Spark-gap transmitter

thumb|upright=1.35|Low-power inductively coupled spark-gap transmitter on display in Electric Museum, [[Frastanz, Austria. The spark gap is inside the box with the transparent cover at top center.]]

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. 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.

thumb|upright=1.5|Pictorial diagram of a simple spark-gap transmitter from a 1917 boy's hobby book, showing examples of the early electronic components used. It is typical of the low-power transmitters homebuilt by thousands of amateurs during this period to explore the exciting new technology of radio.

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. 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.

Induction coil

An induction coil (Ruhmkorff coil) was used in low-power transmitters, usually less than 500 watts, often battery-powered. An induction coil is a type of transformer powered by DC, in which a vibrating arm switch contact on the coil called an interrupter repeatedly breaks the circuit that provides current to the primary winding, causing the coil to generate pulses of high voltage. When the primary current to the coil is turned on, the primary winding creates a magnetic field in the iron core which pulls the springy interrupter arm away from its contact, opening the switch and cutting off the primary current. Then the magnetic field collapses, creating a pulse of high voltage in the secondary winding, and the interrupter arm springs back to close the contact again, and the cycle repeats. Each pulse of high voltage charged up the capacitor until the spark gap fired, resulting in one spark per pulse. Interrupters were limited to low spark rates of 20–100 Hz, sounding like a low buzz in the receiver. In powerful induction coil transmitters, instead of a vibrating interrupter, a mercury turbine interrupter was used. This could break the current at rates up to several thousand hertz, and the rate could be adjusted to produce the best tone.

AC transformer

In higher power transmitters powered by AC, a transformer steps the input voltage up to the high voltage needed. The sinusoidal voltage from the transformer is applied directly to the capacitor, so the voltage on the capacitor varies from a high positive voltage, to zero, to a high negative voltage. The spark gap is adjusted so sparks only occur near the maximum voltage, at peaks of the AC sine wave, when the capacitor was fully charged. Since the AC sine wave has two peaks per cycle, ideally two sparks occurred during each cycle, so the spark rate was equal to twice the frequency of the AC power Thomas Edison had come close to discovering radio in 1875; he had generated and detected radio waves which he called "etheric currents" experimenting with high-voltage spark circuits, but due to lack of time did not pursue the matter. Fitzgerald in a brief note published in 1883 suggested that electromagnetic waves could be generated practically by discharging a capacitor rapidly; the method used in spark transmitters, however there is no indication that this inspired other inventors.

The division of the history of spark transmitters into the different types below follows the organization of the subject used in many wireless textbooks.

Hertzian oscillators

German physicist Heinrich Hertz in 1887 built the first experimental spark gap transmitters during his historic experiments to demonstrate the existence of electromagnetic waves predicted by James Clerk Maxwell in 1864, in which he discovered radio waves,

Due to the influence of Maxwell's theory, their thinking was dominated by the similarity between radio waves and light waves; they thought of radio waves as an invisible form of light. As late as 1894 Oliver Lodge speculated that the maximum distance Hertzian waves could be transmitted was a half mile.

The "spark" era

The first application of radio was on ships, to keep in touch with shore, and send out a distress call if the ship were sinking. The Marconi Company built a string of shore stations and in 1904 established the first Morse code distress call, the letters CQD, used until the Second International Radiotelegraphic Convention in 1906 at which SOS was agreed on. The first significant marine rescue due to radiotelegraphy was the 23 January 1909 sinking of the luxury liner RMS Republic, in which 1500 people were saved.

{| style="background:#e0e0e0;" align="right" border="1" cellpadding="5" cellspacing="0"

! colspan="4" style="background:#c0c0c0; text-align:center;" |Radio frequencies used by spark transmitters during the wireless telegraphy era

! style="background:#d0d0d0; text-align:center;" | Uses

! style="background:#d0d0d0; text-align:center;" border="1" | Frequency<br/>(kilohertz)

! style="background:#d0d0d0; text-align:center;" border="1" | Wavelength<br/>(meters)

! style="background:#d0d0d0; text-align:center;" | Typical power<br/> range (kW)

| valign="top" | Amateur || > 1500 || < 200 || 0.25 - 0.5

| Ships || 500, 660, 1000 || 600, 450, 300 || 1 - 10

| Navy || 187.5 - 500 || 1600 - 600 || 5 - 20

| Moderate size land stations || 187.5 - 333 || 1600 - 900 || 5 - 20

| Transoceanic stations || 15 - 187.5 || 20,000 - 1600 || 20 - 500

Spark transmitters and the crystal receivers used to receive them were simple enough that they were widely built by hobbyists. Low-power amateur transmitters ("squeak boxes") were often built with "trembler" ignition coils from early automobiles such as the Ford Model T.

The sinking of the on 15 April 1912 increased public appreciation for the role of radio, but the loss of life brought attention to the disorganized state of the new radio industry, and prompted regulation which corrected some abuses. In response Congress passed the 1912 Radio Act, in which licenses were required for all radio transmitters, maximum damping of transmitters was limited to a decrement of 0.2 to get old noisy non-syntonic transmitters off the air, and amateurs were mainly restricted to the unused frequencies above 1.5 MHz and output power of 1 kilowatt. Beginning about 1910, industrial countries built global networks of these stations to exchange commercial and diplomatic telegram traffic with other countries and communicate with their overseas colonies. During World War I, radio became a strategic defensive technology, as it was realized a nation without long distance radiotelegraph stations could be isolated by an enemy cutting its submarine telegraph cables. With a spark transmitter, when the telegraph key was up between Morse symbols the carrier wave was turned off and the receiver was turned on, so the operator could listen for an incoming message. This allowed the receiving station, or a third station, to interrupt or "break in" to an ongoing transmission. In contrast, these early CW transmitters had to operate continuously; the carrier wave was not turned off between Morse code symbols, words, or sentences but just detuned, so a local receiver could not operate as long as the transmitter was powered up. Therefore, these stations could not receive messages until the transmitter was turned off.

Obsolescence

thumb|upright= 1.0|Marconi 2 kilowatt ship spark transmitter, 1920.

All these early technologies were superseded by the vacuum tube feedback electronic oscillator, invented in 1912 by Edwin Armstrong and Alexander Meissner, which used the triode vacuum tube invented in 1906 by Lee de Forest. Spark transmitters were obsolete at this point, and broadcast radio audiences and aviation authorities were complaining of the disruption to radio reception that noisy legacy marine spark transmitters were causing. But shipping interests vigorously fought a blanket prohibition on damped waves, due to the capital expenditure that would be required to replace spark equipment that was still being used on older ships. The Convention prohibited licensing of new land spark transmitters after 1929. Damped wave radio emission, called Class B, was banned after 1934 except for emergency use on ships. High oscillating voltages of hundreds of thousands of volts at frequencies of 0.1 - 1 MHz from a Tesla coil were applied directly to the patient's body. The treatment was not painful, because currents in the radio frequency range do not cause the physiological reaction of electric shock. In 1926 William T. Bovie discovered that RF currents applied to a scalpel could cut and cauterize tissue in medical operations, and spark oscillators were used as electrosurgery generators or "Bovies" as late as the 1980s.

In the 1950s a Japanese toy company, Matsudaya, produced a line of cheap remote control toy trucks, boats and robots called Radicon, which used a low-power spark transmitter in the controller as an inexpensive way to produce the radio control signals. The signals were received in the toy by a coherer receiver.

Spark gap oscillators are still used to generate high-frequency high voltage needed to initiate welding arcs in gas tungsten arc welding. Powerful spark gap pulse generators are still used to simulate EMPs.

References

External links

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 Sparks Telegraph Key Review
Radio Technology in common use circa 1914
Spark gap transmitter history & operation