dBi

dBi stands for decibels relative to an isotropic radiator. It is a unit of antenna gain that expresses how much power an antenna directs in a particular direction compared with an ideal isotropic antenna that radiates equally in all directions. Gain expressed in dBi is a logarithmic ratio: G(dBi) = 10 log10(G_linear), where G_linear is the linear gain relative to an isotropic source. An antenna with 0 dBi is, in principle, equivalent to isotropic in its reference, while positive dBi values indicate more focused radiation in specific directions.

There is a related unit, dBd, decibels relative to a half-wave dipole. Since a half-wave dipole has about 2.15 dBi of gain, G(dBi) = G(dBd) + 2.15. When comparing antennas, it is important to note which reference is used, because the numerical value depends on the reference. Many vendors specify gain in dBi or provide a conversion to dBd.

Gain reflects the shape of the radiation pattern and not the antenna’s efficiency. A higher dBi often accompanies a narrower beam and greater directivity, but it does not by itself indicate higher total radiated power if efficiency is low. In wireless design, antenna gain combines with transmitter power, cable and connector losses, and environmental factors to determine effective coverage and link performance. The term is widely used for Wi‑Fi, cellular, satellite, and other RF links to compare potential coverage and link margin. Real-world results depend on frequency, installation, polarization, and surroundings.