What is antenna efficiency?

Antenna efficiency is a measure of how effectively an antenna converts the electrical power delivered to it into radio waves radiated out into space. It is calculated by dividing the radiation resistance by the total resistance of the antenna system. Total resistance is the sum of the antenna's radiation resistance (the part that actually radiates energy) and its ohmic or loss resistance (which simply converts power into waste heat). Therefore, an antenna with a high radiation resistance relative to its loss resistance will have a high efficiency. Distractors suggesting effective radiated power divided by transmitter output confuse fundamental antenna efficiency with overall system gain.