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Since the directive antenna radiates the same total power within a small angle along the z axis, it can have a higher signal strength in that direction than the isotropic antenna, and so a gain greater than one. In electromagnetics, an antenna's gain is a key performance parameter which combines the antenna's directivity and radiation efficiency.
The difference between ERP and EIRP is that antenna gain has traditionally been measured in two different units, comparing the antenna to two different standard antennas; an isotropic antenna and a half-wave dipole antenna: Isotropic gain is the ratio of the power density (signal strength in watts per square meter) received at a point far from ...
Antenna gain , which is equal to the antenna's directivity multiplied by the antenna efficiency, is defined as the ratio of the intensity (power per unit area) of the radio power received at a given distance from the antenna (in the direction of maximum radiation) to the intensity received from a perfect lossless isotropic antenna at the same ...
Paradoxically, every antenna of any type, shorter than ~ 1 / 10 wave in its longest dimension is approximately isotropic, but no real antenna can ever be exactly isotropic. An antenna that is exactly isotropic is only a mathematical model, used as the base of comparison to calculate either the directivity or gain of real antennas.
The aperture of an isotropic antenna, the basis of the definition of gain above, can be derived on the basis of consistency with thermodynamics. [6] [7] [8] Suppose that an ideal isotropic antenna A with a driving-point impedance of R sits within a closed system CA in thermodynamic equilibrium at temperature T.
The antenna gain, or power gain of an antenna is defined as the ratio of the intensity (power per unit surface area) radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna which radiates equal power in all directions.
We write "hypothetical" because a perfect isotropic antenna cannot be constructed. Gain is a dimensionless number (without units). The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna.
Hence, an isotropic antenna would have a noise temperature that is the average of all temperatures around the antenna; for a perfectly directional antenna (with a pencil beam), the antenna temperature will only depend on the temperature in which the antenna is "looking".