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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 two decades of radio (1885-1906) and the first demonstrations of practical radio were carried out using them.

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, and in 1888 physicist Heinrich Hertz was the first 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.

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.

Nikola 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. Guglielmo Marconi, inspired by the various experimenters, began developing wireless telegraphy using high power spark transmitters. Marconi was able to establish a successful 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.

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.

A typical spark transmitter circuit.
Legend:
capacitor - C₁ and C₂;
resistor - R;
inductor - L.

The spark transmitter is very simple in operation, but it presented significant technical problems mostly due to very large induced EMF when the spark struck, which caused breakdown of the insulation in the primary transformer. To overcome this the construction of even low-power sets was very solid. The damped wave output was very wasteful of bandwidth, and this limited the number of stations that could communicate effectively without interfering with each other.

In its simplest form, a spark-gap transmitter consists of a spark gap connected across an oscillatory circuit consisting of a capacitor and an inductor in series or parallel. In a typical transmitter circuit, a high voltage source (shown in the schematic as a battery, but usually a high voltage transformer) charges a capacitor (C1 in figure) through a resistor until the spark gap discharges, then a pulse of current passes through the capacitor (C2 in figure). The inductor and capacitor after the gap form a resonant circuit. After being excited by the current pulse, the oscillation rapidly decays because energy is radiated from the antenna. Because of the rapid onset and decay of the oscillation, the RF pulse occupies a large band of frequencies.

The function of the spark gap is to present initially a high resistance to the circuit to allow the capacitor to charge. When the breakdown voltage of the gap is reached, it then presents a low resistance to the circuit causing the capacitor to discharge. The discharge through the conducting spark takes the form of a damped oscillation, at a frequency determined by the resonant frequency of the LC circuit.

Main article: Spark gap

A simple spark gap consists of two conducting electrodes separated by a gap immersed within a gas (typically air). When a sufficiently high voltage is applied, a spark will bridge the gap, ionizing the gas and drastically reducing its electrical resistance. An electric current then flows until the path of ionized gas is broken or the current is reduced below a minimum value called the 'holding current'. This usually occurs when the voltage across the gap drops sufficiently, but the process may also be assisted by cooling the spark channel or by physically separating the electrodes. This breaks the conductive filament of ionized gas, allowing the capacitor to recharge, and permitting the recharging/discharging cycle to repeat. The action of ionizing the gas is quite sudden and violent (disruptive), and it creates a sharp sound (ranging from a snap for a spark plug, a loud bang for a wider gap, to thunder for a lightning discharge). The spark gap also liberates light and heat.

Spark gaps used in early radio transmitters varied in construction, depending on the power to be handled. Some were fairly simple, consisting of one or more fixed (static) gaps connected in series, while others were significantly more complex. Because sparks were quite hot and erosive, electrode wear and cooling were constant problems. As transmitter power was increased, the problem of quenching also arose.

Quenching refers to the act of extinguishing the previously established arc within the spark gap. This is considerably more difficult than initiating spark breakdown in the gap. A cold, non-firing spark gap contains no ionized gases. Once the voltage across the gap reaches its breakdown voltage, gas molecules in the gap are very quickly ionized along a path, creating a hot electric arc, or plasma, that consists of large numbers of ions and free electrons between the electrodes. The arc also heats part of the electrodes to incandescence. The incandescent regions contribute free electrons via thermionic emission, and (easily ionized) metal vapor. The mixture of ions and free electrons in the plasma is highly conductive, resulting in a sharp drop in the gap's electrical resistance. This highly conductive arc supports efficient tank circuit oscillations. However, the oscillating current also sustains the arc and, until it can be extinguished, the tank capacitor cannot be recharged for the next pulse.

Several methods were applied to quench the arc.

• Jets of air that cool, stretch, and literally 'blow out' the plasma,
• multi-plate discharger of Max Wien to cool the arcs in medium power spark sets, known as the "whistling spark" for its distinctive signal,
• using a different gas, such as hydrogen, that quenches more efficiently by providing more effective electrode cooling,
• a magnetic field (from a pair of permanent magnets or poles of an electromagnets) oriented at right angles to the gap to stretch and cool the arc.

The need to extinguish arcs in increasingly higher power transmitters led to the development of the rotating spark gap. These devices were used with an alternating current power supply, produced a more regular spark, and could handle more power than conventional static spark gaps. The inner rotating metal disc typically had a number of studs on its outer edge. A discharge would take place when two of the studs lined up with the two outer contacts which carried the high voltage. The resulting arcs were rapidly stretched, cooled, and broken as the disk rotated.

Rotary gaps were operated in two modes, synchronous and asynchronous. A synchronous gap was driven by a synchronous AC motor so that it ran at a fixed speed, and the gap fired in direct relation to the waveform of the A.C. supply that recharged the tank capacitor. The point in the waveform where the gaps were closest was changed by adjusting the rotor position on the motor shaft relative to the stator's studs. By properly adjusting the synchronous gap, it was possible to have the gap fire only at the voltage peaks of the input current. This technique allowed the tank circuit to fire only at successive voltage peaks, thereby delivering maximum energy from the fully charged tank capacitor each time the gap fired. The break rate was thus fixed at twice the incoming power frequency (typically, 100 to 120 breaks/second). When properly engineered and adjusted, synchronous spark gap systems delivered the largest amount of power to the antenna. However, electrode wear would progressively change the gap's firing point, so synchronous gaps were somewhat temperamental and difficult to maintain.

Asynchronous gaps were considerably more common. In an asynchronous gap, the rotation of the motor had no fixed relationship relative to the incoming AC waveform. Asynchronous gaps worked quite well and were much easier to maintain. By using a larger number of rotating studs or a higher rotational speed, many asynchronous gaps operated at break rates in excess of 400 breaks/second. Since the gap could be fired more often than the input waveform switched polarity, the tank capacitor was charged and discharged more rapidly than a synchronous gap. However, each discharge would occur at a varying voltage that was almost always lower than the consistent peak voltage obtained from a synchronous gap.

Rotary gaps also served to alter the tone of the transmitter, since changing either the number of studs or the rotational speed changed the spark discharge frequency which was audiable in receivers with detectors that could detect the modulation on the spark signal. This enabled listeners to distinguish between different transmitters that were nominally tuned to the same frequency. A typical high-power multiple spark system (as it was also called) used a 9 to 24 inch diameter rotating commutator with six to twelve studs per wheel, typically switching several thousand volts. The output of rotary spark gap transmitter was turned on and off by the operator using a special kind of telegraph key that switched power going to the high voltage power supply. The key was designed with large contacts to carry the heavy current that flowed into the low voltage (primary) side of the high voltage transformer (often in excess of 20 amps).

• Alternator, Arc and Spark
• Fessenden and the Early History of Radio Science
• Brief history of spark
• Massie Spark Transmitter The new England Wireless and Steam Museum
• The Sounds of a Spark Transmitter with audio
• Russ Kleinman's web site devoted to Spark Keys and Spark Transmitters - has virtual spark transmitter