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In
electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplification and rectifie ...
and
telecommunications Telecommunication is the transmission of information by various types of technologies over wire, radio, Optical system, optical, or other Electromagnetism, electromagnetic systems. It has its origin in the desire of humans for communication ove ...
, a radio transmitter or just transmitter is an
electronic device Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplification and rectifie ...
which produces
radio wave Radio waves are a type of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ma ...
s with an
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
. The transmitter itself generates a
radio frequency Radio frequency (RF) is the oscillation rate of an Alternating_current, alternating electric current or voltage or of a Magnetic_field, magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around ...
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...
, which is applied to the
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary component parts of all electronic devices that communicate by
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...
, such as
radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device ...
and
television broadcasting A television network or broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid-1980s, te ...

television broadcasting
stations,
cell phone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunications Appliance (disambiguation), devic ...
s,
walkie-talkie A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer Alfred J. Gross, He ...
s, wireless computer networks,
Bluetooth Bluetooth is a short-range wireless technology standard used for exchanging data between fixed and mobile devices over short distances using Ultra high frequency, UHF radio waves in the ISM band, ISM bands, from 2.402 GHz to 2.48GHz, and buildin ...

Bluetooth
enabled devices,
garage door opener A residential garage door opener. The motor is in the box on the upper-right. A garage door opener is a motorized device that opens and closes garage doors controlled by switches on the garage wall. Most also include a handheld radio remote contr ...

garage door opener
s,
two-way radio station, a two-way radio used for recreational purposes by hobbyists called radio amateurs File:P25 hand-held radios.jpg, Several portable two-way radios designed for public services (police, fire, rescue) compatible with the Project 25 digital ...
s in aircraft, ships, spacecraft,
radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Ear ...

radar
sets and navigational beacons. The term ''transmitter'' is usually limited to equipment that generates radio waves for
communication Communication (from Latin ''communicare'', meaning "to share" or "to be in relation with") is "an apparent answer to the painful divisions between self and other, private and public, and inner thought and outer world." As this definition indica ...
purposes; or
radiolocation Radiolocation, also known as radiolocating or radiopositioning, is the process of finding the location of something through the use of radio wave Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spec ...
, such as
radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Ear ...

radar
and navigational transmitters. Generators of radio waves for heating or industrial purposes, such as
microwave oven A microwave oven (commonly referred to as a microwave) is an electric oven that heats and cooks food by exposing it to electromagnetic spectrum, electromagnetic radiation in the microwave frequency range. This induces Dipole#Molecular dipoles, ...

microwave oven
s or
diathermy Diathermy is electrically induced heat or the use of high-frequency electromagnetic currents as a form of physical therapy and in surgical procedures. The earliest observations on the reactions of high-frequency electromagnetic currents upon the ...
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 transmitterA broadcast transmitter is an electronic device which radiates radio wave Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as ...
, a transmitter used in
broadcasting Broadcasting is the distribution (business), distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications medium, but typically one using the electromagnetic spectrum (radio ...
, as in ''FM radio transmitter'' or ''
television transmitterA television transmitter is a transmitter In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to con ...
''. This usage typically includes both the transmitter proper, the antenna, and often the building it is housed in.


Description

A transmitter can be a separate piece of electronic equipment, or an
electrical circuit An electrical network is an interconnection of electronic component, electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of el ...
within another electronic device. A transmitter and a
receiver
receiver
combined in one unit is called a
transceiver In Radio, radio communication, a transceiver is a device that is able to both transmit and receive information through a transmission medium. It is a combination of a transmitter, ''trans''mitter and a Radio receiver, re''ceiver'', hence the name ' ...

transceiver
. The term transmitter is often abbreviated "XMTR" or "TX" in technical documents. The purpose of most transmitters is
radio communication Radio is the technology of signaling and communicating using radio wave Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as hig ...
of information over a distance. The information is provided to the transmitter in the form of an electronic signal, such as an
audio Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion ( ...
(sound) signal from a microphone, a
video Video is an Electronics, electronic medium for the recording, copying, playback, broadcasting, and display of moving picture, moving image, visual Media (communication), media. Video was first developed for mechanical television systems, wh ...
(TV) signal from a video camera, or in
wireless networking A wireless network is a computer network A computer network is a group of computers that use a set of common communication protocols over digital signal, digital interconnections for the purpose of sharing resources located on or provided by ...

wireless networking
devices, a
digital signal A digital signal is a signal that is being used to represent data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values. This contrasts with an analog signal, which represents cont ...
from a computer. The transmitter combines the information signal to be carried with the radio frequency signal which generates the radio waves, which is called the
carrier signal upright=1.4, The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal amplitude or power. It consists of a signal (C) at the carrier wave frequency ''f''C, wi ...
. This process is called ''
modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains information ...

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 The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelength, spatial period). There are various definitions of amplitude ...
. In a
frequency modulation Frequency modulation (FM) is the encoding of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of its characteristic ...

frequency modulation
(FM) transmitter, it is added by varying the radio signal's
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

frequency
slightly. Many other types of modulation are also used... The radio signal from the transmitter is applied to the
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
, 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 A residential garage door opener. The motor is in the box on the upper-right. A garage door opener is a motorized device that opens and closes garage doors controlled by switches on the garage wall. Most also include a handheld radio remote contr ...

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 In a radio antenna, the feed line (feedline), or feeder, is the cable or other transmission line . In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a con ...
, that is a
transmission line . In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

transmission line
.


Operation

Electromagnetic wave Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically charged particles. The electromagnetic force is carried by electromagneti ...

Electromagnetic wave
s are radiated by
electric charge Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respectively). Like ch ...
s when they are accelerated.
Radio wave Radio waves are a type of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies ma ...
s, electromagnetic waves of radio
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

frequency
, are generated by time-varying
electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor or space. It is measured as the net rate of flow of electric charge through a surface or into a control volume. The moving part ...
s, consisting of
electron The electron is a subatomic particle In physical sciences, subatomic particles are smaller than atom An atom is the smallest unit of ordinary matter In classical physics and general chemistry, matter is any substance that has ma ...
s flowing through a metal conductor called an
antenna Antenna (pl. antennas or antennae) may refer to: Science and engineering * Antenna (radio), also known as an aerial, a transducer designed to transmit or receive electromagnetic (e.g., TV or radio) waves * Antennae Galaxies, the name of two coll ...
which are changing their velocity and thus accelerating. An
alternating current Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which ...
flowing back and forth in an antenna will create an oscillating
magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to the ...

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 An electric field (sometimes E-field) is the physical field that surrounds electrically-charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' ' ...
around the conductor. If the
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

frequency
of the oscillations is high enough, in the
radio frequency Radio frequency (RF) is the oscillation rate of an Alternating_current, alternating electric current or voltage or of a Magnetic_field, magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around ...
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 an
electronic circuit File:PExdcr01CJC.jpg, 200px, A circuit built on a printed circuit board (PCB). An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or ...
which transforms
electric power Electric power is the rate, per unit time, at which electrical energy is transferred by an electric circuit. The SI unit of power is the watt The watt (symbol: W) is a unit of Power (physics), power or radiant flux. In the International Sy ...
from a power source, a battery or mains power, into a
radio frequency Radio frequency (RF) is the oscillation rate of an Alternating_current, alternating electric current or voltage or of a Magnetic_field, magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around ...
alternating current to apply to the antenna, and the antenna radiates the energy from this current as radio waves. The transmitter also encodes information such as an
audio Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion ( ...
or
video signal Video is an electronic medium for the recording, copying, playback, broadcasting Broadcasting is the distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications med ...
into the radio frequency current to be carried by the radio waves. When they strike the antenna of a
radio receiver radio in the 1940s. During the golden age of radio, 1925–1955, families gathered to listen to the home radio receiver in the evening In radio, radio communications, a radio receiver, also known as a receiver, a wireless or simply a radio, is ...

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#REDIRECT Power supply A power supply is an electrical device that supplies electric power to an electrical load. The primary function of a power supply is to convert electric current from a source to the correct voltage, electric current, current ...

power supply
circuit to transform the input electrical power to the higher
voltage Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is the ...

voltage
s needed to produce the required power output. *An
electronic oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillation, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillation, Oscillators convert direct current (DC) from a power suppl ...
circuit to generate the
radio frequency Radio frequency (RF) is the oscillation rate of an Alternating_current, alternating electric current or voltage or of a Magnetic_field, magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around ...
signal. This usually generates a
sine wave A sine wave or sinusoid is a curve, mathematical curve that describes a smooth periodic oscillation. A sine wave is a continuous wave. It is named after the function sine, of which it is the graph of a function, graph. It occurs often in both p ...

sine wave
of constant
amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelength, spatial period). There are various definitions of amplitude ...
called the
carrier wave Image:Modulated radio signal frequency spectrum.svg, upright=1.4, The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. The horizontal axis is frequency; the vertical axis is signal amplitude or power. It consists of a ...
, because it generates the radio waves which "carry" the information through space. In most modern transmitters, this is a
crystal oscillator A crystal oscillator is an electronic oscillator An electronic oscillator is an electronic circuit that produces a periodic, oscillation, oscillating electronic signal, often a sine wave or a square wave or a triangle wave. Oscillation, Oscillat ...

crystal oscillator
in which the frequency is precisely controlled by the vibrations of a
quartz crystal Quartz is a hard, crystalline mineral In geology Geology (from the Ancient Greek γῆ, ''gē'' ("earth") and -λoγία, ''-logia'', ("study of", "discourse")) is an Earth science concerned with the solid Earth, the rock (geology), ro ...

quartz crystal
. The
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

frequency
of the carrier wave is considered the frequency of the transmitter. *A
modulator In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplifi ...
circuit to add the information to be transmitted to the carrier wave produced by the
oscillator Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of Mechanical equilibrium, equilibrium) or between two or more different states. The term ''vibration'' is precisely used to describe ...

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 . The modulation signal may be an
audio signal Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion ( ...
, which represents
sound In physics, sound is a vibration that propagates as an acoustic wave, through a transmission medium such as a gas, liquid or solid. In human physiology and psychology, sound is the ''reception'' of such waves and their ''perception'' by the br ...

sound
, a
video signal Video is an electronic medium for the recording, copying, playback, broadcasting Broadcasting is the distribution of sound, audio or video content to a dispersed audience via any electronic medium (communication), mass communications med ...
which represents moving images, or for data in the form of a
binary Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics, a binary number is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal ...
digital signal A digital signal is a signal In signal processing Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as audio signal processing, sound, image processing, images ...
which represents a sequence of bits, a
bitstream A bitstream (or bit stream), also known as binary sequence, is a sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order theory, order matters. Like a Set (mathematics), set, it conta ...

bitstream
. Different types of transmitters use different
modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains information ...

modulation
methods to transmit information: **In an AM ( amplitude modulation) transmitter the
amplitude The amplitude of a Periodic function, periodic Variable (mathematics), variable is a measure of its change in a single Period (mathematics), period (such as frequency, time or Wavelength, spatial period). There are various definitions of amplitude ...
(strength) of the carrier wave is varied in proportion to the modulation signal. **In an FM (
frequency modulation Frequency modulation (FM) is the encoding of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of its characteristic ...

frequency modulation
) transmitter the
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

frequency
of the carrier is varied by the modulation signal. **In an FSK (
frequency-shift keying Frequency-shift keying (FSK) is a frequency modulation Frequency modulation (FM) is the encoding of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus de ...
) transmitter, which transmits digital data, the frequency of the carrier is shifted between two frequencies which represent the two
binary digit Binary may refer to: Science and technology Mathematics * Binary number In mathematics and digital electronics, a binary number is a number A number is a mathematical object used to counting, count, measurement, measure, and nominal ...
s, 0 and 1. **OFDM (
orthogonal frequency division multiplexing In telecommunications Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of ...
) is a family of complicated
digital modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains information ...
methods very widely used in high bandwidth systems such as
Wi-Fi Wi-Fi () is a family of wireless network Communication protocol, protocols, based on the IEEE 802.11 family of standards, which are commonly used for Wireless LAN, local area networking of devices and Internet access, allowing nearby digital de ...

Wi-Fi
networks,
cellphone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunications Appliance (disambiguation), devic ...
s,
digital television Digital television (DTV) is the transmission of television audiovisual Audiovisual (AV) is electronic media possessing both a sound and a visual component, such as slide-tape presentations, films, television programs, corporate conferencing, c ...
broadcasting, and
digital audio broadcasting Digital Audio Broadcasting (DAB) is a digital radio International standard, standard for broadcasting digital audio radio services in many countries around the world, defined and promoted by the WorldDAB forum. The standard is dominant in Europe ...

digital audio broadcasting
(DAB) to transmit digital data using a minimum of
radio spectrum The radio spectrum is the part of the electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of ...
bandwidth. OFDM has higher
spectral efficiency Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandw ...
and more resistance to
fading In wireless communications, fading is variation of the attenuation of a signal with various variables. These variables include time, geographical position, and radio frequency. Fading is often modeled as a random process. A fading channel is a ...

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 A bitstream (or bit stream), also known as binary sequence, is a sequence In mathematics, a sequence is an enumerated collection of objects in which repetitions are allowed and order theory, order matters. Like a Set (mathematics), set, it conta ...

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. :Many other types of
modulation In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains information ...

modulation
are also used. In large transmitters the oscillator and modulator together are often referred to as the ''exciter''. *A radio frequency (RF)
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power (physics), power of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It is a two-port networ ...

amplifier
to increase the power of the signal, to increase the range of the radio waves. *An
impedance matching In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplific ...
(
antenna tuner Antenna tuner, matching network, matchbox, transmatch, antenna tuning unit (ATU), antenna coupler, and feedline coupler are all equivalent names for a device connected between a radio transmitter and its antenna, to improve power transfer bet ...
) circuit to match the impedance of the transmitter to the impedance of the antenna (or the
transmission line . In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmi ...

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 In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, space and time, and the related entities of energy and force. "Ph ...

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
UHF Ultra high frequency (UHF) is the ITU designation for radio frequencies in the range between 300 megahertz The hertz (symbol: Hz) is the SI derived unit, derived unit of frequency in the International System of Units (SI) and is defin ...

UHF
and
microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...

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 In electronics Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. It uses active devices to control electron flow by amplifier, amplificat ...
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.
Interference Interference is the act of interfering, invading, or poaching. Interference may also refer to: Communications * Interference (communication), anything which alters, modifies, or disrupts a message * Adjacent-channel interference, caused by extran ...
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 Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purp ...

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 Broadcasting is the distributionDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized functions, are objects that generalize the classical notion of functi ...

broadcast
,
marine radio Marine VHF radio is a worldwide system of two way radio transceivers on ships and watercraft used for bidirectional voice communication from ship-to-ship, ship-to-shore (for example with harbormasters), and in certain circumstances ship-to-ai ...
,
Airband Airband or aircraft band is the name for a group of frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency an ...
,
Amateur An amateur (; ; ) is generally considered a person who pursues an avocation independent from their source of income. Amateurs and their pursuits are also described as popular, informal, autodidacticism, self-taught, user-generated, do it yourself, ...
and are restricted to certain frequencies and power levels. A body called the
International Telecommunication Union 260px, ITU Monument, Bern The International Telecommunication Union is a specialized agency of the United Nations United Nations Specialized Agencies are autonomous organizations working with the United Nations and each other through the ...

International Telecommunication Union
(ITU) allocates the
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

frequency
bands in the
radio spectrum The radio spectrum is the part of the electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of ...
to various classes of users. In some classes, each transmitter is given a unique
call sign In broadcasting and radio communications, a call sign (also known as a call name or call letters—and historically as a call signal—or abbreviated as a call) is a Identifier, unique designation for a transmitter station. In the United States of ...
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 licenseThe General Radiotelephone Operator License (GROL) is a license granted by the U.S. Federal Communications Commission (FCC), which is required to operate certain radio equipment. It is required by any persons who adjust, maintain, or internally repai ...
, 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 A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunications Appliance (disambiguation), devic ...
s,
cordless telephone A cordless telephone or portable telephone is a telephone A telephone is a telecommunications Appliance (disambiguation), device that permits two or more users to conduct a conversation when they are too far apart to be heard directly. A ...
s,
wireless microphone A wireless microphone, or cordless microphone, is a microphone A microphone, colloquially called a mic or mike (), is a device – a transducer – that converts sound In physics Physics (from grc, φυσική (ἐπιστή ...
s,
walkie-talkie A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer Alfred J. Gross, He ...
s,
Wi-Fi Wi-Fi () is a family of wireless network Communication protocol, protocols, based on the IEEE 802.11 family of standards, which are commonly used for Wireless LAN, local area networking of devices and Internet access, allowing nearby digital de ...

Wi-Fi
and
Bluetooth Bluetooth is a short-range wireless technology standard used for exchanging data between fixed and mobile devices over short distances using Ultra high frequency, UHF radio waves in the ISM band, ISM bands, from 2.402 GHz to 2.48GHz, and buildin ...

Bluetooth
devices,
garage door opener A residential garage door opener. The motor is in the box on the upper-right. A garage door opener is a motorized device that opens and closes garage doors controlled by switches on the garage wall. Most also include a handheld radio remote contr ...

garage door opener
s, and
baby monitor 230px, Audio baby monitor A baby monitor, also known as a baby alarm, is a radio system used to remotely listen to sounds made by an infant. An audio monitor consists of a transmitter unit, equipped with a microphone, placed near to the child. It t ...
s. In the US, these fall under
Part 15 Code of Federal Regulations The ''Code of Federal Regulations'' (''CFR'') is the codification of the general and permanent regulations published in the ''Federal Register'' by the executive departments and agencies of the federal government of t ...
of the
Federal Communications Commission The Federal Communications Commission (FCC) is an Independent agencies of the United States government, independent agency of the United States government that regulates communications by radio, television, wire, satellite, and cable across the ...
(FCC) regulations. Although they can be operated without a license, these devices still generally must be type-approved before sale.


History

The first primitive radio transmitters (called
spark gap transmitter A spark-gap transmitter is an obsolete type of transmitter, radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the ma ...

spark gap transmitter
s) were built by German physicist
Heinrich Hertz Heinrich Rudolf Hertz ( ; ; 22 February 1857 – 1 January 1894) was a German physicist who first conclusively proved the existence of the electromagnetic waves predicted by James Clerk Maxwell's Maxwell's equations, equations of electroma ...

Heinrich Hertz
in 1887 during his pioneering investigations of radio waves. These generated radio waves by a high voltage spark between two conductors. Beginning in 1895,
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of ...

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 Audio most commonly refers to sound In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, its Motion ( ...
(sound) and instead transmitted information by
radiotelegraphy Wireless telegraphy or radiotelegraphy is transmission of electric telegraph, telegraph signals by radio waves. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for transmitting telegraph sign ...
, the operator tapped on a
telegraph key A telegraph key is a specialized electrical switch used by a trained operator to transmit text messages in Morse code in a telegraphy system. Keys are used in all forms of electrical telegraph systems, including landline (also called wire) te ...
which turned the transmitter on and off to produce pulses of radio waves spelling out text messages in
Morse code Morse code is a method used in telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines bot ...
. At the receiver, these pulses were audible as "beeps" in the receiver's loudspeaker and 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 Wireless telegraphy or radiotelegraphy is transmission of telegraph Telegraphy is the long-distance transmission of textual messages where the sender uses a semaphore system, known to the recipient, rather than a physical exchange of an ...
or "spark" era. Because they generated
damped wave Damping is an influence within or upon an oscillatory system that has the effect of reducing or preventing its oscillation. In physical systems, damping is produced by processes that dissipate the energy stored in the oscillation. Examples inclu ...
s, spark transmitters were electrically "noisy". Their energy was spread over a broad band of
frequencies Frequency is the number of occurrences of a repeating event per unit of time. It is also referred to as temporal frequency, which emphasizes the contrast to spatial frequency and angular frequency. Frequency is measured in Hertz (unit), hertz ( ...

frequencies
, creating
radio noise In radio Radio is the technology of signaling and telecommunication, communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electroni ...
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 A continuous wave or continuous waveform (CW) is an electromagnetic wave Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically ...
transmitters: the
arc converter The arc converter, sometimes called the arc transmitter, or Poulsen arc after Danish engineer Valdemar Poulsen who invented it in 1903, was a variety of spark transmitter used in early wireless telegraphy Wireless telegraphy or radiotelegraphy ...
(
Poulsen arc The arc converter, sometimes called the arc transmitter, or Poulsen arc after Danish engineer Valdemar Poulsen who invented it in 1903, was a variety of spark transmitter used in early wireless telegraphy. The arc converter used an electric arc ...
) in 1904 and the
Alexanderson alternator An Alexanderson alternator is a alternator, rotating machine invented by Ernst Alexanderson in 1904 for the generation of high-frequency alternating current for use as a Transmitter, radio transmitter. It was one of the first devices capable of ge ...

Alexanderson alternator
around 1910, which were used into the 1920s. All these early technologies were replaced by
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor o ...
transmitters in the 1920s, which used the 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 A continuous wave or continuous waveform (CW) is an electromagnetic wave Electromagnetism is a branch of physics involving the study of the electromagnetic force, a type of physical interaction that occurs between electric charge, electrically ...
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 Frequency modulation (FM) is the encoding of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines both its essence and the nature of its characteristic ...

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 A television network or broadcaster is a telecommunications network for distribution of television program content, where a central operation provides programming to many television stations or pay television providers. Until the mid-1980s, te ...

television broadcasting
didn't begin until the late 1930s. The development of
radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Ear ...

radar
during World War II motivated the evolution of high frequency transmitters in the Ultrahigh frequency, UHF and
microwave Microwave is a form of electromagnetic radiation In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies mat ...

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 A wireless microphone, or cordless microphone, is a microphone A microphone, colloquially called a mic or mike (), is a device – a transducer – that converts sound In physics Physics (from grc, φυσική (ἐπιστή ...
s,
garage door opener A residential garage door opener. The motor is in the box on the upper-right. A garage door opener is a motorized device that opens and closes garage doors controlled by switches on the garage wall. Most also include a handheld radio remote contr ...

garage door opener
s and
walkie-talkie A walkie-talkie, more formally known as a handheld transceiver (HT), is a hand-held, portable, two-way radio transceiver. Its development during the Second World War has been variously credited to Donald Hings, radio engineer Alfred J. Gross, He ...
s. The development of the integrated circuit (IC) in the 1970s made possible the current proliferation of wireless devices, such as
cell phone A mobile phone, cellular phone, cell phone, cellphone, handphone, or hand phone, sometimes shortened to simply mobile, cell or just phone, is a portable telephone A telephone is a telecommunications Appliance (disambiguation), devic ...
s and
Wi-Fi Wi-Fi () is a family of wireless network Communication protocol, protocols, based on the IEEE 802.11 family of standards, which are commonly used for Wireless LAN, local area networking of devices and Internet access, allowing nearby digital de ...

Wi-Fi
networks, in which integrated digital transmitters and receivers (wireless modems) in portable devices operate automatically, in the background, to exchange data with wireless networks. The need to conserve bandwidth in the increasingly congested
radio spectrum The radio spectrum is the part of the electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of ...
is driving the development of new types of transmitters such as spread spectrum, trunked radio systems 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 Spectral efficiency, spectrum efficiency or bandwidth efficiency refers to the information rate that can be transmitted over a given bandwidth Bandwidth commonly refers to: * Bandwidth (signal processing) or ''analog bandwidth'', ''frequency bandw ...
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 circuits. File:Marconi 1897 spark gap transmitter.jpg,
Guglielmo Marconi Guglielmo Giovanni Maria Marconi, 1st Marquis of Marconi (; 25 April 187420 July 1937) was an Italian Italian may refer to: * Anything of, from, or related to the country and nation of Italy ** Italians, an ethnic group or simply a citizen of ...

Guglielmo Marconi
's
spark gap transmitter A spark-gap transmitter is an obsolete type of transmitter, radio transmitter which generates radio waves by means of an electric spark."Radio Transmitters, Early" in Spark-gap transmitters were the first type of radio transmitter, and were the ma ...

spark gap transmitter
, with which he performed the first experiments in practical
Morse code Morse code is a method used in telecommunication Telecommunication is the transmission of information Information can be thought of as the resolution of uncertainty; it answers the question of "What an entity is" and thus defines bot ...
radiotelegraphy Wireless telegraphy or radiotelegraphy is transmission of electric telegraph, telegraph signals by radio waves. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for transmitting telegraph sign ...
communication in 1895-1897 File:Powerful spark gap transmitter.png, High power spark gap
radiotelegraphy Wireless telegraphy or radiotelegraphy is transmission of electric telegraph, telegraph signals by radio waves. Before about 1910, the term ''wireless telegraphy'' was also used for other experimental technologies for transmitting telegraph sign ...
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 An Alexanderson alternator is a alternator, rotating machine invented by Ernst Alexanderson in 1904 for the generation of high-frequency alternating current for use as a Transmitter, radio transmitter. It was one of the first devices capable of ge ...

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 A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current An electric current is a stream of charged particles, such as electrons or ions, moving through an electrical conductor o ...
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 MHz at a power of 2 kW. File:Magnetron radar assembly 1947.jpg, Transmitter assembly of a 20 kW, 9.375 GHz
air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through controlled airspace, and can provide advisory services to aircraft in non-controlled airspace. The primary purp ...

air traffic control
radar Radar is a detection system that uses radio waves to determine the range, angle, or velocity of objects. It can be used to detect aircraft An aircraft is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Ear ...

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.


See also

*List of transmission sites *Radio transmitter design *Repeater *Transmitter station *Transposer *Television transmitter *Fiber-optic communication#Transmitters, Fiber-optic transmitters


References


External links


International Telecommunication Union
*[http://www.wolfbane.com/ukdtt.htm Details of UK digital television transmitters] {{Telecommunications Broadcast transmitters, Telecommunications equipment Radar