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{{short description|Technology of using radio waves to carry information}}
{{About|science and technology|its use in audio distribution|Radio broadcasting|the user device|Radio receiver|other uses}}
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[[File:Radio towers on Sandia Peak - closeup.jpg|thumb|upright=1.3|A variety of radio [[antenna (radio)|antenna]]s on [[Sandia Mountains|Sandia Peak]] near [[Albuquerque, New Mexico|Albuquerque]], New Mexico, US]]

'''Radio''' is the technology of [[telecommunication|communicating]] using [[radio wave]]s.<ref name="OED">{{cite web|title=Radio|work=Oxford Living Dictionaries|publisher=Oxford University Press|date=2019|url=https://en.oxforddictionaries.com/definition/radio|archive-url=https://web.archive.org/web/20190324025858/https://en.oxforddictionaries.com/definition/radio|url-status=dead|archive-date=24 March 2019|access-date=26 February 2019}}</ref><ref name="PCMag">{{cite encyclopedia|title=Definition of radio|encyclopedia=Encyclopedia|publisher=PCMagazine website, Ziff-Davis|date=2018|url=https://www.pcmag.com/encyclopedia/term/50130/radio|access-date=26 February 2019}}</ref><ref name="Ellingson1">{{cite book|last1=Ellingson|first1=Steven W.|title=Radio Systems Engineering|publisher=Cambridge University Press|date=2016|pages=1–4|url=https://books.google.com/books?id=QMKSDQAAQBAJ&pg=PA1|isbn=978-1316785164}}</ref> Radio waves are [[electromagnetic wave]]s of [[frequency]] between 3&nbsp;[[hertz]] (Hz) and 300&nbsp;[[gigahertz]] (GHz). They are generated by an electronic device called a [[transmitter]] connected to an [[antenna (radio)|antenna]] which radiates the waves. They are received by another antenna connected to a [[radio receiver]]. In addition to communication, radio is used for [[radar]], [[radio navigation]], [[radio control|remote control]], [[remote sensing]], and other applications.<!--[[User:Kvng/RTH]]-->

In '''radio communication''', used in [[radio broadcasting|radio]] and [[television broadcasting]], [[cell phone]]s, [[two-way radio]]s, [[wireless network]]ing, and [[satellite communication]], among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, by [[Modulation|modulating]] the radio signal (impressing an information signal on the radio wave by varying some aspect of the wave) in the transmitter. In [[radar]], used to locate and track objects like aircraft, ships, spacecraft and missiles, a beam of radio waves emitted by a radar transmitter reflects off the target object, and the reflected waves reveal the object's location. In [[radio navigation]] systems such as [[Global Positioning System|GPS]] and [[VHF omnidirectional range|VOR]], a mobile navigation instrument receives radio signals from [[radio beacon|navigational radio beacons]] whose position is known, and by precisely measuring the arrival time of the radio waves the receiver can calculate its position on Earth. In wireless [[radio control|radio remote control]] devices like [[drone (aircraft)|drone]]s, [[garage door opener]]s, and [[keyless entry system]]s, radio signals transmitted from a controller device control the actions of a remote device.

The noun ''radio'' is also used to mean a [[radio receiver|broadcast radio receiver]].

The existence of radio waves was first proven by German physicist [[Heinrich Hertz]] on 11 November 1886.<ref name="KIT"/> In the mid 1890s, building on techniques physicists were using to study electromagnetic waves, [[Guglielmo Marconi]] developed the first apparatus for long-distance radio communication,<ref name="ieeexplore.ieee.org">Bondyopadhyay, Prebir K.  (1995) "[https://ieeexplore.ieee.org/document/4137304/?tp=&arnumber=4137304 Guglielmo Marconi – The father of long distance radio communication – An engineer's tribute"], ''25th European Microwave Conference: Volume 2'', pp. 879–85</ref> sending a wireless [[Morse Code]] message to a recipient over a kilometer away in 1895,<ref name="1890s – 1930s: Radio">{{cite web|url=https://www.elon.edu/u/imagining/time-capsule/150-years/back-1890-1930/|title=1890s – 1930s: Radio|publisher=[[Elon University]]|language=en-US|url-status=live|accessdate=14 July 2022|archivedate=8 June 2022|archiveurl=https://web.archive.org/web/20220608035814/https://www.elon.edu/u/imagining/time-capsule/150-years/back-1890-1930/}}</ref> and the first transatlantic signal on 12 December 1901.<ref name="IEEEatlantic">{{Cite web |last=Belrose |first=John S. |date=5–7 September 1995 |title=Radio's First Message -- Fessenden and Marconi |url=https://ewh.ieee.org/reg/7/millennium/radio/radio_differences.html |access-date=6 November 2022 |website=[[Institute of Electrical and Electronics Engineers]]}}</ref>  The first commercial radio broadcast was transmitted on 2 November 1920, when the live returns of the [[Harding-Cox presidential election]] were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign [[KDKA (AM)|KDKA]].<ref name="History of Commercial Radio">{{cite web|url=https://www.fcc.gov/media/radio/history-of-commercial-radio|title=History of Commercial Radio|date=23 October 2020 |publisher=[[Federal Communications Commission]]|language=en-US|url-status=live|accessdate=14 July 2022|archivedate=1 January 2022|archiveurl=https://web.archive.org/web/20220101085855/https://www.fcc.gov/media/radio/history-of-commercial-radio}}</ref>

The emission of radio waves is regulated by law, coordinated by the [[International Telecommunication Union]] (ITU), which allocates frequency bands in the [[radio spectrum]] for various uses.

==Etymology==
The word "radio" is derived from the Latin word "radius", meaning "spoke of a wheel, beam of light, ray". It was first applied to communications in 1881 when, at the suggestion of French scientist {{ill|Ernest Mercadier|fr}}, [[Alexander Graham Bell]] adopted "radiophone" (meaning "radiated sound") as an alternate name for his [[photophone]] optical transmission system.<ref>{{Cite web|title=radio (n.) |url=https://www.etymonline.com/word/radio|website=Online Etymology Dictionary|language=en|access-date=13 July 2022}}</ref><ref>{{cite magazine|title=Production of Sound by Radiant Energy|first=Alexander Graham|last=Bell|authorlink=Alexander Graham Bell|magazine=Popular Science Monthly|date=July 1881|pages=329–330|quote="[W]e have named the apparatus for the production and reproduction of sound in this way the "photophone", ''because an ordinary beam of light contains the rays which are operative.'' To avoid in future any misunderstandings upon this point, we have decided to adopt the term "''radiophone''", proposed by M. Mercadier, as a general term signifying the production of sound by any form of radiant energy..."}}</ref>

Following [[Heinrich Hertz]]'s discovery of the existence of [[radio waves]] in 1886, the term "Hertzian waves" was initially used for this radiation.<ref>{{cite book|first=Trevor|last=Manning|title=Microwave Radio Transmission Design Guide|publisher=Artech House|date=2009|page=2}}</ref> The first practical radio communications systems, developed by [[Guglielmo Marconi]] in 1894–1895, transmitted [[telegraph]] signals by radio waves,<ref name=KIT /> so radio communication was first called "[[wireless telegraphy]]". Up until about 1910 the term "wireless telegraphy" also included a variety of other experimental systems for transmitting telegraph signals without wires, including [[electrostatic induction]], [[electromagnetic induction]] and [[Electrical conductor|aquatic and earth conduction]], so there was a need for a more precise term referring exclusively to electromagnetic radiation.<ref name="Maver">{{cite book | last = Maver | first = William Jr. | title = American Telegraphy and Encyclopedia of the Telegraph: Systems, Apparatus, Operation | publisher = Maver Publishing Co. | date = 1903 | location = New York | page = [https://archive.org/details/americantelegra02mavegoog/page/n391 333] | url =https://archive.org/details/americantelegra02mavegoog | quote = wireless telegraphy. }}</ref><ref name="Steuart">{{cite book | last = Steuart | first = William Mott | title = Special Reports: Telephones and Telegraphs 1902 | publisher = U.S. Bureau of the Census | date = 1906 | location = Washington D.C. | pages = 118–119 | url =https://books.google.com/books?id=x-cpAAAAYAAJ&q=%22wireless+telegraphy&pg=PA118 |display-authors=etal}}</ref>

The French physicist [[Édouard Branly]], who in 1890 developed the radio wave detecting [[coherer]], called it in French a ''[[:fr:radioconducteur|radio-conducteur]]''.<ref name="earlyradiohistory.us">https://earlyradiohistory.us/sec022.htm Thomas H. White, United States Early Radio History, Section 22</ref><ref>{{cite magazine|title=The Genesis of Wireless Telegraphy|first=A. Frederick|last=Collins|authorlink=Archie Frederick Collins|magazine=Electrical World and Engineer|date=10 May 1902|page=811}}</ref> The ''radio-'' prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe. For example, in early 1898 the British publication ''The Practical Engineer'' included a reference to "the radiotelegraph" and "radiotelegraphy".<ref name="earlyradiohistory.us"/><ref>{{cite magazine|title=Wireless Telegraphy|magazine=The Practical Engineer|date=25 February 1898|page=174|quote="Dr. O. J. Lodge, who preceded Marconi in making experiments in what may be called "ray" telegraphy or radiotelegraphy by a year or two, has devised a new method of sending and receiving the messages. The reader will understand that in the radiotelegraph electric waves forming the signals of the message starting from the sending instrument and travel in all directions like rays of light from a lamp, only they are invisible."}}</ref>

The use of "radio" as a standalone word dates back to at least 30 December 1904, when instructions issued by the British Post Office for transmitting telegrams specified that "The word 'Radio'... is sent in the Service Instructions".<ref name="earlyradiohistory.us"/><ref>"Wireless Telegraphy", ''The Electrical Review'' (London), 20 January 1905, page 108, quoting from the British Post Office's 30 December 1904 ''Post Office Circular''.</ref> This practice was universally adopted, and the word "radio" introduced internationally, by the 1906 Berlin Radiotelegraphic Convention, which included a Service Regulation specifying that "Radiotelegrams shall show in the preamble that the service is 'Radio{{'"}}.<ref name="earlyradiohistory.us"/>

The switch to "radio" in place of "wireless" took place slowly and unevenly in the English-speaking world. [[Lee de Forest]] helped popularize the new word in the United States—in early 1907, he founded the DeForest Radio Telephone Company, and his letter in the 22 June 1907 ''Electrical World'' about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced".<ref>"Interference with Wireless Messages", ''Electrical World'', 22 June 1907, page 1270.</ref> The United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the terms "wireless telegraph" and "wireless telegram", by 1912 it began to promote the use of "radio" instead. The term started to become preferred by the general public in the 1920s with the introduction of broadcasting.

==History==
:''See'' [[History of radio]], [[Invention of radio]], [[Timeline of radio]], [[History of broadcasting]]
[[Electromagnetic wave]]s were predicted by [[James Clerk Maxwell]] in his 1873 theory of [[electromagnetism]], now called [[Maxwell's equations]], who proposed that a coupled oscillating [[electric field]] and [[magnetic field]] could travel through space as a wave, and proposed that light consisted of electromagnetic waves of short [[wavelength]].  On 11 November 1886, German physicist [[Heinrich Hertz]], attempting to confirm Maxwell's theory, first observed radio waves he generated using a primitive [[spark gap transmitter]].<ref name=KIT>{{cite web|url=https://www.kit.edu/kit/english/pi_2011_8434.php|title=125 Years Discovery of Electromagnetic Waves|publisher=[[Karlsruhe Institute of Technology]]|language=en-US|url-status=live|date=16 May 2022|accessdate=14 July 2022|archivedate=14 July 2022|archiveurl=https://web.archive.org/web/20220714163329/https://www.kit.edu/kit/english/pi_2011_8434.php}}</ref> Experiments by Hertz and physicists [[Jagadish Chandra Bose]], [[Oliver Lodge]], [[Lord Rayleigh]], and [[Augusto Righi]], among others, showed that radio waves like light demonstrated reflection, [[refraction]], [[diffraction]], [[Polarization (waves)|polarization]], [[standing wave]]s, and traveled at the same speed as light, confirming that both light and radio waves were electromagnetic waves, differing only in frequency.<ref>Sungook Hong (2001), ''Wireless: From Marconi's Black-box to the Audion'', MIT Press, pp. 5–10</ref> In 1895, [[Guglielmo Marconi]] developed the first radio communication system, using a [[spark gap transmitter]] to send [[Morse code]] over long distances.  By December 1901, he had transmitted across the Atlantic ocean.<ref name=KIT /><ref name="ieeexplore.ieee.org"/><ref name="1890s – 1930s: Radio"/><ref name="IEEEatlantic"/>  Marconi and [[Karl Ferdinand Braun]] shared the 1909 Nobel Prize in Physics "for their contributions to the development of wireless telegraphy".<ref>{{Cite web |title=The Nobel Prize in Physics 1909|url=https://www.nobelprize.org/prizes/physics/1909/summary/ |publisher=[[NobelPrize.org]]|language=en|url-status=live|date=2023|access-date=31 July 2023|archive-date=31 July 2023|archive-url=https://archive.today/20230731180825/https://www.nobelprize.org/prizes/physics/1909/summary/}}</ref> 

During radio's first two decades, called the [[radiotelegraphy]] era, the primitive [[damped wave]] radio transmitters could only transmit pulses of radio waves, not the continuous waves which were needed for audio [[modulation]], so radio was used for person-to-person commercial, diplomatic and military text messaging.  Starting around 1908 industrial countries built worldwide networks of powerful transoceanic spark transmitters to exchange [[telegram]] traffic between continents and communicate with their colonies and naval fleets.  During [[World War 1]] the development of [[continuous wave]] radio transmitters, [[rectifying]] electrolytic, and crystal [[detector (radio)|radio receiver detectors]] enabled [[amplitude modulation]] (AM) [[radiotelephony]] to be achieved by [[Reginald Fessenden]] and others, allowing sound ([[audio signal|audio]]) to be transmitted. On 2 November 1920, the first commercial radio broadcast was transmitted by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call sign [[KDKA (AM)|KDKA]] featuring live coverage of the [[Harding-Cox presidential election]].<ref name="History of Commercial Radio"/>

==Technology==
Radio waves are radiated by [[electric charge]]s undergoing [[acceleration]].<ref name="Kraus">{{cite book| last1=Kraus| first1 = John D.| title  = Antennas|edition=2nd| publisher = Tata-McGraw Hill| date   = 1988| page=50|url= https://archive.org/details/Antennas2ndbyjohnD.Kraus1988/page/n75| isbn= 0074632191}}</ref><ref name="Serway">{{cite book|last1=Serway|first1=Raymond|last2=Faughn|first2=Jerry|last3=Vuille|first3=Chris|title=College Physics, 8th Ed.|publisher=Cengage Learning|date=2008|page=714|url=https://books.google.com/books?id=CX0u0mIOZ44C&q=%22electromagnetic+wave%22+charge+acceleration&pg=PA714|isbn=978-0495386933}}</ref> They are generated artificially by time varying [[electric current]]s, consisting of [[electron]]s flowing back and forth in a metal conductor called an antenna.<ref name="Balanis">{{cite book| last1  = Balanis| first1 = Constantine A.| title  = Antenna theory: Analysis and Design, 3rd Ed.| publisher = John Wiley and Sons|date= 2005|page=[https://archive.org/details/Antenna.Theory.Analysis.and.Design3rd.Edition/page/n22 10]| url=https://archive.org/details/Antenna.Theory.Analysis.and.Design3rd.Edition|isbn= 978-1118585733}}</ref><ref name="Ellingson2">{{cite book|last1=Ellingson|first1=Steven W.|title=Radio Systems Engineering|publisher=Cambridge University Press|date=2016|pages=16–17|url=https://books.google.com/books?id=QMKSDQAAQBAJ&q=%22radio+wave%22+time+varying+electric+current&pg=PA16|isbn=978-1316785164}}</ref>

As they travel farther from the transmitting antenna, radio waves spread out so their [[signal strength]] ([[intensity (physics)|intensity]] in watts per square meter) decreases, so radio transmissions can only be received within a limited range of the transmitter, the distance depending on the transmitter power, the antenna [[radiation pattern]], receiver sensitivity, noise level, and presence of obstructions between transmitter and receiver.  An [[omnidirectional antenna]] transmits or receives radio waves in all directions, while a [[directional antenna]] or [[high-gain antenna]] transmits radio waves in a beam in a particular direction, or receives waves from only one direction.<ref name="Visser">{{cite book |last1=Visser |first1=Hubregt J. |title=Antenna Theory and Applications |year=2012 |url=https://books.google.com/books?id=rEs5dz1Zu7AC&q=radio+antenna+theory |publisher=[[Wiley (publisher)|John Wiley & Sons]] |isbn=978-1119990253 |access-date=29 August 2022}}</ref><ref>{{cite book |title=Proceedings of the 11th National Technical Seminar on Unmanned System Technology 2019: NUSYS'19 |author1=Zainah Md Zain |author2=Hamzah Ahmad |author3=Dwi Pebrianti |author4=Mahfuzah Mustafa |author5=Nor Rul Hasma Abdullah |author6=Rosdiyana Samad |author7=Maziyah Mat Noh |edition= |publisher=Springer Nature |year=2020 |isbn=978-9811552816 |page=535 |url=https://books.google.com/books?id=t_LvDwAAQBAJ}} [https://books.google.com/books?id=t_LvDwAAQBAJ&pg=PA535 Extract of pp. 535–536]</ref><ref>{{Cite web |title=Omnidirectional Antenna - an overview {{!}} ScienceDirect Topics |url=https://www.sciencedirect.com/topics/computer-science/omnidirectional-antenna |access-date=5 September 2022 |website=www.sciencedirect.com}}</ref>

Radio waves travel at the speed of light in vacuum.<ref>{{cite web|url=https://lambda.gsfc.nasa.gov/product/suborbit/POLAR/cmb.physics.wisc.edu/tutorial/light.html#:~:text=Generally%20speaking%2C%20we%20say%20that,than%20the%20speed%20of%20light.|title=Electromagnetic Radiation|publisher=[[NASA]]|language=en-US|url-status=live|access-date=18 August 2022|archivedate=23 May 2016|archiveurl=https://web.archive.org/web/20160523101442/https://lambda.gsfc.nasa.gov/product/suborbit/POLAR/cmb.physics.wisc.edu/tutorial/light.html}}</ref><ref>{{cite web|url=https://physics.stackexchange.com/questions/488705/how-far-can-radio-waves-travel-in-vacuum-and-light-waves|title=How far can radio waves travel in vacuum? and light waves?|publisher=[[Physics Stack Exchange]]|language=en-US|url-status=live|date=July 2019|access-date=18 August 2022|archivedate=18 August 2022|archiveurl=https://web.archive.org/web/20220818150851/https://physics.stackexchange.com/questions/488705/how-far-can-radio-waves-travel-in-vacuum-and-light-waves}}</ref>

The other types of [[electromagnetic wave]]s besides radio waves, [[infrared]], [[visible light]], [[ultraviolet]], [[X-ray]]s and [[gamma rays]], can also carry information and be used for communication.  The wide use of radio waves for telecommunication is mainly due to their desirable [[radio propagation|propagation]] properties stemming from their large wavelength.<ref name="Ellingson2"/>

==Radio communication==
[[File:Signal processing system.png|thumb|upright=1.4|Radio communication.  Information such as sound is converted by a transducer such as a [[microphone]] to an electrical signal, which modulates a [[radio wave]] produced by the [[transmitter]]. A receiver intercepts the radio wave and extracts the information-bearing modulation signal, which is converted back to a human usable form with another transducer such as a [[loudspeaker]].]]
[[File:Amfm3-en-de.gif|thumb|Comparison of AM and FM modulated radio waves]]

In radio communication systems, information is carried across space using radio waves.  At the sending end, the information to be sent is converted by some type of [[transducer]] to a time-varying [[electrical signal]] called the modulation signal.<ref name="Ellingson2" /><ref name="HowStuffWorks">{{cite web|url=http://electronics.howstuffworks.com/radio8.htm|title=How Radio Works|first=Marshall|last=Brain|publisher=HowStuffWorks.com|date=7 December 2000|access-date=11 September 2009}}</ref> The modulation signal may be an [[audio signal]] representing sound from a [[microphone]], a [[video signal]] representing moving images from a [[video camera]], or a [[digital signal]] consisting of a sequence of [[binary digit|bits]] representing binary data from a computer.  The modulation signal is applied to a [[radio transmitter]].  In the transmitter, an [[electronic oscillator]] generates an [[alternating current]] oscillating at a [[radio frequency]], called the ''[[carrier wave]]'' because it serves to "carry" the information through the air.  The information signal is used to [[modulation|modulate]] the carrier, varying some aspect of the carrier wave, impressing the information on the carrier. Different radio systems use different [[modulation]] methods:<ref name="Faruque">{{cite book |last1=Faruque |first1=Saleh |title=Radio Frequency Modulation Made Easy |year=2016 |url=https://books.google.com/books?id=AT3ADAAAQBAJ&q=radio+types+of+modulation |publisher=[[Springer Publishing]] |isbn=978-3319412023 |access-date=29 August 2022}}</ref>
* AM ([[amplitude modulation]]) – in an AM transmitter, the [[amplitude]] (strength) of the radio carrier wave is varied by the modulation signal;<ref name="Faruque" />{{rp|3}}
* FM ([[frequency modulation]]) – in an FM transmitter, the [[frequency]] of the radio carrier wave is varied by the modulation signal;<ref name="Faruque" />{{rp|33}}
* FSK ([[frequency-shift keying]]) – used in wireless digital devices to transmit [[digital signal]]s, the frequency of the carrier wave is shifted between frequencies.<ref name="Faruque" />{{rp|58}}
* OFDM ([[orthogonal frequency-division multiplexing]]) – a family of [[digital modulation]] methods widely used in high bandwidth systems such as [[Wi-Fi]] networks, [[cellphone]]s, [[digital television]] broadcasting, and [[digital audio broadcasting]] (DAB) to transmit digital data using a minimum of [[radio spectrum]] bandwidth.  It has higher [[spectral efficiency]] and more resistance to [[fading]] than AM or FM.  In OFDM, multiple radio carrier waves closely spaced in frequency are transmitted within the radio channel, with each carrier modulated with bits from the incoming [[bitstream]] so multiple bits are being sent simultaneously, in parallel.  At the receiver, the carriers are demodulated and the bits are combined in the proper order into one bitstream.<ref name="ergen">{{cite book |title= Mobile Broadband: including WiMAX and LTE |publisher= Springer Science+Business Media |year= 2009 |author= Mustafa Ergen |isbn= 978-0387681894 |doi= 10.1007/978-0-387-68192-4 }}</ref>

Many other types of modulation are also used. In some types, a carrier wave is not transmitted but just one or both modulation [[sideband]]s.<ref>Tony Dorbuck (ed.), ''The Radio Amateur's Handbook, Fifty-Fifth Edition'', American Radio Relay League, 1977, p. 368</ref>

The modulated carrier is [[amplifier|amplified]] in the transmitter and applied to a transmitting [[antenna (radio)|antenna]] which radiates the energy as radio waves.  The radio waves carry the information to the receiver location.<ref>John Avison, The World of Physics, Nelson · 2014, page 367</ref>
At the receiver, the radio wave induces a tiny oscillating [[voltage]] in the receiving antenna which is a weaker replica of the current in the transmitting antenna.<ref name="Ellingson2" /><ref name="HowStuffWorks" /> This voltage is applied to the [[radio receiver]], which [[amplifier|amplifies]] the weak radio signal so it is stronger, then [[demodulate]]s it, extracting the original modulation signal from the modulated carrier wave.  The modulation signal is converted by a [[transducer]] back to a human-usable form: an audio signal is converted to [[sound wave]]s by a loudspeaker or earphones, a [[video signal]] is converted to images by a [[display device|display]], while a digital signal is applied to a computer or microprocessor, which interacts with human users.<ref name="Faruque" />

The radio waves from many transmitters pass through the air simultaneously without interfering with each other because each transmitter's radio waves oscillate at a different rate, in other words, each transmitter has a different [[frequency]], measured in [[hertz]] (Hz), [[kilohertz]] (kHz), [[megahertz]] (MHz) or [[gigahertz]] (GHz).  The receiving antenna typically picks up the radio signals of many transmitters.  The receiver uses ''[[tuned circuit]]s'' to select the radio signal desired out of all the signals picked up by the antenna and reject the others.  A [[tuned circuit]] (also called resonant circuit or tank circuit) acts like a [[resonator]], similar to a [[tuning fork]].<ref name="HowStuffWorks" /> It has a natural [[resonant frequency]] at which it oscillates.  The resonant frequency of the receiver's tuned circuit is adjusted by the user to the frequency of the desired radio station; this is called "tuning".  The oscillating radio signal from the desired station causes the tuned circuit to [[resonance|resonate]], oscillate in sympathy, and it passes the signal on to the rest of the receiver.  Radio signals at other frequencies are blocked by the tuned circuit and not passed on.<ref>''C-W and A-M Radio Transmitters and Receivers'', United States. Department of the Army – 1952, pp. 167–168</ref>

===Bandwidth===
[[File:Modulated radio signal frequency spectrum.svg|thumb|[[Frequency]] spectrum of a typical modulated AM or FM radio signal.  It consists of a component ''C'' at the [[carrier wave]] frequency <math>f_c</math> with the information ([[modulation]]) contained in two narrow bands of frequencies called [[sideband]]s (''SB'') just above and below the carrier frequency.]]
A modulated radio wave, carrying an information signal, occupies a range of [[frequency|frequencies]]. The information ([[modulation]]) in a radio signal is usually concentrated in narrow frequency bands called [[sideband]]s (''SB'') just above and below the [[carrier wave|carrier]] frequency. The width in [[hertz]] of the frequency range that the radio signal occupies, the highest frequency minus the lowest frequency, is called its [[bandwidth (signal processing)|bandwidth]] (''BW'').<ref name="Faruque" /><ref name="Spectrum101">{{cite web| title = Spectrum 101| publisher = US National Aeronautics and Space Administration (NASA)|date = February 2016|url=https://www.nasa.gov/sites/default/files/atoms/files/spectrum_101.pdf|access-date = 2 December 2019|archive-url = https://web.archive.org/web/20170211001857/https://www.nasa.gov/sites/default/files/atoms/files/spectrum_101.pdf|archive-date = 11 February 2017|url-status = live}}, p. 6</ref> For any given [[signal-to-noise ratio]], an amount of bandwidth can carry the same amount of information ([[data signaling rate|data rate]] in [[binary digit|bits]] per second) regardless of where in the radio frequency spectrum it is located, so bandwidth is a measure of [[channel capacity|information-carrying capacity]]. The bandwidth required by a radio transmission depends on the data rate of the information (modulation signal) being sent, and the [[spectral efficiency]] of the [[modulation]] method used; how much data it can transmit in each kilohertz of bandwidth.  Different types of information signals carried by radio have different data rates.  For example, a television (video) signal has a greater data rate than an [[audio signal]].<ref name="Faruque" /><ref name="Pogorel-Girard">{{cite book |last1=Pogorel |first1=Girard |last2=Chaduc |first2=Jean-Marc |title=The Radio Spectrum: Managing a Strategic Resource |year= 2010 |url=https://books.google.com/books?id=BGjfWBG33t4C&q=radio+spectrum+management |publisher=[[Wiley (publisher)|Wiley]]) |isbn=978-0470393529 |access-date=29 August 2022}}</ref>

The [[radio spectrum]], the total range of radio frequencies that can be used for communication in a given area, is a limited resource.<ref name="Spectrum101" /><ref name="Ellingson1" /> Each radio transmission occupies a portion of the total bandwidth available.  Radio bandwidth is regarded as an [[economic good]] which has a monetary cost and is in increasing demand.  In some parts of the radio spectrum, the right to use a frequency band or even a single radio channel is bought and sold for millions of dollars.  So there is an incentive to employ technology to minimize the bandwidth used by radio services.<ref name="Pogorel-Girard" />

A slow transition from [[analog signal|analog]] to [[digital signal|digital]] radio transmission technologies began in the late 1990s.<ref>{{cite web|url=https://www.theledger.com/story/news/2002/11/28/digital-radio-is-coming-but-analog-isnt-dead-yet/26029858007/|author=Norberg, Bob|title=Digital Radio Is Coming, But Analog Isn't Dead Yet|language=en-US|url-status=live|newspaper=[[The Ledger]]|date=27 November 2022|access-date=3 September 2022|archivedate=3 September 2022|archiveurl=https://web.archive.org/web/20220903133917/https://www.theledger.com/story/news/2002/11/28/digital-radio-is-coming-but-analog-isnt-dead-yet/26029858007/}}</ref><ref>{{cite web|url=https://www.financialexpress.com/archive/analogue-to-digital-radio-slow-to-tune-into-transition/96138/|title=Analogue To Digital: Radio Slow To Tune Into Transition|newspaper=[[The Financial Express (India)|Financial Express]]|language=en|url-status=live|date=13 October 2005|access-date=3 September 2022|archivedate=3 September 2022|archiveurl=https://web.archive.org/web/20220903133628/https://www.financialexpress.com/archive/analogue-to-digital-radio-slow-to-tune-into-transition/96138/}}</ref>  Part of the reason for this is that [[digital modulation]] can often transmit more information (a greater data rate) in a given bandwidth than [[analog modulation]], by using [[data compression]] algorithms, which reduce redundancy in the data to be sent, and more efficient modulation.  Other reasons for the transition is that digital modulation has greater [[noise immunity]] than analog, [[digital signal processing]] chips have more power and flexibility than analog circuits, and a wide variety of types of information can be transmitted using the same digital modulation.<ref name="Faruque" />

Because it is a fixed resource which is in demand by an increasing number of users, the [[radio spectrum]] has become increasingly congested in recent decades, and the need to use it more effectively is driving many additional radio innovations such as [[trunked radio system]]s, [[spread spectrum]] (ultra-wideband) transmission, [[frequency reuse]], [[dynamic spectrum management]], frequency pooling, and [[cognitive radio]].<ref name="Pogorel-Girard" />

===ITU frequency bands===
The [[International Telecommunication Union|ITU]] arbitrarily divides the [[radio spectrum]] into 12 bands, each beginning at a wavelength which is a power of ten (10<sup>n</sup>)&nbsp;metres, with corresponding frequency of 3 times a power of ten, and each covering a decade of frequency or wavelength.<ref name="Ellingson1"/><ref name="ITU2016">{{cite web| title = Radio Regulations, 2016 Edition| publisher = International Telecommunication Union| date = 3 November 2016| url = https://www.itu.int/dms_pub/itu-r/opb/reg/R-REG-RR-2016-ZPF-E.zip| format = PDF| access-date = 9 November 2019 }} Article 2, Section 1, p.27</ref>  Each of these bands has a traditional name:<ref name="Nomenclature">{{Cite report |title=Nomenclature of the frequency and wavelength bands used in telecommunications |docket=ITU-R V.431-8 |date=2015 |publisher=International Telecommunications Union |location=Geneva |url=https://www.itu.int/dms_pubrec/itu-r/rec/v/R-REC-V.431-8-201508-I!!PDF-E.pdf |access-date=6 April 2023}}</ref>
<div style=display:inline-table>
:{| class="wikitable"
|- style="text-align:left;"
! Band name !! Abbreviation !! Frequency !! Wavelength
|- style="text-align:center;"
| [[Extremely low frequency|Extremely<br />low frequency]] || ELF || 3–30&nbsp;Hz || 100,000–<br />10,000&nbsp;km
|- style="text-align:center;"
| [[Super low frequency|Super<br />low frequency]] || SLF || 30–300&nbsp;Hz || 10,000 –<br />1,000&nbsp;km
|- style="text-align:center;"
| [[Ultra low frequency|Ultra<br />low frequency]] || ULF || 300–<br />3,000&nbsp;Hz || 1,000–<br />100&nbsp;km
|- style="text-align:center;"
| [[Very low frequency|Very<br />low frequency]] || VLF || 3–30&nbsp;kHz || 100–10&nbsp;km
|- style="text-align:center;"
| [[Low frequency|Low<br />frequency]] || LF || 30–300&nbsp;kHz || 10–1&nbsp;km
|- style="text-align:center;"
| [[Medium frequency|Medium<br />frequency]] || MF || 300–<br />3,000&nbsp;kHz || 1,000–<br />100&nbsp;m
|}
</div>
<div style=display:inline-table>
:{| class="wikitable"
|- style="text-align:left;"
! Band name !! Abbreviation !! Frequency !! Wavelength
|- style="text-align:center;"
| [[High frequency|High<br />frequency]] || HF || 3–30&nbsp;MHz || 100–10&nbsp;m
|- style="text-align:center;"
| [[Very high frequency|Very<br />high frequency]] || VHF || 30–300&nbsp;MHz || 10–1&nbsp;m
|- style="text-align:center;"
| [[Ultra high frequency|Ultra<br />high frequency]] || UHF || 300–<br />3,000&nbsp;MHz || 100–10&nbsp;cm
|- style="text-align:center;"
| [[Super high frequency|Super<br />high frequency]] || SHF || 3–30&nbsp;GHz || 10–1&nbsp;cm
|- style="text-align:center;"
| [[Extremely high frequency|Extremely<br />high frequency]] || EHF || 30–300&nbsp;GHz || 10–1&nbsp;mm
|- style="text-align:center;"
| [[Tremendously high frequency|Tremendously<br />high frequency]] || THF || 300–3,000&nbsp;GHz<br />(0.3–3.0&nbsp;THz) || 1.0–0.1&nbsp;mm
|}</div>
It can be seen that the [[bandwidth (signal processing)|bandwidth]], the range of frequencies, contained in each band is not equal but increases exponentially as the frequency increases; each band contains ten times the bandwidth of the preceding band.<ref>{{cite report |publisher=United States Department of the Army |title=Communications-electronics Management of the Electromagnetic Spectrum |series=Headquarters, Department of the Army |year=1973 |page=2}}</ref>

The term "tremendously low frequency" (TLF) has been used for wavelengths from 1–3&nbsp;Hz (300,000–100,000&nbsp;km),<ref>{{Cite book |last1=Duncan |first1=Christopher |last2=Gkountouna |first2=Olga |last3=Mahabir |first3=Ron |title=Advances in Computer Vision and Computational Biology |chapter=Theoretical Applications of Magnetic Fields at Tremendously Low Frequency in Remote Sensing and Electronic Activity Classification |series=Transactions on Computational Science and Computational Intelligence |date=2021 |editor-last=Arabnia |editor-first=Hamid R. |editor2-last=Deligiannidis |editor2-first=Leonidas |editor3-last=Shouno |editor3-first=Hayaru |editor4-last=Tinetti |editor4-first=Fernando G. |editor5-last=Tran |editor5-first=Quoc-Nam |chapter-url=https://link.springer.com/10.1007/978-3-030-71051-4_18 |language=en |location=Cham |publisher=Springer International Publishing |pages=235–247 |doi=10.1007/978-3-030-71051-4_18 |isbn=978-3030710507|s2cid=238934419 }}</ref> but the term has not been defined by the ITU.<ref name="Nomenclature"/>

==Regulation==
{{further|Radio regulation|Radio communication service}}
The airwaves are a resource shared by many users.  Two radio transmitters in the same area that attempt to transmit on the same frequency will interfere with each other, causing garbled reception, so neither transmission may be received clearly.<ref name="Spectrum101"/> [[radio frequency interference|Interference]] with radio transmissions can not only have a large economic cost, but it can also be life-threatening (for example, in the case of interference with emergency communications or [[air traffic control]]).<ref name="CISA">{{cite web |title=Radio Frequency Interference Best Practices Guidebook - CISA - Feb. 2020 |url=https://www.cisa.gov/sites/default/files/publications/safecom-ncswic_rf_interference_best_practices_guidebook_2.7.20_-_final_508c.pdf |website=Cybersecurity and Infrastructure Security Agency SAFECOM/National Council of Statewide Interoperability Coordinators |publisher=USDepartment of Homeland Security |access-date=29 August 2022}}</ref><ref name="Mazar">{{cite book |last1=Mazar (Madjar) |first1=Haim |title=Radio Spectrum Management: Policies, Regulations and Techniques |year= 2016 |url=https://books.google.com/books?id=ysqOCQAAQBAJ&q=radio+spectrum+management |publisher=[[Wiley (publisher)|Wiley]] |isbn=978-1118511794 |access-date=29 August 2022}}</ref>

To prevent interference between different users, the emission of radio waves is strictly regulated by national laws, coordinated by an international body, the [[International Telecommunication Union]] (ITU), which allocates bands in the [[radio spectrum]] for different uses.<ref name="Spectrum101" /><ref name="Ellingson1" /> Radio transmitters must be licensed by governments, under a variety of license classes depending on use, and are restricted to certain frequencies and power levels.  In some classes, such as radio and television broadcasting stations, the transmitter is given a unique identifier consisting of a string of letters and numbers called a ''[[call sign]]'', which must be used in all transmissions.<ref name="ITU1">{{cite web |title=ARTICLE 19 Identification of stations |url=https://life.itu.int/radioclub/rr/art19.pdf |publisher=International Telecommunication Union |access-date=29 August 2022}}</ref> In order to adjust, maintain, or internally repair radiotelephone transmitters, individuals must hold a government license, such as the [[general radiotelephone operator license]] in the US, obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation.<ref name="FCC2">{{cite web |title=Commercial Radio Operator Types of Licenses |url=https://www.fcc.gov/wireless/bureau-divisions/mobility-division/commercial-radio-operator-license-program/commercial-0 |website=Federal Communications Commission |date=6 May 2016 |access-date=29 August 2022}}</ref>

Exceptions to the above rules allow the unlicensed operation by the public of low power short-range transmitters in consumer products such as cell phones, [[cordless phone]]s, [[wireless device]]s, [[walkie-talkie]]s, [[citizens band radio]]s, [[wireless microphone]]s, [[garage door opener]]s, and [[baby monitor]]s. In the US, these fall under [[Part 15]] of the [[Federal Communications Commission]] (FCC) regulations.  Many of these devices use the [[ISM band]]s, a series of frequency bands throughout the radio spectrum reserved for unlicensed use.  Although they can be operated without a license, like all radio equipment these devices generally must be [[Type approval|type-approved]] before the sale.<ref name="FCC4">{{cite web |title=FCC Basics of Unlicensed Transmitters |url=https://transition.fcc.gov/oet/ea/presentations/files/oct07/Oct_07-Basics_of_Unlicensed_Trans-JD.pdf |website=Federal Communications Commission |date=October 9, 2007 |first1=Joe |last1=Dichoso |access-date=29 August 2022}}</ref>

==Applications==
Below are some of the most important uses of radio, organized by function.

===Broadcasting===
{{further|Broadcasting}}
[[Broadcasting]] is the one-way transmission of information from a transmitter to receivers belonging to a public audience.<ref name="Pizzi">{{cite book
 | last1  = Pizzi
 | first1 = Skip
 | last2  = Jones
 | first2 = Graham
 | title  = A Broadcast Engineering Tutorial for Non-Engineers, 4th Ed.
 | publisher = National Association of Broadcasters, Taylor and Francis
 | date   = 2014
 | pages  =
 | url    = https://books.google.com/books?id=DaVwAwAAQBAJ
 | doi    =
 | id     =
 | isbn   = 978-0415733397
 }}</ref> Since the radio waves become weaker with distance, a [[broadcasting station]] can only be received within a limited distance of its transmitter.<ref name="Witten">{{cite book |last1=Witten |first1=Alan Joel |title=Handbook of Geophysics and Archaeology |year=2017 |url=https://books.google.com/books?id=VzQrDwAAQBAJ&dq=radio+waves+become+weaker+with+distance&pg=PA162 |publisher=[[Routledge]] |isbn=978-1351564588 |access-date=30 August 2022}}</ref> Systems that broadcast from [[communication satellite|satellites]] can generally be received over an entire country or continent.  Older terrestrial radio and television are paid for by [[commercial advertising]] or governments.  In subscription systems like [[satellite television]] and [[satellite radio]] the customer pays a monthly fee. In these systems, the radio signal is [[encryption|encrypted]] and can only be decrypted by the receiver, which is controlled by the company and can be deactivated if the customer does not pay.<ref name="Bonsor">{{cite web |last1=Bonsor |first1=Kevin |title=How Satellite Radio Works |url=https://electronics.howstuffworks.com/satellite-radio.htm |website=howstuffworks.com |date=26 September 2001 |publisher=HowStuffWorks |access-date=30 August 2022}}</ref>

Broadcasting uses several parts of the radio spectrum, depending on the type of signals transmitted and the desired target audience. [[Longwave]] and [[medium wave]] signals can give reliable coverage of areas several hundred kilometers across, but have a more limited information-carrying capacity and so work best with audio signals (speech and music), and the sound quality can be degraded by [[radio noise]] from natural and artificial sources.  The [[shortwave]] bands have a greater potential range but are more subject to interference by distant stations and varying atmospheric conditions that affect reception.<ref name="Gosling2">{{cite book |last1=Gosling |first1=William |title=Radio Antennas and Propagation: Radio Engineering Fundamentals |year= 1998 |url=https://books.google.com/books?id=jqj2k9EXgioC&q=longwave |publisher=Newnes |isbn=978-0750637411 |access-date=30 August 2022}}</ref><ref name="Griffin">{{cite book |last1=Griffin |first1=B. Whitfield |title=Radio-electronic Transmission Fundamentals |year=2000 |url=https://books.google.com/books?id=m5DIroWLw2EC&q=radio+fundamentals |publisher=SciTech Publishing/Noble |isbn=978-1884932137 |access-date=30 August 2022}}</ref>

In the [[very high frequency]] band, greater than 30 megahertz, the Earth's atmosphere has less of an effect on the range of signals, and [[line-of-sight propagation]] becomes the principal mode. These higher frequencies permit the great bandwidth required for television broadcasting.  Since natural and artificial noise sources are less present at these frequencies, high-quality audio transmission is possible, using [[frequency modulation]].<ref name="Pizzi-Jones">{{cite book |last1=Pizzi |first1=Skip |last2=Jones |first2=Graham |title=A Broadcast Engineering Tutorial for Non-Engineers |year= 2014 |url=https://books.google.com/books?id=sQ1xAwAAQBAJ&dq=vhf+propagation+television&pg=PR9 |publisher=CRC Press/Focal Press |isbn=978-1317906834 |access-date=30 August 2022}}</ref><ref name="Perez">{{cite book |last1=Perez |first1=Reinaldo |title=Handbook of Electromagnetic Compatibility |year= 2013 |url=https://books.google.com/books?id=dPBGBQAAQBAJ&dq=vhf+uhf+noise&pg=PA756 |publisher=Academic Press |isbn=978-1483288970 |access-date=30 August 2022}}</ref>

====Audio: Radio broadcasting====
{{main|Radio broadcasting}}
[[Radio broadcasting]] means transmission of [[audio signal|audio]] (sound) to [[radio receiver]]s belonging to a public audience.  Analog audio is the earliest form of radio broadcast. [[AM broadcasting]] began around 1920. [[FM broadcasting]] was introduced in the late 1930s with improved [[fidelity]].  A broadcast radio receiver is called a ''radio''.  Most radios can receive both AM and FM.<ref>{{cite book |first1=Clarence R. |last1=Green |first2=Robert M. |last2=Bourque |year=1980 |title=The Theory and Servicing of AM, FM, and FM Stereo Receivers |publisher=Prentice-Hall |page=6}}</ref>

{{multiple image
| align= right
| direction= horizontal
| image1= Transmisor de bulbos AM Elcom Bauer 701 B XEQK.jpg
| caption1= 1100 W AM broadcasting transmitter
| width1= 133
| image2= 2008-07-28 Mast radiator.jpg
| caption2= [[Mast radiator]] antenna of AM radio station
| width2= 118
| image3= Vintage Panasonic Table Top Transistor Radio, Model R-8, AM Band, 6 Transistors, Made In Japan, Circa 1964 (49305570428).jpg
| caption3= [[Panasonic]] AM radio from 1964
| width3= 266
| footer=
}}
* AM ([[amplitude modulation]]) – in AM, the [[amplitude]] (strength) of the radio carrier wave is varied by the audio signal. [[AM broadcasting]], the oldest broadcasting technology, is allowed in the [[AM broadcast band]]s, between 148–283&nbsp;kHz in the [[low frequency]] (LF) band for longwave broadcasts and between 526–1706&nbsp;kHz in the [[medium frequency]] (MF) band for medium-wave broadcasts.<ref>{{cite report |section-url=https://www.ofcom.org.uk/__data/assets/pdf_file/0020/33392/appendix_c.pdf |title=Radio – Preparing for the Future |section=Appendix&nbsp;C: Glossary |date=October 2005 |publisher=[[Ofcom]] |place=London |page=2}}</ref> Because waves in these bands travel as [[ground wave]]s following the terrain, [[AM station|AM radio stations]] can be received beyond the horizon at hundreds of miles distance, but AM has lower fidelity than FM. Radiated power ([[effective radiated power|ERP]]) of AM stations in the US is usually limited to a maximum of 10&nbsp;kW, although a few ([[clear-channel stations]]) are allowed to transmit at 50&nbsp;kW. AM stations broadcast in [[monaural]] audio; [[AM stereo]] broadcast standards exist in most countries, but the radio industry has failed to upgrade to them, due to lack of demand.<ref name=Gupta/>

:*[[Shortwave listening|Shortwave broadcasting]] – AM broadcasting is also allowed in the [[shortwave]] bands by legacy radio stations. Since radio waves in these bands can travel intercontinental distances by reflecting off the [[ionosphere]] using [[skywave]] or "skip" propagation, shortwave is used by international stations, broadcasting to other countries.<ref name=Gupta>{{cite book |last1=Gupta |first1=Rakesh |year= 2021 |title=Education Technology in Physical Education and Sports |series=Audio Visual Media in Physical Education |publisher=Friends Publications |place=India |isbn=978-9390649808 |url=https://books.google.com/books?id=KjgvEAAAQBAJ&dq=am+mode+broadcasting&pg=PA84 |access-date=30 August 2022}}</ref><ref name="Berg1" />

{{multiple image
| align= right
| direction= horizontal
| image1= KWNR Continental 816R-5B SN 247.jpg
| caption1= FM broadcast transmitter of radio station [[W:KWNR|KWNR]], Las Vegas, with a power of 35&nbsp;kW on 95.5&nbsp;MHz
| width1= 190
| image2= FM broadcasting antenna Willans Hill.jpg
| caption2= FM broadcasting antenna
| width2= 82
| image3= Klaudia 801.JPG
| caption3= AM/FM boombox radio with FM whip antenna
| width3= 172
| image4= BP74-Servicemenu-2008-05-31-03.jpg
| caption4 = FM [[car radio]]'s interface display
| width4= 264
| footer=
}}
* FM ([[frequency modulation]]) – in FM the [[frequency]] of the radio carrier signal is varied slightly by the audio signal. [[FM broadcasting]] is permitted in the [[FM broadcast band]]s between about 65 and 108&nbsp;MHz in the [[very high frequency]] (VHF) range. Radio waves in this band travel by [[line-of-sight propagation|line-of-sight]] so FM reception is limited by the visual [[horizon]] to about {{convert|30-40|miles|km|sigfig=2|abbr=on}}, and can be blocked by hills. However it is less susceptible to interference from [[radio noise]] ([[Radio frequency interference|RFI]], [[sferics]], static), and has higher [[fidelity]], better [[frequency response]], and less [[audio distortion]] than AM. In the US, radiated power ([[effective radiated power|ERP]]) of FM stations varies from 6–100&nbsp;kW.<ref name="Sterling-Kieth">{{cite book |last1=Sterling |first1=Christopher H. |last2=Kieth |first2=Michael C. |year= 2009 |title=Sounds of Change: A history of FM broadcasting in America |publisher=University of North Carolina Press |isbn=978-0807877555 |url=https://books.google.com/books?id=Ik-3w8ZwwqkC&q=propagation |access-date=30 August 2022}}</ref>
* [[Digital radio]] involves a variety of standards and technologies for broadcasting digital radio signals over the air. Some systems, such as [[HD Radio]] and [[Digital Radio Mondiale|DRM]], operate in the same wavebands as analog broadcasts, either as a replacement for analog stations or as a complementary service. Others, such as [[Digital Audio Broadcasting|DAB/DAB+]] and [[ISDB]]_Tsb, operate in wavebands traditionally used for television or satellite services.<ref>{{cite report |title=Digital Radio Guide |year=2017 |publisher=World Broadcasting Unions |place=Switzerland |url=https://tech.ebu.ch/docs/digitalradio/WBU%20Radio%20Techologies%20Guide.pdf}}</ref>

[[File:Portable radio receiving DAB%2B transmission in UK.jpg|thumb|upright=0.7|"Roberts" radio for DAB]]
:* [[Digital Audio Broadcasting]] (DAB) debuted in some countries in 1998. It transmits audio as a [[digital signal]] rather than an [[analog signal]] as AM and FM do.<ref name=Baker>{{cite magazine | last = Baker | first = William | date = 2020 | title = DAB vs. FM: The differences between analog and digital radio | magazine = Radio Fidelity online magazine
| url = https://radiofidelity.com/dab-vs-fm-radio/ |access-date = 14 September 2020}}</ref> DAB has the potential to provide higher quality sound than FM (although many stations do not choose to transmit at such high quality), has greater immunity to [[radio noise]] and interference, makes better use of scarce [[radio spectrum]] bandwidth and provides advanced user features such as [[electronic program guide]]s. Its disadvantage is that it is incompatible with previous radios so that a new DAB receiver must be purchased.<ref name=Hoeg-Lauterbach/> Several nations have set dates to switch off analog FM networks in favor of DAB / DAB+, notably Norway in 2017<ref>{{cite news |last=Revel |first=Timothy |date=10 January 2017 |title=Norway is first country to turn off FM radio and go digital-only |language=en-US |magazine=[[New Scientist]] |url=https://www.newscientist.com/article/2117569-norway-is-first-country-to-turn-off-fm-radio-and-go-digital-only/ |access-date=4 September 2022}}</ref> and Switzerland in 2024.<ref>{{cite news |last=McLane |first=Paul |date=30 August 2021 |title=Swiss FM shutdown reverts to original 2024 date |language=en-US |magazine=Radio World |url=https://www.radioworld.com/global/swiss-fm-shutdown-reverts-to-original-2024-date |access-date=4 September 2022}}</ref>

:: A single DAB station transmits a {{frequency|1,500|kHz}} bandwidth signal that carries from 9–12&nbsp;channels of digital audio modulated by [[orthogonal frequency-division multiplexing|OFDM]] from which the listener can choose. Broadcasters can transmit a channel at a range of different [[bit rate]]s, so different channels can have different audio quality.  In different countries DAB stations broadcast in either [[Band III]] (174–240&nbsp;MHz) or [[L band]] (1.452–1.492&nbsp;GHz) in the UHF range, so like FM reception is limited by the visual horizon to about {{convert|40|miles|km|sigfig=2}}.<ref>{{cite book |title=Trends in Radio Research: Diversity, innovation, and policies |publisher=Cambridge Scholars Publishing |year=2018 |page=263}}</ref><ref name=Hoeg-Lauterbach>{{cite book |last1=Hoeg |first1=Wolfgang |last2=Lauterbach |first2=Thomas |year= 2004 |title=Digital Audio Broadcasting: Principles and applications of digital radio |publisher=John Wiley & Sons |isbn=978-0470871423 |url=https://books.google.com/books?id=xp8Cd8NxhBUC&q=DAB+broadcasting |access-date=30 August 2022}}</ref>

:* [[HD Radio]] is an alternative digital radio standard widely implemented in North America.<ref>{{cite report |last=Bortzfield |first=Bill |date=27 November 2017 |title=The state of HD&nbsp;Radio in Jacksonville and nationwide |lang=en |website=[[WJCT]] Public Media |url=https://news.wjct.org/news/2017-11-27/the-state-of-hd-radio-in-jacksonville-and-nationwide |access-date=4 September 2022}}</ref> An [[in-band on-channel]] technology, HD&nbsp;Radio broadcasts a digital signal in a subcarrier of a station's analog FM or AM signal. Stations are able to [[multicast]] more than one audio signal in the subcarrier, supporting the transmission of multiple audio services at varying bitrates.<ref>{{cite report |first=Marty |last=Hadfield |date=15 August 2016 |title=Transmitter & programming considerations for HD&nbsp;Radio |website=RBR + TVBR (rbr.com) |url=https://www.rbr.com/mib0816/ |access-date=4 September 2022}}</ref> The digital signal is transmitted using [[OFDM]] with the HDC ([[High-Definition Coding]]) proprietary [[audio compression format]]. HDC is based on, but not compatible with, the [[MPEG-4]] standard [[HE-AAC]].<ref>{{cite web |title=Receiving NRSC‑5 |website=theori.io |date=9 June 2017 |url=http://theori.io/research/nrsc-5-c |access-date=14 April 2018 |url-status=live |archive-url=https://web.archive.org/web/20170820185827/http://theori.io/research/nrsc-5-c |archive-date=20 August 2017}}</ref> It uses a [[modified discrete cosine transform]] (MDCT) [[audio data compression]] algorithm.<ref>{{cite book |last1=Jones |first1=Graham A. |last2=Layer |first2=David H. |last3=Osenkowsky |first3=Thomas G. |year=2013 |title=NAB Engineering Handbook |publisher=[[National Association of Broadcasters]] / [[Taylor & Francis]] |isbn=978-1136034107 |pages=558–559 |url=https://books.google.com/books?id=K9N1TVhf82YC&pg=PA558}}</ref>

:* [[Digital Radio Mondiale]] (DRM) is a competing digital terrestrial radio standard developed mainly by broadcasters as a higher [[spectral efficiency]] replacement for legacy AM and FM broadcasting. ''Mondiale'' means "worldwide" in French and Italian; DRM was developed in 2001, and is currently supported by 23&nbsp;countries, and adopted by some European and Eastern broadcasters beginning in 2003. The ''DRM30'' mode uses the commercial broadcast bands below 30&nbsp;MHz, and is intended as a replacement for standard AM broadcast on the [[longwave#longwave broadcasting anchor|longwave]], [[AM broadcasting|mediumwave]], and [[shortwave radio#shortwave broadcasting anchor|shortwave]] bands. The ''DRM+'' mode uses [[very high frequency|VHF]] frequencies centered around the FM broadcast band, and is intended as a replacement for FM broadcasting. It is incompatible with existing radio receivers, so it requires listeners to purchase a new DRM receiver. The modulation used is a form of [[orthogonal frequency-division multiplexing|OFDM]] called [[COFDM]] in which, up to 4&nbsp;carriers are transmitted on a channel formerly occupied by a single AM or FM signal, modulated by [[quadrature amplitude modulation]] (QAM).<ref name=ETSI-201-980/><ref name=Berg1/>
:: The DRM system is designed to be as compatible as possible with existing AM and FM radio transmitters, so that much of the equipment in existing radio stations can continue in use, augmented with DRM modulation equipment.<ref name=ETSI-201-980>{{cite report |title=DRM System Specification |date=January 2021 |via=ETSI.org |type=vers.&nbsp;4.2.1 |id=ETSI ES 201 980 |publisher=European Broadcasting Union |place=Geneva, CH |page=178 |url=https://www.etsi.org/deliver/etsi_es/201900_201999/201980/04.02.01_60/es_201980v040201p.pdf |access-date=19 April 2018 }}</ref><ref name=Berg1>{{cite book |last=Berg |first=Jerome S. |year= 2008 |title=Broadcasting on the Short Waves: 1945 to today |publisher=McFarland |isbn=978-0786451982 |url=https://books.google.com/books?id=Ux9fZj6izuEC&dq=DRM+broadcasting&pg=PA51 |access-date=30 August 2022}}</ref>

[[File:SiriusXM Display on Volkswagen's RNS-510 Receiver.png|thumb|Volkswagen's RNS-510 receiver supports [[Sirius Satellite Radio]].]]
* [[Satellite radio]] is a subscription radio service that broadcasts CD quality [[digital audio]] direct to subscribers' receivers using a [[microwave]] [[downlink]] signal from a [[communications satellite|direct broadcast communication satellite]] in [[geostationary]] orbit {{convert|22,000|miles}} above the Earth. It is mostly intended for [[car radio|radios in vehicles]]. Satellite radio uses the 2.3&nbsp;GHz [[S band]] in North America, in other parts of the world, it uses the 1.4&nbsp;GHz {{nobr|[[L band]]}} allocated for DAB.<ref name="csgnetwork.com">{{cite report |title=Satellite S‑band radio frequency table |date=15 August 2011 |via=CSG Network |url=http://www.csgnetwork.com/satradiofreq.html |access-date=23 April 2013}}</ref><ref name="kbonsor">{{cite web |first=Kevin |last=Bonsor |date=26 September 2001 |title=How satellite radio works |website=[[HowStuffWorks]] |url=http://electronics.howstuffworks.com/satellite-radio.htm |access-date=1 May 2013}}</ref>

====Video: Television broadcasting====
{{main|Television broadcasting}}
[[Television broadcasting]] is the transmission of moving images by radio, which consist of sequences of still images, which are displayed on a screen on a [[television receiver]] (a "television" or TV) along with a synchronized audio (sound) channel. Television ([[video signal|video]]) signals occupy a wider [[bandwidth (signal processing)|bandwidth]] than broadcast radio ([[audio signal|audio]]) signals.  [[Analog television]], the original television technology, required 6&nbsp;MHz, so the television frequency bands are divided into 6&nbsp;MHz channels, now called "RF channels".<ref name="Enticknap">{{cite book |last1=Enticknap |first1=Leo Douglas Graham |title=Moving Image Technology: From Zoetrope to Digital |year=2005 |url=https://books.google.com/books?id=ghnGAwholkcC&dq=Television+broadcasting+is+the+transmission+of+moving+images+by+radio%2C&pg=PA163 |publisher=Wallflower Press ([[Columbia University Press]]) |isbn=978-1904764069 |access-date=31 August 2022}}</ref>

The current television standard, introduced beginning in 2006, is a digital format called [[high-definition television]] (HDTV), which transmits pictures at higher resolution, typically 1080 [[pixel]]s high by 1920 pixels wide, at a rate of 25 or 30 frames per second. [[Digital television]] (DTV) transmission systems, which replaced older analog television in a [[Digital television transition|transition]] beginning in 2006, use [[image compression]] and high-efficiency digital modulation such as [[orthogonal frequency-division multiplexing|OFDM]] and [[8VSB]] to transmit HDTV video within a smaller bandwidth than the old analog channels, saving scarce [[radio spectrum]] space.  Therefore, each of the 6&nbsp;MHz analog RF channels now carries up to 7 DTV channels – these are called "virtual channels". Digital television receivers have different behavior in the presence of poor reception or noise than analog television, called the "[[digital cliff]]" effect.  Unlike analog television, in which increasingly poor reception causes the picture quality to gradually degrade, in digital television picture quality is not affected by poor reception until, at a certain point, the receiver stops working and the screen goes black.<ref name="Starks">{{cite book |last1=Starks |first1=M. |title=The Digital Television Revolution: Origins to Outcomes |year=2013 |url=https://books.google.com/books?id=DLssmgEACAAJ|publisher=Springer |isbn=978-1137273345 |access-date=31 August 2022}}</ref><ref name="Brice">{{cite book |last1=Brice |first1=Richard |title=Newnes Guide to Digital TV |year= 2002 |url=https://books.google.com/books?id=Tm_U-Qd58RgC&dq=digital+tv+digital+cliff&pg=PA81 |publisher=Newnes |isbn=978-0750657211 |access-date=31 August 2022}}</ref>

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* [[Terrestrial television]], ''over-the-air (OTA) television'', or ''broadcast television'' – the oldest television technology, is the transmission of television signals from land-based [[television station]]s to [[television receiver]]s (called ''televisions'' or TVs) in viewer's homes.  Terrestrial television broadcasting uses the bands 41 – 88&nbsp;MHz ([[VHF]] low band or [[Band I]], carrying RF channels 1–6), 174 – 240&nbsp;MHz, (VHF high band or [[Band III]]; carrying RF channels 7–13), and 470 – 614&nbsp;MHz ([[UHF]] [[Band IV]] and [[Band V]]; carrying RF channels 14 and up).<ref name="NAB1">{{cite book
 | last1  = Bartlet
 | first1 = George W., Ed.
 | title  = NAB Engineering Handbook, 6th Ed.
 | publisher = National Association of Broadcasters
 | date   = 1975
 | location = Washington, D.C.
 | page  = 21
 | url    = https://books.google.com/books?id=eRquswEACAAJ
 | doi    =
 | id     =
 | isbn   =
 }}</ref>  The exact frequency boundaries vary in different countries.<ref name="Lundstrom">{{cite book |last1=Lundstrom |first1=Lars-Ingemar |title=Understanding Digital Television: An Introduction to DVB Systems with Satellite, Cable, Broadband and Terrestrial TV Distribution |year= 2012 |url=https://books.google.com/books?id=upjcAwAAQBAJ&dq=television+channel+frequencies+europe&pg=PA290 |publisher=CRC Press|isbn=978-1136032820 }}</ref>  Propagation is by [[line-of-sight propagation|line-of-sight]], so reception is limited by the visual horizon.<ref name="Ingram">{{cite book |last1=Ingram |first1=Dave |title=Video Electronics Technology |year=1983 |url=https://books.google.com/books?id=QbcZTrYhheoC&q=television+propagation+line+of+sight+rabbit+ears |publisher=TAB Books |isbn=978-0830614745 |access-date=1 September 2022}}</ref>  In the US, the [[effective radiated power]] (ERP) of television transmitters is regulated according to [[HAAT|height above average terrain]].<ref name="ProStar">{{cite book |title=Federal Communications Commission (Parts 20 - 39) |url=https://books.google.com/books?id=25usF6PafMcC&dq=television+tramsitter+ERP+limits&pg=PA70 |publisher=ProStar Publications| isbn=9781577858461 }}</ref> Viewers closer to the television transmitter can use a simple "rabbit ears" [[dipole antenna]] on top of the TV, but viewers in [[fringe reception]] areas typically require an outdoor antenna mounted on the roof to get adequate reception.<ref name="Ingram" />

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* [[Satellite television]] – a [[set-top box]] which receives subscription [[direct-broadcast satellite television]], and displays it on an ordinary [[television]]. A [[direct broadcast satellite]] in [[geostationary]] orbit {{convert|22200|mi|km|sigfig=3}} above the Earth's [[equator]] transmits many channels (up to 900) modulated on a 12.2 to 12.7&nbsp;GHz [[Ku band|K<sub>u</sub> band]] [[microwave]] downlink signal to a rooftop [[satellite dish]] antenna on the subscriber's residence.  The microwave signal is converted to a lower [[intermediate frequency]] at the dish and conducted into the building by a coaxial cable to a [[set-top box]] connected to the subscriber's TV, where it is demodulated and displayed. The subscriber pays a monthly fee.<ref name="Benoit">{{cite book |last1=Benoit |first1=Herve |title=Satellite Television: Analogue and Digital Reception Techniques |year= 1999 |url=https://books.google.com/books?id=4MLy2qXARXYC&q=Satellite+television |publisher=Butterworth-Heinemann/Arnold |isbn=978-0340741085 |access-date=1 September 2022}}</ref><ref name="Long">{{cite book |last1=Long |first1=Mark |title=The Digital Satellite TV Handbook |year=1999 |url=https://books.google.com/books?id=L4yQ0iztvQEC&q=Satellite+television |publisher=Newnes |isbn=978-0750671712 |access-date=1 September 2022}}</ref>

====Time====
Government [[standard frequency and time signal service]]s operate time radio stations which continuously broadcast extremely accurate time signals produced by [[atomic clock]]s, as a reference to synchronize other clocks.<ref name="Weik">{{cite book |last1=Weik |first1=Martin H. |title=Computer Science and Communications Dictionary |chapter=standard frequency and time signal |year=2000 |page=1649 |chapter-url=https://link.springer.com/referenceworkentry/10.1007/1-4020-0613-6_18062 |publisher=Computer Science and Communications Dictionary. Springer |doi=10.1007/1-4020-0613-6_18062 |isbn=978-0792384250 |access-date=1 September 2022}}</ref>  Examples are [[BPC (time signal)|BPC]], [[DCF77]], [[JJY]], [[Time from NPL (MSF)|MSF]], [[RTZ (radio station)|RTZ]], [[TDF time signal|TDF]], [[WWV (radio station)|WWV]], and [[YVTO]].<ref name="RadioNavAids">{{cite book |title=Radio Aids to Navigation, Publication 117, Chapter 2, Radio Time Signals | year=2005 |url=https://books.google.com/books?id=cU9k_GDAzDAC&q=Radio+Aids+to+Navigation%2C+Publication+117 |publisher=Lighthouse Press | isbn=978-1577855361 |access-date=1 September 2022}}</ref>  One use is in [[radio clock]]s and watches, which include an automated receiver that periodically (usually weekly) receives and decodes the time signal and resets the watch's internal [[quartz clock]] to the correct time, thus allowing a small watch or desk clock to have the same accuracy as an atomic clock.  Government time stations are declining in number because [[global positioning system|GPS]] satellites and the Internet [[Network Time Protocol]] (NTP) provide equally accurate time standards.<ref name="NPR">{{cite web |title=What Closing A Government Radio Station Would Mean For Your Clocks |url=https://www.npr.org/2018/08/25/641835302/what-closing-a-government-radio-station-would-mean-for-your-clocks |publisher=National Public Radio, Weekend Edition |access-date=1 September 2022}}</ref>

===Two-way voice communication===
{{main|Two-way radio}}
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A [[two-way radio]] is an [[audio signal|audio]] [[transceiver]], a [[radio receiver|receiver]] and [[transmitter]] in the same device, used for bidirectional person-to-person voice communication with other users with similar radios. An older term for this mode of communication is ''[[radiotelephony]]''.  The radio link may be [[half-duplex]], as in a [[walkie-talkie]], using a single radio channel in which only one radio can transmit at a time, so different users take turns talking, pressing a "[[push to talk]]" button on their radio which switches off the receiver and switches on the transmitter.  Or the radio link may be [[full duplex]], a bidirectional link using two radio channels so both people can talk at the same time, as in a cell phone.<ref name="Frenzel">{{cite book |last1=Frenzel |first1=Louis |title=Electronics Explained: Fundamentals for Engineers, Technicians, and Makers |year= 2017 |url=https://books.google.com/books?id=uqjRDgAAQBAJ&dq=two+way+radio+duplex&pg=PA161 |publisher=Newnes |isbn=978-0128118795 |access-date=2 September 2022}}</ref>
* [[Cell phone]] – a portable wireless [[telephone]] that is connected to the [[public switched telephone network|telephone network]] by radio signals exchanged with a local antenna at a [[cellular base station]] ([[cell tower]]).<ref name="HowCellPhonesWork">{{cite web| last1 = Brain| first1 = Marshall |first2=Jeff |last2=Tyson |first3=Julia |last3=Layton| title = How Cell Phones Work| work = How Stuff Works| publisher =  InfoSpace Holdings LLC| date = 2018|url= https://electronics.howstuffworks.com/cell-phone.htm| access-date = 31 December 2018}}</ref> The service area covered by the provider is divided into small geographical areas called "cells", each served by a separate base station antenna and multichannel [[transceiver]].  All the cell phones in a cell communicate with this antenna on separate frequency channels, assigned from a common pool of frequencies. The purpose of cellular organization is to conserve radio bandwidth by [[frequency reuse]].  Low power transmitters are used so the radio waves used in a cell do not travel far beyond the cell, allowing the same frequencies to be reused in geographically separated cells.  When a user carrying a cellphone crosses from one cell to another, his phone is automatically "handed off" seamlessly to the new antenna and assigned new frequencies.  Cellphones have a highly automated [[full duplex]] digital [[transceiver]] using [[orthogonal frequency-division multiplexing|OFDM]] modulation using two digital radio channels, each carrying one direction of the bidirectional conversation, as well as a control channel that handles dialing calls and "handing off" the phone to another cell tower.  Older [[2G]], [[3G]], and [[4G]] networks use frequencies in the [[ultrahigh frequency|UHF]] and low microwave range, between 700&nbsp;MHz and 3&nbsp;GHz.  The cell phone transmitter adjusts its power output to use the minimum power necessary to communicate with the cell tower; 0.6&nbsp;W when near the tower, up to 3&nbsp;W when farther away.  Cell tower channel transmitter power is 50&nbsp;W.  Current generation phones, called [[smartphone]]s, have many functions besides making telephone calls, and therefore have several other radio transmitters and receivers that connect them with other networks: usually a [[wireless modem|Wi-Fi modem]], a [[Bluetooth]] modem, and a [[GPS receiver]].<ref name="Lawson">{{cite web |last1=Lawson |first1=Stephen |title=Ten Ways Your Smartphone Knows Where You Are |url=https://www.pcworld.com/article/469718/ten_ways_your_smartphone_knows_where_you_are.html |publisher=[[PCWorld]] |access-date=2 September 2022}}</ref><ref name=Zander>{{cite book|author1=Guowang Miao|author2=Jens Zander|author3=Ki Won Sung|author4=Ben Slimane|title=Fundamentals of Mobile Data Networks|publisher=[[Cambridge University Press]]|isbn=978-1107143210|year=2016|author1-link=Guowang Miao}}</ref><ref>{{cite web|url=http://www.privateline.com/Cellbasics/Cellbasics02.html|title=Cellular Telephone Basics|date=1 January 2006|publisher=Privateline.com|page=2|access-date=2 September 2022|url-status=dead|archive-url=https://web.archive.org/web/20120417220750/http://www.privateline.com/Cellbasics/Cellbasics02.html|archive-date=17 April 2012}}</ref>

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* [[5G|5G cellular network]] – next-generation cellular networks which began deployment in 2019.  Their major advantage is much higher [[data signaling rate|data rates]] than previous cellular networks, up to 10&nbsp;[[bits per second|Gbps]]; 100 times faster than the previous cellular technology, [[4G LTE]]. The higher data rates are achieved partly by using higher frequency radio waves, in the higher microwave band 3–6&nbsp;GHz, and [[millimeter wave]] band, around 28 and 39&nbsp;GHz.  Since these frequencies have a shorter range than previous cellphone bands, the cells will be smaller than the cells in previous cellular networks which could be many miles across.  Millimeter-wave cells will only be a few blocks long, and instead of a [[cell site|cell base station]] and antenna tower, they will have many small antennas attached to utility poles and buildings.<ref name="S.Brown">{{cite web |last1=Brown |first1=Sara |title=5G, explained |url=https://mitsloan.mit.edu/ideas-made-to-matter/5g-explained |website=mitsloan.mit.edu |publisher=MIT Sloan School of Management |access-date=2 September 2022}}</ref><ref name="Osseiran-Monserrat-Marsch">{{cite book |last1=Osseiran |first1=Afif |last2=Monserrat |first2=Jose F. |last3=Marsch |first3=Patrick |title=5G Mobile and Wireless Communications Technology |year=2016 |url=https://books.google.com/books?id=qnMjDAAAQBAJ&q=5G+explained |publisher=Cambridge University Press |isbn=978-1107130098 |access-date=2 September 2022}}</ref>

[[Image:Zivile Satellitentelefone.jpg|thumb|upright=0.7|Satellite phones, showing the large antennas needed to communicate with the satellite]]
* [[Satellite phone]] (''satphone'') – a portable wireless [[telephone]] similar to a cell phone, connected to the [[public switched telephone network|telephone network]] through a radio link to an orbiting [[communications satellite]] instead of through [[cell tower]]s.  They are more expensive than cell phones; but their advantage is that, unlike a cell phone which is limited to areas covered by cell towers, satphones can be used over most or all of the geographical area of the Earth.  In order for the phone to communicate with a satellite using a small [[omnidirectional antenna]], first-generation systems use satellites in [[low Earth orbit]], about {{convert|400-700|mi|km|sigfig=2}} above the surface.  With an orbital period of about 100 minutes, a satellite can only be in view of a phone for about 4 – 15 minutes, so the call is "handed off" to another satellite when one passes beyond the local horizon. Therefore, large numbers of satellites, about 40 to 70, are required to ensure that at least one satellite is in view continuously from each point on Earth.  Other satphone systems use satellites in [[geostationary orbit]] in which only a few satellites are needed, but these cannot be used at high latitudes because of terrestrial interference.<ref name="Chandler">{{cite web |last1=Chandler |first1=Nathan |title=How Satellite Phones Work |url=https://electronics.howstuffworks.com/gadgets/travel/satellite-phone.htm |website=howstuffworks.com |date=13 February 2013 |publisher=HowStuffWorks |access-date=2 September 2022}}</ref><ref name="TW">{{cite web |title=Satellite Phone : Functioning/Working Of Satellite Phone |url=https://www.tutorialsweb.com/satellite-phones/satellite-phones-2.htm |website=tutorialsweb.com |publisher=Tutorials Web |access-date=2 September 2022}}</ref>
* [[Cordless phone]] – a [[landline telephone]] in which the [[handset]] is portable and communicates with the rest of the phone by a short-range [[duplex (telecommunications)|full duplex]] radio link, instead of being attached by a cord.  Both the handset and the base station have low-power radio transceivers that handle the short-range bidirectional radio link.<ref>{{cite magazine|title=Never Miss a Call: PS Buyer's Guide to Cordless Phones|date=October 1982|last=McComb|first=Gordon|magazine=Popular Science|pages=84–85|url=https://books.google.com/books?id=ia91vNwYgLIC|via=Google Books}}</ref> {{As of|2022}}, cordless phones in most nations use the [[Digital enhanced cordless telecommunications|DECT]] transmission standard.<ref>{{Cite news |last=Guy |first=Nick |date=13 July 2022 |title=Wirecutter: The Best Cordless Phone |language=en-US |work=The New York Times |url=https://www.nytimes.com/wirecutter/reviews/best-cordless-phone/ |access-date=7 September 2022 |issn=0362-4331}}</ref>
[[File:Portable radio SCR536.png|thumb|left|upright=0.6|Motorola SCR-536 from WW2, the first walkie-talkie]]
* [[Land mobile radio system]] – short-range mobile or portable [[half-duplex]] radio transceivers operating in the VHF or UHF band that can be used without a license.  They are often installed in vehicles, with the mobile units communicating with a dispatcher at a fixed [[base station]]. Special systems with reserved frequencies are used by [[first responder]] services; police, fire, ambulance, and emergency services, and other government services.  Other systems are made for use by commercial firms such as taxi and delivery services.  VHF systems use channels in the range 30–50&nbsp;MHz and 150–172&nbsp;MHz.  UHF systems use the 450–470&nbsp;MHz band and in some areas the 470–512&nbsp;MHz range.  In general, VHF systems have a longer range than UHF but require longer antennas. AM or FM modulation is mainly used, but digital systems such as [[Digital mobile radio|DMR]] are being introduced.  The radiated power is typically limited to 4 watts.<ref name="HowCellPhonesWork"/> These systems have a fairly limited range, usually {{convert|3 to 20|miles|km|sigfig=2}} depending on terrain. [[Repeater]]s installed on tall buildings, hills, or mountain peaks are often used to increase the range when it is desired to cover a larger area than line-of-sight.  Examples of land mobile systems are [[Citizens Band|CB]], [[Family Radio Service|FRS]], [[General Mobile Radio Service|GMRS]], and [[Multi-Use Radio Service|MURS]].  Modern digital systems, called [[trunked radio system]]s, have a digital channel management system using a control channel that automatically assigns frequency channels to user groups.<ref>{{cite report|title=Voice Radio Communications Guide for the Fire Service|date=June 2016|author=U.S. Fire Administration|publisher=Federal Emergency Management Agency|location=Washington, D.C.|pages=33–34|url=https://www.usfa.fema.gov/downloads/pdf/publications/voice_radio_communications_guide_for_the_fire_service.pdf|access-date=7 September 2022}}</ref>
[[Image:Miramar Fire Department Firefighters Conduct Prescribed Burns 140612-M-RB277-066.jpg|thumb|Firefighter using modern walkie-talkie]]
** [[Walkie-talkie]] – a battery-powered portable handheld half-duplex two-way radio, used in land mobile radio systems.<ref>{{cite book|first=Christopher H.|last=Sterling|title=Military Communications: From Ancient Times to the 21st Century|url=https://books.google.com/books?id=RBC2nY1rp5MC&pg=PA504|year=2008|publisher=ABC-CLIO|isbn=978-1851097326|pages=503–504}}</ref>
* [[Airband]] – Half-duplex radio system used by aircraft pilots to talk to other aircraft and ground-based [[air traffic controller]]s. This vital system is the main communication channel for [[air traffic control]]. For most communication in overland flights in [[air corridor]]s a VHF-AM system using channels between 108 and 137&nbsp;MHz in the [[very high frequency|VHF]] band is used.  This system has a typical transmission range of {{convert|200|miles|km|sigfig=2}} for aircraft flying at cruising altitude.<ref>{{Cite report |url=https://us-afc.org/wp-content/uploads/2017/01/AFC-Manual-Rev-B-May-20-2012.pdf |title=Aeronautical Frequency Committee Manual |publisher=Aviation Spectrum Resources Inc. |issue=61604 Rev. B |publication-date=2012}}</ref><ref name="freq">{{cite web|url=http://www.smeter.net/spectrum/aviation.php |archive-url=https://web.archive.org/web/20040212142911/http://www.smeter.net/spectrum/aviation.php |url-status=dead |archive-date=12 February 2004 |title=Aviation Radio Bands and Frequencies |publisher=Smeter network 2011 |access-date=16 February 2011 }}</ref> For flights in more remote areas, such as transoceanic airline flights, aircraft use the [[high frequency|HF]] band or channels on the [[Inmarsat]] or [[Iridium]] satphone satellites.<ref>{{Cite report |url=https://www.icao.int/WACAF/Documents/Meetings/2019/SAT24/SAT%2024%20WP%2017e%20_APP%20E_NAT%20Doc%20007%20%20(EN)%20-%20Edition%20V.2019-2_eff.pdf |title=North Atlantic Operations and Airspace Manual |date=28 March 2019 |publisher=ICAO European and North Atlantic Office |issue=V.2019–2}}</ref>  Military aircraft also use a dedicated UHF-AM band from 225.0 to 399.95&nbsp;MHz.<ref>{{Cite magazine |last=Van Horn |first=Larry |title=The Military VHF/UHF Spectrum |url=http://www.monitoringtimes.com/html/mtMilVHF.html |magazine=Monitoring Times}}</ref>

[[Image:Maritime VHF Sailor type.jpg|thumb|VHF marine radio on a ship]]
* [[Marine radio]] – medium-range transceivers on ships, used for ship-to-ship, ship-to-air, and ship-to-shore communication with [[harbormaster]]s They use FM channels between 156 and 174&nbsp;MHz in the [[very high frequency|VHF]] band with up to 25 watts power, giving them a range of about {{convert|60|miles|km|sigfig=2}}. Some channels are [[half-duplex]] and some are [[full-duplex]], to be compatible with the telephone network, to allow users to make telephone calls through a marine operator.<ref>{{Cite book |last=Fletcher |first=Sue |url=https://www.worldcat.org/oclc/48674566 |title=A Boater's Guide to VHF and GMDSS |publisher=International Marine/McGraw-Hill |year=2002 |isbn=0071388028 |location=Camden, Maine |oclc=48674566}}</ref>
* [[Amateur radio]] – long-range half-duplex two-way radio used by hobbyists for non-commercial purposes: recreational radio contacts with other amateurs, volunteer emergency communication during disasters, contests, and experimentation. [[Radio amateur]]s must hold an [[amateur radio license]] and are given a unique [[callsign]] that must be used as an identifier in transmissions.  Amateur radio is restricted to small frequency bands, the [[amateur radio band]]s, spaced throughout the radio spectrum starting at 136&nbsp;kHz.  Within these bands, amateurs are allowed the freedom to transmit on any frequency using a wide variety of voice modulation methods, along with other forms of communication, such as [[slow-scan television]] (SSTV), and [[radioteletype]] (RTTY).  Additionally, amateurs are among the only radio operators still using [[Morse code]] [[radiotelegraphy]].<ref>{{Cite book |url=https://www.worldcat.org/oclc/961215964 |title=The ARRL Handbook for Radio Communications 2017 |date=2016 |publisher=American Radio Relay League |isbn=978-1625950628 |edition=94th |location=Newington, Connecticut |oclc=961215964}}</ref>

===One-way voice communication===
One way, unidirectional radio transmission is called ''[[simplex communication|simplex]]''. 
* [[Baby monitor]] – a crib-side appliance for parents of infants that transmits the baby's sounds to a receiver carried by the parent, so they can monitor the baby while they are in other parts of the house.<ref name="HSW1">{{cite web | last = Brain | first = Marshall | title = Radio basics: Real life examples | work = How radio works | publisher = How Stuff Works website | date = 11 February 2021 | url = https://electronics.howstuffworks.com/radio.htm#pt4 | accessdate = 27 August 2022}}</ref> The wavebands used vary by region, but analog baby monitors generally transmit with low power in the 16, 9.3–49.9 or 900&nbsp;MHz wavebands, and digital systems in the 2.4&nbsp;GHz waveband.<ref>{{Cite report |url=http://www.bccdc.ca/resource-gallery/Documents/Educational%20Materials/EH/Radiofrequency-Toolkit.pdf |title=Radiofrequency Toolkit for Environmental Health Practitioners |publisher=British Columbia Centre for Disease Control/National Collaborating Centre for Environmental Health |location=Vancouver, British Columbia, Canada |page=26 |isbn=978-1926933481}}</ref> Many baby monitors have duplex channels so the parent can talk to the baby, and cameras to show video of the baby.<ref>{{Cite magazine |date=24 April 2016 |title=Best Baby Monitor Buying Guide |url=https://www.consumerreports.org/babies-kids/baby-monitors/buying-guide/ |magazine=Consumer Reports |language=en-US |access-date=9 September 2022}}</ref>
* [[Wireless microphone]] – a battery-powered microphone with a short-range transmitter that is handheld or worn on a person's body which transmits its sound by radio to a nearby receiver unit connected to a sound system.  Wireless microphones are used by public speakers, performers, and television personalities so they can move freely without trailing a microphone cord. Traditionally, analog models transmit in FM on unused portions of the television broadcast frequencies in the VHF and UHF bands.  Some models transmit on two frequency channels for [[diversity reception]] to prevent [[null (radio)|nulls]] from interrupting transmission as the performer moves around.<ref>{{Cite book |last=Eargle |first=John |url=https://books.google.com/books?id=JnkqBgAAQBAJ |title=The Microphone Book |date=2005 |publisher=Focal Press |edition=2nd |location=Oxford|pages=142–151 |chapter=Overview of Wireless Microphone Technology |isbn=978-1136118067 |via=Google Books}}</ref> Some models use digital modulation to prevent unauthorized reception by scanner radio receivers; these operate in the 900&nbsp;MHz, 2.4&nbsp;GHz or 6&nbsp;GHz [[ISM bands]].<ref>{{Cite news |last=Bell |first=Dee Ana |date=1 November 2012 |title=Avoiding Audio Problems with Wireless Microphone Systems |language=en |work=TV Technology |url=https://www.tvtechnology.com/opinions/avoiding-audio-problems-with-wireless-microphone-systems |access-date=10 September 2022}}</ref> European standards also support wireless multichannel audio systems (WMAS) that can better support the use of large numbers of wireless microphones at a single event or venue. {{As of|2021}}, U.S. regulators were considering adopting rules for WMAS.<ref>{{Cite news |last=Vernon |first=Tom |date=28 August 2021 |title=Wireless Mic Industry Debates WMAS Technology |language=en-US |work=Radio World |url=https://www.radioworld.com/tech-and-gear/wireless-mic-industry-debates-wmas-technology |access-date=10 September 2022}}</ref>

===Data communication===
{{further|Data transmission|Telemetry|Wireless}}
* [[Wireless networking]] – automated radio links which transmit digital data between [[computer]]s and other wireless devices using radio waves, linking the devices together transparently in a [[computer network]].  Computer networks can transmit any form of data: in addition to email and web pages, they also carry [[phone call]]s ([[VoIP]]), audio, and video content (called [[streaming media]]).  Security is more of an issue for wireless networks than for wired networks since anyone nearby with a wireless modem can access the signal and attempt to log in. The radio signals of wireless networks are [[encryption|encrypted]] using [[Wi-Fi Protected Access|WPA]].<ref name="Lewis-Davis">{{cite book |last1=Lewis |first1=Barry D. |last2=Davis |first2=Peter T. |title=Wireless Networks For Dummies |year=2004 |url=https://books.google.com/books?id=a3_S-c8nfqsC&q=Wireless+networking+WPA |publisher=John Wiley & Sons |isbn=978-0764579776 |access-date=12 September 2022}}</ref>
[[File:Wireless network.jpg|thumb|upright=0.8|A [[laptop]] (with [[Wi-Fi]] module) and a typical home [[wireless router]] ''(on the right)'' connecting it to the Internet. The laptop [[Droste effect|shows its own photo]]]]
** [[Wireless LAN]] (''wireless local area network'' or ''[[Wi-Fi]]'') – based on the [[IEEE 802.11]] standards, these are the most widely used computer networks, used to implement [[local area network]]s without cables, linking computers, laptops, cell phones, [[video game console]]s, [[smart TV]]s and [[computer printer|printers]] in a home or office together, and to a [[wireless router]] connecting them to the [[Internet]] with a wire or cable connection.  Wireless routers in public places like libraries, hotels and coffee shops create [[wireless access points]] ([[hotspot (Wi-Fi)|hotspots]]) to allow the public to access the Internet with portable devices like [[smartphone]]s, [[tablet computer|tablets]] or [[laptop computer|laptops]].  Each device exchanges data using a [[wireless network interface controller|wireless modem]] (wireless network interface controller), an automated [[microwave]] transmitter and receiver with an omnidirectional antenna that works in the background, exchanging [[data packet]]s with the router.  Wi-Fi uses channels in the 2.4&nbsp;GHz and 5&nbsp;GHz [[ISM band]]s with OFDM ([[orthogonal frequency-division multiplexing]]) modulation to transmit data at high rates.  The transmitters in Wi-Fi modems are limited to a radiated power of 200&nbsp;mW to 1&nbsp;watt, depending on country. They have a maximum indoor range of about {{convert|150|ft|m|sigfig=1|abbr=on}} on 2.4&nbsp;GHz and {{convert|50|ft|m|sigfig=1|abbr=on}} on 5&nbsp;GHz.<ref name="Lowe" />
[[File:USI router.jpg|thumb|upright=0.8|Neighborhood wireless WAN router on telephone pole]]
** [[Wireless WAN]] (wireless wide area network, WWAN) – a variety of technologies that provide wireless internet access over a wider area than Wi-Fi networks do – from an office building to a campus to a neighborhood, or to an entire city.  The most common technologies used are: [[cellular modem]]s, that exchange computer data by radio with [[cell tower]]s; satellite internet access; and lower frequencies in the UHF band, which have a longer range than Wi-Fi frequencies.  Since WWAN networks are much more expensive and complicated to administer than Wi-Fi networks, their use so far has generally been limited to private networks operated by large corporations.<ref name="Lowe">{{cite book |last1=Lowe |first1=Doug |title=Networking For Dummies |year= 2020 |url=https://books.google.com/books?id=LiHxDwAAQBAJ&q=Wireless+WAN+basics |publisher=John Wiley & Sons |isbn=978-1119748670 |access-date=12 September 2022}}</ref>
** [[Bluetooth]] – a very short-range wireless interface on a portable wireless device used as a substitute for a wire or cable connection, mainly to exchange files between portable devices and connect cellphones and music players with wireless [[headphone]]s.  In the most widely used mode, transmission power is limited to 1&nbsp;milliwatt, giving it a very short range of up to 10 m (30 feet).  The system uses [[frequency-hopping spread spectrum]] transmission, in which successive [[data packet]]s are transmitted in a pseudorandom order on one of 79 1&nbsp;MHz Bluetooth channels between 2.4 and 2.83&nbsp;GHz in the [[ISM band]]. This allows Bluetooth networks to operate in the presence of [[radio noise|noise]], other wireless devices and other Bluetooth networks using the same frequencies, since the chance of another device attempting to transmit on the same frequency at the same time as the Bluetooth modem is low.  In the case of such a "collision", the Bluetooth modem just retransmits the data packet on another frequency.<ref name="Muller">{{cite book
 |last1        = Muller
 |first1       = Nathan J.
 |title        = Networking A to Z
 |publisher    = McGraw-Hill Professional
 |date         = 2002
 |pages        = 45–47
 |url          = https://books.google.com/books?id=0qv4KbasX7wC&q=bluetooth&pg=PA45
 |doi          =
 |id           =
 |isbn         = 978-0071429139
 |access-date  = 12 September 2022
|archive-date = 24 June 2021
|archive-url  = https://web.archive.org/web/20210624205933/https://books.google.com/books?id=0qv4KbasX7wC&q=bluetooth&pg=PA45
 |url-status   = live
}}</ref>
** [[Packet radio]] – a long-distance [[peer-to-peer]] [[wireless ad-hoc network]] in which [[data packet]]s are exchanged between computer-controlled [[radio modem]]s (transmitter/receivers) called nodes, which may be separated by miles, and maybe mobile.  Each node only communicates with neighboring nodes, so packets of data are passed from node to node until they reach their destination using the [[X.25]] network protocol. Packet radio systems are used to a limited degree by commercial telecommunications companies and by the [[amateur radio]] community.<ref name="Silver">{{cite book |last1=Silver |first1=H. Ward |title=The ARRL Extra Class License Manual for Ham Radio |year=2008 |url=https://books.google.com/books?id=EcbjKvhgPLsC&dq=Packet+radio&pg=SA2-PA11 |publisher=American Radio Relay League |isbn=978-0872591356 |access-date=12 September 2022}}</ref>
* [[Text messaging]] (texting) – this is a service on [[cell phone]]s, allowing a user to type a short alphanumeric message and send it to another phone number, and the text is displayed on the recipient's phone screen.  It is based on the [[Short Message Service]] (SMS) which transmits using spare bandwidth on the control radio channel used by cell phones to handle background functions like dialing and cell handoffs. Due to technical limitations of the channel, text messages are limited to 160 alphanumeric characters.<ref name="Hillebrand">{{cite book |last1=Hillebrand |first1=Friedhelm |title=Short Message Service (SMS): The Creation of Personal Global Text Messaging |year= 2010 |url=https://books.google.com/books?id=YPgfNaoYHUsC&q=Text+messaging++160+char |publisher=John Wiley & Sons |isbn=978-0470689936 |access-date=12 September 2022}}</ref>

[[Image:Parabolic antennas.JPG|thumb|upright=0.8|[[Parabolic antenna]]s of microwave relay links on tower in Australia]]
* [[Microwave relay]] – a long-distance high bandwidth point-to-point digital data transmission link consisting of a microwave transmitter connected to a [[parabolic antenna|dish antenna]] that transmits a beam of [[microwaves]] to another dish antenna and receiver. Since the antennas must be in line-of-sight, distances are limited by the visual horizon to {{convert|30-40|miles|km|sigfig=2}}. Microwave links are used for private business data, wide area computer networks (WANs), and by telephone companies to transmit long-distance phone calls and television signals between cities.<ref name="McGregor-Driscoll-Mcdowell">{{cite book |last1=McGregor |first1=Michael A. |last2=Driscoll |first2=Paul D. |last3=Mcdowell |first3=Walter |title=Head's Broadcasting in America: A Survey of Electronic Media |year= 2016 |url=https://books.google.com/books?id=m6dYCwAAQBAJ&dq=Microwave+relay+%E2%80%93&pg=PT224 |publisher=Routledge |isbn=978-1317347927 |access-date=12 September 2022}}</ref><ref name="EIA">{{cite web |last1=Radio-Electronics-Television Manufacturers Association. Engineering Department |title=Microwave Relay Systems for Communications |year=1955 |url=https://books.google.com/books?id=-OP2xwEACAAJ&q=Microwave+relay+%E2%80%93 |publisher=Electronic Industries Association |access-date=12 September 2022}}</ref>
* [[Telemetry]] – automated one-way (simplex) transmission of measurements and operation data from a remote process or device to a receiver for monitoring.  Telemetry is used for in-flight monitoring of missiles, drones, satellites, and [[weather balloon]] [[radiosonde]]s, sending scientific data back to Earth from interplanetary spacecraft, communicating with electronic biomedical sensors implanted in the human body, and [[well logging]]. Multiple channels of data are often transmitted using [[frequency-division multiplexing]] or [[time-division multiplexing]].<ref name="Bailey">{{cite book |last1=Bailey |first1=David |title=Practical Radio Engineering and Telemetry for Industry |year=2003 |url=https://books.google.com/books?id=K-wxbN3rjMwC&dq=radio+telemetry+fundamentals&pg=PR4 |publisher=Elsevier |isbn=978-0080473895 |access-date=12 September 2022}}</ref> Telemetry is starting to be used in consumer applications such as: 
** [[Automated meter reading]] – [[electricity meter|electric power meters]], [[water meter]]s, and [[gas meter]]s that, when triggered by an interrogation signal, transmit their readings by radio to a utility reader vehicle at the curb, to eliminate the need for an employee to go on the customer's property to manually read the meter.<ref name="Arafath-Mazumder-Hassan">{{cite book |last1=Arafath |first1=Yeasin |last2=Mazumder |first2=Debabrata |last3=Hassan |first3=Rakib |title=Automatic Meter Reading by Radio Frequency Technology |year=2012 |url=https://books.google.com/books?id=OuSNtgAACAAJ&q=RF+Automated+meter+reading |publisher=Lap Lambert Academic Publishing GmbH KG |isbn=978-3847372219 |access-date=12 September 2022}}</ref>
** [[Electronic toll collection]] – on [[toll road]]s, an alternative to manual collection of tolls at a toll booth, in which a [[transponder]] in a vehicle, when triggered by a roadside transmitter, transmits a signal to a roadside receiver to register the vehicle's use of the road, enabling the owner to be billed for the toll.<ref name="Bonsor2">{{cite web |last1=Bonsor |first1=Kevin |title=How E-ZPass Works |url=https://auto.howstuffworks.com/e-zpass.htm |website=howstuffworks.com |date=28 August 2001 |publisher=HowStuff Works |access-date=12 September 2022}}</ref>
[[Image:RFID Chip 001.JPG|thumb|upright=0.6|RFID tag from a DVD]]
* [[Radio Frequency Identification]] (RFID) – identification tags containing a tiny radio transponder ([[Radio receiver|receiver]] and [[transmitter]]) which are attached to merchandise.  When it receives an interrogation pulse of radio waves from a nearby reader unit, the tag transmits back an ID number, which can be used to inventory goods.  Passive tags, the most common type, have a chip powered by the radio energy received from the reader, rectified by a diode, and can be as small as a grain of rice.  They are incorporated in products, clothes, railroad cars, library books, airline baggage tags and are implanted under the skin in pets and livestock ([[microchip implant (animal)|microchip implant]]) and even people. Privacy concerns have been addressed with tags that use [[encryption|encrypted]] signals and [[authentication|authenticate]] the reader before responding.  Passive tags use 125–134&nbsp;kHz, 13, 900&nbsp;MHz and 2.4 and 5&nbsp;GHz [[ISM band]]s and have a short range.  Active tags, powered by a battery, are larger but can transmit a stronger signal, giving them a range of hundreds of meters.<ref name="Hunt-Puglia-Puglia">{{cite book |last1=Hunt |first1=V. Daniel |last2=Puglia |first2=Albert |last3=Puglia |first3=Mike |title=RFID: A Guide to Radio Frequency Identification |year=2007 |url=https://books.google.com/books?id=ZCHiHqL6QKkC&q=Radio+Frequency+Identification |publisher=John Wiley & Sons |isbn=978-0470112243 |access-date=12 September 2022}}</ref>
* [[Communication with submarines|Submarine communication]] – When submerged, [[submarine]]s are cut off from all ordinary radio communication with their military command authorities by the conductive seawater.  However radio waves of low enough frequencies, in the [[very low frequency|VLF]] (30 to 3&nbsp;kHz) and [[extremely low frequency|ELF]] (below 3&nbsp;kHz) bands are able to penetrate seawater.  Navies operate large shore transmitting stations with power output in the megawatt range to transmit [[encryption|encrypted]] messages to their submarines in the world's oceans.  Due to the small bandwidth, these systems cannot transmit voice, only text messages at a slow data rate. The communication channel is one-way, since the long antennas needed to transmit VLF or ELF waves cannot fit on a submarine.  [[Very low frequency|VLF]] transmitters use miles long wire antennas like [[umbrella antenna]]s.  A few nations use ELF transmitters operating around 80&nbsp;Hz, which can communicate with submarines at lower depths.  These use even larger antennas called [[ground dipole]]s, consisting of two [[ground (electricity)|ground]] (Earth) connections {{convert|23-60|km|miles|sigfig=2|abbr=on}} apart, linked by overhead transmission lines to a power plant transmitter.<ref name="White">{{cite web |last1=White |first1=Ryan |title=How do submarines communicate with the outside world? |url=https://navalpost.com/how-do-submarines-communicate-with-the-outside-world/ |website=navalpost.com |date=17 December 2021 |publisher=Naval Post |access-date=12 September 2022}}</ref><ref name="NRR">{{cite web |title=Naval Research Reviews, Vol. 27 | year=1974 |url=https://books.google.com/books?id=QANVAAAAMAAJ&dq=Submarine+communication+grounded+dipole&pg=PA5 |publisher=Superintendent of Government Documents |access-date=12 September 2022}}</ref>

===Space communication===
[[Image:ЦКС Дубна ГПКС -6.jpg|thumb|upright=0.8|[[Earth station|Satellite Communications Center]] [[Dubna]] in Russia<ref>{{cite web|url=https://eng.rscc.ru/space/centry-kosmicheskoj-svyazi/cks-dubna/|title=Ground infrastructure|website=[[Russian Satellite Communications Company]]}}</ref>]]
This is radio communication between a [[spacecraft]] and an Earth-based ground station, or another spacecraft.  Communication with spacecraft involves the longest transmission distances of any radio links, up to billions of kilometers for [[interplanetary spaceflight|interplanetary spacecraft]].  In order to receive the weak signals from distant spacecraft, [[satellite ground station]]s use large [[parabolic antenna|parabolic "dish" antennas]] up to {{convert|25|m|ft|sigfig=2}} in diameter and extremely sensitive receivers. High frequencies in the [[microwave]] band are used, since microwaves pass through the [[ionosphere]] without [[refraction]], and at microwave frequencies the [[high-gain antenna]]s needed to focus the radio energy into a narrow beam pointed at the receiver are small and take up a minimum of space in a satellite. Portions of the [[ultrahigh frequency|UHF]], [[L band|L]], [[C band (IEEE)|C]], [[S band|S]], [[ku band|k<sub>u</sub>]] and [[ka band|k<sub>a</sub> band]] are allocated for space communication.  A radio link that transmits data from the Earth's surface to a spacecraft is called an [[uplink]], while a link that transmits data from the spacecraft to the ground is called a downlink.<ref name="NASA3">{{cite web |title=State-of-the-Art of Small Spacecraft Technology, 9.0 - Communications |url=https://www.nasa.gov/smallsat-institute/sst-soa/communications |website=nasa.gov |date=16 October 2021 |publisher=National Aeronautics and Space Administration |access-date=11 September 2022}}</ref>

[[Image:Azərbaycan peyki - VOA.jpg|thumb|upright=0.8|Communications satellite belonging to [[Azerbaijan]] ]]
* [[Communication satellite]] – an [[artificial satellite]] used as a telecommunications relay to transmit data between widely separated points on Earth.  These are used because the [[microwave]]s used for telecommunications travel by [[Line-of-sight propagation|line of sight]] and so cannot propagate around the curve of the Earth. {{as of|2021|1|1}}, there were 2,224 communications satellites in Earth orbit.<ref name="UCSdatabase">{{cite web| url= https://www.ucsusa.org/resources/satellite-database#.W7WcwpMza9Y |title= UCS Satellite Database|publisher= Union of Concerned Scientists |date=1 January 2021|access-date=21 May 2021}}</ref>  Most are in [[geostationary]] orbit {{convert|22200|mi|km|sigfig=3}} above the [[equator]], so that the satellite appears stationary at the same point in the sky, so the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track it.  In a [[satellite ground station]] a microwave transmitter and large satellite dish antenna transmit a microwave uplink beam to the satellite.  The uplink signal carries many channels of telecommunications traffic, such as long-distance telephone calls, television programs, and internet signals, using a technique called [[frequency-division multiplexing]] (FDM).  On the satellite, a [[transponder (satellite communications)|transponder]] receives the signal, translates it to a different downlink frequency to avoid interfering with the uplink signal, and retransmits it down to another ground station, which may be widely separated from the first.  There the downlink signal is demodulated and the telecommunications traffic it carries is sent to its local destinations through landlines.  Communication satellites typically have several dozen transponders on different frequencies, which are leased by different users.<ref name="Marsten">{{cite book |last1=Marsten |first1=Richard B. |title=Communication Satellite Systems Technology |year= 2014 |publisher=Academic Press |isbn=978-1483276816 |url=https://books.google.com/books?id=NEWoBQAAQBAJ&q=Communication+satellite |access-date=11 September 2022}}</ref>
* [[Direct broadcast satellite]] – a geostationary communication satellite that transmits retail programming directly to receivers in subscriber's homes and vehicles on Earth, in [[satellite radio]] and TV systems.  It uses a higher transmitter power than other communication satellites, to allow the signal to be received by consumers with a small unobtrusive antenna.  For example, [[satellite television]] uses downlink frequencies from 12.2 to 12.7&nbsp;GHz in the [[ku band|k<sub>u</sub> band]] transmitted at 100 to 250 watts, which can be received by relatively small {{convert|43-80|cm|inch|sigfig=2|abbr=on}} [[satellite dish]]es mounted on the outside of buildings.<ref name="rfwirelessworld">{{cite web |title=Satellite TV-Direct Broadcast Satellite System, DBS TV |url=https://www.rfwireless-world.com/Tutorials/satellite-TV-basics.html |website=rfwireless-world.com |publisher=RF Wireless World |access-date=11 September 2022}}</ref>

===Radar===
{{main|Radar}}
[[Image:US Navy 120208-N-TU894-022 Air-Traffic Controller 2nd Class Gregory Clemmons stands the departure position watch as Air-Traffic Controller 3rd Clas.jpg|thumb|upright=0.8|Military [[air traffic controller]] on US Navy aircraft carrier monitors aircraft on radar screen]]
[[Radar]] is a [[radiolocation]] method used to locate and track aircraft, spacecraft, missiles, ships, vehicles, and also to map weather patterns and terrain. A radar set consists of a transmitter and receiver.<ref name="HowRadarWorks">{{cite web
  | last = Brain
  | first = Marshall
  | title = How radar works
  | publisher = How Stuff Works
  | date = 2020
  | url = https://science.howstuffworks.com/radar.htm?srch_tag=swsu5cd23kj6x6xnw4jfrts6awdrk5sg
  | format =
  | doi =
  | accessdate = 3 September 2022}}</ref><ref name="Skolnik">{{cite encyclopedia
  | last = Skolnik
  | first = Merrill
  | title = Radar
  | encyclopedia = Encyclopædia Britannica online
  | publisher = Encyclopædia Britannica Inc.
  | date = 2021
  | url = https://www.britannica.com/technology/radar
  | format =
  | doi =
  | accessdate = 3 September 2022}}</ref>  The transmitter emits a narrow beam of radio waves which is swept around the surrounding space.  When the beam strikes a target object, radio waves are reflected back to the receiver.  The direction of the beam reveals the object's location.  Since radio waves travel at a constant speed close to the [[speed of light]], by measuring the brief time delay between the outgoing pulse and the received "echo", the range to the target can be calculated.  The targets are often displayed graphically on a map display called a ''radar screen''.  [[Doppler radar]] can measure a moving object's velocity, by measuring the change in frequency of the return radio waves due to the [[Doppler effect]].<ref>{{Cite web|url=https://www.noaa.gov/jetstream|title=JetStream|website=www.noaa.gov}}</ref>

Radar sets mainly use high frequencies in the [[microwave]] bands, because these frequencies create strong reflections from objects the size of vehicles and can be focused into narrow beams with compact antennas.<ref name="Skolnik" />  [[Parabolic antenna|Parabolic (dish) antennas]] are widely used.  In most radars the transmitting antenna also serves as the receiving antenna; this is called a ''[[monostatic radar]]''.  A radar which uses separate transmitting and receiving antennas is called a ''[[bistatic radar]]''.<ref Name="Chernyak">{{cite book|title=Fundamentals of multisite radar systems: multistatic radars and multiradar systems|last= Chernyak|first= Victor S.|year= 1998|publisher= CRC Press|isbn= 9056991655|url=https://books.google.com/books?id=0bEU2oBzMJEC&q=Bistatic+&pg=PA149|pages=3, 149}}</ref>

[[Image:ASR-9 Radar Antenna.jpg|thumb|upright=0.8|ASR-8 airport surveillance radar antenna.  It rotates once every 4.8 seconds.  The rectangular antenna on top is the secondary radar.]]
* [[Airport surveillance radar]] – In [[aviation]], radar is the main tool of [[air traffic control]].  A rotating dish antenna sweeps a vertical fan-shaped beam of microwaves around the airspace and the radar set shows the location of aircraft as "blips" of light on a display called a radar screen. Airport radar operates at 2.7 – 2.9&nbsp;GHz in the microwave [[S band]].  In large airports the radar image is displayed on multiple screens in an operations room called the TRACON ([[Terminal Radar Approach Control]]), where [[air traffic controller]]s direct the aircraft by radio to maintain safe aircraft separation.<ref name="FAA">{{cite web
  | title = Airport Surveillance Radar
  | work = Air traffic control, technology
  | publisher = US Federal Aviation Administration website
  | date = 2020
  | url = https://www.faa.gov/air_traffic/technology/asr-11/
  | format =
  | doi =
  | accessdate = 3 September 2022}}</ref>
** [[Secondary surveillance radar]] – Aircraft carry [[transponder (aeronautics)|radar transponders]], transceivers which when triggered by the incoming radar signal transmit a return microwave signal.  This causes the aircraft to show up more strongly on the radar screen.  The radar which triggers the transponder and receives the return beam, usually mounted on top of the primary radar dish, is called the [[secondary surveillance radar]].  Since radar cannot measure an aircraft's altitude with any accuracy, the transponder also transmits back the aircraft's altitude measured by its [[altimeter]], and an ID number identifying the aircraft, which is displayed on the radar screen.<ref name="Binns">{{cite book |last1=Binns |first1=Chris |title=Aircraft Systems: Instruments, Communications, Navigation, and Control |year= 2018 |url=https://books.google.com/books?id=N5lxDwAAQBAJ&q=Secondary+surveillance+radar |publisher=Wiley |isbn=978-1119259541 |access-date=11 September 2022}}</ref>
* [[Electronic countermeasures]] (ECM) – Military defensive electronic systems designed to degrade enemy radar effectiveness, or deceive it with false information, to prevent enemies from locating local forces.  It often consists of powerful microwave transmitters that can mimic enemy radar signals to create false target indications on the enemy radar screens.<ref name="Artech-Horizon House">{{cite book |title=International Electronic Countermeasures Handbook | year=2004 |url=https://books.google.com/books?id=Y8kePYFK1L8C&q=Electronic+countermeasures |publisher=Artech/Horizon House | isbn=978-1580538985 |access-date=11 September 2022}}</ref>

[[File:Rotating marine radar - rotating waveguide antenna.gif|thumb|upright=0.7|Rotating marine radar antenna on a ship]]
* [[Marine radar]] – an [[S band|S]] or [[X band]] radar on ships used to detect nearby ships and obstructions like bridges.<ref name="Bhattacharjee">{{cite web
  | last = Bhattacharjee
  | first = Shilavadra
  | title = Marine Radars and Their Use in the Shipping Industry
  | publisher = Marine Insight website
  | date = 2021
  | url = https://www.marineinsight.com/marine-navigation/marine-radars-and-their-use-in-the-shipping-industry/
  | format =
  | doi =
  | accessdate = 3 September 2022}}</ref>  A rotating antenna sweeps a vertical fan-shaped beam of microwaves around the water surface surrounding the craft out to the horizon. 
* [[Weather radar]] – A [[Doppler radar]] which maps weather precipitation intensities and wind speeds with the echoes returned from raindrops and their [[radial velocity]] by their [[Doppler shift]].<ref name="NWS">{{cite web
  | title = Using and Understanding Doppler Radar
  | work = US National Weather Service website
  | publisher = US National Weather Service, NOAA
  | date = 2020
  | url = https://www.weather.gov/mkx/using-radar
  | format =
  | doi =
  | accessdate = 3 September 2022}}</ref>
* [[Phased-array radar]] – a radar set that uses a [[phased array]], a computer-controlled antenna that can steer the radar beam quickly to point in different directions without moving the antenna.  Phased-array radars were developed by the military to track fast-moving missiles and aircraft.  They are widely used in military equipment and are now spreading to civilian applications.<ref name="Fenn">{{cite book |last1=Fenn |first1=Alan J. |title=Adaptive Antennas and Phased Arrays for Radar and Communications |year=2007 |url=https://books.google.com/books?id=k1MmAgAAQBAJ&q=Phased-array+radar |publisher=Artech House |isbn=978-1596932739 |access-date=11 September 2022}}</ref>
* [[Synthetic aperture radar]] (SAR) – a specialized airborne radar set that produces a high-resolution map of ground terrain.  The radar is mounted on an aircraft or spacecraft and the radar antenna radiates a beam of radio waves sideways at right angles to the direction of motion, toward the ground.  In processing the return radar signal, the motion of the vehicle is used to simulate a large antenna, giving the radar a higher resolution.<ref name="Teeuw">{{cite book |last1=Teeuw |first1=R.M. |title=Mapping Hazardous Terrain Using Remote Sensing |year=2007 |url=https://books.google.com/books?id=Zy-94WdP3dgC&dq=Synthetic+aperture+radar+ground+terrain&pg=PA45 |publisher=Geological Society of London |isbn=978-1862392298 |access-date=11 September 2022}}</ref>
* [[Ground-penetrating radar]] – a specialized radar instrument that is rolled along the ground surface in a cart and transmits a beam of radio waves into the ground, producing an image of subsurface objects.  Frequencies from 100&nbsp;MHz to a few GHz are used.  Since radio waves cannot penetrate very far into earth, the depth of GPR is limited to about 50 feet.<ref name="Jol">{{cite book |last1=Jol |first1=Harry M. |title=Ground Penetrating Radar Theory and Applications |year= 2008 |url=https://books.google.com/books?id=y__uIi-5RvgC&q=Ground+Penetrating+Radar+Theory+and+Applications |publisher=Elsevier |isbn=978-0080951843 |access-date=10 September 2022}}</ref>
* [[Collision avoidance system]] – a short range radar or [[LIDAR]] system on an automobile or vehicle that detects if the vehicle is about to collide with an object and applies the brakes to prevent the collision.<ref name="Grosch">{{cite journal |last1=Grosch |first1=Theodore O. |editor-first1=Jacques G. |editor-last1=Verly |title=Radar sensors for automotive collision warning and avoidance |journal=Synthetic Vision for Vehicle Guidance and Control |date=30 June 1995 |volume=2463 |pages=239–247 |url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/2463/0000/Radar-sensors-for-automotive-collision-warning-and-avoidance/10.1117/12.212749.short?SSO=1 |publisher=Society of Photo-Optical Instrumentation Engineers |doi=10.1117/12.212749 |bibcode=1995SPIE.2463..239G |s2cid=110665898 |access-date=10 September 2022}}</ref>
* [[Fuze#Proximity fuze|Radar fuze]] – a [[detonator]] for an [[aerial bomb]] which uses a radar altimeter to measure the height of the bomb above the ground as it falls and detonates it at a certain altitude.<ref name="Brodie-Brodie">{{cite book |last1=Brodie |first1=Bernard |last2=Brodie |first2=Fawn McKay |title=From Crossbow to H-bomb |year=1973 |url=https://books.google.com/books?id=mAsrwMrfaoQC&dq=radar+fuse+bomb&pg=PA213 |publisher=Indiana University Press |isbn=0253201616 |access-date=10 September 2022}}</ref>

===Radiolocation===
{{main|Radiolocation}}
Radiolocation is a generic term covering a variety of techniques that use radio waves to find the location of objects, or for navigation.<ref name="Sharp-Yu">{{cite book |last1=Sharp |first1=Ian |last2=Yu |first2=Kegen |title=Wireless Positioning: Principles and Practice, Navigation: Science and Technology |year= 2018 |url=https://books.google.com/books?id=pHFdDwAAQBAJ&q=Wireless+Positioning%3A+Principles+and+Practice |publisher=Springer |isbn=978-9811087912 |access-date=10 September 2022}}</ref>
[[File:HA0478-006_(6011470974).jpg|thumb|left|An early [[iPhone]] with its GPS navigation app in use.]]
* [[GNSS|Global Navigation Satellite System]] (GNSS) or ''satnav system'' – A system of satellites which allows geographical location on Earth ([[latitude]], [[longitude]], and altitude/elevation) to be determined to high precision (within a few metres) by small portable navigation instruments, by timing the arrival of radio signals from the satellites.  These are the most widely used navigation systems today.  The main satellite navigation systems are the US [[Global Positioning System]] (GPS), [[Russia]]'s [[GLONASS]], [[China]]'s [[BeiDou Navigation Satellite System]] (BDS) and the [[European Union]]'s [[Galileo (satellite navigation)|Galileo]].<ref name="Teunissen-Montenbruck">{{cite book |last1=Teunissen |first1=Peter |last2=Montenbruck |first2=Oliver |title=Springer Handbook of Global Navigation Satellite Systems |year= 2017 |url=https://books.google.com/books?id=93goDwAAQBAJ&q=Global+Navigation+Satellite+System |publisher=Springer |isbn=978-3319429281 |access-date=10 September 2022}}</ref>
[[Image:Paris-PorteMolitor-GPS.jpg|thumb|A ''[[personal navigation assistant]]'' by [[Garmin]], which uses GPS to give driving directions to a destination.]]
** [[Global Positioning System]] (GPS) – The most widely used satellite navigation system, maintained by the US Air Force, which uses a constellation of 31 satellites in [[low Earth orbit]].  The orbits of the satellites are distributed so at any time at least four satellites are above the horizon over each point on Earth.  Each satellite has an onboard [[atomic clock]] and transmits a continuous radio signal containing a precise time signal as well as its current position. Two frequencies are used, 1.2276 and 1.57542&nbsp;GHz. Since the velocity of radio waves is virtually constant, the delay of the radio signal from a satellite is proportional to the distance of the receiver from the satellite.  By receiving the signals from at least four satellites a [[GPS receiver]] can calculate its position on Earth by comparing the arrival time of the radio signals.  Since each satellite's position is known precisely at any given time, from the delay the position of the receiver can be calculated by a microprocessor in the receiver. The position can be displayed as latitude and longitude, or as a marker on an electronic map.  GPS receivers are incorporated in almost all cellphones and in vehicles such as automobiles, aircraft, and ships, and are used to guide [[Unmanned aerial vehicle|drones]], [[missile]]s, [[cruise missile]]s, and even [[artillery shell]]s to their target, and handheld GPS receivers are produced for hikers and the military.<ref name="El-Rabbany">{{cite book |last1=El-Rabbany |first1=Ahmed |title=Introduction to GPS: The Global Positioning System |year=2002 |url=https://books.google.com/books?id=U2JmghrrB8cC&q=how+GPS+works |publisher=Artech House |isbn=978-1580531832 |access-date=10 September 2022}}</ref><ref name="Kiland-Silverstein Gray">{{cite book |last1=Kiland |first1=Taylor Baldwin |last2=Silverstein Gray |first2=Judy |title=The Military GPS: Cutting Edge Global Positioning System |date=15 July 2016 |url=https://books.google.com/books?id=uAhiDwAAQBAJ&q=how+GPS+works+military |publisher=Enslow Publishing |isbn=978-0766075184 |access-date=10 September 2022}}</ref>
* [[Radio beacon]] – a fixed location terrestrial radio transmitter which transmits a continuous radio signal used by aircraft and ships for [[navigation]].  The locations of beacons are plotted on navigational maps used by aircraft and ships.<ref name="Deltour">{{cite web |last1=Deltour |first1=B.V. |title=A Guide To Nav-Com Equipment |date=August 1960 |url=https://books.google.com/books?id=QETktYruMwsC&dq=radio+beacon+continuous+radio+signal&pg=PA44 |publisher=Flying Magazine Aug 1960 |access-date=10 September 2022}}</ref>
** [[VHF omnidirectional range]] (VOR) – a worldwide aircraft [[radio navigation]] system consisting of fixed ground [[radio beacon]]s transmitting between 108.00 and 117.95&nbsp;MHz in the [[very high frequency]] (VHF) band.  An automated navigational instrument on the aircraft displays a [[bearing (navigation)|bearing]] to a nearby VOR transmitter. A VOR beacon transmits two signals simultaneously on different frequencies.  A [[directional antenna]] transmits a beam of radio waves that rotates like a lighthouse at a fixed rate, 30 times per second. When the directional beam is facing north, an [[omnidirectional antenna]] transmits a pulse. By measuring the difference in [[phase (waves)|phase]] of these two signals, an aircraft can determine its [[bearing (navigation)|bearing]] (or "radial") from the station accurately. By taking a bearing on two VOR beacons an aircraft can determine its position (called a "fix") to an accuracy of about {{convert|90|m|ft|sigfig=2}}.  Most VOR beacons also have a distance measuring capability, called [[distance measuring equipment]] (DME); these are called VOR/DME's.  The aircraft transmits a radio signal to the VOR/DME beacon and a [[transponder]] transmits a return signal.  From the [[propagation delay]] between the transmitted and received signal the aircraft can calculate its distance from the beacon.  This allows an aircraft to determine its location "fix" from only one VOR beacon.  Since line-of-sight VHF frequencies are used VOR beacons have a range of about 200 miles for aircraft at cruising altitude. [[TACAN]] is a similar military radio beacon system which transmits in 962–1213&nbsp;MHz, and a combined VOR and TACAN beacon is called a [[VORTAC]]. The number of VOR beacons is declining as aviation switches to the [[RNAV]] system that relies on [[Global Positioning System]] satellite navigation.<ref name="USDOD">{{cite web |title=2008 Federal Radionavigation Plan | year=2009 |url=https://books.google.com/books?id=x5sRi3P-B-EC&q=2008+Federal+Radionavigation+Plan |publisher=U.S. Department of Defense |access-date=10 September 2022}}</ref><ref name="Martin">{{cite web |last1=Martin |first1=Swayne |title=How A VOR Works |url=https://www.boldmethod.com/learn-to-fly/navigation/how-a-vor-works/ |website=boldmethod.com |publisher=Boldmethod -Digital Aviation Content |access-date=10 September 2022}}</ref>
** [[Non-directional beacon]] (NDB) – Legacy fixed radio beacons used before the VOR system that transmit a simple signal in all directions for aircraft or ships to use for [[radio direction finding]].  Aircraft use [[automatic direction finder]] (ADF) receivers which use a [[directional antenna]] to determine the [[bearing (navigation)|bearing]] to the beacon.  By taking bearings on two beacons they can determine their position.  NDBs use frequencies between 190 and 1750&nbsp;kHz in the [[Low frequency|LF]] and [[Medium frequency|MF]] bands which propagate beyond the horizon as [[ground wave]]s or [[skywave]]s much farther than VOR beacons.  They transmit a [[callsign]] consisting of one to 3 [[Morse code]] letters as an identifier.<ref name="SI">{{cite web |title=Non-Directional Beacon (NDB) |url=https://www.systemsinterface.com/products/navaids/ndb/ |website=systemsinterface.com |publisher=Systems Interface |access-date=10 September 2022}}</ref>
[[Image:EPIRB (1).jpg|thumb|upright=0.7|EPIRB emergency locator beacon on a ship]]
* [[Emergency locator beacon]] – a portable battery powered [[radio transmitter]] used in emergencies to locate airplanes, vessels, and persons in distress and in need of immediate rescue.  Various types of emergency locator beacons are carried by aircraft, ships, vehicles, hikers and cross-country skiers.  In the event of an emergency, such as the aircraft crashing, the ship sinking, or a hiker becoming lost, the transmitter is deployed and begins to transmit a continuous radio signal, which is used by [[search and rescue]] teams to quickly find the emergency and render aid.  The latest generation [[Emergency Position Indicating Rescue Beacon]]s (EPIRBs) contain a [[GPS receiver]], and broadcast to rescue teams their exact location within 20 meters.<ref name="CBC">{{cite web |title=How does an emergency beacon work? |url=https://www.cbc.ca/news/science/how-does-an-emergency-beacon-work-1.1327070 |website=cbc.ca |publisher=CBC News |access-date=10 September 2022}}</ref>
** [[International Cospas-Sarsat Programme|Cospas-Sarsat]] – an international humanitarian consortium of governmental and private agencies which acts as a dispatcher for [[search and rescue]] operations.  It operates a network of about 47 [[satellite]]s carrying radio receivers, which detect distress signals from [[emergency locator beacon]]s anywhere on Earth transmitting on the international Cospas distress frequency of 406&nbsp;MHz.  The satellites calculate the geographic location of the beacon within 2&nbsp;km by measuring the [[Doppler shift|Doppler frequency shift]] of the radio waves due to the relative motion of the transmitter and the satellite, and quickly transmit the information to the appropriate local [[first responder]] organizations, which perform the [[search and rescue]].<ref name="cospas-sarsat">{{cite web |title=What is a Cospas-Sarsat Beacon? |url=http://www.cospas-sarsat.int/en/18-frontpage-articles/603-what-is-a-cospas-sarsat-beacon |website=cospas-sarsat.int |publisher=International Cospas-Sarsat Programme |access-date=10 September 2022}}</ref><ref name="NASA1985">{{cite web |title=Scientific and Technical Aerospace Reports, Volume 23, Issue 20 | year=1985 |url=https://books.google.com/books?id=wg50RfqjLWMC&dq=cospas-sarsat.int+2km&pg=PA3400 |publisher=NASA, Office of Scientific and Technical Information |access-date=10 September 2022}}</ref>
[[File:Tracking Mountain Lions.jpg|thumb|upright=0.7|Wildlife officer tracking radio-tagged mountain lion]]
* [[Radio direction finding]] (RDF) – this is a general technique, used since the early 1900s, of using specialized radio receivers with [[directional antenna]]s (RDF receivers) to determine the exact [[bearing (navigation)|bearing]] of a radio signal, to determine the location of the transmitter.  The location of a terrestrial transmitter can be determined by simple [[triangulation]] from bearings taken by two RDF stations separated geographically, as the point where the two bearing lines cross, this is called a "fix".  Military forces use RDF to locate enemy forces by their tactical radio transmissions, [[counterintelligence]] services use it to locate clandestine transmitters used by [[espionage|espionage agent]]s, and governments use it to locate unlicensed transmitters or interference sources.  Older RDF receivers used rotatable [[loop antenna]]s, the antenna is rotated until the radio signal strength is weakest, indicating the transmitter is in one of the antenna's two [[null (radio)|null]]s.  The nulls are used since they are sharper than the antenna's [[main lobe|lobes]] (maxima).  More modern receivers use [[phased array]] antennas which have a much greater angular resolution.<ref name="defenceweb">{{cite web |title=An Introduction to Radio Direction Finding |url=https://www.defenceweb.co.za/virtual-press-offices/111485/ |website=defenceweb.co.za |date=8 January 2021 |publisher=defenceWeb |access-date=10 September 2022}}</ref><ref name="Moell-Curlee">{{cite book |last1=Moell |first1=Joseph D. |last2=Curlee |first2=Thomas N. |title=Transmitter Hunting: Radio Direction Finding Simplified |year=1987 |url=https://books.google.com/books?id=RfzF2-fHJ6MC&q=Radio+direction+finding |publisher=McGraw Hill Professional |isbn=978-0830627011 |access-date=10 September 2022}}</ref>
** [[Animal migration tracking]] – a widely used technique in [[wildlife biology]], [[conservation biology]], and [[wildlife management]] in which small battery-powered radio transmitters are attached to [[wild animal]]s so their movements can be tracked with a directional [[radio direction finding|RDF]] receiver. Sometimes the transmitter is implanted in the animal.  The VHF band is typically used since antennas in this band are fairly compact.  The receiver has a [[directional antenna]] (typically a small [[Yagi antenna|Yagi]]) which is rotated until the received signal is strongest; at this point the antenna is pointing in the direction of the animal.  Sophisticated systems used in recent years use satellites to track the animal, or [[geolocation]] tags with [[global positioning system|GPS]] receivers which record and transmit a log of the animal's location.<ref name="Smithsonian">{{cite web |title=Radio telemetry |url=http://migratoryconnectivityproject.org/vhf-radios/ |publisher=Migratory Connectivity Project, Smithsonian Migratory Bird Center |access-date=10 September 2022}}</ref>

===Remote control===
{{main|Radio control}}
[[Image:MQ-1 Predator unmanned aircraft.jpg|thumb|upright=0.8|US Air Force [[MQ-1 Predator]] drone flown remotely by a pilot on the ground]]
Radio remote control is the use of electronic [[control signal]]s sent by radio waves from a transmitter to control the actions of a device at a remote location.  Remote control systems may also include telemetry channels in the other direction, used to transmit real-time information on the state of the device back to the control station. Uncrewed [[spacecraft]] are an example of remote-controlled machines, controlled by commands transmitted by [[satellite ground station]]s.  Most handheld [[remote control]]s used to control [[consumer electronics]] products like televisions or DVD players actually operate by [[infrared light]] rather than radio waves, so are not examples of radio remote control.  A security concern with remote control systems is [[spoofing attack|spoofing]], in which an unauthorized person transmits an imitation of the control signal to take control of the device.<ref name="Layton">{{cite web |last1=Layton |first1=Julia |title=How Remote Controls Work |date=10 November 2005 |url=https://electronics.howstuffworks.com/remote-control3.htm |publisher=HowStuff Works |access-date=10 September 2022}}</ref>  Examples of radio remote control:
* [[Unmanned aerial vehicle]] (UAV, drone) – A drone is an aircraft without an onboard pilot, flown by remote control by a pilot in another location, usually in a piloting station on the ground.  They are used by the military for reconnaissance and ground attack, and more recently by the civilian world for news reporting and [[aerial photography]].  The pilot uses aircraft controls like a joystick or steering wheel, which create control signals which are transmitted to the drone by radio to control the flight surfaces and engine.  A telemetry system transmits back a video image from a camera in the drone to allow the pilot to see where the aircraft is going, and data from a [[GPS receiver]] giving the real-time position of the aircraft.  UAVs have sophisticated onboard [[automatic pilot]] systems that maintain stable flight and only require manual control to change directions.<ref name="Sadraey">{{cite book |last1=Sadraey |first1=Mohammad H. |title=Design of Unmanned Aerial Systems |year= 2020 |url=https://books.google.com/books?id=zlbSDwAAQBAJ&dq=UAV+telemetry&pg=PA535 |publisher=Wiley |isbn=978-1119508694 |access-date=10 September 2022}}</ref>

[[Image:Automobile remote keyless entry transmitter.jpg|thumb|upright=0.5|Remote keyless entry fob for a car]]
* [[Keyless entry system]] – a short-range handheld battery powered [[key fob]] transmitter, included with most modern cars, which can lock and unlock the doors of a vehicle from outside, eliminating the need to use a key.  When a button is pressed, the transmitter sends a coded radio signal to a receiver in the vehicle, operating the locks.  The fob must be close to the vehicle, typically within 5 to 20 meters.  North America and Japan use a frequency of 315&nbsp;MHz, while Europe uses 433.92 and 868&nbsp;MHz.  Some models can also remotely start the engine, to warm up the car.  A security concern with all keyless entry systems is a [[replay attack]], in which a thief uses a special receiver ("code grabber") to record the radio signal during opening, which can later be replayed to open the door.  To prevent this, keyless systems use a [[rolling code]] system in which a [[pseudorandom number generator]] in the remote control generates a different random key each time it is used.  To prevent thieves from simulating the pseudorandom generator to calculate the next key, the radio signal is also [[encryption|encrypted]].<ref name="Smith">{{cite book |last1=Smith |first1=Craig |title=The Car Hacker's Handbook: A Guide for the Penetration Tester |date=2016 |url=https://books.google.com/books?id=vAYvDwAAQBAJ&dq=keyless+entry+system+315+MHz&pg=PA216 |publisher=No Starch Press |isbn=978-1593277703 |access-date=10 September 2022}}</ref>
** [[Garage door opener]] – a short-range handheld transmitter which can open or close a building's electrically operated [[garage door]] from outside, so the owner can open the door upon arrival, and close it after departure.  When a button is pressed the control transmits a coded [[Frequency-shift keying|FSK]] radio signal to a receiver in the opener, raising or lowering the door.  Modern openers use 310, 315 or 390&nbsp;MHz.  To prevent a thief using a [[replay attack]], modern openers use a [[rolling code]] system.<ref name="Pinkerton">{{cite book |last1=Pinkerton |first1=Alasdair |title=Radio: Making Waves in Sound |date=15 June 2019 |url=https://books.google.com/books?id=cRieDwAAQBAJ&dq=Garage+door+opener++390+mhz&pg=PT24 |publisher=Reaktion Books |isbn=978-1789140996 |access-date=9 September 2022}}</ref><ref name="Biffl-Eckhart-Luder-Weippl">{{cite book |last1=Biffl |first1=Stefan |last2=Eckhart |first2=Matthias |last3=Lüder |first3=Arndt |last4=Weippl |first4=Edgar |title=Security and Quality in Cyber-Physical Systems Engineering |year=2019 |url=https://books.google.com/books?id=c1S9DwAAQBAJ&dq=Garage+door+opener++replay+attack&pg=PA435 |publisher=Springer Nature |isbn=978-3030253127 |access-date=9 September 2022}}</ref>

[[Image:Md4-200.jpg|thumb|upright=0.7|[[Quadcopter]], a popular remote-controlled toy]]
* [[Radio-controlled model]]s – a popular hobby is playing with radio-controlled model boats, cars, airplanes, and helicopters ([[quadcopter]]s) which are controlled by radio signals from a handheld console with a [[joystick]].  Most recent transmitters use the 2.4&nbsp;GHz [[ISM band]] with multiple control channels modulated with [[pulse-width modulation|PWM]], [[PCM]] or FSK.<ref name="Boukerche">{{cite book |last1=Boukerche |first1=Azzedine |title=Algorithms and Protocols for Wireless and Mobile Ad Hoc Networks |year= 2008 |url=https://books.google.com/books?id=bI05X9-4tw8C&dq=radio+controlled+models+2.4+ghz+ism&pg=PA46 |publisher=Wiley |isbn=978-0470396377 |access-date=9 September 2022}}</ref>
* [[Doorbell|Wireless doorbell]] – A residential [[doorbell]] that uses wireless technology to eliminate the need to run wires through the building walls.  It consists of a doorbell button beside the door containing a small battery powered transmitter.  When the doorbell is pressed it sends a signal to a receiver inside the house with a speaker that sounds chimes to indicate someone is at the door.  They usually use the 2.4&nbsp;GHz ISM band.  The frequency channel used can usually be changed by the owner in case another nearby doorbell is using the same channel.<ref name="Wonning">{{cite web |last1=Wonning |first1=Paul R. |title=A Guide to the Home Electric System |date=12 May 2021 |url=https://books.google.com/books?id=INctEAAAQBAJ&dq=wireless+doorbell&pg=PT34 |publisher=Mossy Feet Books |access-date=9 September 2022}}</ref><ref name="Chatterjee-Kumar-Jain-Rathore">{{cite book |last1=Chatterjee |first1=Jyotir Moy |last2=Kumar |first2=Abhishek |last3=Jain |first3=Vishal |last4=Rathore |first4=Pramod Singh |title=Internet of Things and Machine Learning in Agriculture: Technological Impacts and Challenges |year= 2021 |url=https://books.google.com/books?id=-9MWEAAAQBAJ&dq=wireless+doorbells+2.4+ghz+ism&pg=PR7 |publisher=Walter de Gruyter GmbH & Co KG |isbn=978-3110691283 |access-date=9 September 2022}}</ref>

===Jamming===
[[Radio jamming]] is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals.  Jamming devices are called "signal suppressors" or "interference generators" or just jammers.<ref>{{Cite web|url=https://ajax.systems/blog/what-is-jamming/|title=What jamming of a wireless security system is and how to resist it {{!}} Ajax Systems Blog|website=Ajax Systems|language=en|access-date=18 January 2020}}</ref>

During wartime, militaries use jamming to interfere with enemies' tactical radio communication. Since radio waves can pass beyond national borders, some [[totalitarian]] countries which practice [[censorship]] use jamming to prevent their citizens from listening to broadcasts from radio stations in other countries. Jamming is usually accomplished by a powerful transmitter which generates noise on the same frequency as the target transmitter.<ref>{{Cite report|title=FM 24-33 — Communications Techniques: Electronic Counter-Countermeasures|date=July 1990|publisher=Department of the Army|chapter=Remedial Electronic Counter-Countermeasures Techniques|chapter-url=https://irp.fas.org/doddir/army/fm24-33/fm243_4.htm#REF16h2}}</ref><ref>{{Cite journal |last=Varis |first=Tapio |date=1970 |title=The Control of Information by Jamming Radio Broadcasts |url=https://www.jstor.org/stable/45083158 |journal=Cooperation and Conflict |volume=5 |issue=3 |pages=168–184 |doi=10.1177/001083677000500303 |issn=0010-8367 |jstor=45083158|s2cid=145418504 }}</ref>

US Federal law prohibits the nonmilitary operation or sale of any type of jamming devices, including ones that interfere with GPS, cellular, Wi-Fi and police radars.<ref>{{Cite web|url=https://www.fcc.gov/general/jammer-enforcement|title=Jammer Enforcement|date=3 March 2011|website=Federal Communications Commission|language=en|access-date=18 January 2020}}</ref>

===Scientific research===
* [[Radio astronomy]] is the scientific study of radio waves emitted by astronomical objects.  Radio astronomers use [[radio telescope]]s, large radio antennas and receivers, to receive and study the radio waves from [[astronomical radio source]]s.  Since astronomical radio sources are so far away, the radio waves from them are extremely weak, requiring extremely sensitive receivers, and radio telescopes are the most sensitive radio receivers in existence.  They use large [[parabolic antenna|parabolic (dish) antennas]] up to {{convert|500|meters|feet|sigfig=1}} in diameter to collect enough radio wave energy to study.  The [[RF front end]] electronics of the receiver is often cooled by [[liquid nitrogen]] to reduce [[thermal noise]].  Multiple antennas are often linked together in arrays which function as a single antenna, to increase collecting power.  In [[VLBI|Very Long Baseline Interferometry]] (VLBI) radio telescopes on different continents are linked, which can achieve the resolution of an antenna thousands of miles in diameter.<ref name="Yeap-Hirisawa">{{cite book |last1=Yeap |first1=Kim Ho |last2=Hirasawa |first2=Kazuhiro |title=Analyzing the Physics of Radio Telescopes and Radio Astronomy |year= 2020 |url=https://books.google.com/books?id=VwfYDwAAQBAJ&q=Analyzing+the+Physics+of+Radio+Telescopes+and+Radio+Astronomy |publisher=IG Global |isbn=978-1799823834 |access-date=9 September 2022}}</ref><ref name="Joardar-Clayomb">{{cite book |last1=Joardar |first1=Shubhendu |last2=Claycomb |first2=J. R. |title=Radio Astronomy: An Introduction |year= 2015 |url=https://books.google.com/books?id=cjW_DgAAQBAJ&q=Radio+Astronomy%3A+An+Introduction |publisher=Mercury Learning and Information|isbn=978-1937585624 }}</ref>
* [[Remote sensing]] – in radio, remote sensing is the reception of electromagnetic waves radiated by natural objects or the atmosphere for scientific research.  All warm objects emit [[microwave]]s and the spectrum emitted can be used to determine temperature. [[Microwave radiometer]]s are used in [[meteorology]] and [[earth science]]s to determine temperature of the atmosphere and earth surface, as well as chemical reactions in the atmosphere.<ref name="Chapman-Gasparovic">{{cite book |last1=Chapman |first1=Rick |last2=Gasparovic |first2=Richard |title=Remote Sensing Physics: An Introduction to Observing Earth from Space |year=2022 |url=https://books.google.com/books?id=B8Z6EAAAQBAJ&q=Remote+Sensing+Physics%3A+An+Introduction+to+Observing+Earth+from+Space |publisher=Wiley |isbn=978-1119669074 |access-date=9 September 2022}}</ref><ref name="Pampaloni-Paloscia">{{cite book |last1=Pampaloni |first1=Paulo |last2=Paloscia |first2=S. |title=Microwave Radiometry and Remote Sensing of the Earth's Surface and Atmosphere |year=2000 |isbn=9067643181 |url=https://books.google.com/books?id=IwIIhhMxcJwC&q=Remote+Sensing+radiometer |access-date=9 September 2022}}</ref>

==See also==
{{Portal|Radio}}
* [[Outline of radio]]
* [[Electromagnetic radiation and health]]
* {{annotated link|List of radios}}
* [[Radio quiet zone]]

==References==
{{Reflist}}

==General references==
* [https://books.google.com/books?id=vaUGlb_Gd18C&q=radio+basic+text Basic Radio Principles and Technology – Elsevier Science]
* [https://books.google.com/books?id=203BJeFu5qQC&q=cover+of+The+Electronics+of+Radio+The+Electronics+of+Radio+by+David+Rutledge The Electronics of Radio – Cambridge University Press]
* [https://books.google.com/books?id=C8fxDAAAQBAJ&q=radio+textbook Radio Systems Engineering – Cambridge University Press]
* [https://books.google.com/books?id=m5DIroWLw2EC&q=Radio-Electronic+Transmission+Fundamentals Radio-Electronic Transmission Fundamentals – SciTech Publishing]
* [https://books.google.com/books?id=l-AgBQAAQBAJ&dq=radio++textbook+many+kinds+of+modulation&pg=PA183 Analog Electronics, Analog Circuitry Explained – Elsevier Science]

==External links==
{{Commons and category|Radio|Radio}}
{{Wikiquote}}
* {{Merriam-Webster|Radio}}

{{Morse code}}
{{Telecommunications}}
{{Audio broadcasting}}
{{Radio spectrum}}
{{Media culture}}

{{Authority control}}

[[Category:Radio| ]]
[[Category:British inventions]]
[[Category:Italian inventions]]
[[Category:Media formats]]
[[Category:Promotion and marketing communications]]
[[Category:Infrastructure]]
[[Category:Communication]]