Antenna gain is a measure of how strongly an antenna concentrates radio energy in its best direction, compared with a reference antenna. An antenna cannot add power to a signal. A higher-gain antenna takes the same energy and puts more of it in some directions, at the cost of less in others.

Antenna gain is quoted in dBi or dBd. dBi is gain compared with an isotropic antenna, a theoretical source that radiates equally in all directions. dBd is gain compared with a half-wave dipole, which itself has a gain of 2.15 dBi, so adding 2.15 to a figure in dBd gives the figure in dBi. An antenna rated at 3 dBd is therefore a 5.15 dBi antenna, and comparing a dBi figure with a dBd figure without converting makes one antenna look 2.15 dB better than it is.

Every 3 dB of antenna gain doubles the power in the antenna's best direction, and every 10 dB multiplies it by 10. That gain comes from a narrower beamwidth. A high-gain directional antenna has a tight beam that must be aimed, and a high-gain omnidirectional antenna has a flatter pattern that reaches less far up and down.

Antenna gain varies with frequency, so a wideband cellular antenna usually has a different figure for each band it covers. Datasheets may quote peak gain, the figure in the antenna's best direction, or average gain, taken across all directions. Peak gain is the larger number and the one most often printed on packaging, so compare like with like and check the figure for the bands the site will use.

Higher antenna gain is not always better. More gain can lift RSRP at a weak site, but it also narrows the area the antenna covers and can bring in more interference if the beam faces several masts, which lowers SINR. Cable loss between the antenna and the cellular router also subtracts directly from antenna gain, so a high-gain antenna on a long run of thin cable can deliver less than a modest antenna mounted close to the router.

‍