Radiation efficiency
Ratio of radiated power to accepted power, frequency dependent.
Radiation efficiency describes how effectively a radio antenna converts the radio-frequency power it receives at its terminals into power that is actually radiated. For a receiving antenna, it indicates the fraction of the radio wave's power intercepted by the antenna that ends up as an electrical signal. This concept should not be confused with antenna efficiency, which is used for aperture antennas like parabolic reflectors or phased arrays, nor with antenna or aperture illumination efficiency, which compares an antenna's maximum directivity to its standard directivity.
The formal definition is the ratio of the total power radiated by an antenna to the net power accepted from the connected transmitter. This value is often expressed as a percentage (always less than 100) and varies with frequency; it can also be given in decibels. An antenna's gain in a specific direction equals its directivity in that direction multiplied by its radiation efficiency. For wire antennas with a defined radiation resistance, radiation efficiency is the ratio of that radiation resistance to the total resistance of the antenna, which includes ground loss and conductor resistance. In practice, resistive losses from tuning or matching networks are often included, though such network losses are not strictly an antenna property. For other antenna types, radiation efficiency is harder to calculate and is typically determined through measurements.
For antennas or antenna arrays with multiple ports, radiation efficiency depends on the excitation—specifically, the relative phases and amplitudes of the signals applied to the ports. This dependence is always present but easier to interpret when port interactions are small. In real configurations, such as a mobile phone antenna array for spatial diversity or spatial multiplexing, these interactions can be large. In this context, one can define a metric as the minimum radiation efficiency across all possible excitations, as well as a maximum radiation efficiency across all possible excitations. The minimum may be especially relevant for a multiport array used in MIMO transmission with spatial multiplexing, while the maximum is more relevant for beamforming in a single direction or over a small solid angle.
Measuring radiation efficiency is challenging.
- Definition
- The ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter.
- Expression
- Sometimes expressed as a percentage (less than 100) or in decibels.
- Frequency dependence
- Frequency dependent.
- Gain relation
- Gain = radiation efficiency × directivity.
- Wire antenna formula
- Radiation efficiency = radiation resistance / total resistance (including ground loss and conductor resistance).
- Multiport dependence
- Depends on relative phases and amplitudes of signals applied to different ports.
Lore & Background
For wire antennas with a defined radiation resistance, the radiation efficiency is the ratio of the radiation resistance to the total resistance of the antenna, including ground loss 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—specifically on the relative phases and the relative amplitudes of the signals applied to the different ports. This dependence is always present but easier to interpret when interactions between ports are small. These interactions may be large in many actual configurations, such as in an antenna array built in a mobile phone to provide spatial diversity and/or spatial multiplexing. Two metrics can be defined: the minimum radiation efficiency for all possible excitations, and the maximum radiation efficiency for all possible excitations.
Measurements of radiation efficiency are difficult. Classical techniques include the Wheeler method (using a metallic cap) and the Q factor method. The Wheeler method uses two impedance measurements, one with the antenna in a metallic box, but the cap may significantly modify the current distribution, making accuracy difficult to determine. The Q factor method does not use a metallic enclosure but assumes the Q factor of an ideal antenna is known, making it only semi-experimental and its accuracy also difficult to determine. Other techniques include pattern integration and reverberation chamber methods.
Reader's Guide
Radiation efficiency is a fundamental parameter in antenna design, directly linking the power accepted by an antenna to the power it radiates. It is distinct from antenna efficiency and aperture illumination efficiency, which apply to different antenna types. The relationship gain = radiation efficiency × directivity shows that radiation efficiency directly affects the realized gain of an antenna. For wire antennas, the efficiency can be calculated from resistance ratios, but for other types it must be measured. The difficulty of accurate measurement—whether by Wheeler cap, Q factor, pattern integration, or reverberation chamber—means that reported efficiencies often carry uncertainty. In multiport antennas, such as those used in MIMO systems, the efficiency depends on the excitation, leading to the use of minimum and maximum efficiency metrics for different applications: minimum efficiency for spatial multiplexing, maximum efficiency for beamforming. Losses are typically divided into ohmic loss (in conductors and feed cables) and ground loss (in soil and nearby metal objects), though ground losses can be ignored for antennas mounted several wavelengths above the earth on non-conducting masts.
Did You Know?
- Radiation efficiency is frequency dependent and can be expressed as a percentage or in decibels.
- For wire antennas, radiation efficiency equals radiation resistance divided by total resistance including ground and conductor losses.
- The Wheeler method for measuring radiation efficiency uses a metallic cap, but the cap may alter the antenna's current distribution.
- In multiport antennas, radiation efficiency depends on the relative phases and amplitudes of signals applied to the ports.
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