Editing Radio wave (section)

{{short description|Type of electromagnetic radiation}}
{{About|the radiation|the generic oscillation|Radio frequency|the electronics|Radio frequency engineering|the British broadcasting station|Radio Wave 96.5|other uses|Radio Waves (disambiguation){{!}}Radio Waves}}

[[File:Dipole xmting antenna animation 4 408x318x150ms.gif|thumb|upright=1.0|Animation of a [[half-wave dipole]] [[antenna (radio)|antenna]] radiating radio waves, showing the [[electric field]] lines.  The antenna in the center is two vertical metal rods connected to a [[radio transmitter]] (not shown). The transmitter applies an [[alternating current|alternating electric current]] to the rods, which charges them alternately [[positive charge|positive]] (+) and [[negative charge|negative]] (−).  Loops of electric field leave the antenna and travel away at the [[speed of light]]; these are the radio waves. In this animation the action is shown slowed down tremendously.]]

'''Radio waves''' (formerly called '''Hertzian waves''') are a type of [[electromagnetic radiation]] with the lowest [[frequencies]] and the longest [[wavelength]]s in the [[electromagnetic spectrum]], typically with frequencies below 300 [[gigahertz]] (GHz) and wavelengths greater than {{convert|1|mm|in|abbr=off|sp=us|frac=64}}, about the diameter of a grain of rice.  Radio waves with frequencies above about 1&nbsp;GHz and wavelengths shorter than 30&nbsp;centimeters are called [[microwave]]s.<ref name="Kumar">
{{cite book
 | last1  = Kumar
 | first1 = Sanjay
 | last2  = Shukla
 | first2 = Saurabh
 | title  = Concepts and Applications of Microwave Engineering
 | publisher = PHI Learning Pvt. Ltd
 | date   = 2014
 | pages  = 3
 | url    = https://books.google.com/books?id=GY9eBAAAQBAJ&q=microwave&pg=PA3
 | isbn   = 978-8120349353
}}</ref>  Like all electromagnetic waves, radio waves in vacuum travel at the [[speed of light]], and in the Earth's atmosphere at a slightly lower speed. Radio waves are generated by [[charged particle]]s undergoing [[acceleration]], such as time-varying [[electric current]]s.<ref name="Ellingson">{{cite book |last=Ellingson |first=Steven W. |url=https://books.google.com/books?id=QMKSDQAAQBAJ&q=%22radio+wave%22+time+varying+electric+current&pg=PA16 |title=Radio Systems Engineering |publisher=Cambridge University Press |year=2016 |isbn=978-1316785164 |pages=16–17 |access-date=2020-11-19 |archive-url=https://web.archive.org/web/20240922182239/https://books.google.com/books?id=QMKSDQAAQBAJ&q=%22radio+wave%22+time+varying+electric+current&pg=PA16#v=snippet&q=%22radio%20wave%22%20time%20varying%20electric%20current&f=false |archive-date=2024-09-22 |url-status=live}}</ref> Naturally occurring radio waves are emitted by [[lightning]] and [[astronomical radio source|astronomical objects]], and are part of the [[blackbody radiation]] emitted by all warm objects.<ref>{{cite web |date=2024-07-26 |title=Radio wave {{!}} Examples, Uses, Facts, & Range {{!}} Britannica |url=https://www.britannica.com/science/radio-wave |url-status=live |archive-url=https://web.archive.org/web/20240913234804/https://www.britannica.com/science/radio-wave |archive-date=2024-09-13 |access-date=2024-09-22 |website=www.britannica.com |language=en}}</ref>

Radio waves are generated artificially by an electronic device called a [[transmitter]], which is connected to an [[antenna (radio)|antenna]], which radiates the waves. They are received by another antenna connected to a [[radio receiver]], which processes the received signal. Radio waves are very commonly used in modern technology for fixed and mobile [[radio communication]], [[broadcasting]], [[radar]] and [[radio navigation]] systems, [[communications satellite]]s, [[wireless network|wireless computer networks]] and many other applications. Different frequencies of radio waves have different propagation characteristics in the Earth's atmosphere; long waves can [[diffract]] around obstacles like mountains and follow the contour of the Earth ([[ground wave]]s), shorter waves can reflect off the [[ionosphere]] and return to Earth beyond the horizon ([[skywave]]s), while much shorter wavelengths bend or diffract very little and travel on a [[line-of-sight propagation|line of sight]], so their propagation distances are limited to the visual horizon.

To prevent [[electromagnetic interference|interference]] between different users, the artificial generation and use of radio waves is strictly regulated by law, coordinated by an international body called the [[International Telecommunication Union]] (ITU), which defines radio waves as "[[electromagnetic wave]]s of [[frequencies]] arbitrarily lower than {{val|3,000|ul=GHz}}, propagated in space without artificial guide".<ref>{{cite book |url=http://search.itu.int/history/HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf |title=Radio Regulations |publisher=[[ITU]] |year=2016 |isbn=9789261191214 |location=Geneva, CH |pages=7 |chapter=Ch. 1: Terminology and technical characteristics - Terms and definitions |archive-url=https://web.archive.org/web/20170829072058/http://search.itu.int/history/HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf |archive-date=2017-08-29 |url-status=live}}</ref> The [[radio spectrum]] is divided into a number of radio bands on the basis of frequency, allocated to different uses. Higher-frequency, shorter-wavelength radio waves are called [[microwaves]].

[[File:Radio waves.svg|right|300px|thumb|Diagram of the [[electric field]]s (E) and [[magnetic field]]s (H) of radio waves emitted by a [[Monopole antenna|monopole]] radio transmitting [[antenna (radio)|antenna]] (small dark vertical line in the center). The E and H fields are perpendicular, as implied by the phase diagram in the lower right.]]