Editing Spark-gap transmitter (section)

==Theory of operation==
[[Electromagnetic wave]]s are radiated by [[electric charge]]s when they are [[acceleration|accelerated]].<ref name="Serway">{{cite book |last1= Serway |first1= Raymond |last2= Faughn |first2= Jerry |last3= Vuille |first3= Chris |title= College Physics |edition= 8th |publisher= Cengage Learning |date= 2008 |pages= 714 |url= https://books.google.com/books?id=CX0u0mIOZ44C&q=%22electromagnetic+wave%22+charge+acceleration&pg=PA714 |isbn= 978-0495386933}}</ref><ref name="Ellingson">{{cite book
| last1= Ellingson
| first1= Steven W.
| title= Radio Systems Engineering
| publisher= Cambridge University Press
| date= 2016
| pages= 16–17
| url= https://books.google.com/books?id=QMKSDQAAQBAJ&q=%22radio+wave%22+time+varying+electric+current&pg=PA16
| isbn= 978-1316785164
}}</ref> [[Radio wave]]s, electromagnetic waves of radio [[frequency]], can be generated by time-varying [[electric current]]s, consisting of [[electron]]s flowing through a conductor which suddenly change their velocity, thus accelerating.<ref name="Ellingson"/><ref name="Nahin1">{{cite book
| last1= Nahin
| first1= Paul J.
| title= The Science of Radio: with MATLAB and Electronics Workbench demonstrations
| edition= 2nd
| publisher= Springer Science and Business Media
| date= 2001
| pages= 27–28
| url= https://books.google.com/books?id=V1GBW6UD4CcC&q=%22electromagnetic+waves%22+accelerated+&pg=PA27
| isbn= 978-0387951508
}}</ref> 

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.<ref name="Aitken2014" />{{rp|p.3}} The spark itself doesn't produce the radio waves, it merely serves as a fast acting [[switch]] to excite [[resonant]] [[radio frequency]] oscillating [[electric current]]s 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.<ref name="CodellaSparkRadio">{{cite web
| last= Codella
| first= Christopher F.
| title= Spark Radio
| work= Ham Radio History
| publisher= C. F. Codella's private website
| date= 2016
| url= http://w2pa.net/HRH/spark-radio/
| access-date= 22 May 2018}}</ref><ref name="Fleming2">{{cite book
| last1= Fleming
| first1= John Archibald
| title= The Principles of Electric Wave Telegraphy
| publisher= Longmans Green and Co.
| date= 1906
| location= London
| pages= 15–16
| url= https://archive.org/stream/cu31924074412739#page/n45/mode/2up/
}}</ref>

[[File:Spark gap transmitter diagram (alt).png|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:<ref name="CodellaSparkRadio"/><ref name="Kennedy">{{cite web
| last= Kennedy
| first= Hal
| title= How spark transmitters work
| work= The history of QST Vol. 1 - Technology
| publisher= American Radio Relay League
| date= 1990
| url= http://www.arrl.org/files/file/History/History%20of%20QST%20Volume%201%20-%20Technology/Kennedy%20N4GG.pdf
| access-date= 27 March 2018}}</ref><ref name="Morecroft">{{cite book
| last1= Morecroft
| first1= John H.
| title= Principles of Radio Communication
| publisher= John Wiley and Sons
| date= 1921
| location= New York
| pages= [https://archive.org/details/principlesofradi00moreuoft/page/275 275]–279
| url= https://archive.org/details/principlesofradi00moreuoft
}}</ref><ref name="Hyder">{{cite journal
  | last1  = Hyder
  | first1 = Harry R.
  | title = The final days of ham spark
  | journal = QST
  | volume = 
  | issue = 
  | pages = 29–32
  | publisher = American Radio Relay League
  | date = March 1992
  | url = http://rfcec.com/RFCEC/Section-3%20-%20Fundamentals%20of%20RF%20Communication-Electronics/13%20-%20HISTORY/Radio%20History%2002%20-%20The%20Final%20Days%20of%20Ham%20Spark%20(By%20Harry%20Hydir%20W7IV).pdf
  | issn = 
  | doi = 
  | id = 
  | accessdate = 5 February 2022}}</ref>
*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 [[kilovolt]]s 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 circuit]]s (tuned circuits or tank circuits) which create [[radio frequency]] electrical [[oscillation]]s 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''.<ref name="CodellaSparkRadio"/> 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 (signal processing)|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 (radio)|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 wave]]s.
*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.<ref name="CodellaSparkRadio"/><ref name="Nahin4">Nahin, Paul J. (2001) ''[https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA45&dq=spark The Science of Radio: with MATLAB and Electronics Workbench demonstrations, 2nd Ed.]'', p. 38-43</ref> 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.<ref name="CodellaSparkRadio"/> 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]].

[[File:Massie spark gap.jpg|thumb|right|Demonstration of the restored 1907 [[Massie Wireless Station]] spark gap transmitter 
{{Listen
| filename    = Massie Wireless Station "PJ" Spark Sound.ogg
| title       = Audio of Massie spark gap transmission
| description = [[Morse code]] of "[[CQ (call)|CQ DE]] PJ"
| pos         = left
| plain       = yes}}
]]
#The cycle begins when current from the transformer charges up the capacitor, storing positive electric charge on one of its plates and negative charge on the other.  While the capacitor is charging the spark gap is in its nonconductive state, preventing the charge from escaping through the coil.
#When the voltage on the capacitor reaches the [[breakdown voltage]] of the spark gap, the air in the gap [[ionization|ionizes]], starting an [[electric spark]], reducing its [[Electrical resistance|resistance]] to a very low level (usually less than one [[ohm]]). This closes the circuit between the capacitor and the coil.
#The charge on the capacitor discharges as a current through the coil and spark gap. Due to the [[inductance]] of the coil when the capacitor voltage reaches zero the current doesn't stop but keeps flowing, charging the capacitor plates with an opposite polarity, until the charge is stored in the capacitor again, on the opposite plates. Then the process repeats, with the charge flowing in the opposite direction through the coil. This continues, resulting in oscillating currents flowing rapidly back and forth between the plates of the capacitor through the coil and spark gap.
#The resonant circuit is connected to the antenna, so these oscillating currents also flow in the antenna, charging and discharging it. The current creates an oscillating [[magnetic field]] around the antenna, while the voltage creates an oscillating [[electric field]]. These oscillating fields radiate away from the antenna into space as an [[electromagnetic wave]]; a radio wave.
#The energy in the resonant circuit is limited to the amount of energy originally stored in the capacitor. The radiated radio waves, along with the heat generated by the spark, uses up this energy, causing the oscillations to decrease quickly in [[amplitude]] to zero. When the oscillating electric current in the primary circuit has decreased to a point where it is insufficient to keep the air in the spark gap ionized, the spark stops, opening the resonant circuit, and stopping the oscillations. In a transmitter with two resonant circuits, the oscillations in the secondary circuit and antenna may continue some time after the spark has terminated. Then the transformer begins charging the capacitor again, and the whole cycle repeats.

The cycle is very rapid, taking less than a millisecond. With each spark, this cycle produces a radio signal consisting of an oscillating [[sinusoidal]] wave that increases rapidly to a high [[amplitude]] and decreases [[exponential decay|exponentially]] to zero, called a [[Damped wave (radio transmission)|damped wave]].<ref name="CodellaSparkRadio"/> The [[frequency]] <math>f</math> of the oscillations, which is the frequency of the emitted radio waves, is equal to the [[resonant frequency]] of the resonant circuit, determined by the [[capacitance]] <math>C</math> of the capacitor and the [[inductance]] <math>L</math> of the coil:
:<math>f = \frac {1}{2 \pi} \sqrt { \frac {1}{LC}} \,</math>
The transmitter repeats this cycle rapidly, so the output is a repeating string of damped waves. This is equivalent to a radio signal [[amplitude modulation|amplitude modulated]] with a steady frequency, so it could be [[demodulation|demodulated]] in a radio receiver by a [[Rectifier|rectifying]] [[amplitude modulation|AM]] [[detector (radio)|detector]], such as the [[crystal detector]] or [[Fleming valve]] used during the wireless telegraphy era. The [[frequency]] of repetition (spark rate) is in the [[audio frequency|audio]] range, typically 50 to 1000 sparks per second, so in a receiver's [[earphone]]s the signal sounds like a steady tone, whine, or buzz.<ref name="Kennedy"/>

In order to transmit information with this signal, the operator turns the transmitter on and off rapidly by tapping on a [[switch]] called a [[telegraph key]] in the primary circuit of the transformer, producing sequences of short (dot) and long (dash) strings of damped waves, to spell out messages in [[Morse code]]. As long as the key is pressed the spark gap fires repetitively, creating a string of pulses of radio waves, so in a receiver the keypress sounds like a buzz; the entire Morse code message sounds like a sequence of buzzes separated by pauses. In low-power transmitters the key directly breaks the primary circuit of the supply transformer, while in high-power transmitters the key operates a heavy duty [[relay]] that breaks the primary circuit.

===Charging circuit and spark rate===
The circuit which charges the capacitors, along with the spark gap itself, determines the ''spark rate'' of the transmitter, the number of sparks and resulting damped wave pulses it produces per second, which determines the tone of the signal heard in the receiver. The spark rate should not be confused with the ''frequency'' of the transmitter, which is the number of sinusoidal oscillations per second in each damped wave. Since the transmitter produces one pulse of radio waves per spark, the output power of the transmitter was proportional to the spark rate, so higher rates were favored. Spark transmitters generally used one of three types of power circuits:<ref name="CodellaSparkRadio"/><ref name="Kennedy"/><ref name="Sarkar">{{cite book
| last1= Sarkar
| first1= T. K.
| last2= Mailloux
| first2= Robert
| last3= Oliner
| first3= Arthur A.
| title= History of Wireless
| publisher= John Wiley and Sons
| date= 2006
| url= https://archive.org/stream/HistoryOfWireless#page/n260/mode/2up
| isbn= 978-0471783015
}}</ref>{{rp|p.359–362}}

====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&nbsp;Hz, sounding like a low buzz in the receiver. In powerful induction coil transmitters, instead of a vibrating interrupter, a [[Induction coil#Mercury and electrolytic interrupters|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<ref name="Hyder" /> (often multiple sparks occurred during the peak of each half cycle). The spark rate of transmitters powered by 50 or 60&nbsp;Hz mains power was thus 100 or 120&nbsp;Hz. However higher audio frequencies cut through interference better, so in many transmitters the transformer was powered by a [[motor–generator|motor–alternator]] set, an [[electric motor]] with its shaft turning an [[alternator]], that produced AC at a higher frequency, usually 500&nbsp;Hz, resulting in a spark rate of 1000&nbsp;Hz.<ref name="Hyder" />

====Quenched spark gap====
The speed at which signals may be transmitted is naturally limited by the time taken for the spark to be extinguished. If, as described above, the conductive plasma does not, during the zero points of the alternating current, cool enough to extinguish the spark, a 'persistent spark' is maintained until the stored energy is dissipated, permitting practical operation only up to around 60 signals per second.{{citation needed|date=September 2024}} If active measures are taken to break the arc (either by blowing air through the spark or by lengthening the spark gap), a much shorter "quenched spark" may be obtained.{{citation needed|date=September 2024}} A simple quenched spark system still permits several oscillations of the capacitor circuit in the time taken for the spark to be quenched. With the spark circuit broken, the transmission frequency is solely determined by the antenna resonant circuit, which permits simpler tuning.

====Rotary spark gap====
In a transmitter with a "rotary" spark gap ''(below)'', the capacitor was charged by AC from a high-voltage transformer as above, and discharged by a spark gap consisting of electrodes spaced around a wheel which was spun by an electric motor, which produced sparks as they passed by a stationary electrode.<ref name="CodellaSparkRadio"/><ref name="Hyder" /> The spark rate was equal to the rotations per second times the number of spark electrodes on the wheel. It could produce spark rates up to several thousand hertz, and the rate could be adjusted by changing the speed of the motor. The rotation of the wheel was usually synchronized to the AC [[sine wave]] so the moving electrode passed by the stationary one at the peak of the sine wave, initiating the spark when the capacitor was fully charged, which produced a musical tone in the receiver. When tuned correctly in this manner, the need for external cooling or quenching airflow was eliminated, as was the loss of power directly from the charging circuit (parallel to the capacitor) through the spark.