A spark-gap transmitter is a device for generating radio frequency electromagnetic waves. These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio (1887–1916) and the first demonstrations of practical radio were carried out using them. In later years more efficient transmitters were developed based on high-speed Alexanderson alternators and Poulsen Arc generators, but spark transmitters were still preferred by most operators. This was because of both their uncomplicated design and the fact that the transmitter stopped generating a carrier as soon as the Morse Code key was released, allowing the operator to "listen through" for a reply. With the other types of transmitter, the carrier wave could not be controlled so easily, and elaborate measures were required both to modulate the carrier and to separate the receiving antenna from the transmitting antenna. After WWI, greatly improved vacuum tube transmitters became available which overcame these problems, and by the late 1920s the only spark transmitters still in operation were "legacy" installations on Naval vessels. Even when vacuum tube based transmitters had been installed, many vessels retained their crude but reliable spark transmitters as an emergency backup, but by 1940, the technology was no longer used. Use of the spark-gap transmitter has led to many radio operators being nicknamed "Sparks" even long after the transmitters were no longer in use.
History
The history of radio shows that the spark gap transmitter was the product of many people, often working in competition. In 1862 James Clerk Maxwell predicted the propagation of electromagnetic waves through a vacuum.
In 1887, David E. Hughes used a spark gap to generate radio signals, achieving a range of approximately 500 metres.
In 1888 physicist Heinrich Hertz set out to verify Maxwell's predictions. Hertz used a tuned spark gap transmitter and a tuned spark gap detector (consisting of a loop of wire connected to a small spark gap) located a few meters away. In a series of UHF experiments, Hertz verified that electromagnetic waves were being produced by the transmitter. When the transmitter sparked, small sparks also appeared across the receiver's spark gap, which could be seen under a microscope.
Nikola Tesla introduced his radio system in 1893 and later developed the so-called "loose coupler" system which produced a far more coherent carrier wave, produced far less interference, worked with much greater efficiency, and could be operated in any weather conditions.
One form of Nikola Tesla's Spark-gap transmitter
Source: H. S. Norrie, "Induction coils: how to make, use, and repair them". Norman H. Schneider, 1907, 4th edition, New York.Tesla pursued the application of his high voltage high frequency technology to radio. By tuning a receiving coil to the specific frequency used in the transmitting coil, he showed that the radio receiver's output could be greatly magnified through resonant action. Tesla was one of the first to patent a means to reliably produce radio frequencies (eg., U.S. Patent 447,920 , "Method of Operating Arc-Lamps" (March 10, 1891)). Tesla also invented a variety of rotary, cooled, and quenched spark gaps capable of handling high power.
Marconi began experimenting with wireless telegraphy in the early 1890s. In 1895 he succeeded in transmitting over a distance of 1 1/4 miles. His first transmitter consisted of an induction coil connected between a wire antenna and ground, with a spark gap across it. Every time the induction coil pulsed, the antenna would be momentarily charged up to tens (sometimes hundreds) of thousands of volts until the spark gap started to arc over. This acted as a switch, essentially connecting the charged antenna to ground, producing a very brief burst of electromagnetic radiation.
While this worked well enough to prove the concept of wireless telegraphy, it had some severe shortcomings. The biggest problem was that the maximum power that could be transmitted was directly determined by how much electrical charge the antenna could hold. Because the capacitance of practical antennas is quite small, the only way to get a reasonable power output was to charge it up to very high voltages. However, this made transmission impossible in rainy or even damp conditions. Also, it necessitated a quite wide spark gap, with a very high electrical resistance, with the result that most of the electrical energy was used simply to heat up the air in the spark gap.
The other problem was that, due to the very brief duration of each burst of electromagnetic radiation, the system radiated an extremely "dirty" signal sideband-wise, which was almost impossible to tune out if the listener wanted to monitor a different station. Despite this, Marconi was able to establish a commercial wireless telegraph service that served the United States and Europe.
Reginald Fessenden's first attempts to transmit voice employed a spark transmitter operating at approximately 10,000 sparks/second. To modulate this transmitter he inserted a carbon microphone in series with the supply lead. He experienced great difficulty in achieving Intelligible sound.
In 1905 a "state of the art" spark gap transmitter generated a signal having a wavelength between 250 meters (1.2 MHz) and 550 meters (545 kHz). 600 meters (500 kHz) became the International distress frequency. The receivers were simple unamplified detectors, usually coherers (small quantity of metal filings lying loosely between metallic electrodes). This later gave way to the famous and more sensitive galena crystal sets. Tuners were primitive or nonexistent. Early amateur radio operators built low power spark gap transmitters using the spark coil from Ford Model T automobiles. But a typical commercial station in 1916 might include a 1/2 kW transformer that supplied 14,000 volts, an eight section condenser, and a rotary gap capable of handling a peak current of several hundred amperes.
Shipboard installations usually used a DC motor (usually run off the ship's DC lighting supply) to drive an alternator whose output was then stepped up to 10,000–14,000 volts by a transformer.
Spark gap transmitters generate fairly broad signals. As the more efficient transmission mode of continuous waves (CW) became easier to produce and band crowding and interference worsened, spark-gap transmitters and damped waves were legislated off the new shorter wavelengths by international treaty, and replaced by Poulsen arc converters and high frequency alternators which developed a sharply defined transmitter frequency. These approaches later yielded to vacuum tube technology and the 'electric age' of radio would end. Long after they stopped being used for communications, spark gap transmitters were employed for radio jamming. Spark gap oscillators are still used to generate high frequency high voltage to initiate welding arcs in gas tungsten arc welding[1]. Powerful spark gap pulse generators are still used to simulate EMP. Most high power gas-discharge street lamps (mercury and sodium vapor) still use modified spark transmitters as switch-on ignitors.
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