In radio frequency (RF) 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 (IF) filter of the radio receiver. Thus, we can define the noise temperature as:

: <math>T = \frac{P_\text{noise{kB}=\frac{1}{k}\frac{P_\text{noise{B}</math>

Because k is a constant, we can effectively think of T as noise power spectral density (with unit W/Hz) 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 T<sub>A</sub>. It is added directly to the effective noise temperature of the receiver to obtain the overall system noise temperature:

: <math>T_S=T_\text{A}+T_\text{E}</math>

Sources of antenna noise

Antenna noise temperature has contributions from many sources, including:

  • Cosmic microwave background radiation
  • Galactic radiation
  • Earth heating
  • The Sun
  • The Moon
  • Electrical devices
  • The antenna itself

Galactic noise is high below 1000&nbsp;MHz. At around 150&nbsp;MHz, it is approximately 1000&nbsp;K. At 2500&nbsp;MHz, it has leveled off to around 10&nbsp;K .

Earth has an accepted standard temperature of 288 K.

The level of the Sun's contribution depends on the solar flux. It is given by

: <math>T_\text{A}=3.468\,F