How is antenna gain defined?

Antenna gain is defined as a measure of how effectively an antenna converts input power into radio waves in a specified direction compared to a hypothetical isotropic radiator. This definition encapsulates the performance of an antenna as both its directivity and efficiency are taken into account.

Directivity refers to how focused the antenna's radiation pattern is in a particular direction, while efficiency measures how well the antenna converts the input power into radiated power. Therefore, the performance ratio derived from directivity and efficiency is a key component in understanding the gain of an antenna.

This choice provides a comprehensive view of antenna gain by linking it to both physical characteristics (how the antenna radiates energy) and operational performance (how efficiently it uses power). In contrast, the other options do not capture this essential framework: while one option discusses total energy output, another talks about effectiveness in a less precise manner, and the last option refers to frequency response quality, which is not a direct measure of gain.