Antenna Types

Antenna Types

Dipoles Two of the simplest and most basic antennas are the half-wave dipole, or
Hertz, antenna (Figure 5.2a) and the quarter-wave vertical, or Marconi, antenna.
The half-wave dipole consists of two straight collinear conductors of
equal length, separated by a small gap. The length of the antenna is one-heM the
wavelength of the signal that can be transmitted most efficiently.A vertical quarterwave
antenna is the type commonly used for automobile radios and portable radios.
A half-wave dipole has a uniform or omnidirectional radiation pattern in one
dimension and a figure eight pattern in the other two dimensions.
More complex antenna configurations can be used to produce a directional beam.

A typical directional radiation pattern is shown in Figure 5.3b. In this case the main
strength of the antenna is in the x-direction.

Parabolic Reflective Antenna An important type of antenna is the parabolic
reflective antenna, which is used in terrestrial microwave and satellite applications.
A parabola is the locus of all points equidistant from a fixed line and a fixed point
not on the line. The fixed point is called the focus and the fixed line is called the
directrix (Figure 5.4a). If a parabola is revolved about its axis, the surface generated
is called a paraboloid. A cross section through the paraboloid parallel to its axis
forms a parabola and a cross section perpendicular to the axis forms a circle. Such
surfaces are used in automobile headlights, optical and radio telescopes, and
microwave antennas because of the following property: If a source of electromagnetic
energy (or sound) is placed at the focus of the paraboloid, and if the paraboloid
is a reflecting surface, then the wave will bounce back in lines parallel to the
axis of the paraboloid; Figure 5.4b shows this effect in cross section. In theory, this
effect creates a parallel beam without dispersion. In practice, there will be some dispersion,
because the source of energy must occupy more than one point. The converse
is also true. If incoming waves are parallel to the axis of the reflecting
paraboloid, the resulting signal will be concentrated at the focus.
 shows a typical radiation pattern for the parabolic reflective
antenna, and Table 5.1 lists beam widths for antennas of various sizes at a frequency
of 12 GHz. Note that the larger the diameter of the antenna, the more
tightly directional is the beam.