Isotropic radiator
An isotropic radiator (English isotropic antenna ), also called spherical radiator or isotropic antenna is a hypothetical model or idealization of a point reflector which (ie uniformly in all directions ) and lossless sends or receives isotropic. The isotropic radiator emits transverse waves may be polarized differently depending on the application. For the characterization of the directional effect of antennas whose radiation patterns are compared to the radiation diagram of Isotropstrahlers. For this comparison, it is assumed for the shot peening, that it with the exception of the efficiency and the directivity has the same characteristics as the antenna with which it is compared. An ideal isotropic radiator is not feasible in practice, all antennas have in reality a more or less pronounced directivity.
In the isotropic radiator, the total transmit power is distributed evenly on the surface of a sphere. The power density at a distance of:
Because of the sometimes very large numerical values of the comparison is that given almost exclusively in a logarithmic measure, the decibel ( dB). The dBi value is the antenna gain of an antenna based on the isotropic radiator as a reference antenna. For example, it is 1.8 dBi for a λ/2-Dipolantenne in the direction perpendicular to the antenna axis 2.15 dBi, for a Hertzian dipole.
In some measurement methods for antennas, as the Sunstrobe recording, the widespread of solar radiation spectrum can be used as an approximation to a point source. The sun emits not only visible light but transmits in all frequency ranges with for the period of measurement rather stable services. The point source approximation is admissible despite the huge size of the solar surface, because the sun is very far from the measuring antenna.
An isotropic radiator can be computationally used as a receiving antenna. However, a receiving antenna needs a certain aperture efficiency (aperture) in order to draw power from a field with a given power density (power per unit area) can. A point has no area and would be able to see any output in the sequence and the receiving antenna would not work. Thus an isotropic receiving antenna requires a minimum area that is based on the wavelength to be received. Their dimensions and their effective antenna area AW is dependent on the wavelength: