This article is about the technology as a whole. For the receiver commonly referred to as a "radio", seeRadio receiver. For other uses, seeRadio (disambiguation).
Inradio communication, used in radio andtelevision broadcasting, cell phones,two-way radios,wireless networking, andsatellite communication, among numerous other uses, radio waves are used to carry information across space from a transmitter to a receiver, bymodulating 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 to a receiver that is typically colocated with the transmitter. In radio navigation systems such asGPS andVOR, a mobile navigation instrument receives radio signals from multiplenavigational 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 wirelessradio remote control devices likedrones,garage door openers, andkeyless entry systems, radio signals transmitted from a controller device control the actions of a remote device.
The existence of radio waves was first proven by German physicistHeinrich Hertz on 11 November 1886.[4] In the mid-1890s, building on techniques physicists were using to study electromagnetic waves, Italian physicistGuglielmo Marconi developed the first apparatus for long-distance radio communication,[5] sending a wirelessMorse Code message to a recipient over a kilometer away in 1895,[6] and the first transatlantic signal on 12 December 1901.[7] The first commercial radio broadcast was transmitted on 2 November 1920, when the live returns of the1920 United States presidential election were broadcast by Westinghouse Electric and Manufacturing Company in Pittsburgh, under the call signKDKA.[8]
The wordradio is derived from the Latin wordradius, meaning "spoke of a wheel, beam of light, ray." It was first applied to communications in 1881 when, at the suggestion of French scientistErnest Mercadier [fr],Alexander Graham Bell adoptedradiophone (meaning "radiated sound") as an alternate name for hisphotophone optical transmission system.[9][10]
Following Hertz's discovery of the existence ofradio waves in 1886, the termHertzian waves was initially used for this radiation.[11] The first practical radio communication systems, developed byMarconi in 1894–1895, transmittedtelegraph signals by radio waves,[4] so radio communication was first calledwireless telegraphy. Up until about 1910 the termwireless telegraphy also included a variety of other experimental systems for transmitting telegraph signals without wires, includingelectrostatic induction,electromagnetic induction andaquatic and earth conduction, so there was a need for a more precise term referring exclusively to electromagnetic radiation.[12][13]
The French physicistÉdouard Branly, who in 1890 developed the radio wave detectingcoherer, called it in French aradio-conducteur.[14][15] Theradio- prefix was later used to form additional descriptive compound and hyphenated words, especially in Europe. For example, in early 1898 the British publicationThe Practical Engineer included a reference tothe radiotelegraph andradiotelegraphy.[14][16]
The use ofradio 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."[14][17] 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'".[14]
The switch toradio in place ofwireless 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 1907Electrical World about the need for legal restrictions warned that "Radio chaos will certainly be the result until such stringent regulation is enforced."[18] The United States Navy would also play a role. Although its translation of the 1906 Berlin Convention used the termswireless telegraph andwireless telegram, by 1912 it began to promote the use ofradio instead. The term started to become preferred by the general public in the 1920s with the introduction of broadcasting.
During radio's first two decades, called theradiotelegraphy era, the primitive radio transmitters could only transmit pulses of radio waves, not the continuous waves which were needed for audiomodulation, 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 transmitters to exchangetelegram traffic between continents and communicate with their colonies and naval fleets. DuringWorld War I the development ofcontinuous wave radio transmitters,rectifying electrolytic, and crystalradio receiver detectors enabledamplitude modulation (AM)radiotelephony to be achieved byReginald Fessenden and others, allowingaudio 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 signKDKA featuring live coverage of the1920 United States presidential election.[8]
Radio waves travel at thespeed of light in vacuum[29] and at slightly lower velocity in air.[30]
The other types ofelectromagnetic waves besides radio waves,infrared,visible light,ultraviolet,X-rays andgamma rays, can also carry information and be used for communication. The wide use of radio waves for telecommunication is mainly due to their desirablepropagation properties stemming from their longer wavelength.[24] Radio waves have the ability to pass through the atmosphere in any weather, foliage, and at longer wavelengths through most building materials. Bydiffraction, longer wavelengths can bend around obstructions, and unlike other electromagnetic waves they tend to be scattered rather than absorbed by objects larger than their wavelength.
Radio communication. Information such as sound is converted by a transducer such as amicrophone to an electrical signal, which modulates aradio wave produced by thetransmitter. 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 aloudspeaker.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 oftransducer to a time-varyingelectrical signal called the modulation signal.[24][31] The modulation signal may be anaudio signal representing sound from amicrophone, avideo signal representing moving images from avideo camera, or adigital signal consisting of a sequence ofbits representing binary data from a computer. The modulation signal is applied to aradio transmitter. In the transmitter, anelectronic oscillator generates analternating current oscillating at aradio frequency, called thecarrier wave because it serves to generate the radio waves thatcarry the information through the air. The modulation signal is used tomodulate the carrier, varying some aspect of the carrier wave, impressing the information in the modulation signal onto the carrier. Different radio systems use different modulation methods:[32]
Amplitude modulation (AM) – in an AM transmitter, theamplitude (strength) of the radio carrier wave is varied by the modulation signal;[32]: 3
Frequency modulation (FM) – in an FM transmitter, the frequency of the radio carrier wave is varied by the modulation signal;[32]: 33
Frequency-shift keying (FSK) – used in wireless digital devices to transmitdigital signals, the frequency of the carrier wave is shifted between frequencies.[32]: 58
Orthogonal frequency-division multiplexing (OFDM) – a family ofdigital modulation methods widely used in high-bandwidth systems such asWi-Fi networks, cellphones,digital television broadcasting, anddigital audio broadcasting (DAB) to transmit digital data using a minimum ofradio spectrum bandwidth. It has higherspectral efficiency and more resistance tofading 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 incomingbitstream 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.[33]
Many other types of modulation are also used. In some types, the carrier wave is suppressed, and only one or both modulationsidebands are transmitted.[34]
The modulated carrier isamplified in the transmitter and applied to a transmittingantenna which radiates the energy as radio waves. The radio waves carry the information to the receiver location.[35] At the receiver, the radio wave induces a tiny oscillatingvoltage in the receiving antenna – a weaker replica of the current in the transmitting antenna.[24][31] This voltage is applied to theradio receiver, whichamplifies the weak radio signal so it is stronger, thendemodulates it, extracting the original modulation signal from the modulated carrier wave. The modulation signal is converted by atransducer back to a human-usable form: an audio signal is converted tosound waves by a loudspeaker or earphones, avideo signal is converted to images by adisplay, while a digital signal is applied to a computer or microprocessor, which interacts with human users.[32]
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 frequency, measured inhertz (Hz),kilohertz (kHz),megahertz (MHz) orgigahertz (GHz). The receiving antenna typically picks up the radio signals of many transmitters. The receiver usestuned circuits to select the radio signal desired out of all the signals picked up by the antenna and reject the others. A tuned circuit acts like aresonator, similar to atuning fork.[31] It has a naturalresonant 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 calledtuning. The oscillating radio signal from the desired station causes the tuned circuit to 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.[36]
Frequency spectrum of a typical modulated AM or FM radio signal. It consists of a componentC at thecarrier wave frequency with themodulated information contained in two narrow bands of frequencies calledsidebands (SB) just above and below the carrier frequency. The bandwidth (BW) is the amount of spectrum occupied by the sidebands.
A modulated radio wave, carrying an information signal, occupies a range of frequencies. The information in a radio signal is usually concentrated in narrow frequency bands calledsidebands (SB) just above and below thecarrier frequency. The width inhertz of the frequency range that the radio signal occupies, the highest frequency minus the lowest frequency, is called itsbandwidth (BW).[32][37] For any givensignal-to-noise ratio, a given bandwidth can carry the same amount of information regardless of where in the radio frequency spectrum it is located; bandwidth is a measure ofinformation-carrying capacity. The bandwidth required by a radio transmission depends on the data rate of the information being sent, and thespectral efficiency of the modulation method used; how much data it can transmit in each unit of bandwidth. Different types of information signals carried by radio have different data rates. For example, a television signal has a greater data rate than anaudio signal.[32][38]
Theradio spectrum, the total range of radio frequencies that can be used for communication in a given area, is a limited resource.[37][3] Each radio transmission occupies a portion of the total spectrum available. Radio spectrum is regarded as aneconomic 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 spectrum used by radio services.[38]
A slow transition from analog todigital radio transmission technologies began in the late 1990s.[39][40] Part of the reason for this is thatdigital modulation can transmit more information in a given bandwidth thananalog modulation; the modulation itself is more efficient andloss compression further improves efficiency. Digital modulation also has greaternoise immunity than analog, associateddigital signal processors have more power and flexibility than analog circuits, and a wide variety of information can be transmitted using the same digital modulation.[32]
TheITU arbitrarily divides theradio spectrum into 12 bands, each beginning at a wavelength which is a power of ten (10n) metres, with corresponding frequency of 3 times a power of ten, and each covering a decade of frequency or wavelength.[3][41] Each of these bands has a traditional name:[42]
It can be seen that thebandwidth, the absolute 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.[43]
Though not defined by the ITU,[42] the termtremendously low frequency (TLF) has been used for wavelengths from 1–3 Hz (300,000–100,000 km).[44]
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, often to the extent that neither transmission may be received clearly.[37]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 orair traffic control).[45][46]
To prevent interference between different users, the emission of radio waves is strictly regulated by national laws, and coordinated by an international body, theInternational Telecommunication Union (ITU), which allocates bands in theradio spectrum for different uses.[37][3] 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 acall sign, which must be used in all transmissions.[47] In order to adjust, maintain, or internally repair radio transmitters, individuals must hold a government license, such as thegeneral radiotelephone operator license in the US, obtained by taking a test demonstrating adequate technical and legal knowledge of safe radio operation.[48]
Radio jamming is the deliberate radiation of radio signals designed to interfere with the reception of other radio signals. Jamming devices are calledsignal suppressors orinterference generators or justjammers.[9] During wartime, militaries use jamming to interfere with enemies' tactical radio communication. Since radio waves can pass beyond national borders, sometotalitarian countries that 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 that generates noise on the same frequency as the target transmitter.[10][11] 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.[15]
^ab"radio (n.)".Online Etymology Dictionary. Retrieved13 July 2022.
^abBell, Alexander Graham (July 1881). "Production of Sound by Radiant Energy".Popular Science Monthly. pp. 329–330.[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...
^abManning, Trevor (2009).Microwave Radio Transmission Design Guide. Artech House. p. 2.
^abCollins, A. Frederick (10 May 1902). "The Genesis of Wireless Telegraphy".Electrical World and Engineer. p. 811.
^"Wireless Telegraphy".The Practical Engineer. 25 February 1898. p. 174.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.
^"Wireless Telegraphy",The Electrical Review (London), 20 January 1905, page 108, quoting from the British Post Office's 30 December 1904Post Office Circular.
^"Interference with Wireless Messages",Electrical World, 22 June 1907, page 1270.
^Sungook Hong (2001),Wireless: From Marconi's Black-box to the Audion, MIT Press, pp. 5–10
^Tony Dorbuck (ed.),The Radio Amateur's Handbook, Fifty-Fifth Edition, American Radio Relay League, 1977, p. 368
^John Avison, The World of Physics, Nelson · 2014, page 367
^C-W and A-M Radio Transmitters and Receivers, United States. Department of the Army – 1952, pp. 167–168
^abcd"Spectrum 101"(PDF). US National Aeronautics and Space Administration (NASA). February 2016.Archived(PDF) from the original on 11 February 2017. Retrieved2 December 2019., p. 6
^"Radio Regulations, 2016 Edition"(PDF). International Telecommunication Union. 3 November 2016.Archived from the original on 13 December 2021. Retrieved9 November 2019. Article 2, Section 1, p.27
^Communications-electronics Management of the Electromagnetic Spectrum (Report). Headquarters, Department of the Army. United States Department of the Army. 1973. p. 2.
