What is meant by "antenna gain"?

Antenna gain is specifically defined as the measure of how effectively an antenna converts input power into radio waves in a specified direction. This concept is crucial because it helps in understanding the antenna's performance and its ability to focus energy in a specific direction, which enhances the overall transmission and reception capabilities of radio signals.

Gain is often expressed in decibels (dB) and indicates how much stronger the signal can be in a certain direction compared to a reference antenna, typically an isotropic radiator. A higher gain indicates a more focused and powerful transmission in that direction, leading to better communication range and quality.

The other options do not accurately capture the meaning of antenna gain. While signal distortion refers to the quality and shape of the transmitted signal, it does not measure how effectively an antenna radiates energy. Efficiency in cooling doesn’t pertain to the gain of an antenna but rather its thermal management, which is not a factor in the antenna's gain measurement. Likewise, total power transmitted relates to the overall power output of the antenna rather than its directional effectiveness or gain. Thus, the emphasis is on the directional efficacy in converting power to radio waves, which is succinctly captured in the correct choice.