Editing Spark-gap transmitter (section)

===Syntonic transmitters===
[[File:British patent 11575 Lodge IMPROVEMENTS IN SYNTONIZED TELEGRAPHY May 10 1897 fig 3 & 4.png|thumb|Transmitter ''(bottom)'' and receiver ''(top)'' of the first "syntonic" radio system, from Lodge's 1897 patent<ref name="Patent11575"/>]]

It became clear that for multiple transmitters to operate, some system of "selective signaling"<ref name="Ashley2">{{cite book
| last1= Ashley
| first1= Charles Grinnell
| last2= Hayward
| first2= Charles Brian
| title= Wireless telegraphy and wireless telephony: An understandable presentation of the science of wireless transmission of intelligence
| publisher= American School of Correspondence
| date= 1912
| pages= [https://archive.org/details/wirelesstelegra00haywgoog/page/n52 38]
| url= https://archive.org/details/wirelesstelegra00haywgoog
| quote= selective signaling.
}}</ref><ref name="Kennelly">{{cite book
| last1= Kennelly
| first1= Arthur Edwin
| title= Wireless Telegraphy: An Elementary Treatise
| publisher= Moffat, Yard, and Co.
| date= 1906
| location= New York
| pages= [https://archive.org/details/wirelesstelegra01kenngoog/page/n184 173]–180
| url= https://archive.org/details/wirelesstelegra01kenngoog
| quote= selective signaling.
}}</ref> had to be devised to allow a receiver to select which transmitter's signal to receive, and reject the others. In 1892 [[William Crookes]] had given an influential<ref name="Aitken">"Crookes’s article was read very widely—and more than that, attended to and remembered—both in Europe and in the United States; there is hardly one figure important in the early days of radio who does not at some point in his memoirs or correspondence refer to the article of 1892 as having made a difference."   [https://books.google.com/books?id=Mez_AwAAQBAJ&pg=PA115&dq=vertical+antenna Aitken, Hugh (2014) ''Syntony and Spark: The origins of radio'', p. 111-116]</ref> lecture<ref name="Crookes">{{cite journal
| last1= Crookes
| first1= William
| title= Some Possibilities of Electricity
| journal= The Fortnightly Review
| volume= 51
| pages= 174–176
| date= February 1, 1892
| url= http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
| access-date= August 19, 2015
| archive-date= September 29, 2018
| archive-url= https://web.archive.org/web/20180929160315/http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
| url-status= dead
}}</ref> on radio in which he suggested using [[resonance]] (then called ''syntony'') to reduce the bandwidth of transmitters and receivers.<ref name="Sarkar" />{{rp|p.352-353, 355-358}} Using a [[resonant circuit]] (also called tuned circuit or tank circuit) in transmitters would narrow the [[bandwidth (signal processing)|bandwidth]] of the radiated signal, it would occupy a smaller range of frequencies around its center frequency, so that the signals of transmitters "tuned" to transmit on different frequencies would no longer overlap. A receiver which had its own resonant circuit could receive a particular transmitter by "tuning" its [[resonant frequency]] to the frequency of the desired transmitter, analogously to the way one musical instrument could be tuned to resonance with another.<ref name="Ashley2"/> This is the system used in all modern radio.

During the period 1897 to 1900 wireless researchers realized the advantages of "syntonic" or "tuned" systems, and added [[capacitor]]s ([[Leyden jar]]s) and [[inductor]]s (coils of wire) to transmitters and receivers, to make resonant circuits (tuned circuits, or tank circuits).<ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} [[Oliver Lodge]], who had been researching electrical resonance for years,<ref name="Aitken1985" />{{rp|p.108-109}}<ref name="Thrower"/> patented the first "syntonic" transmitter and receiver in May 1897<ref name="Patent11575">British patent GB189711575 Lodge, O. J. ''[https://worldwide.espacenet.com/publicationDetails/biblio?CC=GB&NR=189711575A&KC=A&FT=D# Improvements in Syntonized Telegraphy without Line Wires]'' filed: May 10, 1897, granted: August 10, 1898</ref><ref name="Lodge1900">Lodge's explanation of his syntonic radio system is in {{cite book
| last1= Lodge
| first1= Oliver
| title= Signaling through space without wires
| publisher= The Electrician Publishing Co.
| date= 1900
| location= London
| pages= 50–58
| url= https://archive.org/stream/cu31924012383992#page/n61/mode/2up
}}</ref><ref name="Lee1"/><ref name="Aitken1985" />{{rp|p.130–143}}
<ref name="Hong" />{{rp|p.90-93}} Lodge added an [[inductor]] (coil) between the sides of his dipole antennas, which resonated with the capacitance of the antenna to make a tuned circuit.<ref name="Thrower"/><ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} Although his complicated circuit did not see much practical use, Lodge's "syntonic" patent was important because it was the first to propose a radio transmitter and receiver containing resonant circuits which were tuned to resonance with each other.<ref name="Thrower"/><ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} In 1911 when the patent was renewed the Marconi Company was forced to buy it to protect its own syntonic system against infringement suits.<ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}}

The resonant circuit functioned analogously to a [[tuning fork]], storing oscillating electrical energy, increasing the [[Q factor]] of the circuit so the oscillations were less damped.<ref name="Aitken1985"/>{{rp|p. 125-136, 254-255, 259}} Another advantage was the frequency of the transmitter was no longer determined by the length of the antenna but by the resonant circuit, so it could easily be changed by adjustable taps on the coil. The antenna was brought into resonance with the tuned circuit using [[loading coil]]s. The energy in each spark, and thus the power output, was no longer limited by the capacitance of the antenna but by the size of the capacitor in the resonant circuit.<ref name="Sarkar" />{{rp|p.352-353, 355-358}} In order to increase the power very large capacitor banks were used. The form that the resonant circuit took in practical transmitters was the inductively-coupled circuit described in the next section.

<gallery mode="packed" heights="160">
File:Small spark gap transmitter.jpg|Demonstration inductively coupled spark transmitter 1909, with parts labeled
File:Spark gap wireless station 1910.jpg|Amateur inductively coupled spark transmitter and receiver, 1910. The spark gap is in glass bulb ''(center right)'' next to tuning coil, on top of box containing glass plate capacitor
File:Marconi spark transmitter on SS Minnetonka 1902.jpg|Standard Marconi inductively coupled transmitter on ship 1902. Spark gap is in front of induction coil, lower right. The spiral oscillation transformer is in the wooden box on the wall above the Leyden jars.
File:Leyden jars Nauen wireless telegraphy station 1908.jpg|Telefunken 25&nbsp;kW long distance transmitter built 1906 at [[Nauen Transmitter Station]], Nauen, Germany, showing large 360 Leyden jar 400&nbsp;μF capacitor bank ''(rear)'' and vertical spark gaps ''(right)''
</gallery>

====Inductive coupling====
In developing these syntonic transmitters, researchers found it impossible to achieve low damping with a single resonant circuit. A [[resonant circuit]] can only have low damping (high Q, narrow bandwidth) if it is a "closed" circuit, with no energy dissipating components.<ref name="Marconi1"/><ref name="Hong" />{{rp|p.90-93}}<ref name="Aitken1985" />{{rp|p.108-109}} But such a circuit does not produce radio waves. A resonant circuit with an antenna radiating radio waves (an "open" tuned circuit) loses energy quickly, giving it high damping (low Q, wide bandwidth). There was a fundamental tradeoff between a circuit which produced persistent oscillations which had narrow bandwidth, and one which radiated high power.<ref name="CodellaSparkRadio"/>

[[File:Inductively coupled spark gap transmitter circuit.svg|thumb|upright=1.3|Inductively coupled spark transmitter. ''C2'' is not an actual capacitor but represents the capacitance between the antenna ''A'' and ground.]]

The solution found by a number of researchers was to use two resonant circuits in the transmitter, with their coils [[inductive coupling|inductively (magnetically) coupled]], making a [[resonant transformer]] (called an ''oscillation transformer'');<ref name="CodellaSparkRadio"/><ref name="Ashley1"/><ref name="Sarkar" />{{rp|p.352-353, 355-358}} this was called an "''inductively coupled''", "''coupled circuit''"<ref name="Marriott1"/> or "''two circuit''" transmitter.<ref name="Beauchamp1"/><ref name="Jansky1"/><ref name="Hong" />{{rp|p.98-100}} See circuit diagram. The [[primary winding]] of the oscillation transformer (''L1'') with the capacitor (''C1'') and spark gap (''S'') formed a "closed" resonant circuit which generated the oscillations, while the secondary winding (''L2'') was connected to the wire antenna (''A'') and ground, forming an "open" resonant circuit with the capacitance of the antenna (''C2'').<ref name="Sarkar" />{{rp|p.352-353, 355-358}} Both circuits were tuned to the same [[resonant frequency]].<ref name="Sarkar" />{{rp|p.352-353, 355-358}} The advantage of the inductively coupled circuit was that the "loosely coupled" transformer transferred the oscillating energy of the tank circuit to the radiating antenna circuit gradually, creating long "ringing" waves.<ref name="Ashley1"/><ref name="CodellaSparkRadio"/> A second advantage was that it allowed a large primary capacitance ''(C1)'' to be used which could store a lot of energy, increasing the power output enormously.<ref name="Ashley1"/><ref name="Sarkar" />{{rp|p.352-353, 355-358}} Powerful transoceanic transmitters often had huge [[Leyden jar]] capacitor banks filling rooms ''(see pictures above)''. The receiver in most systems also used two inductively coupled circuits, with the antenna an "open" resonant circuit coupled through an oscillation transformer to a "closed" resonant circuit containing the [[detector (radio)|detector]]. A radio system with a "two circuit" (inductively coupled) transmitter and receiver was called a "four circuit" system.

The first person to use resonant circuits in a radio application was [[Nikola Tesla]], who invented the [[resonant transformer]] in 1891.<ref name="Wheeler">"''Tesla is entitled to either distinct priority or independent discovery of''" three concepts in wireless theory: "''(1) the idea of inductive coupling between the driving and the working circuits (2) the importance of tuning both circuits, i.e. the idea of an 'oscillation transformer' (3) the idea of a capacitance loaded open secondary circuit''" {{cite journal
| last= Wheeler
| first= L. P.
| title= Tesla's contribution to high frequency
| journal= Electrical Engineering
| volume= 62
| issue= 8
| pages= 355–357
| date= August 1943
| issn= 0095-9197
| doi= 10.1109/EE.1943.6435874
| s2cid= 51671246
}}</ref> At a March 1893 St. Louis lecture<ref name="Martin3">Tesla, N., "''On light and other high frequency phenomena''", in Thomas Cummerford Martin (1894) ''[https://archive.org/details/inventionsresea00teslgoog/page/n317 <!-- pg=294 --> The Inventions, Researches and Writings of Nikola Tesla, 2nd Ed.]'', p. 294-373</ref> he had demonstrated a wireless system that, although it was intended for [[wireless power transmission]], had many of the elements of later radio communication systems.<ref name="Sterling">{{cite book
| last1= Sterling
| first1= Christopher H.
| title= Biographical Encyclopedia of American Radio
| publisher= Routledge
| date= 2013
| pages= 382–383
| url= https://books.google.com/books?id=mdQq-663faAC&q=tesla&pg=PA383
| isbn= 978-1136993756
}}</ref><ref name="Uth">{{cite book
| last1= Uth
| first1= Robert
| title= Tesla, Master of Lightning
| publisher= Barnes and Noble Publishing
| date= 1999
| pages= 65–70
| url= https://books.google.com/books?id=3W6_h6XG6VAC&q=tesla&pg=PA66
| isbn= 978-0760710050
}}</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Aitken1985" />{{rp|p. 125-136, 254-255, 259}}<ref name="Rockman">{{cite book
| last1= Rockman
| first1= Howard B.
| title= Intellectual Property Law for Engineers and Scientists
| publisher= John Wiley and Sons
| date= 2004
| pages= 196–199
| url= https://books.google.com/books?id=6B88P-32IIQC&pg=PA196
| isbn= 978-0471697398
}}</ref> A grounded capacitance-loaded spark-excited [[resonant transformer]] (his ''[[Tesla coil]]'') attached to an elevated wire monopole antenna transmitted radio waves, which were received across the room by a similar wire antenna attached to a receiver consisting of a second grounded resonant transformer tuned to the transmitter's frequency, which lighted a [[Geissler tube]].<ref name="Regal">{{cite book
| last1= Regal
| first1= Brian
| title= Radio: The Life Story of a Technology
| publisher= Greenwood Publishing Group
| date= 2005
| pages= 21–23
| url= https://books.google.com/books?id=N2rNO6FX8o4C&q=Tesla&pg=PA22
| isbn= 978-0313331671
}}</ref><ref name="Rockman"/><ref name="Cheney4">[https://books.google.com/books?id=HIuK7iLO9zgC&pg=PA96 Cheney, Margaret (2011) ''Tesla: Man Out Of Time'', p. 96-97]</ref> This system, patented by Tesla 2 September 1897,<ref name="Patent645576">US Patent No. 645576, Nikola Tesla, ''[https://patents.google.com/patent/US645576 System of transmission of electrical energy]'', filed: 2 September 1897; granted: 20 March 1900</ref> 4 months after Lodge's "syntonic" patent, was in effect an inductively coupled radio transmitter and receiver, the first use of the "four circuit" system claimed by Marconi in his 1900 patent ''(below)''.<ref name="Wunsch">{{cite journal
| last1= Wunsch
| first1= A. David
| title= Misreading the Supreme Court: A Puzzling Chapter in the History of Radio
| journal= Antenna
| volume= 11
| issue= 1
| date= November 1998
| url= http://www.mercurians.org/1998_Fall/misreading.htm
| access-date= 3 December 2018}}</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Rockman"/><ref name="Sterling"/> However, Tesla was mainly interested in [[wireless power]] and never developed a practical radio ''communication'' system.<ref name="Coe2">{{cite book
| last1= Coe
| first1= Lewis
| title= Wireless Radio: A History
| publisher= McFarland
| date= 2006
| pages= 111–113
| url= https://books.google.com/books?id=W1JAeg1PiWIC&pg=PA112
| isbn= 978-0786426621
}}</ref><ref name="Smith">{{cite book |last=Smith |first=Craig B. |title=Lightning: Fire from the Sky|publisher=Dockside Consultants Inc. |date=2008 |url=https://books.google.com/books?id=iGZw0qPDk8YC&q=%22tesla+coil%22&pg=PT11 |isbn=978-0-615-24869-1}}</ref><ref name="Regal"/><ref name="Sarkar" />{{rp|p.352-353, 355-358}}

In addition to Tesla's system, inductively coupled radio systems were patented by [[Oliver Lodge]] in February 1898,<ref name="Patent609154">US Patent no. 609,154 Oliver Joseph Lodge, [https://patents.google.com/patent/US609154 Electric Telegraphy], filed: 1 February 1898, granted: 16 August 1898</ref><ref name="White1"/> [[Karl Ferdinand Braun]],<ref name="Hong" />{{rp|p.98-100}}<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Nahin5"/><ref name="PatentGB189922020">British patent no. 189922020 Karl Ferdinand Braun, [https://worldwide.espacenet.com/publicationDetails/biblio?DB=en.worldwide.espacenet.com&II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=19000922&CC=GB&NR=189922020A&KC=A# ''Improvements in or related to telegraphy without the use of continuous wires''], applied: 3 November 1899, complete specification: 30 June 1900, granted: 22 September 1900</ref> in November 1899, and [[John Stone Stone]] in February 1900.<ref name="Patent714756">US Patent no. 714,756, John Stone Stone [https://patents.google.com/patent/US714756 Method of electric signaling], filed: 8 February 1900, granted: 2 December 1902</ref><ref name="White1"/> Braun made the crucial discovery that low damping required "loose coupling" (reduced [[mutual inductance]]) between the primary and secondary coils.<ref name="Orton">{{cite book
| last1= Orton
| first1= John W.
| title= Semiconductors and the Information Revolution: Magic Crystals that made IT Happen
| publisher= Academic Press
| date= 2009
| pages= 37
| url= https://books.google.com/books?id=6YLL9197NfMC&q=Braun+Tesla+Marconi+coupling&pg=PA37
| isbn= 978-0080963907
}}</ref><ref name="Sarkar" />{{rp|p.352-353, 355-358}}

<gallery mode="packed" heights="170">
Image:US Patent 645576 Nikola Tesla 1897 System of transmission of electrical energy.png|Tesla's inductively coupled power transmitter ''(left)'' patented 2 September 1897<ref name="Patent645576"/>
Image:British patent 22,020-Karl Ferdinand Braun-filed 3 November 1899-fig. 2.png|Braun's inductively coupled transmitter patented 3 November 1899<ref name="PatentGB189922020"/>
Image:US Patent 714756-John Stone Stone-Method of selective electric signaling 1900 figs 5&6.png|Stone's inductively coupled transmitter ''(left)'' and receiver ''(right)'' patented 8 February 1900<ref name="Patent714756"/>
Image:Spark gap transmitter-Marconi patent 763772 fig 1.png|Marconi's inductively coupled transmitter patented 26 April 1900.<ref name="Patent763772"/>
</gallery>

Marconi at first paid little attention to syntony, but by 1900 developed a radio system incorporating features from these systems,<ref name="Orton"/><ref name="Nahin5"/> with a two circuit transmitter and two circuit receiver, with all four circuits tuned to the same frequency, using a resonant transformer he called the "jigger".<ref name="Marconi1">{{cite journal
| last1= Marconi
| first1= Guglielmo
| title= Syntonic Wireless Telegraphy
| journal= The Electrician
| date= May 24, 1901
| volume= 13
| issue= 335
| page= 874
| doi= 10.1126/science.13.335.874
| bibcode= 1901Sci....13..874.
| url= http://teslacollection.com/tesla_articles/1901/the_electrician/guglielmo_marconi/syntonic_wireless_telegraphy
| access-date= April 8, 2017| url-access= subscription
}}</ref><ref name="Beauchamp1"/><ref name="Hong" />{{rp|p.98-100}} In spite of the above prior patents, Marconi in his 26 April 1900 "four circuit" or "master tuning" patent<ref name="Patent763772">British patent no. 7777, Guglielmo Marconi, [https://worldwide.espacenet.com/publicationDetails/biblio?CC=GB&NR=190007777&KC=&FT=E&locale=en_EP Improvements in apparatus for wireless telegraphy], filed: 26 April 1900, granted: 13 April 1901. Corresponding US Patent no. 763,772, Guglielmo Marconi, [https://patents.google.com/patent/US763772 Apparatus for wireless telegraphy], filed: 10 November 1900, granted: 28 June 1904.</ref> on his system claimed rights to the inductively coupled transmitter and receiver.<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="White1"/><ref name="Rockman"/> This was granted a British patent, but the US patent office twice rejected his patent as lacking originality. Then in a 1904 appeal a new patent commissioner reversed the decision and granted the patent,<ref name="PBS">{{cite web
| title= Who invented radio?
| work= Tesla: Master of Lightning - companion site for 2000 PBS television documentary
| publisher= PBS.org, Public Broadcasting Service website
| date= 2000
| url= http://www.pbs.org/tesla/ll/ll_whoradio.html
| access-date= 9 April 2018}}</ref><ref name="Rockman"/> on the narrow grounds that Marconi's patent by including an antenna [[loading coil]] ''(J in circuit above)'' provided the means for tuning the four circuits to the same frequency, whereas in the Tesla and Stone patents this was done by adjusting the length of the antenna.<ref name="White1"/><ref name="Rockman"/> This patent gave Marconi a near monopoly of syntonic wireless telegraphy in England and America.<ref name="Morse3">[https://archive.org/stream/radiobeamandbroa029214mbp#page/n35/mode/2up/ Morse (1925) ''Radio: Beam and Broadcast'', p. 30]</ref><ref name="Beauchamp1"/> Tesla sued Marconi's company for patent infringement but didn't have the resources to pursue the action. In 1943 the [[US Supreme Court]] invalidated the inductive coupling claims of Marconi's patent<ref name="Findlaw">{{cite web
| title= No. 369 (1943) Marconi Wireless Co. of America v. United States
| work= United States Supreme Court decision
| publisher= Findlaw.com website
| date= June 21, 1943
| url= https://caselaw.findlaw.com/us-supreme-court/320/1.html
| access-date= March 14, 2017}}</ref> due to the prior patents of Lodge, Tesla, and Stone, but this came long after spark transmitters had become obsolete.<ref name="White1">{{cite web
| last= White
| first= Thomas H.
| title= Nikola Tesla: The Guy Who DIDN'T "Invent Radio"
| work= United States Early Radio History
| publisher= T. H. White's personal website
| date= 1 November 2012
| url= https://earlyradiohistory.us/tesla.htm
| access-date= 20 June 2018}}</ref><ref name="Rockman"/>

The inductively coupled or "syntonic" spark transmitter was the first type that could communicate at intercontinental distances, and also the first that had sufficiently narrow bandwidth that interference between transmitters was reduced to a tolerable level. It became the dominant type used during the "spark" era.<ref name="Beauchamp1"/> A drawback of the plain inductively coupled transmitter was that unless the primary and secondary coils were very loosely coupled it radiated on two frequencies.<ref name="Sarkar" />{{rp|p.352-353, 355-358}}<ref name="Beauchamp3"/> This was remedied by the quenched-spark and rotary gap transmitters'' (below)''.

In recognition of their achievements in radio, Marconi and Braun shared the 1909 [[Nobel Prize in physics]].<ref name="Sarkar" />{{rp|p.352-353, 355-358}}