The corner-reflector antenna consists of two flat conducting sheets that meet at an angle to form a
corner, as shown in view F of figure 3-19. This reflector is normally driven by a half-wave radiator
located on a line which bisects the angle formed by the sheet reflectors.
The desired beam widths are provided for some vhf radars by a broadside array, such as the one
shown in figure 3-20. The broadside array consists of two or more half-wave dipole elements and a flat
reflector. The elements are placed one-half wavelength apart and parallel to each other. Because they are
excited in phase, most of the radiation is perpendicular or broadside to the plane of the elements. The flat
reflector is located approximately one-eighth wavelength behind the dipole elements and makes possible
the unidirectional characteristics of the antenna system.
Figure 3-20.Broadside array.
Horn radiators, like parabolic reflectors, may be used to obtain directive radiation at microwave
frequencies. Because they do not involve resonant elements, horns have the advantage of being usable
over a wide frequency band.
The operation of a horn as an electromagnetic directing device is analogous to that of acoustic horns.
However, the throat of an acoustic horn usually has dimensions much smaller than the sound wavelengths
for which it is used, while the throat of the electromagnetic horn has dimensions that are comparable to
the wavelength being used.
Horn radiators are readily adaptable for use with waveguides because they serve both as an
impedance-matching device and as a directional radiator. Horn radiators may be fed by coaxial or other
types of lines.
Horns are constructed in a variety of shapes as illustrated in figure 3-21. The shape of the horn, along
with the dimensions of the length and mouth, largely determines the field-pattern shape. The ratio of the
horn length to mouth opening size determines the beam angle and thus the directivity. In general, the
larger the opening of the horn, the more directive is the resulting field pattern.