a transmitter or radio transmitter is an electronic device
which produces radio wave
s with an antenna
. The transmitter itself generates a radio frequency alternating current
, which is applied to the antenna
. When excited by this alternating current, the antenna radiates radio waves.
Transmitters are necessary component parts of all electronic devices that communicate by radio
, such as radio
and television broadcasting
stations, cell phone
s, wireless computer networks
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
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.
A transmitter can be a separate piece of electronic equipment, or an electrical circuit
within another electronic device. A transmitter and a receiver
combined in one unit is called a transceiver
. The term transmitter is often abbreviated "XMTR" or "TX" in technical documents. 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, such as an audio
(sound) signal from a microphone, a video
(TV) signal from a video camera, or in wireless networking
devices, a digital signal
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
. This process is 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
, 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
s are radiated by electric charge
s when they are accelerated
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
which are changing their velocity and thus accelerating.
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 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. The transmitter also impresses information such as an audio
or video signal
onto 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.
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 generates 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.
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 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 digital signal
which represents a sequence of 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
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 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''.
*A radio frequency (RF) amplifier
to increase the power of the signal, to increase the range of the radio waves.
*An impedance matching
) circuit to match the impedance
of the transmitter to 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 UHF
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.
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 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
and are restricted to certain frequencies and power levels. A body called the International Telecommunications 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
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-approved
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 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
(sound) and instead transmitted information by radiotelegraphy
, the operator tapped on a telegraph key
which turned the transmitter on and off to produce pulses of radio waves spelling out text messages in 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 frequencies
, creating 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
) 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 feedback oscillator
invented by Edwin Armstrong
and Alexander Meissner
around 1912, based on the Audion
) vacuum tube invented by Lee De Forest
in 1906. Vacuum tube transmitters were inexpensive and produced continuous wave
s, and could be easily 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 UHF
ranges, using new active devices such as the magnetron
, 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
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
radio transmission methods. Digital modulation
can have greater spectral efficiency
than analog modulation
; that is it can often transmit more information (data rate
) in a given 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
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 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 radar, 1947. The magnetron tube mounted between two magnets ''(right)'' produces microwaves which pass from the aperture ''(left)'' into a waveguide which conducts them to the dish antenna.
*List of transmission sites
*Radio transmitter design
International Telecommunication Union
*ttp://www.wolfbane.com/ukdtt.htm Details of UK digital television transmitters