An antenna is a device that converts electrical signals into electromagnetic waves during transmission and converts electromagnetic waves into electrical signals during reception. Thus, it acts as an interface between the transmission line and free space.
The radiation pattern of an antenna represents the variation of radiated power with direction at a constant distance from the antenna. Although radiation patterns are three-dimensional, they are usually represented as two-dimensional plots in the horizontal or vertical plane for practical analysis.
Antennas are broadly classified into isotropic antennas, omnidirectional antennas, and directional antennas, based on their radiation characteristics.
An isotropic antenna is a theoretical and ideal antenna that radiates power uniformly in all directions. It has a perfect spherical radiation pattern and a gain of 1 (0 dB).
It is not physically realizable but is used as a reference antenna for comparing the performance and gain of practical antennas.
Omnidirectional antennas radiate power equally in all directions in the horizontal plane and have limited radiation in the vertical plane.
They are also called non-directional antennas and are widely used for broadcasting.
The Hertzian dipole is the most commonly used omnidirectional antenna. It consists of two collinear conductors of equal length, separated by a small gap. The length of a half-wave dipole is λ/2.
Its radiation pattern is figure-of-eight in the vertical plane and circular in the horizontal plane.
A monopole antenna is a vertical antenna of length λ/4, mounted over a ground plane. It is also known as the Marconi antenna.
A monopole over an infinite ground plane behaves similarly to a dipole antenna and is commonly used in vehicular communication.
A directional antenna radiates or receives maximum power in a specific direction, thereby improving signal strength and reducing interference.
These antennas must be properly aligned with the transmitter or receiver.
Examples include: