Editing Transmitter (section)

==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''.
*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.