View source for Radio noise

{{Short description|Unwanted radio frequency signals}}
{{No footnotes|date=September 2011}}
In [[radio]] reception, '''radio noise''' (commonly referred to as '''radio static''') is unwanted random [[radio frequency]] electrical signals, fluctuating voltages, always present in a [[radio receiver]] in addition to the desired radio signal.

[[File:Atmosphericnoise.PNG|thumb|upright=1.2|right| [[Atmospheric noise]] and human-caused noise as a function of frequency in the LF, MF, and HF radio spectrum according to CCIR&nbsp;322.<ref name=CCIR-322/> The vertical axis is in decibels above the thermal noise floor. The graph shows that as frequency drops atmospheric noise dominates other sources, and that as frequency rises, although all noise quiets, human-caused noise exceeds atmospheric noise.]]
Radio noise is a combination of natural electromagnetic [[atmospheric noise]] ("spherics", static) created by electrical processes in the atmosphere like [[lightning]]; human-made [[radio frequency interference]] ({{sc|RFI}}) from other electrical devices picked up by the receiver's [[antenna (radio)|antenna]]; and [[thermal noise]] present in the receiver input circuits, mostly caused by the random thermal motion of molecules inside [[resistor]]s.

== Effects of noise on radio ==
Radio noise near in [[frequency]] to a received radio signal (in the receiver's [[passband]]) [[radio frequency interference|interferes]] ([[radio frequency interference|{{sc|RFI}}]]) 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 [[low-noise amplifier|amplification]], with low enough [[Johnson-Nyquist noise|internal-noise]].

The limiting noise source in a receiver depends on the frequency range in use: At frequencies below about 40&nbsp;MHz but above about 20&nbsp;MHz, nearby [[electromagnetic interference|radio frequency interference]] caused by human-made devices is the main issue, and [[lightning]] in occasional storms that pass within line-of-sight of the [[radio antenna|antenna]]. [[Atmospheric noise]] is variably comparable to human-caused noise below 20&nbsp;MHz, and particularly severe in the [[mediumwave]] and [[longwave]] bands, or [[VLF|even lower frequencies]]; below about 200&nbsp;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]] ({{sc|SNR}}, {{small|{{math|{{sfrac| S | N }}}}}}), 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&nbsp;[[decibel|dB]]) the noise is greater than the signal, requiring special processing to recover the information, if that is even possible.

== Atmospheric noise ==
{{main|atmospheric noise}}
[[Atmospheric noise]] ("spherics", static, or [[Q-codes|{{sc|QRN}}]]) created by natural electrical events in the atmosphere, principally [[lightning]] in tropical storms. At frequencies below about 20&nbsp;MHz the [[ionosphere]] traps radio waves inside the atmosphere – the same phenomenon that enables continent-wide up to [[radio propagation|world-wide communication]] in the [[shortwave]]s. Above about 30&nbsp;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 ==
{{main|electromagnetic interference}}
Human-caused radio frequency interference ({{sc|RFI}}, {{sc|EMI}}, or [[Q-codes|{{sc|QRM}}]]) arises from [[switch|electrical switches]], [[electric motor|motors]], vehicle [[ignition system|ignition circuits]], [[computer]]s, and other man-made sources tend to be above the [[thermal noise]] floor in the receiver's circuits. These noises are often also referred to as ''"static"''.

Human-caused electromagnetic interference ({{sc|EMI}}) 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]] 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]] ({{sc|EMC}}).

== Astronomical noise sources ==
{{main|astronomical radio source}}
[[cosmic noise|Cosmic background noise]] is experienced at frequencies above about 15&nbsp;MHz when [[radio telescope|highly directional antennas]] are pointed toward the [[Solar radio emission|Sun]] or to certain other [[astronomical radio source|radio-bright objects in the sky]], such as the [[Galactic Center|center of the Milky Way Galaxy]], or the planet [[magnetosphere of Jupiter#Jupiter-radio-source-anchor|Jupiter]].

== Thermal noise ==
{{main|Johnson–Nyquist noise}}
At [[very high frequency]] ({{sc|VHF}}) and [[ultra high frequency]] ({{sc|UHF}}) and above, atmospheric noise and human-made noise are often low, and [[thermal noise]] generated within the radio's own circuitry is usually the limiting factor. In the most sensitive receivers at these frequencies – [[radio telescope]]s and [[NASA Deep Space Network|spacecraft communication]] – thermal noise is reduced by cooling the [[RF front end]] of the receiver to [[cryogenic]] temperatures.

== See also ==
{{div col begin|colwidth=13em}}
* [[astronomical radio source]]
* [[atmospheric noise]]
* [[cosmic noise]]
* [[electromagnetic interference]]
* [[magnetosphere of Jupiter#Jupiter-radio-source-anchor|Jupiter radio source]]
* [[Johnson–Nyquist noise|Johnson–Nyquist (thermal) noise]]
* [[noise]]
* [[radio propagation]]
* [[shot noise]]
* [[Solar radio emission]]
{{div col end}}

== References ==
{{reflist|25em|refs=

<ref name=CCIR-322>
{{cite report
 |title = Characteristics and Applications of Atmospheric Radio Noise Data
 |year = 1968
 |series = International Radio Consultative Committee (CCIR)
 |place = Geneva, CH
 |publisher = [[International Telecommunication Union]] (ITU)
 |id = CCIR Report&nbsp;322-3
}}; first CCIR Report&nbsp;322 was 1963; [https://books.google.com/books?id=1ls2AQAAIAAJ revised first ed.]; second is {{ISBN|92-61-01741-X}}.
</ref>

}} <!-- end "refs=" -->

== Further reading ==
{{refbegin|25em|small=yes}}

* {{cite report
 |title=Radio noise
 |series=ITU-R Recommendation |volume=P.372
 |publisher=[[International Telecommunication Union]]
 |place=Geneva, CH
 |url=http://www.itu.int/rec/R-REC-P.372/en |lang=en
}}

* {{cite journal
 |last1=Blackard  |first1=K.L.
 |last2=Rappaport |first2=T.S.
 |last3=Bostian   |first3=C.W.
 |display-authors=etal
 |date=30 September 1993
 |title=Measurements and models of radio frequency impulsive noise for indoor wireless communications
 |journal=[[IEEE Journal on Selected Areas in Communications]]
 |volume=11  |issue=7  |pages=991–1001
 |doi=10.1109/49.233212  |issn=0733-8716
}}

* {{cite conference
 |first1=R. |last1=Dalke     |first2=R. |last2=Achatz
 |first3=Y. |last3=Lo        |first4=P. |last4=Papazian
 |first5=G. |last5=Hufford
 |orig-date=11–13 August 1997
 |title=Measurement and analysis of man-made noise in VHF and UHF bands
 |conference=1997 Wireless Communications Conference
 |place=Boulder, CO
 |series=IEEE Xplore
 |date=6 August 2002 
 |doi=10.1109/WCC.1997.622284 |isbn=0-7803-4194-5
 |publisher=[[IEEE]]
 |publication-date=1997
}}

* {{cite journal
 |last1=Lauber   |first1=W.R.
 |last2=Bertrand |first2=J.M.
 |date=August 1999
 |title=Statistics of motor vehicle ignition noise at VHF / UHF
 |journal=[[IEEE Transactions on Electromagnetic Compatibility]]
 |volume=41  |issue=3  |pages=257–259
 |doi=10.1109/15.784164  |issn=0018-9375
}}

* {{cite journal
 |first1=M.G. |last1=Sanchez    |first2=L. |last2=de Haro
 |first3=M.C. |last3=Ramon      |first4=A. |last4=Mansilla
 |first5=C.M. |last5=Ortega     |first6=D. |last6=Oliver
 |date=31 May 1999
 |title=Impulsive noise measurements and characterization in a UHF digital TV channel
 |journal=[[IEEE Transactions on Electromagnetic Compatibility]]
 |volume=41  |issue=2  |pages=124–136
 |doi=10.1109/15.765101 |issn=0018-9375
}}

* {{cite journal
 |last1=Blankenship |first1=T.K.
 |last2=Rappaport   |first2=T.S.
 |date=28 February 1998
 |title=Characteristics of impulsive noise in the 450&nbsp;MHz band in hospitals and clinics
 |journal=[[IEEE Transactions on Antennas and Propagation]]
 |volume=46  |issue=2  |pages=194–203
 |doi=10.1109/8.660963  |issn=0018-926X
}}

{{refend}}

{{Noise}}

[[Category:Noise (electronics)|Radio]]

[[tr:Parazit (elektronik)]]