There are two types of antenna are:
1. Dipoles Antenna
2. Parabolic Reflective Antenna
1. Dipoles Antenna
Two of the simplest and most basic antennas are the half-wave dipole antenna 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-half the wavelength of the signal that can be transmitted most efficiently.
A vertical quarter wave 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.
Fig. Type of Antenna
2. 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 a).
Figure 4: Parabolic Antenna
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. Figure b shows this effect in cross section.
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