Directive gain

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In telecommunications, the term directive gain has the following meanings:

1. Of an antenna, the ratio of 4π times the radiant intensity in a given direction (i.e., power radiated per unit solid angle), to the total power.

The directive gain is usually expressed in dBi, and expresses the performance of the antenna relative to an isotropic antenna. Note that 4π steradians is the solid angle subtended by a complete sphere, so the total power radiated by the antenna is the power radiated into 4π sr.

2. Of an antenna, for a given direction, the ratio of the radiance produced in the given direction to the average value of the radiance in all directions.

If the direction is not specified, the direction of maximum radiance is assumed. The directive gain is usually expressed in dB.

Source: from Federal Standard 1037C and from MIL-STD-188

3. The gain (G) for a parabolic antenna of effective diameter D at wavelength λ and efficiency μ can be can calculated as follows

$G = \mu (\pi D / \lambda)^2 \,$

A typical satellite antenna (satellite dish) has an efficiency in the range 55% to 65%, this depends on antenna diameter and geometry. Typical units used are dBi (dB over isotropic) where the value is

$G = 10 \ \log_{10} G \,$