Noise temperature (antenna)
In radio frequency applications such as radio, radar and telecommunications, noise temperature of an antenna is a measure of the noise power density contributed by the antenna to the overall RF receiver system. It is defined as "the temperature of a resistor having an available thermal noise power per unit bandwidth equal to that at the antenna's output at a specified frequency". In other words, antenna noise temperature is a parameter that describes how much noise an antenna produces in a given environment. This temperature is not the physical temperature of the antenna. Moreover, an antenna does not have an intrinsic "antenna temperature" associated with it; rather the temperature depends on its gain pattern, pointing direction, and the thermal environment that it is placed in.
Mathematics
In RF applications, noise power is defined using the relationship, where k is the Boltzmann constant, T is the noise temperature, and B is the noise bandwidth. Typically the noise bandwidth is determined by the bandwidth of the intermediate frequency filter of the radio receiver. Thus, we can define the noise temperature as:Because k is a constant, we can effectively think of T as noise power spectral density normalized by k.
Antenna noise is only one of the contributors to the overall noise temperature of an RF receiver system, so it is typically subscripted, such as TA. It is added directly to the effective noise temperature of the receiver to obtain the overall system noise temperature: