Inelectronics andtelecommunications, aradio transmitter or justtransmitter (often abbreviated asXMTR orTX in technical documents) is anelectronic device which producesradio waves with anantenna with the purpose ofsignal transmission up to aradio receiver. The transmitter itself generates aradio frequencyalternating current, which is applied to theantenna. When excited by this alternating current, the antennaradiates radio waves.

Transmitters are necessary component parts of all electronic devices that communicate byradio, such asradio (audio) andtelevision broadcasting stations,cell phones,walkie-talkies,wireless computer networks,Bluetooth enabled devices,garage door openers,two-way radios in aircraft, ships, spacecraft,radar sets and navigational beacons. The termtransmitter is usually limited to equipment that generates radio waves forcommunication purposes; orradiolocation, such asradar and navigational transmitters. Generators of radio waves for heating or industrial purposes, such asmicrowave ovens ordiathermy equipment, are not usually called transmitters, even though they often have similar circuits.

The term is popularly used more specifically to refer to abroadcast transmitter, a transmitter used inbroadcasting, as inFM radio transmitter ortelevision transmitter. This usage typically includes both the transmitter proper, the antenna, and often the building it is housed in.

A transmitter can be a separate piece of electronic equipment, or anelectrical circuit within another electronic device. A transmitter and areceiver combined in one unit is called atransceiver. The purpose of most transmitters isradio 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 anaudio (sound) signal from a microphone, avideo (TV) signal from a video camera, or inwireless networking devices, adigital signal from a computer. The transmitter generates aradio frequency signal which when applied to the antenna produces the radio waves, called thecarrier signal. It combines the carrier with the modulation signal, a process calledmodulation. The information can be added to the carrier in several different ways, in different types of transmitters. In anamplitude modulation (AM) transmitter, the information is added to the radio signal by varying itsamplitude. In afrequency modulation (FM) transmitter, it is added by varying the radio signal'sfrequency slightly. Many other types of modulation are also used.

The radio signal from the transmitter is applied to theantenna, 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, andgarage door openers. 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 afeed line, that is atransmission line.

Electromagnetic waves are radiated byelectric charges when they areaccelerated.^[1][2]Radio waves, electromagnetic waves of radiofrequency, are generated by time-varyingelectric currents, consisting ofelectrons flowing through a metal conductor called anantenna which are changing their velocity and thus accelerating.^[3][2] Analternating current flowing back and forth in an antenna will create an oscillatingmagnetic field around the conductor. The alternating voltage will also charge the ends of the conductor alternately positive and negative, creating an oscillatingelectric field around the conductor. If thefrequency of the oscillations is high enough, in theradio frequency range above about 20 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 anelectronic circuit which transformselectric power from a power source, a battery or mains power, into aradio frequency alternating current to apply to the antenna, and the antenna radiates the energy from this current as radio waves.^[4] The transmitter also encodes information such as anaudio orvideo signal into the radio frequency current to be carried by the radio waves. When they strike the antenna of aradio receiver, the waves excite similar (but less powerful) radio frequency currents in it. The radio receiver extracts the information from the received waves.

A practical radio transmitter mainly consists of the following parts:

• In high power transmitters, apower supply circuit to transform the input electrical power to the highervoltages needed to produce the required power output.
• Anelectronic oscillator circuit to generate theradio frequency signal. This usually generates asine wave of constantamplitude, called thecarrier wave because it produces the radio waves which "carry" the information through space. In most modern transmitters, this is acrystal oscillator in which the frequency is precisely controlled by the vibrations of aquartz crystal. Thefrequency of the carrier wave is considered the frequency of the transmitter.
• Amodulator 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 themodulation signal. The modulation signal may be anaudio signal, which representssound, avideo signal which represents moving images, or for data in the form of abinarydigital signal which represents a sequence ofbits, abitstream. Different types of transmitters use differentmodulation methods to transmit information:
  • In an AM (amplitude modulation) transmitter theamplitude (strength) of the carrier wave is varied in proportion to the modulation signal.
  • In an FM (frequency modulation) transmitter thefrequency 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 twobinary digits, 0 and 1.
  • OFDM (orthogonal frequency-division multiplexing) is a family of complicateddigital modulation methods very 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. OFDM 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 multiplebits 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 ofmodulation are also used. In large transmitters the oscillator and modulator together are often referred to as theexciter.

• A radio frequency (RF)amplifier to increase the power of the signal, to increase the range of the radio waves.
• Animpedance matching (antenna tuner) circuit to transform the outputimpedance of the transmitter to match the impedance of the antenna (or thetransmission line to the antenna), to transfer power efficiently to the antenna. If these impedances are not equal, it causes a condition calledstanding waves, 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 theUHF andmicrowave range, free running oscillators are unstable at the output frequency. Older designs used an oscillator at a lower frequency, which was multiplied byfrequency multipliers 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.

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.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 orair 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 asbroadcast,marine radio,Airband,Amateur and are restricted to certain frequencies and power levels. A body called theInternational Telecommunication Union (ITU) allocates thefrequency bands in theradio spectrum to various classes of users. In some classes, each transmitter is given a uniquecall 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 ageneral 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 ascell phones,cordless telephones,wireless microphones,walkie-talkies,Wi-Fi andBluetooth devices,garage door openers, andbaby monitors. In the US, these fall underPart 15 of theFederal Communications Commission (FCC) regulations. Although they can be operated without a license, these devices still generally must betype-approved before sale.

The first primitive radio transmitters (calledspark gap transmitters) were built by German physicistHeinrich Hertz in 1887 during his pioneering investigations of radio waves. These generated radio waves by a high voltagespark 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 transmitaudio (sound) and instead transmitted information byradiotelegraphy: the operator tapped on atelegraph key which turned the transmitter on-and-off to produce radio wave pulses spelling out text messages in telegraphic code, usuallyMorse 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 thewireless telegraphy or "spark" era. Because they generateddamped waves, spark transmitters were electrically "noisy". Their energy was spread over a broad band offrequencies, creatingradio 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 firstcontinuous wave transmitters: thearc converter (Poulsen arc) in 1904 and theAlexanderson alternator around 1910, which were used into the 1920s.

All these early technologies were replaced byvacuum tube transmitters in the 1920s, which used thefeedback oscillator invented byEdwin Armstrong andAlexander Meissner around 1912, based on theAudion (triode) vacuum tube invented byLee De Forest in 1906. Vacuum tube transmitters were inexpensive and producedcontinuous waves, and could be easilymodulated to transmit audio (sound) usingamplitude modulation (AM). This made AMradio broadcasting possible, which began in about 1920. Practicalfrequency modulation (FM) transmission was invented byEdwin 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. Experimentaltelevision transmission had been conducted by radio stations since the late 1920s, but practicaltelevision broadcasting didn't begin until the late 1930s. The development ofradar duringWorld War II motivated the evolution of high frequency transmitters in theUHF andmicrowave ranges, using new active devices such as themagnetron,klystron, andtraveling wave tube.

The invention of thetransistor allowed the development in the 1960s of small portable transmitters such aswireless microphones,garage door openers andwalkie-talkies. The development of theintegrated circuit (IC) in the 1970s made possible the current proliferation ofwireless devices, such ascell phones andWi-Fi networks, in which integrated digital transmitters and receivers (wireless modems) in portable devices operate automatically, in the background, to exchange data withwireless networks.

The need to conserve bandwidth in the increasingly congestedradio spectrum is driving the development of new types of transmitters such asspread spectrum,trunked radio systems andcognitive radio. A related trend has been an ongoing transition fromanalog todigital radio transmission methods.Digital modulation can have greaterspectral efficiency thananalog modulation; that is it can often transmit more information (data rate) in a givenbandwidth than analog, usingdata compression algorithms. Other advantages of digital transmission are increasednoise immunity, and greater flexibility and processing power ofdigital signal processingintegrated circuits.

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  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), andtelegraph key(C).
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  Guglielmo Marconi'sspark gap transmitter, with which he performed the first experiments in practicalMorse coderadiotelegraphy communication in 1895–1897
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  High power spark gapradiotelegraphy transmitter in Australia around 1910.
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  1 MW US NavyPoulsen 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
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  AnAlexanderson alternator, a huge rotating machine used as a radio transmitter at very low frequency from about 1910 until World War 2
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  One of the firstvacuum tubeAM radio transmitters, built byLee De Forest in 1914. The earlyAudion (triode) tube is visible at right.
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  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.
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  Armstrong's first experimental FM broadcast transmitter W2XDG, in theEmpire State Building, New York City, used for secret tests 1934–1935. It transmitted on 41 MHz at a power of 2 kW.
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  Transmitter assembly of a 20 kW, 9.375 GHzair traffic controlradar, 1947. Themagnetron tube mounted between two magnets(right) produces microwaves which pass from the aperture(left) into awaveguide which conducts them to the dish antenna.

• List of transmission sites
• List of radios – List of specific models of radios
• Radio transmitter design
• Repeater
• Transmitter station
• Transposer
• Television transmitter

• International Telecommunication Union
• Jim Hawkins' Radio and Broadcast Technology Page
• WCOV-TV's Transmitter Technical Website
• Major UK television transmitters including change of group information, see Transmitter Planning section.
• Details of UK digital television transmitters