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 ...

reception, radio noise (commonly referred to as radio static) is unwanted random

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the u ...

electrical signals, fluctuating voltages, always present in a

radio receiver In radio communications, a radio receiver, also known as a receiver, a wireless, or simply a radio, is an electronic device that receives radio waves and converts the information carried by them to a usable form. It is used with an antenna. ...

in addition to the desired radio signal. Atmosphericnoise

Radio noise is a combination of natural electromagnetic

atmospheric noise Atmospheric noise is radio noise, or "static", caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, there are about 40 lightning flashes per second, or lightning discharges per day ...

("spherics", static) created by electrical processes in the atmosphere like

lightning Lightning is a natural phenomenon consisting of electrostatic discharges occurring through the atmosphere between two electrically charged regions. One or both regions are within the atmosphere, with the second region sometimes occurring on ...

; human-made radio frequency interference () from other electrical devices picked up by the receiver's antenna; and

thermal noise A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...

present in the receiver input circuits, mostly caused by the random thermal motion of molecules inside

resistor A resistor is a passive two-terminal electronic component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active e ...

Effects of noise on radio

Radio noise near in

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

to a received radio signal (in the receiver's

passband A passband is the range of frequency, frequencies or wavelengths that can pass through a Filter (signal processing), filter. For example, a radio receiver contains a bandpass filter to select the frequency of the desired radio signal out of all t ...

) interferes ( ) with the operation of the receiver's circuitry. The level of noise determines the maximum sensitivity and reception range of a radio receiver; if no noise were picked up with radio signals, even weak transmissions could be received at virtually any distance by making a radio receiver that had high enough amplification, with low enough internal-noise. The limiting noise source in a receiver depends on the frequency range in use: At frequencies below about 40 MHz but above about 20 MHz, nearby radio frequency interference caused by human-made devices is the main issue, and

in occasional storms that pass within line-of-sight of the antenna.

Atmospheric noise Atmospheric noise is radio noise, or "static", caused by natural atmospheric processes, primarily lightning discharges in thunderstorms. On a worldwide scale, there are about 40 lightning flashes per second, or lightning discharges per day ...

is variably comparable to human-caused noise below 20 MHz, and particularly severe in the

mediumwave Medium wave (MW) is a part of the medium frequency (MF) radio band used mainly for AM broadcasting, AM radio broadcasting. The spectrum provides about 120 channels with more limited sound quality than FM stations on the FM broadcast band. Duri ...

and

longwave In radio, longwave (also spelled long wave or long-wave and commonly abbreviated LW) is the part of the radio spectrum with wavelengths longer than what was originally called the medium-wave (MW) broadcasting band. The term is historic, dati ...

bands, or even lower frequencies; below about 200 kHz atmospheric noise typically dominates. With radio noise present, if a radio source is so weak and far away that the radio signal in the receiver has a lower amplitude than the average noise, the noise will drown out the signal. The level of noise in a communications circuit is measured by the

signal-to-noise ratio Signal-to-noise ratio (SNR or S/N) is a measure used in science and engineering that compares the level of a desired signal to the level of background noise. SNR is defined as the ratio of signal power to noise power, often expressed in deci ...

(, ), the ratio of the average amplitude of the signal voltage to the average amplitude of the noise voltage. When this ratio is below one (0 dB) the noise is greater than the signal, requiring special processing to recover the information, if that is even possible.

Atmospheric noise

("spherics", static, or ) created by natural electrical events in the atmosphere, principally

in tropical storms. At frequencies below about 20 MHz the

ionosphere The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays ...

traps radio waves inside the atmosphere – the same phenomenon that enables continent-wide up to world-wide communication in the

shortwave Shortwave radio is radio transmission using radio frequencies in the shortwave bands (SW). There is no official definition of the band range, but it always includes all of the high frequency band (HF), which extends from 3 to 30 MHz (app ...

s. Above about 30 MHz any noise freely radiates through the ionosphere and dissipates into space; at those higher frequencies naturally-caused noise only troubles radio receivers within line-of-sight of a nearby lightning storm. The same effect applies to human-caused radio interference.

Human-caused radio interference

Human-caused radio frequency interference (, , or ) arises from electrical switches, motors, vehicle ignition circuits,

computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...

s, and other man-made sources tend to be above the

floor in the receiver's circuits. These noises are often also referred to as ''"static"''. Human-caused electromagnetic interference () can disrupt the operation of any electronic equipment in general, not just radios, causing malfunction. In recent years standards have been developed for the levels of

electromagnetic radiation In physics, electromagnetic radiation (EMR) is a self-propagating wave of the electromagnetic field that carries momentum and radiant energy through space. It encompasses a broad spectrum, classified by frequency or its inverse, wavelength ...

that electronic equipment is permitted to radiate, and normal levels which equipment is expected to tolerate. These standards are aimed at ensuring what is referred to as electromagnetic compatibility ().

Astronomical noise sources

Cosmic background noise is experienced at frequencies above about 15 MHz when highly directional antennas are pointed toward 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 ...

or to certain other radio-bright objects in the sky, such as the center of the Milky Way Galaxy, or the planet

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a Jupiter mass, mass more than 2.5 times that of all the other planets in the Solar System combined a ...

Thermal noise

At very high frequency () and

ultra high frequency Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter ...

() and above, atmospheric noise and human-made noise are often low, and

generated within the radio's own circuitry is usually the limiting factor. In the most sensitive receivers at these frequencies –

radio telescope A radio telescope is a specialized antenna (radio), antenna and radio receiver used to detect radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the r ...

s and

spacecraft communication A spacecraft is a vehicle that is designed to fly and operate in outer space. Spacecraft are used for a variety of purposes, including communications, Earth observation, meteorology, navigation, space colonization, planetary exploration, ...

– thermal noise is reduced by cooling the RF front end of the receiver to

cryogenic In physics, cryogenics is the production and behaviour of materials at very low temperatures. The 13th International Institute of Refrigeration's (IIR) International Congress of Refrigeration (held in Washington, DC in 1971) endorsed a univers ...

temperatures.

Effects of noise on radio

Atmospheric noise

Human-caused radio interference

Astronomical noise sources

Thermal noise

See also

References

Further reading