Spark-gap transmitter
Encyclopedia
A spark-gap transmitter is a device for generating radio frequency
electromagnetic waves
using a spark gap
.
These devices served as the transmitter
s 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 (prior to the development of practical vacuum tube
systems) somewhat more efficient transmitters were developed based on high-speed Alexanderson alternator
s and Poulsen Arc generators, but spark transmitters were still preferred by most operators. This was both because of 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, to keep transmitter leakage from de-sensitizing the receiver. After WWI
, greatly improved transmitters based on vacuum tubes became available, which overcame these problems, and by the late 1920s the only spark transmitters still in regular 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. However, by 1940, the technology was no longer used for communication. Use of the spark-gap transmitter led to many radio operators being nicknamed "Sparks" long after spark transmitters ceased to be used. Even today, the German verb "funken", literally, "to spark", also means "to send a radio message/signal".
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 detectable range of approximately 500 metres.
In 1888 physicist Heinrich Hertz set out to scientifically 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 was a major technological breakthrough. It produced a far more coherent
carrier wave, generated far less interference, worked with much greater efficiency, required much lower operating voltages and could be operated in any weather conditions.
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 (e.g., , "Method of Operating Arc-Lamps" (March 10, 1891)). Tesla also invented a variety of rotary, cooled, and quenched spark gap
s 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 the various early systems of spark transmitters worked well enough to prove the concept of wireless telegraphy, the primitive spark gap assemblies used 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. This signal-bleed played a role in the inability of the RMS Titanic and the SS Californian to communicate with each other in 1912 (see Sinking of the RMS Titanic).
Despite these flaws, Marconi was able to generate sufficient interest from the British Admiralty in these originally crude systems to eventually finance the development of a commercial wireless telegraph service between United States
and Europe
using vastly improved equipment.
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
. At least one high-powered audio transmitter used water cooling for the microphone.
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 Magnetic Detectors or electrolytic detector
s. 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
automobile
s. But a typical commercial station in 1916 might include a 1/2 kW transformer that supplied 14,000 volt
s, an eight section capacitor
, 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 AC output was then stepped up to 10,000–14,000 volts by a transformer. This was a very convenient arrangement, since the signal could be easily modulated by simply connecting a relay
between the relatively low voltage alternator output and the transformer's primary winding, and activating it with the morse
key. (Lower-powered units sometimes used the morse key to directly switch the AC, but this required a heavier key making it more difficult to operate).
Spark gap transmitters generate fairly broad-band signals. As the more efficient transmission mode of continuous wave
s (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 converter
s 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
. As late as 1955, a Japanese radio-controlled toy bus used a spark transmitter and coherer receiver; the spark was visible behind a sheet of blue transparent plastic. Spark gap oscillators are still used to generate high frequency high voltage to initiate welding arcs in gas tungsten arc welding
http://www.lincolnelectric.com/knowledge/articles/content/tigseriesreliable.asp. Powerful spark gap pulse generators are still used to simulate EMP
s. Most high power gas-discharge street lamps (mercury and sodium vapor) still use modified spark transmitters as switch-on ignitors.
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 a high resistance
to the circuit initially 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
.
s separated by a gap immersed within a gas
(typically air). When a sufficiently high voltage
is applied, a spark
will bridge the gap, ion
izing 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
, to a loud bang for a wider gap). The spark gap also liberates light
and heat
.
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.
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.
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 audible 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 in (228.6 to 609.6 mm) rotating commutator
with six to twelve studs per wheel, typically switching several thousand volts.
The output of a 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
). Alternatively a relay
was used to do the actual switching.
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
electromagnetic waves
Electromagnetic radiation
Electromagnetic radiation is a form of energy that exhibits wave-like behavior as it travels through space...
using a spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...
.
These devices served as the transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
s for most wireless
Wireless
Wireless telecommunications is the transfer of information between two or more points that are not physically connected. Distances can be short, such as a few meters for television remote control, or as far as thousands or even millions of kilometers for deep-space radio communications...
telegraphy
Telegraphy
Telegraphy is the long-distance transmission of messages via some form of signalling technology. Telegraphy requires messages to be converted to a code which is known to both sender and receiver...
systems for the first three decades of radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
(1887–1916) and the first demonstrations of practical radio were carried out using them. In later years (prior to the development of practical vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
systems) somewhat more efficient transmitters were developed based on high-speed Alexanderson alternator
Alexanderson alternator
An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...
s and Poulsen Arc generators, but spark transmitters were still preferred by most operators. This was both because of their uncomplicated design and the fact that the transmitter stopped generating a carrier as soon as the Morse Code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
key was released, allowing the operator to "listen through" for a reply. With the other types of transmitter, the carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
could not be controlled so easily, and elaborate measures were required both to modulate
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
the carrier and to separate the receiving antenna from the transmitting antenna, to keep transmitter leakage from de-sensitizing the receiver. After WWI
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
, greatly improved transmitters based on vacuum tubes became available, which overcame these problems, and by the late 1920s the only spark transmitters still in regular operation were "legacy" installations on Naval vessels. Even when vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
based transmitters had been installed, many vessels retained their crude but reliable spark transmitters as an emergency backup. However, by 1940, the technology was no longer used for communication. Use of the spark-gap transmitter led to many radio operators being nicknamed "Sparks" long after spark transmitters ceased to be used. Even today, the German verb "funken", literally, "to spark", also means "to send a radio message/signal".
History
The history of radioHistory of radio
The early history of radio is the history of technology that produced radio instruments that use radio waves. Within the timeline of radio, many people contributed theory and inventions in what became radio. Radio development began as "wireless telegraphy"...
shows that the spark gap transmitter was the product of many people, often working in competition. In 1862 James Clerk Maxwell
James Clerk Maxwell
James Clerk Maxwell of Glenlair was a Scottish physicist and mathematician. His most prominent achievement was formulating classical electromagnetic theory. This united all previously unrelated observations, experiments and equations of electricity, magnetism and optics into a consistent theory...
predicted the propagation
Wave propagation
Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....
of electromagnetic waves through a vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
.
In 1887, David E. Hughes
David E. Hughes
David Edward Hughes , was a British scientist and musician. Hughes was co-inventor of the microphone, a harpist and a professor of music.-Biography:...
used a spark gap to generate radio signals, achieving a detectable range of approximately 500 metres.
In 1888 physicist Heinrich Hertz set out to scientifically verify Maxwell's predictions. Hertz used a tuned spark gap transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
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
Nikola Tesla
Nikola Tesla was a Serbian-American inventor, mechanical engineer, and electrical engineer...
introduced his radio system in 1893 and later developed the so-called "loose coupler" system which was a major technological breakthrough. It produced a far more coherent
Coherence (physics)
In physics, coherence is a property of waves that enables stationary interference. More generally, coherence describes all properties of the correlation between physical quantities of a wave....
carrier wave, generated far less interference, worked with much greater efficiency, required much lower operating voltages and could be operated in any weather conditions.
Tesla pursued the application of his high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
high frequency
High frequency
High frequency radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters . Frequencies immediately below HF are denoted Medium-frequency , and the next higher frequencies are known as Very high frequency...
technology to radio. By tuning a receiving coil to the specific frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
used in the transmitting coil, he showed that the radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
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 (e.g., , "Method of Operating Arc-Lamps" (March 10, 1891)). Tesla also invented a variety of rotary, cooled, and quenched spark gap
Spark gap
A spark gap consists of an arrangement of two conducting electrodes separated by a gap usually filled with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the gap's breakdown voltage, a spark forms,...
s capable of handling high power.
Marconi
Marconi Company
The Marconi Company Ltd. was founded by Guglielmo Marconi in 1897 as The Wireless Telegraph & Signal Company...
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 the various early systems of spark transmitters worked well enough to prove the concept of wireless telegraphy, the primitive spark gap assemblies used 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
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
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. This signal-bleed played a role in the inability of the RMS Titanic and the SS Californian to communicate with each other in 1912 (see Sinking of the RMS Titanic).
Despite these flaws, Marconi was able to generate sufficient interest from the British Admiralty in these originally crude systems to eventually finance the development of a commercial wireless telegraph service between United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
and Europe
Europe
Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
using vastly improved equipment.
Reginald Fessenden
Reginald Fessenden
Reginald Aubrey Fessenden , a naturalized American citizen born in Canada, was an inventor who performed pioneering experiments in radio, including early—and possibly the first—radio transmissions of voice and music...
'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
Carbon microphone
The carbon microphone, also known as a carbon button microphone or a carbon transmitter, is a sound-to-electrical signal transducer consisting of two metal plates separated by granules of carbon. One plate faces outward and acts as a diaphragm...
in series with the supply lead. He experienced great difficulty in achieving intelligible
Intelligibility
In phonetics, Intelligibility is a measure of how comprehendible speech is, or the degree to which speech can be understood. Intelligibility is affected by spoken clarity, explicitness, lucidity, comprehensibility, perspicuity, and precision.-Noise levels:...
sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
. At least one high-powered audio transmitter used water cooling for the microphone.
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
International distress frequency
Since early in the 20th century, the radio frequency of 500 kilohertz has been an international calling and distress frequency for Morse code maritime communication. The unit kilohertz was not introduced until 1960...
. The receivers were simple unamplified Magnetic Detectors or electrolytic detector
Electrolytic detector
The electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of wet demodulator used in early radio receivers. This form of detector was in extensive use between the years 1902 and 1913, after which the superior vacuum tube diode became available...
s. This later gave way to the famous and more sensitive galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
crystal sets. Tuners
Tuner (radio)
A radio tuner is a subsystem that receives radio broadcasts and converts them into audio-frequency signals which can be fed into an amplifier driving a loudspeaker. FM tuner, AM tuner, Digital Audio Broadcasting DAB tuner, etc. are types of radio tuner dealing with transmissions using different...
were primitive or nonexistent. Early amateur radio operators built low power spark gap transmitters using the spark coil from Ford Model T
Ford Model T
The Ford Model T is an automobile that was produced by Henry Ford's Ford Motor Company from September 1908 to May 1927...
automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s. But a typical commercial station in 1916 might include a 1/2 kW transformer that supplied 14,000 volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s, an eight section capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, 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
Alternator
An alternator is an electromechanical device that converts mechanical energy to electrical energy in the form of alternating current.Most alternators use a rotating magnetic field but linear alternators are occasionally used...
whose AC output was then stepped up to 10,000–14,000 volts by a transformer. This was a very convenient arrangement, since the signal could be easily modulated by simply connecting a relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
between the relatively low voltage alternator output and the transformer's primary winding, and activating it with the morse
Morse
Morse can refer to:* Morse code, a method of coding messages into long and short beeps-Places:Canada* Morse , Saskatchewan* Morse, Saskatchewan, a hamlet* Morse No...
key. (Lower-powered units sometimes used the morse key to directly switch the AC, but this required a heavier key making it more difficult to operate).
Spark gap transmitters generate fairly broad-band signals. As the more efficient transmission mode of continuous wave
Continuous wave
A continuous wave or continuous waveform is an electromagnetic wave of constant amplitude and frequency; and in mathematical analysis, of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a carrier wave is switched on and off...
s (CW) became easier to produce and band crowding and interference
Interference (communication)
In communications and electronics, especially in telecommunications, interference is anything which alters, modifies, or disrupts a signal as it travels along a channel between a source and a receiver. The term typically refers to the addition of unwanted signals to a useful signal...
worsened, spark-gap transmitters and damped waves were legislated off the new shorter wavelengths by international treaty
Treaty
A treaty is an express agreement under international law entered into by actors in international law, namely sovereign states and international organizations. A treaty may also be known as an agreement, protocol, covenant, convention or exchange of letters, among other terms...
, and replaced by Poulsen
Valdemar Poulsen
Valdemar Poulsen was a Danish engineer who developed a magnetic wire recorder in 1899.-Biography:He was born on 23 November 1869 in Copenhagen...
arc converter
Arc converter
The arc converter, sometimes called the arc transmitter or Poulsen arc after its inventor Danish engineer Valdemar Poulsen, is a device that used an electric arc to convert direct current electricity into radio frequency alternating current...
s and high frequency alternators
Alexanderson alternator
An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...
which developed a sharply defined transmitter frequency. These approaches later yielded to vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
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
Radio jamming
Radio jamming is the transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. Unintentional jamming occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency...
. As late as 1955, a Japanese radio-controlled toy bus used a spark transmitter and coherer receiver; the spark was visible behind a sheet of blue transparent plastic. Spark gap oscillators are still used to generate high frequency high voltage to initiate welding arcs in gas tungsten arc welding
Gas tungsten arc welding
Gas tungsten arc welding , also known as tungsten inert gas welding, is an arc welding process that uses a nonconsumable tungsten electrode to produce the weld...
http://www.lincolnelectric.com/knowledge/articles/content/tigseriesreliable.asp. Powerful spark gap pulse generators are still used to simulate EMP
Electromagnetic pulse
An electromagnetic pulse is a burst of electromagnetic radiation. The abrupt pulse of electromagnetic radiation usually results from certain types of high energy explosions, especially a nuclear explosion, or from a suddenly fluctuating magnetic field...
s. Most high power gas-discharge street lamps (mercury and sodium vapor) still use modified spark transmitters as switch-on ignitors.
Operation
Faraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
when the spark struck, which caused breakdown of the insulation
Electrical insulation
thumb|250px|[[Coaxial Cable]] with dielectric insulator supporting a central coreThis article refers to electrical insulation. For insulation of heat, see Thermal insulation...
in the primary transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
. To overcome this the construction of even low-power sets was very solid. The damped wave
Damped wave
A damped wave is a wave whose amplitude of oscillation decreases with time, eventually going to zero. This term also refers to an early method of radio transmission produced by spark gap transmitters, which consisted of a series of damped electromagnetic waves...
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
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
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
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
(C1 in figure) through a resistor
Resistor
A linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
until the spark gap discharges, then a pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...
of current passes through the capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
(C2 in figure). The inductor and capacitor after the gap form a resonant circuit. After being excited by the current pulse, the oscillation
Oscillation
Oscillation is the repetitive variation, typically in time, of some measure about a central value or between two or more different states. Familiar examples include a swinging pendulum and AC power. The term vibration is sometimes used more narrowly to mean a mechanical oscillation but sometimes...
rapidly decays because energy is radiated from the antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
. Because of the rapid onset and decay of the oscillation, the RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
pulse occupies a large band of frequencies.
The function of the spark gap is to present a high resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
to the circuit initially to allow the capacitor to charge. When the breakdown voltage
Breakdown voltage
The breakdown voltage of an insulator is the minimum voltage that causes a portion of an insulator to become electrically conductive.The breakdown voltage of a diode is the minimum reverse voltage to make the diode conduct in reverse...
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
LC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
.
Spark gaps
A simple spark gap consists of two conducting electrodeElectrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...
s separated by a gap immersed within a gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
(typically air). When a sufficiently high voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is applied, a spark
Electrostatic discharge
Electrostatic discharge is a serious issue in solid state electronics, such as integrated circuits. Integrated circuits are made from semiconductor materials such as silicon and insulating materials such as silicon dioxide...
will bridge the gap, ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
izing the gas and drastically reducing its electrical resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
. 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
Holding current
The term holding current may refer to:* In electronics, the holding current is the minimum current which must pass through the device in order for it to remain in the 'ON' state. See silicon controlled rectifier....
'. This usually occurs when the voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
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
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
(ranging from a snap for a spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
, to a loud bang for a wider gap). The spark gap also liberates light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
and heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
.
Quenching the arc
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. As transmitter power was increased, the problem of quenching arose.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
Electric arc
An electric arc is an electrical breakdown of a gas which produces an ongoing plasma discharge, resulting from a current flowing through normally nonconductive media such as air. A synonym is arc discharge. An arc discharge is characterized by a lower voltage than a glow discharge, and relies on...
, or plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, 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
Thermionic emission
Thermionic emission is the heat-induced flow of charge carriers from a surface or over a potential-energy barrier. This occurs because the thermal energy given to the carrier overcomes the binding potential, also known as work function of the metal. The charge carriers can be electrons or ions, and...
, 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
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
. 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 WienMax WienMax Wien was a German physicist and the director of the Institute of Physics at the University of Jena. He was born in Königsberg, Prussia.Wien studied under Helmholtz and Kundt. He invented the "Löschfunkensender" during the years 1906 to 1909 and the Wien bridge in 1891...
to cool the arcs in medium power spark sets, known as the "whistling spark" for its distinctive signal, - using a different gas, such as hydrogenHydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, that quenches more efficiently by providing more effective electrode cooling, - a magnetic fieldMagnetic fieldA magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
(from a pair of permanent magnetMagnetA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
s or poles of an electromagnetElectromagnetAn electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
s) oriented at right angles to the gap to stretch and cool the arc.
Magnetic
Rotary gaps
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 currentAlternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
power supply
Power supply
A power supply is a device that supplies electrical energy to one or more electric loads. The term is most commonly applied to devices that convert one form of electrical energy to another, though it may also refer to devices that convert another form of energy to electrical energy...
, 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
Synchronization (alternating current)
-Electricity generation:Electricity generation requires the connection of large numbers of alternators in parallel and additional alternators must be switched in when demand rises....
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
Waveform
Waveform means the shape and form of a signal such as a wave moving in a physical medium or an abstract representation.In many cases the medium in which the wave is being propagated does not permit a direct visual image of the form. In these cases, the term 'waveform' refers to the shape of a graph...
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
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
. 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
Electrical polarity
Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...
, 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
Timbre
In music, timbre is the quality of a musical note or sound or tone that distinguishes different types of sound production, such as voices and musical instruments, such as string instruments, wind instruments, and percussion instruments. The physical characteristics of sound that determine the...
of the transmitter, since changing either the number of studs or the rotational speed changed the spark discharge frequency which was audible 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 in (228.6 to 609.6 mm) rotating commutator
Commutator (electric)
A commutator is a rotary electrical switch in certain types of electric motors or electrical generators that periodically reverses the current direction between the rotor and the external circuit. In a motor, it applies power to the best location on the rotor, and in a generator, picks off power...
with six to twelve studs per wheel, typically switching several thousand volts.
The output of a 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
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
). Alternatively a relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
was used to do the actual switching.
See also
- CohererCohererThe coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
- Crystal Radio
- Spark-gap transmitterSpark-gap transmitterA spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...
- Radio receiverReceiver (radio)A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
- Antique radioAntique radioAn antique radio is a radio receiving set that is collectible because of its age and rarity. Although collectors may differ on the cutoff dates, most would use 50 years old, or the pre-World War II Era, for vacuum tube sets and the first five years of transistor sets.-Morse only sets:The first...
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 Sounds of a Spark Transmitter with audio
- Russ Kleinman's web site devoted to Spark Keys and Spark Transmitters - has virtual spark transmitter