Wikipedia
In electromagnetics, an antenna's power gain or simply gain is a key performance number which combines the antenna's directivity and electrical efficiency 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 When no direction is specified, "gain" is understood to refer to the peak value of the gain, the gain in the direction of the antenna's main lobe A plot of the gain as a function of direction is called the gain pattern or radiation pattern
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