edVirtus Reference

What Is Antenna Gain?

How Does an Antenna Concentrate Radiated Power in Particular Directions?

Antenna gain describes how strongly an antenna radiates or receives in a specified direction compared with a reference antenna supplied with the power needed to produce the same field in that direction. Gain is a ratio and is normally expressed in decibels.

Gain is directional. An antenna can have high gain in its main beam and much lower gain to the sides or rear. A single quoted value normally means maximum gain unless another direction is specified, so it must be interpreted together with the radiation pattern.

Gain expressed in dBi uses an ideal isotropic radiator as the reference. Gain expressed in dBd uses an ideal half-wave dipole. Because a half-wave dipole has about 2.15 dBi maximum gain, a value in dBi is 2.15 dB greater than the corresponding value in dBd.

Directivity describes directional concentration without accounting for antenna losses. Gain includes radiation efficiency and is therefore no greater than directivity for a passive antenna. Neither quantity means that the antenna creates power; it redistributes the available radiated power in space.

Transmitter power, feeder loss, and antenna gain are combined to obtain effective isotropic radiated power or effective radiated power. These equivalent powers are useful in link budgets and far-field estimates, but they do not represent total radiated power in every direction.

Real gain can depend on frequency, mounting, nearby structures, ground, radomes, cables, and interaction with a device or the human body. Manufacturer data, measurement, or modeling must represent the installed or tested configuration and the required polarization.

In RF exposure assessment, antenna gain helps identify beam directions and estimate fields, but gain alone does not determine exposure. Distance, field region, duty cycle, reflections, access, source power, operating state, and the person's location must also be considered.