Editing Transmitter (section)

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

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

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

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

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

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

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