In radio frequency (RF) applications such as

radio Radio is the technology of communicating using radio waves. Radio waves are electromagnetic waves of frequency between 3 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transmitter connec ...

radar Radar is a system that uses radio waves to determine the distance ('' ranging''), direction ( azimuth and elevation angles), and radial velocity of objects relative to the site. It is a radiodetermination method used to detect and track ...

and

telecommunications Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

, 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".IEEE Std 145-2013, IEEE Standard for Definitions of Terms for Antennas, IEEE 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 In telecommunications, the term noise power has the following meanings: # The measured total noise in a given bandwidth at the input or output of a device when the signal is not present; the integral of noise spectral density over the bandwidth # ...

is defined using the relationship , where ''k'' is the

Boltzmann constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative thermal energy of particles in a ideal gas, gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin (K) and the ...

, ''T'' is the

noise temperature In electronics, noise temperature is one way of expressing the level of available noise power introduced by a component or source. The power spectral density of the noise is expressed in terms of the temperature (in kelvins) that would produce ...

, and ''B'' is the noise bandwidth. Typically the noise bandwidth is determined by the bandwidth of the

intermediate frequency In communications and electronic engineering, an intermediate frequency (IF) is a frequency to which a carrier wave is shifted as an intermediate step in Transmission (telecommunications), transmission or reception. The intermediate frequency is ...

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

T = \frac=\frac\frac

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''_A. It is added directly to the effective noise temperature of the receiver to obtain the overall system noise temperature: :

T_S=T_\text+T_\text

Sources of antenna noise

Antenna noise temperature has contributions from many sources, including: *

Cosmic microwave background radiation The cosmic microwave background (CMB, CMBR), or relic radiation, is microwave radiation that fills all space in the observable universe. With a standard optical telescope, the background space between stars and galaxies is almost completely dar ...

* Galactic radiation * Earth heating * The

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

* The

Moon The Moon is Earth's only natural satellite. It Orbit of the Moon, orbits around Earth at Lunar distance, an average distance of (; about 30 times Earth diameter, Earth's diameter). The Moon rotation, rotates, with a rotation period (lunar ...

* Electrical devices * The antenna itself Galactic noise is high below 1000 MHz. At around 150 MHz, it is approximately 1000 K. At 2500 MHz, it has leveled off to around 10 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 :

T_\text=3.468\,F10^

where

F

is the solar flux, :

\lambda

is the wavelength, and

G

is the logarithmic gain of the antenna in decibels. The antenna

depends on antenna

coupling A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mo ...

to all noise sources in its environment as well as on noise generated within the antenna. That is, in a directional antenna, the portion of the noise source that the antenna's main and side lobes intersect contribute proportionally. For example, a satellite antenna may not receive noise contribution from the Earth in its main lobe, but sidelobes will contribute a portion of the 288 K Earth noise to its overall noise temperature.

References

{{cite web , title=ITU P.372 : Radio noise , url=https://www.itu.int/rec/R-REC-P.372 , publisher=ITU , access-date=4 July 2019 Temperature Noise (electronics)

Mathematics

Sources of antenna noise

See also

References