A spark-gap transmitter is an obsolete type of radio transmitter which generates radio waves by means of an electric spark. Spark-gap transmitters were the first type of radio transmitter, and were the main type used during the wireless telegraphy or "spark" era, the first three decades of radio, from 1887 to the end of World War I. German physicist Heinrich Hertz built the first experimental spark-gap transmitters in 1887, with which he proved the existence of radio waves and studied their properties. A fundamental limitation of spark-gap transmitters is that they generate a series of brief transient pulses of radio waves called damped waves; they are unable to produce the continuous waves used to carry audio (sound) in modern AM or FM radio transmission. So spark-gap transmitters could not transmit audio, and instead transmitted information by radiotelegraphy; the operator switched the transmitter on and off with a telegraph key, creating pulses of radio waves to spell out text messages in Morse code. The first practical spark gap transmitters and receivers for radiotelegraphy communication were developed by Guglielmo Marconi around 1896. One of the first uses for spark-gap transmitters was on ships, to communicate with shore and broadcast a distress call if the ship was sinking. They played a crucial role in maritime rescues such as the 1912 RMS Titanic disaster. After World War I, vacuum tube transmitters were developed, which were less expensive and produced continuous waves which had a greater range, produced less interference, and could also carry audio, making spark transmitters obsolete by 1920. The radio signals produced by spark-gap transmitters are electrically "noisy"; they have a wide bandwidth, creating radio frequency interference (RFI) that can disrupt other radio transmissions. This type of radio emission has been prohibited by international law since 1934.
Theory of operation Electromagnetic waves are radiated by electric charges when they are accelerated. Radio waves, electromagnetic waves of radio frequency, can be generated by time-varying electric currents, consisting of electrons flowing through a conductor which suddenly change their velocity, thus accelerating. An electrically charged capacitance discharged through an electric spark across a spark gap between two conductors was the first device known which could generate radio waves. The spark itself does not produce the radio waves; it merely serves as a fast-acting switch to excite resonant radio-frequency oscillating electric currents in the conductors of the attached circuit. The conductors radiate the energy in this oscillating current as radio waves. Due to the inherent inductance of circuit conductors, the discharge of a capacitor through a low enough resistance (such as a spark) is oscillatory; the charge flows rapidly back and forth through the spark gap for a brief period, charging the conductors on each side alternately positive and negative, until the oscillations die away.
A practical spark gap transmitter consists of these parts:
A high-voltage transformer, to transform the low-voltage electricity from the power source, a battery or electric outlet, to a high enough voltage (from a few kilovolts to 75-100 kilovolts in powerful transmitters) to jump across the spark gap. The transformer charges the capacitor. In low-power transmitters powered by batteries, this was usually an induction coil (Ruhmkorff coil). One or more resonant circuits (tuned circuits or tank circuits) which create radio frequency electrical oscillations when excited by the spark. A resonant circuit consists of a capacitor (in early days a type called a Leyden jar) which stores high-voltage electricity from the transformer, and a coil of wire called an inductor or tuning coil, connected together. The values of the capacitance and inductance determine the frequency of the radio waves produced. The earliest spark-gap transmitters before 1897 did not have a resonant circuit; the antenna performed this function, acting as a resonator. However, this meant that the electromagnetic energy produced by the transmitter was dissipated across a wide band, thereby limiting its effective range to a few kilometers at most. Most spark transmitters had two resonant circuits coupled together with an air core transformer called a resonant transformer or oscillation transformer. This was called an inductively-coupled transmitter. The spark gap and capacitor connected to the primary winding of the transformer made one resonant circuit, which generated the oscillating current. The oscillating current in the primary winding created an oscillating magnetic field that induced current in the secondary winding. The antenna and ground were connected to the secondary winding. The capacitance of the antenna resonated with the secondary winding to make a second resonant circuit. The two resonant circuits were tuned to the same resonant frequency. The advantage of this circuit was that the oscillating current persisted in the antenna circuit even after the spark stopped, creating long, ringing, lightly damped waves, in which the energy was concentrated in a narrower bandwidth, creating less interference to other transmitters. A spark gap which acts as a voltage-controlled switch in the resonant circuit, discharging the capacitor through the coil. An antenna, a metal conductor such as an elevated wire, that radiates the power in the oscillating electric currents from the resonant circuit into space as radio waves. A telegraph key to switch the transmitter on and off to communicate messages by Morse code
Operation cycle The transmitter works in a rapid repeating cycle in which the capacitor is charged to a high voltage by the transformer and discharged through the coil by a spark across the spark gap. The impulsive spark excites the resonant circuit to "ring" like a bell, producing a brief oscillating current which is radiated as electromagnetic waves by the antenna. The transmitter repeats this cycle at a rapid rate, so the spark appeared continuous, and the radio signal sounded like a whine or buzz in a radio receiver.
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