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The abbreviation dBi is a measure of sound expressed as acoustical or electrical power relative to an isotropic radiator (i), measured in decibels (dB). It is commonly used to express the gain of an antenna, which is a measure how much the antenna increases the effective signal power. Decibels are a logarithmic representation of magnitudes and are a convenient way to represent large numbers in a way that also corresponds well to the large dynamic range of sounds that humans can perceive.
The dBi is expressed as a ratio of relative gain, comparing the gain of an antenna to a reference standard. The reference standard is an isotropic radiator, a mathematically ideal antenna that uniformly distributes energy equally in all directions. Expressing gain as the dBi ratio standardizes comparisons between different antennas.
The gain of a non-isotropic antenna is a passive gain, because the antenna is not adding power but is redistributing it to provide more radiated power in a particular direction. The focusing aspect of an antenna translates into the increase in signal strength. The higher the dBi rating, the more the antenna preferentially radiates power in a particular direction. Gain is directly related to the directivity or directional properties of an antenna, and it accounts for the efficiency of the antenna.
Antenna directionality is a trade-off between gain and the beamwidth of the antenna. An increase in gain translates to more coverage in a particular direction, with an associated decrease in the coverage area or angle. Antennas with high dBi ratings must be aimed carefully in the desired direction. The orientation of an antenna with a low dBi rating is not as critical, because the antenna comes closer to radiating equally in all directions.
A high gain antenna has a narrow beam with a good signal quality over long distances, but it is not ideal for an application that requires reception over a large geographic area. An antenna with a low dBi rating will have a shorter range with broader coverage. For example, most Wi-Fi antennas are low gain and do not need to be oriented in any particular direction relative to the base station. Higher gain antennas can be useful for improving reception in rural areas where the signal strength is weak.
Gain is just one of the standard parameters used to measure the performance of an antenna. In addition to gain, engineers must consider frequency, bandwidth, impedance and polarization when selecting an antenna for a particular application. The durability of an antenna and its ability to handle extreme weather conditions can also be a concern. All other factors being equal, higher gain antennas are more expensive than lower gain antennas.