|This article needs additional citations for verification. (August 2012)|
|Part of a series on|
In antenna theory, antenna efficiency is a loose term usually meaning radiation efficiency, often abbreviated to efficiency. It is a measure of the efficiency with which a radio antenna converts the radio-frequency power accepted at its terminals into radiated power.
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.
For monopole and other ground-based antennas, ground loss occurs due to ohmic resistance in the antenna's connection to its ground plane/counterpoise, including its mast or stalk and its bonding connections, as well as the ohmic resistance encountered by radio-frequency currents in the ground plane in the vicinity of the antenna.
Other definitions of efficiency in antennas
The IEEE standard defines several other antenna parameters which include the word efficiency, such as
- aperture illumination efficiency for aperture-antennas.
- polarization efficiency; polarization mismatch factor.
These are unconnected and should not be confused with radiation efficiency, which is the most commonly used and implied term.
This is applied to aperture antennas such as a parabolic antenna and is a measure of the reduction in power gain caused by non-uniform aperture illumination. In a typical situation the reflector is illuminated with a reduced power-density at the edge compared with the centre, in order to reduce sidelobes and other effects. This causes a reduction in gain: the ratio of the gain of the tapered aperture distribution to the theoretical gain of a uniformly illuminated aperture is the aperture efficiency.