Gain (antenna)

It is not to be confused with directivity, which does not take an antenna's radiation efficiency into account.

[1] Usually this ratio is expressed in decibels with respect to an isotropic radiator (dBi).

An alternative definition compares the received power to the power received by a lossless half-wave dipole antenna, in which case the units are written as dBd.

Since a lossless dipole antenna has a gain of 2.15 dBi, the relation between these units is

Gain is a unitless measure that multiplies an antenna's radiation efficiency

Losses prior to the antenna terminals are accounted for by separate impedance mismatch factors which are therefore not included in the calculation of radiation efficiency.

Published numbers for antenna gain are almost always expressed in decibels (dB), a logarithmic scale.

dBi is used rather than just dB to emphasize that this is the gain according to the basic definition, in which the antenna is compared to an isotropic radiator.

When actual measurements of an antenna's gain are made by a laboratory, the field strength of the test antenna is measured when supplied with, say, 1 watt of transmitter power, at a certain distance.

That field strength is compared to the field strength found using a so-called reference antenna at the same distance receiving the same power in order to determine the gain of the antenna under test.

That ratio would be equal to G if the reference antenna were an isotropic radiator (irad).

However a true isotropic radiator cannot be built, so in practice a different antenna is used.

This will often be a half-wave dipole, a very well understood and repeatable antenna that can be easily built for any frequency.

The directive gain of a half-wave dipole with respect to the isotropic radiator is known to be 1.64 and it can be made nearly 100% efficient.

The gain relative to a dipole is thus often quoted and is denoted using dBd instead of dBi to avoid confusion.

In any direction, therefore, such numbers are 2.15 dB smaller than the gain expressed in dBi.

corresponding to a given polarization, divided by the total radiation intensity of an isotropic antenna.

The maximum gain is then equal to: Expressed relative to the gain of a half-wave dipole we would find: As an example, consider an antenna that radiates an electromagnetic wave whose electrical field has an amplitude

The intensity of an electromagnetic plane wave is: where and If the resistive part of the series impedance of the antenna is

The intensity of an isotropic antenna is the power so fed divided by the surface of the sphere of radius r: The directive gain is: For the commonly utilized half-wave dipole, the particular formulation works out to the following, including its decibel equivalency, expressed as dBi (decibels referenced to isotropic radiator): Sometimes, the half-wave dipole is taken as a reference instead of the isotropic radiator.

[4] When testing mobile devices, TRP can be measured while in close proximity of power-absorbing losses such as the body and hand of the user.

This article incorporates public domain material from Federal Standard 1037C.

Diagram illustrating how isotropic gain is defined. The axes represent power density in watts per square meter. is the radiation pattern of a directive antenna, which radiates a maximum power density of watts per square meter at some given distance from the antenna. The green ball is the radiation pattern of an isotropic antenna which radiates the same total power, and is the power density it radiates. The gain of the first antenna is . Since the directive antenna radiates the same total power within a small angle along the z axis, it can have a higher signal strength in that direction than the isotropic antenna, and so a gain greater than one.