Radio window

Opacity of the Earth's atmosphere: the radio window spans larger wavelengths.

The radio window is the region of the radio spectrum that penetrate the Earth's atmosphere. Typically, the lower limit of the radio window's range has a value of about 10 MHz (λ ≈ 30 m); the best upper limit achievable from optimal terrestrial observation sites is equal to approximately 1 THz (λ ≈ 0.3 mm).[1][2]

It plays an important role in astronomy; up until the 1940s, astronomers could only use the visible and near infrared spectra for their measurements and observations. With the development of radio telescopes, the radio window became more and more utilizable, leading to the development of radio astronomy that provided astrophysicists with valuable observational data.[3]

## Contents

* 1 Factors affecting lower and upper limits
* 1.1 Absorption of mid-IR
* 1.2 Ionosphere
* 1.3 Troposphere
* 1.4 Radio frequency interference
* 2 See also
* 3 References

## 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 (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.[4][5]

### Ionosphere

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);[6] radio waves with frequencies below the limit of 10 MHz (λ > 30 m) are reflected back into space by the ionosphere.[7] The lower limit is proportional to the density of the ionosphere's free electrons and coincides with the plasma frequency: fp=9Ne, where fp is the plasma frequency in Hz and Ne the electron density in electrons per cubic meter. Since it is highly dependent on sunlight, the value of Ne 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.[8]

### Troposphere

The Atacama Large Millimeter Array, 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.[9] 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.[10] To tackle the issue of water vapour, many observatories are built at high altitudes where the climate is more dry.[11] However, not many things can be done to avoid the oxygen's interference with radio waves propagation.[12]

### Radio frequency interference

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.[13]

## See also

* Infrared window
* Optical window
* Radio propagation

## References

1. ↑ Condon, James J.; Ransom, Scott M. (2016) (in en). Essential Radio Astronomy. Princeton University Press. pp. 1. ISBN 978-0-691-13779-7. https://books.google.com/books?id=vWWYDwAAQBAJ.
2. ↑ "1 Introduction‣ Essential Radio Astronomy". https://www.cv.nrao.edu/~sransom/web/Ch1.html.
3. ↑ Wilson, Thomas; Rohlfs, Kristen; Huettemeister, Susanne (2016) (in English). Tools of Radio Astronomy. Berlin: Springer-Verlag GmbH. pp. 1–2. ISBN 978-3-662-51732-1. OCLC 954868912. http://www.worldcat.org/oclc/954868912.
4. ↑ Liou, Kuo-Nan; Yang, Ping; Takano, Yoshihide (2016) (in English). Light scattering by ice crystals: fundamentals and applications. Cambridge University Press. pp. 251. doi:10.1017/CBO9781139030052. ISBN 978-1-139-03005-2. OCLC 958454932. https://doi.org/10.1017/CBO9781139030052.
5. ↑ Ritchie, Grant (2017) (in English). Atmospheric chemistry: from the surface to the stratosphere. World Scientific. pp. 68. ISBN 978-1-78634-175-4. OCLC 957339640. http://www.worldcat.org/oclc/957339640.
6. ↑ Anderson, John B.; Johannesson, Rolf (2005) (in English). Understanding information transmission. Piscataway, NJ; Hoboken, NJ: IEEE Press, Wiley-Interscience. pp. 110. ISBN 978-0-471-67910-3. OCLC 56103934. http://www.worldcat.org/oclc/56103934.
7. ↑ Torge, Wolfgang; Müller, Jürgen (2012) (in English). Geodesy. Berlin: De Gruyter. pp. 121. ISBN 978-3-11-020718-7. OCLC 987088700. http://www.worldcat.org/oclc/987088700.
8. ↑ Warnick, Karl F.; Maaskant, Rob; Ivashina, Marianna V. (2018) (in English). Phased arrays for radio astronomy, remote sensing and satellite communications. Cambridge University Press. pp. 5. ISBN 978-1-108-42392-2. OCLC 1032582026. http://www.worldcat.org/oclc/1032582026.
9. ↑ Wilson, Thomas; Rohlfs, Kristen; Huettemeister, Susanne (2016) (in English). Tools of Radio Astronomy. Springer-Verlag GmbH. pp. 4. ISBN 978-3-662-51732-1. OCLC 954868912. http://www.worldcat.org/oclc/954868912.
10. ↑ Otung, Ifiok (2021) (in English). Communication engineering principles. Wiley. pp. 390. ISBN 978-1-119-27402-5. OCLC 1225565245. http://www.worldcat.org/oclc/1225565245.
11. ↑ Karttunen, Hannu (2007) (in English). Fundamental astronomy. Berlin: Springer-verlag. pp. 72. ISBN 978-3-540-34143-7. OCLC 860603182. http://www.worldcat.org/oclc/860603182.
12. ↑ (in en) Conference Proceedings. IEEE. 1990. pp. 241. ISBN 978-0-87942-557-9. OCLC 25175353. http://www.worldcat.org/oclc/25175353.
13. ↑ McNally, Derek (1994). McNally, Derek. ed (in English). The vanishing universe: adverse environmental impacts on astronomy: proceedings of the conference sponsored by Unesco. Cambridge; New York: Cambridge University Press. pp. 93. ISBN 978-0-521-45020-1. OCLC 29359179. http://www.worldcat.org/oclc/29359179.

* v
* t
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Radio astronomy

Concepts|

* Units (watt and jansky)
* Radio telescope (Radio window)
* Astronomical interferometer (History)
* Very Long Baseline Interferometry (VLBI)
* Astronomical radio source

Radio telescopes
(List)|

| Individual
telescopes|

* 500 meter Aperture Spherical Telescope (FAST, China)
* Arecibo Observatory (Puerto Rico, US)
* Caltech Submillimeter Observatory (CSO, US)
* Effelsberg Telescope (Germany)
* Galenki RT-70 (Russia)
* Green Bank Telescope (West Virginia, US)
* Large Millimeter Telescope (Mexico)
* Lovell Telescope (UK)
* Ooty Telescope (India)
* Qitai Radio Telescope (China)
* RATAN-600 Radio Telescope (Russia)
* Sardinia Radio Telescope (Italy)
* Suffa RT-70 (Uzbekistan)
* Usuda Telescope (Japan)
* UTR-2 decameter radio telescope (Ukraine)
* Yevpatoria RT-70 (Russia)

Southern Hemisphere
HartRAO (South Africa)
Parkes Observatory (Australia)
Warkworth Radio Astronomical Observatory (NZ)

|

Interferometers|

* Allen Telescope Array (ATA, California, US)
* Atacama Large Millimeter Array (ALMA, Chile)
* Australia Telescope Compact Array (ATCA, Australia)
* Australian Square Kilometre Array Pathfinder (ASKAP, Australia)
* Canadian Hydrogen Intensity Mapping Experiment (CHIME, Canada)
* Combined Array for Research in Millimeter-wave Astronomy (CARMA, California, US)
* European VLBI Network (Europe)
* Event Horizon Telescope (EHT)
* Giant Metrewave Radio Telescope (GMRT, India)
* Green Bank Interferometer (GBI, West Virginia, US)
* Korean VLBI Network (KVN, South Korea)
* Large Latin American Millimeter Array (LLAMA, Argentina/Brazil)
* Long Wavelength Array (LWA, New Mexico, US)
* Low-Frequency Array (LOFAR, Netherlands)
* MeerKAT (South Africa)
* Molonglo Observatory Synthesis Telescope (MOST, Australia)
* Multi-Element Radio Linked Interferometer Network (MERLIN, UK)
* Murchison Widefield Array (MWA, Australia)
* Northern Cross Radio Telescope (Italy)
* Northern Extended Millimeter Array (France)
* One-Mile Telescope (UK)
* Primeval Structure Telescope (PaST, China)
* Square Kilometre Array (SKA, Australia, South Africa)
* Submillimeter Array (SMA, US)
* Very Large Array (VLA, New Mexico, US)
* Very Long Baseline Array (VLBA, US)
* Westerbork Synthesis Radio Telescope (WSRT, Netherlands)

Space-based|

* HALCA (Japan)
* Spektr-R (Russia)

Observatories|

* Algonquin Radio Observatory (Canada)
* Haystack Observatory (US)
* Jodrell Bank Observatory (UK)
* Mullard Radio Astronomy Observatory (UK)
* National Radio Astronomy Observatory (US)
* Nançay Radio Observatory (France)
* Onsala Space Observatory (Sweden)
* Pushchino Radio Astronomy Observatory (PRAO ASC LPI, Russia)
* Special Astrophysical Observatory of the Russian Academy of Science (SAORAS, Russia)
* Warkworth Radio Astronomical Observatory

Multi-use|

* DRAO (Canada)
* ESA New Norcia (Australia)
* PARL (Canada)

People|

* Elizabeth Alexander
* John G. Bolton
* Edward George Bowen
* Ronald Bracewell
* Jocelyn Bell Burnell
* Arthur Covington
* Nan Dieter-Conklin
* Frank Drake
* Antony Hewish
* Karl Guthe Jansky
* Kenneth Kellermann
* Frank J. Kerr
* John D. Kraus
* Bernard Lovell
* Jan Oort
* Joseph Lade Pawsey
* Ruby Payne-Scott
* Arno Penzias
* Grote Reber
* Martin Ryle
* Govind Swarup
* Gart Westerhout
* Paul Wild
* Robert Wilson

Related articles|

* Radio propagation
* Interferometry
* Aperture synthesis
* SETI (Wow! signal, Radio signal from HD 164595)
* Pulsar timing array
* Cosmic microwave background radiation

* Submillimetre astronomy
* Infrared astronomy
* Optical astronomy
* High-energy astronomy
* Gravitational-wave astronomy

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