In antenna theory, radiation efficiency is a measure of how well a radio antenna converts the radio-frequency power accepted at its terminals into radiated power. Likewise, in a receiving antenna it describes the proportion of the radio wave's power intercepted by the antenna which is actually delivered as an electrical signal. It is not to be confused with antenna efficiency, which applies to aperture antennas such as a parabolic reflector or phased array, or antenna/aperture illumination efficiency, which relates the maximum directivity of an antenna/aperture to its standard directivity.

Radiation efficiency is defined 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. The gain of an antenna is the directivity multiplied by the radiation efficiency. Thus, we have

where ${\displaystyle G}$ is the gain of the antenna in a specified direction, ${\displaystyle e_{R}}$ is the radiation efficiency, and ${\displaystyle D}$ is the directivity of the antenna in the specified direction.

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.

In the case of an antenna or antenna array having multiple ports, the radiation efficiency depends on the excitation. More precisely, the radiation efficiency depends on the relative phases and the relative amplitudes of the signals applied to the different ports. This dependence is always present, but it is easier to interpret in the case where the interactions between the ports are sufficiently small. These interactions may be large in many actual configurations, for instance in an antenna array built in a mobile phone to provide spatial diversity and/or spatial multiplexing. In this context, it is possible to define an efficiency metric as the minimum radiation efficiency for all possible excitations, denoted by ${\displaystyle e_{R\,MIN}}$, which is related to the radiation efficiency figure given by ${\displaystyle F_{RE}={\sqrt {1-e_{R\,MIN}}}}$.

Another interesting efficiency metric is the maximum radiation efficiency for all possible excitations, denoted by ${\displaystyle e_{R\,MAX}}$. It is possible to consider that using ${\displaystyle e_{R\,MIN}}$ as design parameter is particularly relevant to a multiport antenna array intended for MIMO transmission with spatial multiplexing, and that using ${\displaystyle e_{R\,MAX}}$ as design parameter is particularly relevant to a multiport antenna array intended for beamforming in a single direction or over a small solid angle.

Measurements of the radiation efficiency are difficult. Classical techniques include the ″Wheeler method″ (also referred to as ″Wheeler cap method″) and the ″Q factor method″. The Wheeler method uses two impedance measurements, one of which with the antenna located in a metallic box (the cap). Unfortunately, the presence of the cap is likely to significantly modify the current distribution on the antenna, so that the resulting accuracy is difficult to determine. The Q factor method does not use a metallic enclosure, but the method is based on the assumption that the Q factor of an ideal antenna is known, the ideal antenna being identical to the actual antenna except that the conductors have perfect conductivity and any dielectrics have zero loss. Thus, the Q factor method is only semi-experimental, because it relies on a theoretical computation using an assumed geometry of the actual antenna. Its accuracy is also difficult to determine. Other radiation efficiency measurement techniques include: the pattern integration method, which requires gain measurements over many directions and two polarizations; and reverberation chamber techniques, which utilize a mode-stirred reverberation chamber.