance
of the waveguide to the impedance of space. At one end, the impedance of the tapered section matches that of
space; at the other end, it matches the impedance of the waveguide.
2-27.
REFLECTOR FEED SYSTEMS
a. Since microwave frequencies have essentially the same behavior as light waves, they can be focused
into beams. One of the reflectors used to focus
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