Editing Radio window

{{Short description|Region of the radio spectrum}}
[[File:Atmospheric electromagnetic opacity.svg|thumb|Opacity of the Earth's atmosphere: the radio window spans larger wavelengths. ]]

The '''radio window''' is the [[spectral region|region]] of the [[radio spectrum]] that [[atmospheric window|penetrate the Earth's atmosphere]]. Typically, the lower limit of the radio window's range has a value of about 10 [[Hertz|MHz]] (λ ≈ 30 m); the best upper limit achievable from optimal terrestrial observation sites is equal to approximately 1 THz (λ ≈ 0.3 mm).<ref>{{Cite book |last1=Condon |first1=James J. |url=https://books.google.com/books?id=vWWYDwAAQBAJ |title=Essential Radio Astronomy |last2=Ransom |first2=Scott M. |date=2016 |publisher=[[Princeton University Press]] |isbn=978-0-691-13779-7 |pages=1 |language=en}}</ref><ref>{{Cite web |title=1 Introduction‣ Essential Radio Astronomy |url=https://www.cv.nrao.edu/~sransom/web/Ch1.html |access-date=2021-12-27 |website=www.cv.nrao.edu}}</ref>

It plays an important role in astronomy; up until the 1940s, astronomers could only use the [[visible spectrum|visible]] and [[infrared|near infrared]] spectra for their measurements and observations. With the development of [[radio telescope]]s, the radio window became more and more utilizable, leading to the development of [[radio astronomy]] that provided [[Astrophysics|astrophysicists]] with valuable observational data.<ref>{{Cite book|last1=Wilson|first1=Thomas|url=http://www.worldcat.org/oclc/954868912|title=Tools of Radio Astronomy|last2=Rohlfs|first2=Kristen|last3=Huettemeister|first3=Susanne|publisher=[[Springer Science+Business Media|Springer-Verlag GmbH]]|year=2016|isbn=978-3-662-51732-1|location=Berlin|pages=1–2|language=English|oclc=954868912}}</ref>

== Factors affecting lower and upper limits ==
The lower and upper limits of the radio window's range of frequencies are not fixed; they depend on a variety of factors.

=== Absorption of mid-IR ===
The upper limit is affected by the vibrational transitions of atmospheric molecules such as [[oxygen]] (O<sub>2</sub>), [[carbon dioxide]] (CO<sub>2</sub>), and [[water]] (H<sub>2</sub>O), whose energies are comparable to the energies of [[Infrared|mid-infrared]] [[photon]]s: these molecules largely absorb the mid-infrared radiation that heads towards Earth.<ref>{{Cite book|last1=Liou|first1=Kuo-Nan|url=https://doi.org/10.1017/CBO9781139030052|title=Light scattering by ice crystals: fundamentals and applications|last2=Yang|first2=Ping|last3=Takano|first3=Yoshihide|date=2016|publisher=[[Cambridge University Press]]|isbn=978-1-139-03005-2|pages=251|doi=10.1017/CBO9781139030052 |language=English|oclc=958454932}}</ref><ref>{{Cite book|last=Ritchie|first=Grant|url=http://www.worldcat.org/oclc/957339640|title=Atmospheric chemistry: from the surface to the stratosphere|publisher=[[World Scientific]]|year=2017|isbn=978-1-78634-175-4|pages=68|language=English|oclc=957339640}}</ref>

=== Ionosphere ===
The radio window's lower frequency limit is greatly affected by the [[Ionospheric absorption|ionospheric refraction]] of the radio waves whose frequencies are approximately below 30 MHz (λ > 10 m);<ref>{{Cite book| last1=Anderson| first1=John B.| url=http://www.worldcat.org/oclc/56103934|title=Understanding information transmission|last2=Johannesson|first2=Rolf|publisher=[[IEEE Press]], [[Wiley-Interscience]]|year=2005|isbn=978-0-471-67910-3| location=Piscataway, NJ; Hoboken, NJ | pages=110|language=English | oclc=56103934}}</ref> radio waves with frequencies below the limit of 10 MHz (λ > 30 m) are reflected back into space by the ionosphere.<ref>{{Cite book| last1=Torge|first1=Wolfgang|url=http://www.worldcat.org/oclc/987088700|title=Geodesy|last2=Müller|first2=Jürgen|publisher=[[De Gruyter]]|year=2012|isbn=978-3-11-020718-7|location=Berlin| pages=121| language=English| oclc=987088700}}</ref> The lower limit is proportional to the density of the ionosphere's free [[electron]]s and coincides with the [[Plasma oscillation|plasma frequency]]:
<math display="block"> f_p = 9 \sqrt{N_e},</math>
where <math>f_p</math> is the plasma frequency in Hz and <math>N_e</math> the electron density in electrons per cubic meter. Since it is highly dependent on sunlight, the value of <math>N_e</math> changes significantly from daytime to nighttime usually being lower during the day, leading to a decrease of the radio window's lower limit and higher during the night, causing an increase of the radio window's lower frequency end. However, this also depends on the [[Solar phenomena|solar activity]] and the geographic position.<ref>{{Cite book |last1=Warnick |first1=Karl F. |url=http://www.worldcat.org/oclc/1032582026 |title=Phased arrays for radio astronomy, remote sensing and satellite communications |last2=Maaskant |first2=Rob |last3=Ivashina |first3=Marianna V. |author-link3=Marianna Ivashina |publisher=[[Cambridge University Press]] |year=2018 |isbn=978-1-108-42392-2 |pages=5 |language=English |oclc=1032582026}}</ref>

=== Troposphere ===
[[File:The Atacama Large Millimeter submillimeter Array (ALMA) by night under the Magellanic Clouds.jpg|thumb|The [[Atacama Large Millimeter Array]], an [[astronomical interferometer]] of 66 [[radio telescope]]s constructed on the 5,000 m (16,000 ft) elevation Chajnantor [[plateau]] in [[Chile]].]]
When performing observations, radio astronomers try to extend the upper limit of the radio window towards the 1 THz optimum, since the [[astronomical object]]s give [[spectral line]]s of greater intensity in the higher frequency range.<ref name=":0">{{Cite book|last1=Wilson|first1=Thomas|url=http://www.worldcat.org/oclc/954868912|title=Tools of Radio Astronomy|last2=Rohlfs|first2=Kristen|last3=Huettemeister|first3=Susanne|publisher=[[Springer Science+Business Media|Springer-Verlag GmbH]]|year=2016|isbn=978-3-662-51732-1|pages=4|language=English|oclc=954868912}}</ref> [[Troposphere|Tropospheric]] water vapour greatly affects the upper limit since its [[Electromagnetic absorption by water|resonant absorption]] frequency bands are 22.3 GHz (λ ≈ 1.32 cm), 183.3 GHz (λ ≈ 1.64 mm) and 323.8 GHz (λ ≈ 0.93 mm). The tropospheric oxygen's bands at 60 GHz (λ ≈ 5.00 mm) and 118.74 GHz (λ ≈ 2.52 mm) also affect the upper limit.<ref>{{Cite book|last=Otung|first=Ifiok|url=http://www.worldcat.org/oclc/1225565245|title=Communication engineering principles|publisher=[[Wiley (publisher)|Wiley]]|year=2021|isbn=978-1-119-27402-5|pages=390|language=English|oclc=1225565245}}</ref> To tackle the issue of water vapour, many observatories are built at high altitudes where the climate is more dry.<ref>{{Cite book|last=Karttunen|first=Hannu|url=http://www.worldcat.org/oclc/860603182|title=Fundamental astronomy|publisher=[[Springer Science+Business Media|Springer-verlag]]|year=2007|isbn=978-3-540-34143-7|location=Berlin|pages=72|language=English|oclc=860603182}}</ref> However, few can be done to avoid the oxygen's interference with radio waves propagation.<ref>{{Cite book|url=http://www.worldcat.org/oclc/25175353|title=Conference Proceedings|publisher=[[IEEE]]|year=1990|isbn=978-0-87942-557-9|pages=241|oclc=25175353 |language=en}}</ref>

=== Radio frequency interference ===
The width of the radio window is also affected by [[Electromagnetic interference|radio frequency interference]] which hinders the observations at certain wavelength ranges and undermines the quality of the observational data of radio astronomy.<ref>{{Cite book|last=McNally|first=Derek|url=http://www.worldcat.org/oclc/29359179|title=The vanishing universe: adverse environmental impacts on astronomy: proceedings of the conference sponsored by Unesco|publisher=[[Cambridge University Press]]|year=1994|isbn=978-0-521-45020-1|editor-last=McNally|editor-first=Derek|location=Cambridge; New York|pages=93|language=English|oclc=29359179|editor-last2=Unesco}}</ref>    

==See also==
* [[Infrared window]]
* [[Optical window]]
* [[Radio propagation]]

== References ==
<references />{{Radio-astronomy}}

{{DEFAULTSORT:Radio Window}}
[[Category:Electromagnetic spectrum]]