The term "directivity" is often used with no particular direction specified. The directivity D( θ, φ) is the ratio of the radiation intensity U in the direction ( θ, φ), to the radiation intensity averaged over all directions. In practice it is impossible to construct such an antenna, but the concept is useful for defining other antenna parameters, such as the antenna directivity and gain.ĭirectivity of the antenna describes how the antenna radiates power in different directions. Where D is the maximum physical dimension of the antenna, and λ 0 is the wavelength corresponding to the operating frequency.Ī theoretical antenna, which radiates its energy uniformly in all directions in space is called an isotropic antenna. The far-field is defined as the electromagnetic field in the region for which the distance r is larger than the far-field distance R ff, given by * The spherical coordinate system with the antenna at origin. This is done due to the fact that the far-field from any antenna is a TEM wave propagating in the r direction of the spherical coordinate system, where the antenna is located in origin, see figure below. When defining the different antenna parameters, usually only the far-field from the antenna is considered.
A number of important characteristics are used to describe an antenna. Many different types of antennas exist, ranging from simple structures consisting of a single straight wire to complex phase controlled antenna arrays with many hundreds of carefully spaced radiating elements. They are used to efficiently transform guided electric signals into freely propagating electromagnetic waves. Antennas are an integral part of any wireless system.