• Radiated power is denoted as P_RAD
• Supplied power of the antenna is denoted as P_S

The efficiency of an antenna is usually measured in an anechoic chamber where an antenna is fed with a certain amount of power and the strength of the radiated electromagnetic field in the surrounding space is measured. A high-efficiency antenna has most of the power present at the antenna's input radiated away. A low-efficiency antenna has most of the power absorbed as losses within the antenna or reflected away due to impedance mismatch.

Antenna efficiency is always denoted in % (percentage).