Isotropic radiators are used as reference radiators with which other sources are compared, for example in determining the gain of antennas.

A hypothetical antenna that radiates equally in all vertical as well as all horizontal angles is called an isotropic radiator however these cannot exist in practice nor would they be particularly desired.

## Dipole antenna

In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna.

A turnstile antenna comprises two dipoles crossed at a right angle and feed system which introduces a quarter-wave phase difference between the currents along the two. With that geometry, the two dipoles do not interact electrically but their fields add in the far-field producing a net radiation pattern which is rather close to isotropic, with horizontal polarization in the plane of the elements and circular or elliptical polarization at other angles. Turnstile antennas can be stacked and fed in phase to realize an omnidirectional broadside array or phased for an end-fire array with circular polarization.

## Omnidirectional antenna

In radio communication, an omnidirectional antenna is a class of antenna which radiates equal radio power in all directions perpendicular to an axis (azimuthal directions), with power varying with angle to the axis (elevation angle), declining to zero on the axis.

Note that this is different from an isotropic antenna, which radiates equal power in all directions, having a spherical radiation pattern.

In the field of antenna design the term radiation pattern (or antenna pattern or far-field pattern) refers to the directional (angular) dependence of the strength of the radio waves from the antenna or other source.

Since electromagnetic radiation is dipole radiation, it is not possible to build an antenna that radiates coherently equally in all directions, although such a hypothetical isotropic antenna is used as a reference to calculate antenna gain.

## Decibel

Relative unit of measurement equal to one tenth of a bel (B).

Relative unit of measurement equal to one tenth of a bel (B).

dBi: dB(isotropic) – the gain of an antenna compared with the gain of a theoretical isotropic antenna, which uniformly distributes energy in all directions. Linear polarization of the EM field is assumed unless noted otherwise.

dBd: dB(dipole) – the gain of an antenna compared with the gain a half-wave dipole antenna. 0 dBd = 2.15 dBi