page_title: Transmitter
section_title: History
hierarchical_section_title: Transmitter / History
caption_attribution_description: English: Artist's conception of German scientist Heinrich Hertz historic discovery of radio waves in 1886. To generate the waves Hertz used a spark gap radio transmitter (rear) consisting of a spark gap between two brass balls attached to a half-wave dipole antenna consisting of two wires with metal plates at the ends, powered by a Ruhmkorff coil with primary current supplied by a set of liquid batteries on the lower table. The Ruhmkorff coil generated pulses of high voltage which caused sparks to jump between the brass balls. Each spark excited oscillating radio frequency currents in the antenna, which were radiated as electromagnetic waves (radio waves). For a receiver, Hertz used a simple loop of wire (in his hands) with a narrow gap between the ends, forming a narrow spark gap (this picture is slightly inaccurate; Hertz actually used a micrometer gap consisting of an adjustable thumbscrew with its end close to the opposite electrode, to precisely measure the spark length). The length of the wire was a quarter wavelength, so the loop formed a resonant loop antenna, and the radio waves excited a voltage in the wire. So each spark of the transmitter excited a spark in the receiver loop. The transmitter shown was one common type Hertz used, but not necessarily the one with which he made his discovery. Hertz experimented with various types of antenna, from a bare spark gap to dipoles 8 feet long, with square metal plates, round metal plates, and metal balls on their ends. Also, the size of the receiver spark gap is exaggerated in the drawing. The gap in the receiving loop was very small, less than a millimeter, to detect the weak voltages induced. Some of his receiving loops used a gap adjusted by a micrometer.
context_page_description: In electronics and telecommunications a transmitter or radio transmitter is an electronic device which produces radio waves 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 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 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 ovens 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.
context_section_description: The first primitive radio transmitters (called spark gap transmitters) 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 waves, 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 (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 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 waves, 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 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 microphones, garage door openers and walkie-talkies. The development of the integrated circuit (IC) in the 1970s made possible the current proliferation of wireless devices, such as cell phones and Wifi 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 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 to digital 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 circuits.
caption: Hertz discovering radio waves in 1887 with his first primitive radio transmitter (background).
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