Power gain (or simply gain) is a unitless measure that combines an antenna's efficiency Eantenna and directivity D:
When considering the power gain for a particular direction given by an elevation (or "altitude") and azimuth, then:
is known as the directive gain. The directive gain signifies the ratio of radiated power in a given direction relative to that of an isotropic radiator which is radiating the same total power as the antenna in question but uniformly in all directions. Note that a true isotropic radiator does not exist in practice.
The power gain, on the other hand, signifies the ratio of radiated power in a given direction relative to that of an isotropic radiator which is radiating the total amount of electrical power received by the antenna in question. This is in contrast to the directive gain which ignores any reduction in efficiency. If only a certain portion of the electrical power received from the transmitter is actually radiated by the antenna (its efficiency) the directive gain compares the power radiated in a given direction to that reduced power, ignoring the inefficiency. By instead comparing the radiated power in a given direction to the actual power that the antenna receives from the transmitter, the power gain takes into account that poorer efficiency, making it a more useful figure of merit for the ability of a transmitter in sending a radio wave toward a receiver.
The radiation intensity expresses the power radiated per solid angle. In terms of the power gain in a specified direction can be calculated:
where Pelec signifies the electrical power received by the antenna from the transmitter.
Read more about this topic: Antenna Gain
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