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What (who) is detector array directivity - definition
![Diagram illustrating how directivity is defined. It shows the [[radiation pattern]] of a [[directional antenna]] ''<span style="color:gray;">(R, grey)</span>'' that radiates maximum power along the z-axis, and the pattern of an [[isotropic antenna]] ''<span style="color:green;">(R<sub>iso</sub>, green)</span>'' with the same total radiated power. The directivity is defined as the ratio of the maximum signal strength ''S'' radiated by the antenna to the signal strength ''S''<sub>iso</sub> radiated by the isotropic antenna <math display="block">G = {S \over S_\text{iso}}</math>
Since the directional antenna radiates most of its power into a small solid angle around the z-axis its maximum signal strength is much larger than the isotropic antenna which spreads the same power in all directions. Thus the directivity <math>G</math> is much greater than one.](https://commons.wikimedia.org/wiki/Special:FilePath/Antenna directive gain diagram.svg?width=200 "Diagram illustrating how directivity is defined. It shows the [[radiation pattern]] of a [[directional antenna]] ''<span style=\"color:gray;\">(R, grey)</span>'' that radiates maximum power along the z-axis, and the pattern of an [[isotropic antenna]] ''<span style=\"color:green;\">(R<sub>iso</sub>, green)</span>'' with the same total radiated power. The directivity is defined as the ratio of the maximum signal strength ''S'' radiated by the antenna to the signal strength ''S''<sub>iso</sub> radiated by the isotropic antenna <math display=\"block\">G = {S \over S_\text{iso}}</math>
Since the directional antenna radiates most of its power into a small solid angle around the z-axis its maximum signal strength is much larger than the isotropic antenna which spreads the same power in all directions. Thus the directivity <math>G</math> is much greater than one.")
Wikipedia
In electromagnetics, directivity is a parameter of an antenna or optical system which measures the degree to which the radiation emitted is concentrated in a single direction. It is the ratio of the radiation intensity in a given direction from the antenna to the radiation intensity averaged over all directions. Therefore, the directivity of a hypothetical isotropic radiator is 1, or 0 dBi.
An antenna's directivity is greater than its gain by an efficiency factor, radiation efficiency. Directivity is an important measure because many antennas and optical systems are designed to radiate electromagnetic waves in a single direction or over a narrow-angle. By the principle of reciprocity, the directivity of an antenna when receiving is equal to its directivity when transmitting.
The directivity of an actual antenna can vary from 1.76 dBi for a short dipole to as much as 50 dBi for a large dish antenna.
