Radio window

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

The upper limit is affected by the vibrational transitions of atmospheric molecules such as oxygen (O₂), carbon dioxide (CO₂), and water (H₂O), whose energies are comparable to the energies of mid-infrared photons: these molecules largely absorb the mid-infrared radiation that heads towards Earth.{{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}}{{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}}

The radio window's lower frequency limit is greatly affected by the ionospheric refraction of the radio waves whose frequencies are approximately below 30 MHz (λ > 10 m);{{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}} radio waves with frequencies below the limit of 10 MHz (λ > 30 m) are reflected back into space by the ionosphere.{{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}} The lower limit is proportional to the density of the ionosphere's free electrons and coincides with the plasma frequency:

$$f\_p\; =\; 9\; \backslash sqrt\{N\_e\},$$

where $f\_p$ is the plasma frequency in Hz and $N\_e$ the electron density in electrons per cubic meter. Since it is highly dependent on sunlight, the value of $N\_e$ 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 activity and the geographic position.{{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}}

File:The Atacama Large Millimeter submillimeter Array (ALMA) by night under the Magellanic Clouds.jpg, an astronomical interferometer of 66 radio telescopes 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 objects give spectral lines of greater intensity in the higher frequency range.{{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-Verlag GmbH|year=2016|isbn=978-3-662-51732-1|pages=4|language=English|oclc=954868912}} Tropospheric water vapour greatly affects the upper limit since its 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.{{Cite book|last=Otung|first=Ifiok|url=http://www.worldcat.org/oclc/1225565245|title=Communication engineering principles|publisher=Wiley|year=2021|isbn=978-1-119-27402-5|pages=390|language=English|oclc=1225565245}} To tackle the issue of water vapour, many observatories are built at high altitudes where the climate is more dry.{{Cite book|last=Karttunen|first=Hannu|url=http://www.worldcat.org/oclc/860603182|title=Fundamental astronomy|publisher=Springer-verlag|year=2007|isbn=978-3-540-34143-7|location=Berlin|pages=72|language=English|oclc=860603182}} However, few can be done to avoid the oxygen's interference with radio waves propagation.{{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}}

The width of the radio window is also affected by radio frequency interference which hinders the observations at certain wavelength ranges and undermines the quality of the observational data of radio astronomy.{{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}}

• Infrared window
• Optical window
• Radio propagation

{{Radio-astronomy}}

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Category:Electromagnetic spectrum