well you need to define the farfield monitor for the desired frequency. after simulation
you can check the farfields folder . when you plot the 3D farfield pattern it will give you both radiated as well as total efficiency
Efficiency = radiated power / power fed to the antenna, but don't get surprised if this goes over 100% sometimes, if the system is lossless. There is always some numerical error, particularly due to the radiation boundaries...
well you need to define the farfield monitor for the desired frequency. after simulation
you can check the farfields folder . when you plot the 3D farfield pattern it will give you both radiated as well as total efficiency