In radio frequency (RF) applications such as
radio
Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...
,
radar
Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
and
telecommunications
Telecommunication is the transmission of information by various types of technologies over wire, radio, optical, or other electromagnetic systems. It has its origin in the desire of humans for communication over a distance greater than tha ...
, 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 is defined using the relationship
, where k is
Boltzmann's constant
The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas 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:
:
Because
is a constant, we can effectively think of
as noise power spectral density (in
,) normalized by
.
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
. It is added directly to the
effective noise temperature of the receiver to obtain the overall
system noise temperature:
:
Sources of antenna noise
Antenna noise temperature has contributions from many sources, including:
*
Cosmic background radiation
Cosmic background radiation is electromagnetic radiation from the Big Bang. The origin of this radiation depends on the region of the spectrum that is observed. One component is the cosmic microwave background. This component is redshifted ph ...
*
Galactic radiation
Galactic is an American jam band from New Orleans, Louisiana.
Origins and background
Formed in 1994 as an octet (under the name Galactic Prophylactic) and including singer Chris Lane and guitarist Rob Gowen, the group was soon pared down to a ...
*Earth heating
*The
sun
*The
moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
*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
:
:where
is the solar flux,
:
is the wavelength,
:and
is the gain of the antenna in decibels.
The antenna
noise temperature depends on antenna
coupling 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 288K earth noise to its overall noise temperature.
See also
*
Noise Temperature
*
Johnson–Nyquist noise
Johnson–Nyquist noise (thermal noise, Johnson noise, or Nyquist noise) is the electronic noise generated by the thermal agitation of the charge carriers (usually the electrons) inside an electrical conductor at equilibrium, which happens rega ...
*
Federal Standard 1037C
Federal Standard 1037C, titled Telecommunications: Glossary of Telecommunication Terms, is a United States Federal Standard issued by the General Services Administration pursuant to the Federal Property and Administrative Services Act of 1949, ...
*
MIL-STD-188
MIL-STD-188 is a series of U.S. military standards relating to telecommunications.
Purpose
Faced with "past technical deficiencies in telecommunications systems and equipment and software…that were traced to basic inadequacies in the applicati ...
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)