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{{short description|Electronic device that emits radio waves}}
{{about||broader coverage of this topic|Signal transmission|the band|The Transmitters}}
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{{More footnotes needed|date=October 2022}}
[[File:WDET-FM transmitter.png|thumb|Commercial [[FM broadcasting]] transmitter at radio station [[WDET-FM]], Wayne State University, Detroit, US. It broadcasts at 101.9 [[Megahertz|MHz]] with a radiated power of 48 [[Kilowatt|kW]].]]

In [[electronics]] and [[telecommunications]], a '''radio transmitter''' or just '''transmitter''' (often abbreviated as '''XMTR''' or '''TX''' in technical documents) is an [[electronic device]] which produces [[radio wave]]s with an [[antenna (radio)|antenna]] with the purpose of [[signal transmission]] to a [[radio receiver]]. The transmitter itself generates a [[radio frequency]] [[alternating current]], which is applied to the [[Antenna (radio)|antenna]]. When excited by this alternating current, the antenna [[Electromagnetic radiation|radiates]] radio waves.

Transmitters are necessary component parts of all electronic devices that communicate by [[radio communication|radio]], such as [[radio broadcasting|radio]] (audio) and [[television broadcasting]] stations, [[cell phone]]s, [[walkie-talkie]]s, [[Wireless LAN|wireless computer networks]], [[Bluetooth]] enabled devices, [[garage door opener]]s, [[two-way radio]]s in aircraft, ships, spacecraft, [[radar]] sets and navigational beacons. The term ''transmitter'' is usually limited to equipment that generates radio waves for [[Communication engineering|communication]] purposes; or [[radiolocation]], such as [[radar]] and navigational transmitters. Generators of radio waves for heating or industrial purposes, such as [[microwave oven]]s or [[diathermy]] equipment, are not usually called transmitters, even though they often have similar circuits.

The term is popularly used more specifically to refer to a [[broadcast transmitter]], a transmitter used in [[broadcasting]], as in ''FM radio transmitter'' or [[television transmitter]]. This usage typically includes both the transmitter proper, the antenna, and often the building it is housed in.

==Description== 
[[File:Signal processing system.png|thumb|upright=1.5|A radio transmitter is usually part of a [[radio communication]] system which uses [[electromagnetic wave]]s ([[radio wave]]s) to transport information (in this case sound) over a distance.]]

A transmitter can be a separate piece of electronic equipment, or an [[Electrical network|electrical circuit]] within another electronic device.  A transmitter and a [[Radio receiver|receiver]] combined in one unit is called a [[transceiver]].  The purpose of most transmitters is [[radio communication]] of information over a distance. The information is provided to the transmitter in the form of an electronic signal called the modulation signal, such as an [[audio signal|audio]] (sound) signal from a microphone, a [[Video signal|video]] (TV) signal from a video camera, or in [[wireless networking]] devices, a [[Digital signal (electronics)|digital signal]] from a computer. The transmitter generates a [[radio frequency]] signal which when applied to the antenna produces the radio waves, called the [[carrier signal]].  It combines the carrier with the modulation signal, a process called [[modulation]]. The information can be added to the carrier in several different ways, in different types of transmitters. In an [[amplitude modulation]] (AM) transmitter, the information is added to the radio signal by varying its [[amplitude]]. In a [[frequency modulation]] (FM) transmitter, it is added by varying the radio signal's [[frequency]] slightly. Many other types of modulation are also used.

The radio signal from the transmitter is applied to the [[antenna (radio)|antenna]], which radiates the energy as radio waves. The antenna may be enclosed inside the case or attached to the outside of the transmitter, as in portable devices such as cell phones, walkie-talkies, and [[garage door opener]]s. In more powerful transmitters, the antenna may be located on top of a building or on a separate tower, and connected to the transmitter by a [[feed line]], that is a [[transmission line]].

{{multiple image
| align = center
| header = Radio transmitters
| image1   = Transmisor de bulbos AM Elcom Bauer 701 B XEQK.jpg
| caption1 = Elcom Bauer model 701B 1100 watt AM broadcast transmitter
| width1   = 100
| image2   = KWNR Continental 816R-5B SN 247.jpg
| caption2 = 35&nbsp;kW, Continental 816R-5B FM transmitter, belonging to American FM radio station KWNR broadcasting on 95.5&nbsp;MHz in Las Vegas 
| width2   = 197
| image3   = Icom IC-746PRO.jpg
| caption3   = Modern [[amateur radio]] [[transceiver]], the ICOM IC-746PRO. It can transmit on the amateur bands from 1.8&nbsp;MHz to 144&nbsp;MHz with an output power of 100&nbsp;W
| width3 =  270
| image4   = CB-Funkgerät im LKW.JPG
| caption4 = A [[citizens band radio|CB radio]] transceiver in a truck, a [[two way radio]] transmitting on 27&nbsp;MHz with a power of 4&nbsp;W, that can be operated without a license
| width4   = 170
| image5   = Miramar Fire Department Firefighters Conduct Prescribed Burns 140612-M-RB277-066.jpg
| caption5 = Firefighter using a [[walkie-talkie]]  
| width5   = 190
}}

{{multiple image
| align = center
| header = Consumer products that contain transmitters
| image1   = HA0478-006 (6011470974).jpg
| caption1 = A cellphone has several transmitters: a duplex cell transceiver, a Wi-Fi modem, and a Bluetooth modem. 
| width1   = 120
| image2   = Cordless phone icon.svg
| caption2 = Both the handset and the base of a [[cordless phone]] contain low power 2.4&nbsp;GHz radio transmitters to communicate with each other.
| width2   = 98
| image3   = Garage-door-opener-remote-control.png
| caption3  = A [[garage door opener]] control contains a low-power 2.4&nbsp;GHz transmitter that sends coded commands to the garage door mechanism to open or close.
| width3 = 122
| width4   = 130
| image5   = Wireless network.jpg
| caption5 = A [[laptop computer]] and home [[wireless router]] (background) which connects it to the Internet, creating a home Wi-Fi network.  Both have Wi-Fi [[wireless network interface controller|modems]], automated microwave transmitters and receivers operating on 2.4&nbsp;GHz which exchange data packets with the [[internet service provider]] (ISP). 
| width5   = 175
| image6   = Plantronics Voyager Legend.JPG
| caption6 = A [[Bluetooth]] earbud with microphone. It has a Bluetooth modem to exchange audio with a cell phone
| width6   = 230
| image7  = Amsterdam RAI METS 2011 (102).JPG
| caption7 = [[Emergency Position Indicating Radio Beacon|Emergency Locator Beacon]] carried by hikers.  
| width7   = 66
}}

==Operation==
{{Main|Radio transmitter design}}
[[File:Dipole xmting antenna animation 4 408x318x150ms.gif|thumb|upright=1.2|Animation of a [[half-wave dipole]] antenna transmitting [[radio waves]], showing the [[electric field]] lines.  The antenna in the center is two vertical metal rods, with an alternating current applied at its center from a radio transmitter (not shown).  The voltage charges the two sides of the antenna alternately positive ''<span style="color:red;">(+)</span>'' and negative  ''<span style="color:blue;">(−)</span>''.  Loops of electric field (black lines) leave the antenna and travel away at the [[speed of light]]; these are the radio waves.  This animation shows the action slowed enormously]]

[[Electromagnetic wave]]s are radiated by [[electric charge]]s when they are [[acceleration|accelerated]].<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| pages  = 714| url    = https://books.google.com/books?id=CX0u0mIOZ44C&q=%22electromagnetic+wave%22+charge+acceleration&pg=PA714| isbn   = 978-0495386933}}</ref><ref name="Ellingson">{{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> [[Radio wave]]s,  electromagnetic waves of radio [[frequency]], are generated by time-varying [[electric current]]s, consisting of [[electron]]s flowing through a metal conductor called an [[antenna (radio)|antenna]] which are changing their velocity and thus accelerating.<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
 | pages  = [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   = 9781118585733
 }}</ref><ref name="Ellingson" />  An [[alternating current]]  flowing back and forth in an antenna will create an oscillating [[magnetic field]] around the conductor.  The alternating voltage will also charge the ends of the conductor alternately positive and negative, creating an oscillating [[electric field]] around the conductor.  If the [[frequency]] of the oscillations is high enough, in the [[radio frequency]] range above about 20&nbsp;kHz, the oscillating coupled electric and magnetic fields will radiate away from the antenna into space as an electromagnetic wave, a radio wave.

A radio transmitter is an [[electronic circuit]] which transforms [[electric power]] from a power source, a battery or mains power, into a [[radio frequency]] alternating current to apply to the antenna, and the antenna radiates the energy from this current as radio waves.<ref name="HowRadioWorks">{{cite web|url=http://electronics.howstuffworks.com/radio8.htm|title=How Radio Works|first=Marshall|last=Brain|publisher=HowStuffWorks.com|date=2000-12-07|access-date=2009-09-11}}</ref>  The transmitter also encodes information such as an [[audio signal|audio]] or [[video signal]] into the radio frequency current to be carried by the radio waves.  When they strike the antenna of a [[radio receiver]], the waves excite similar (but less powerful) radio frequency currents in it. The radio receiver extracts the information from the received waves.

===Components===
A practical radio transmitter mainly consists of the following parts:

*In high power transmitters, a [[power supply]] circuit to transform the input electrical power to the higher [[voltage]]s needed to produce the required power output.
*An [[electronic oscillator]] circuit to generate the [[radio frequency]] signal. This usually generates a [[sine wave]] of constant [[amplitude]], called the [[carrier wave]] because it produces the radio waves which "carry" the information through space. In most modern transmitters, this is a [[crystal oscillator]] in which the frequency is precisely controlled by the vibrations of a [[quartz crystal]].  The [[frequency]] of the carrier wave is considered the frequency of the transmitter.
*A [[modulator]] circuit to add the information to be transmitted to the carrier wave produced by the oscillator. This is done by varying some aspect of the carrier wave. The information is provided to the transmitter as an electronic signal called the [[modulation|modulation signal]].  The modulation signal may be an [[audio signal]], which represents [[sound]], a [[video signal]] which represents moving images, or for data in the form of a [[Binary numeral system|binary]] [[digital signal]] which represents a sequence of [[binary digit|bits]], a [[bitstream]].  Different types of transmitters use different [[modulation]] methods to transmit information: 
**In an AM ([[amplitude modulation]]) transmitter the [[amplitude]] (strength) of the carrier wave is varied in proportion to the modulation signal.
**In an FM ([[frequency modulation]]) transmitter the [[frequency]] of the carrier is varied by the modulation signal.
**In an FSK ([[frequency-shift keying]]) transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two [[binary digit]]s, 0 and 1. 
**OFDM ([[orthogonal frequency-division multiplexing]]) is a family of complicated [[digital modulation]] methods very 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.  OFDM 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 [[binary digit|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. 
:Many other types of [[modulation]] are also used.  In large transmitters the oscillator and modulator together are often referred to as the ''exciter''.<ref name="nab">{{cite book |last1=Welton |first1=Jeff |editor1-last=Cavell |editor1-first=Garrison |title=VHF (FM) Radio Transmitters in National Association of Broadcasters Engineering Handbook, 11th Edition |date=2018 |publisher=Routledge |location=New York |isbn=9781138930513 |pages=1331-1334}}</ref>
*A radio frequency (RF) [[amplifier]] to increase the power of the signal, to increase the range of the radio waves.
*An [[impedance matching]] ([[antenna tuner]]) circuit to transform the output [[Electrical impedance|impedance]] of the transmitter to match the impedance of the antenna (or the [[transmission line]] to the antenna), to transfer power efficiently to the antenna.  If these impedances are not equal, it causes a condition called [[standing wave]]s, in which the power is reflected back from the antenna toward the transmitter, wasting power and sometimes overheating the transmitter.
In higher frequency transmitters, in the [[Ultra high frequency|UHF]] and [[microwave]] range, free running oscillators are unstable at the output frequency. Older designs used an oscillator at a lower frequency, which was multiplied by [[frequency multiplier]]s to get a signal at the desired frequency. Modern designs more commonly use an oscillator at the operating frequency which is stabilized by phase locking to a very stable lower frequency reference, usually a crystal oscillator.

==Regulation==
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. [[radio frequency interference|Interference]] with radio transmissions can not only have a large economic cost, it can be life-threatening (for example, in the case of interference with emergency communications or [[air traffic control]]).
 
For this reason, in most countries, use of transmitters is strictly controlled by law.  Transmitters must be licensed by governments, under a variety of license classes depending on use such as [[broadcast]], [[marine radio]], [[Airband]], [[Amateur radio|Amateur]] and are restricted to certain frequencies and power levels. A body called the [[International Telecommunication Union]] (ITU) allocates the [[frequency]] bands in the [[radio spectrum]] to various classes of users. In some classes, each transmitter is given a unique [[call sign]] consisting of a string of letters and numbers which must be used as an identifier in transmissions. The operator of the transmitter usually must hold a government license, such as a [[general radiotelephone operator license]], which is obtained by passing a test demonstrating adequate technical and legal knowledge of safe radio operation.

Exceptions to the above regulations allow the unlicensed use of low-power short-range transmitters in consumer products such as [[cell phone]]s, [[cordless telephone]]s, [[wireless microphone]]s, [[walkie-talkie]]s, [[Wi-Fi]] and [[Bluetooth]] devices, [[garage door opener]]s, and [[baby monitor]]s. In the US, these fall under [[Part 15]] of the [[Federal Communications Commission]] (FCC) regulations. Although they can be operated without a license, these devices still generally must be [[Type approval|type-approved]] before sale.

==History==
{{Main|History of radio}}
[[File:Heinrich Hertz discovering radio waves.png|thumb|Hertz discovering radio waves in 1887 with his first primitive radio transmitter (background).]]

The first primitive radio transmitters (called [[spark gap transmitter]]s) were built by German physicist [[Heinrich Hertz]] in 1887 during his pioneering investigations of radio waves. These generated radio waves by a high voltage [[Electric spark|spark]] between two conductors. Beginning in 1895, [[Guglielmo Marconi]] developed the first practical radio communication systems using these transmitters, and radio began to be used commercially around 1900.  Spark transmitters could not transmit [[audio signal|audio]] (sound) and instead transmitted information by [[radiotelegraphy]]: the operator tapped on a [[telegraph key]] which turned the transmitter on-and-off to produce radio wave pulses spelling out text messages in telegraphic code, usually [[Morse code]]. At the receiver, these pulses were sometimes directly recorded on paper tapes, but more common was audible reception. The pulses were audible as beeps in the receiver's earphones, which were translated back to text by an operator who knew Morse code. These spark-gap transmitters were used during the first three decades of radio (1887–1917), called the [[wireless telegraphy]] or "spark" era. Because they generated [[damped wave]]s, spark transmitters were electrically "noisy". Their energy was spread over a broad band of [[frequency|frequencies]], creating [[radio frequency interference|radio noise]] which interfered with other transmitters. Damped wave emissions were banned by international law in 1934.

Two short-lived competing transmitter technologies came into use after the turn of the century, which were the first [[continuous wave]] transmitters: the [[arc converter]] ([[Poulsen Arc|Poulsen arc]]) in 1904 and the [[Alexanderson alternator]] around 1910, which were used into the 1920s.

All these early technologies were replaced by [[vacuum tube]] transmitters in the 1920s, which used the [[electronic oscillator#Feedback oscillator|feedback oscillator]] invented by [[Edwin Armstrong]] and [[Alexander Meissner]] around 1912, based on the [[Audion]] ([[triode]]) vacuum tube invented by [[Lee De Forest]] in 1906. Vacuum tube transmitters were inexpensive and produced [[continuous wave]]s, and could be easily [[Modulation|modulated]] to transmit audio (sound) using [[amplitude modulation]] (AM). This made AM [[radio broadcasting]] possible, which began in about 1920. Practical [[frequency modulation]] (FM) transmission was invented by [[Edwin Armstrong]] in 1933, who showed that it was less vulnerable to noise and static than AM.  The first FM radio station was licensed in 1937. Experimental [[television]] transmission had been conducted by radio stations since the late 1920s, but practical [[television broadcasting]] didn't begin until the late 1930s. The development of [[radar]] during [[World War II]] motivated the evolution of high frequency transmitters in the [[Ultrahigh frequency|UHF]] and [[microwave]] ranges, using new active devices such as the [[magnetron]], [[klystron]], and [[traveling wave tube]].

The invention of the [[transistor]] allowed the development in the 1960s of small portable transmitters such as [[wireless microphone]]s, [[garage door opener]]s and [[walkie-talkie]]s. The development of the [[integrated circuit]] (IC) in the 1970s made possible the current proliferation of [[wireless device]]s, such as [[cell phone]]s and [[Wi-Fi]] networks, in which integrated digital transmitters and receivers ([[wireless modem]]s) in portable devices operate automatically, in the background, to exchange data with [[wireless network]]s.

The need to conserve bandwidth in the increasingly congested [[radio spectrum]] is driving the development of new types of transmitters such as [[spread spectrum]], [[trunked radio system]]s and [[cognitive radio]]. A related trend has been an ongoing transition from [[Analog signal|analog]] to [[digital signal (signal processing)|digital]] radio transmission methods. [[Digital modulation]] can have greater [[spectral efficiency]] than [[analog modulation]]; that is it can often transmit more information ([[Bit rate|data rate]]) in a given [[bandwidth (signal processing)|bandwidth]] than analog, using [[data compression]] algorithms.  Other advantages of digital transmission are increased [[noise immunity]], and greater flexibility and processing power of [[digital signal processing]] [[integrated circuit]]s.

<gallery mode="packed" heights="150">
File:Hertzian spark radio transmitter 1902.jpg|Spark oscillator similar to Hertz's, 1902. Visible are antenna consisting of 2 wires ending in metal plates ''(E)'', spark gap ''(D)'', induction coil ''(A)'', auto battery ''(B)'', and [[telegraph key]] ''(C)''.
File:Marconi 1897 spark gap transmitter.jpg|[[Guglielmo Marconi]]'s [[spark gap transmitter]], with which he performed the first experiments in practical [[Morse code]] [[radiotelegraphy]] communication in 1895–1897
File:Powerful spark gap transmitter.png|High power spark gap [[radiotelegraphy]] transmitter in Australia around 1910.
File:Poulsen arc 1MW transmitter.jpg|1 MW US Navy [[Poulsen arc]] transmitter which generated continuous waves using an electric arc in a magnetic field, a technology used for a brief period from 1903 until vacuum tubes took over in the 20s
File:Alexanderson Alternator.jpg|An [[Alexanderson alternator]], a huge rotating machine used as a radio transmitter at very low frequency from about 1910 until World War 2
File:First vacuum tube AM radio transmitter.jpg|One of the first [[vacuum tube]] [[amplitude modulation|AM]] radio transmitters, built by [[Lee De Forest]] in 1914. The early [[Audion]] ([[triode]]) tube is visible at right.
File:Blythe House Science Museum stores tour 99.JPG|One of the BBC's first broadcast transmitters, early 1920s, London.  The 4 triode tubes, connected in parallel to form an oscillator, each produced around 4 kilowatts with 12 thousand volts on their anodes.
File:Armstrong prototype FM transmitter 1935.jpg|Armstrong's first experimental FM broadcast transmitter W2XDG, in the [[Empire State Building]], New York City, used for secret tests 1934–1935. It transmitted on 41&nbsp;MHz at a power of 2&nbsp;kW.
File:Magnetron radar assembly 1947.jpg|Transmitter assembly of a 20&nbsp;kW, 9.375&nbsp;GHz [[air traffic control]] [[radar]], 1947.  The [[magnetron]] tube mounted between two magnets ''(right)'' produces microwaves which pass from the aperture ''(left)'' into a [[waveguide (electromagnetism)|waveguide]] which conducts them to the dish antenna.  
</gallery>

==See also==
*[[List of transmission sites]]
* {{annotated link|List of radios}}
*[[Radio transmitter design]]
*[[Repeater]]
*[[Transmitter station]]
*[[Transposer]]
*[[Television transmitter]]

==References==
{{reflist}}

==External links==
{{Wiktionary}}
*[http://www.itu.int/net/home/index.aspx International Telecommunication Union]
*[http://hawkins.pair.com/radio.html Jim Hawkins' Radio and Broadcast Technology Page]
*[http://www.wcov.com/technical/transmitter.html WCOV-TV's Transmitter Technical Website]
Planning section.]
*[https://vu3dxr.in/build-your-own-grenade-am-transmitter-circuit-a-complete-guide Build/ Your Own Grenade AM Transmitter Circuit: A Complete Guide]
*[http://www.wolfbane.com/ukdtt.htm Details of UK digital television transmitters]

{{Telecommunications}}
{{Authority control}}

[[Category:Broadcast transmitters| ]]
[[Category:Telecommunications equipment]]
[[Category:Radar]]