Antenna gain is a measure of the maximum effectiveness with which an antenna can radiate the power delivered to it by the transmitter towards a target
. It is a key performance parameter in electromagnetics and is used to describe how well an antenna converts input power into radio waves or how well it converts radio waves arriving from a specified direction into electrical power
. Antenna gain is more commonly quoted than directivity in an antenna's specification sheet because it takes into account the actual losses that occur
. In a transmitting antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction
. In a receiving antenna, the gain describes how well the antenna converts radio waves arriving from a specified direction into electrical power
. The gain is usually expressed in decibels (dB)
. For example, a transmitting antenna with a gain of 3 dB means that the power received far from the antenna will be 3 dB higher (twice as much) than what would be received by an isotropic source
. Electrically small antennas (small relative to the wavelength of the frequency that the antenna operates at) can be very inefficient, with antenna gains lower than -10 dB (even without accounting for impedance mismatch loss)
. Antenna gain is related to directivity and radiation efficiency, but it is different from directivity, which does not take an antenna's radiation efficiency into account
. Gain or 'absolute gain' is defined as "The ratio of the radiation intensity in a given direction to the radiation intensity of an isotropic source"
. A plot of the gain as a function of direction is called the antenna pattern or radiation pattern
