Definition
Radiation efficiency is defined by IEEE Std 145-1993 "Standard Definitions of Terms for Antennas" as "The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter." It is sometimes expressed as a percentage (less than 100), and is frequency dependent. It can also be described in decibels.
For wire antennas which have a defined radiation resistance the radiation efficiency is the ratio of the radiation resistance to the total resistance of the antenna including ground loss (see below) and conductor resistance. In practical cases the resistive loss in any tuning and/or matching network is often included, although network loss is strictly not a property of the antenna.
For other types of antenna the radiation efficiency is less easy to calculate and is usually determined by measurements.
The gain of an antenna is the directivity multiplied by the radiation efficiency, as described in Std 145-1993.
Read more about this topic: Antenna Efficiency
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