Editing Spark-gap transmitter (section)

===Hertzian oscillators===
German physicist [[Heinrich Hertz]] in 1887 built the first experimental spark gap transmitters during his historic experiments to demonstrate the existence of [[electromagnetic wave]]s predicted by [[James Clerk Maxwell]] in 1864, in which he discovered [[radio wave]]s,<ref name="Hertz1">Hertz, H., "On very rapid electric oscillations", ''Wiedemann's Annalen'', Vol. 31, p. 421, 1887 reprinted in {{cite book
| last1= Hertz
| first1= Heinrich
| title= Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity Through Space
| publisher= Dover Publications
| date= 1893
| pages= [https://archive.org/details/electricwavesbe00hertgoog/page/n48 29]–53
| url= https://archive.org/details/electricwavesbe00hertgoog
| quote= heinrich hertz .
}} translated to English by D. E. Jones</ref>
<ref name="Hong">{{cite book
| last1= Hong
| first1= Sungook
| title= Wireless: From Marconi's Black-box to the Audion
| publisher= MIT Press
| date= 2010
| url= https://books.google.com/books?id=uJ0jEAAAQBAJ
| isbn= 9780262514194
}}</ref>{{rp|p.3-4}}<ref name="Baird">{{cite book
| last1= Baird
| first1= D.
| last2= Hughes
| first2= R.I.
| last3= Nordmann
| first3= A.
| title= Heinrich Hertz: Classical Physicist, Modern Philosopher
| publisher= Springer Science and Business Media
| date= 2013
| pages= 51–53
| url= https://books.google.com/books?id=9iEyBwAAQBAJ&q=oscillator+transmitter&pg=PA52
| isbn= 978-9401588553
}}</ref><ref name="Sarkar" />{{rp|p.19, 260, 331-332}} which were called "Hertzian waves" until about 1910. Hertz was inspired to try spark excited circuits by experiments with "Reiss spirals", a pair of flat spiral [[inductor]]s with their conductors ending in spark gaps. A [[Leyden jar]] capacitor discharged through one spiral, would cause sparks in the gap of the other spiral.

<gallery mode="packed" heights="150">
File:Heinrich Hertz discovering radio waves.png|Heinrich Hertz discovering radio waves with his spark oscillator ''(at rear)''
File:Hertz drawing of his spark transmitter 1888.png|Hertz's drawing of one of his spark oscillators. ''(A,A')'' antenna, ''(J)'' induction coil
File:Hertzian spark radio transmitter 1902.jpg|Hertzian spark oscillator, 1902. Visible are antenna consisting of 2 wires ending in metal plates ''(E)'', spark gap ''(D)'', induction coil ''(A)'', auto battery ''(B)'', and [[telegraph key]] ''(C)''.
File:Hertz spark gap transmitter and parabolic antenna.png|Hertz's 450&nbsp;MHz transmitter; a 26&nbsp;cm dipole with spark gap at focus of a sheet metal parabolic reflector
File:Microwave Apparatus - Jagadish Chandra Bose Museum - Bose Institute - Kolkata 2011-07-26 4051.JPG|[[Jagadish Chandra Bose]] in 1894 was the first person to produce [[millimeter wave]]s; his spark oscillator ''(in box, right)'' generated 60&nbsp;GHz (5&nbsp;mm) waves using 3&nbsp;mm metal ball resonators.
File:Lodge spark oscillator ball 1894.jpg|Spark oscillator ball used by [[Oliver Lodge]] in 1894 to generate 25&nbsp;cm ( 1.2&nbsp;GHz) microwaves
</gallery>
[[File:Hertz first oscillator.png|thumb|upright=3|center|Hertz's first oscillator: a pair of one meter copper wires with a 7.5 mm spark gap between them, ending in 30 cm zinc spheres. When 20,000 volt pulses from an induction coil ''(not shown)'' was applied, it produced waves at a [[frequency]] of roughly 50 MHz.]]

See circuit diagram. Hertz's transmitters consisted of a [[dipole antenna]] made of a pair of collinear metal rods of various lengths with a [[spark gap]] ''(S)'' between their inner ends and metal balls or plates for [[capacitance]] ''(C)'' attached to the outer ends.<ref name="Hertz1"/><ref name="Sarkar" />{{rp|p.19, 260, 331-332}}<ref name="Baird"/> The two sides of the antenna were connected to an [[induction coil]] (Ruhmkorff coil) ''(T)'' a common lab power source which produced pulses of high voltage, 5 to 30&nbsp;kV. In addition to radiating the waves, the antenna also acted as a [[harmonic oscillator]] ([[resonator]]) which generated the oscillating currents. High-voltage pulses from the induction coil ''(T)'' were applied between the two sides of the antenna. Each pulse stored electric charge in the capacitance of the antenna, which was immediately discharged by a spark across the spark gap. The spark excited brief oscillating [[standing wave]]s of current between the sides of the antenna. The antenna radiated the energy as a momentary pulse of radio waves; a [[Damped wave (radio transmission)|damped wave]]. The frequency of the waves was equal to the [[resonant frequency]] of the antenna, which was determined by its length; it acted as a [[half-wave dipole]], which radiated waves roughly twice the length of the antenna (for example a dipole 1 meter long would generate 150 MHz radio waves). Hertz detected the waves by observing tiny sparks in micrometer spark gaps ''(M)'' in loops of wire which functioned as resonant receiving antennas. [[Oliver Lodge]] was also experimenting with spark oscillators at this time and came close to discovering radio waves before Hertz, but his focus was on waves on wires, not in free space.<ref name="Lee1">{{cite book
| last= Lee
| first= Thomas H.
| title= The Design of CMOS Radio-Frequency Integrated Circuits
| edition= 2nd
| publisher= Cambridge University Press
| date= 2004
| location= UK
| pages= 34–36
| url= https://books.google.com/books?id=io1hL48OqBsC&q=lodge+syntony&pg=PA34
| isbn= 978-0521835398}}</ref><ref name="Sarkar" />{{rp|p.226}}

[[File:Hertz transmitter and receiver - English.svg|thumb|upright=1.3|Circuit of Hertz's spark oscillator and receiver. The interrupter ''(I)'' and capacitor in the primary circuit of the induction coil produced a continuous string of damped waves.  Hertz often just used a pushbutton switch, which created a single spark and pulse of radio waves when pushed, resulting in a single spark in his receiver.]]

Hertz and the first generation of physicists who built these "Hertzian oscillators", such as [[Jagadish Chandra Bose]], [[Lord Rayleigh]], [[George Francis FitzGerald|George Fitzgerald]], [[Frederick Trouton]], [[Augusto Righi]] and [[Oliver Lodge]], were mainly interested in radio waves as a [[scientific phenomenon]], and largely failed to foresee its possibilities as a communication technology.<ref name="McNicol">{{cite book
| last1= Donald
| first1= McNicol
| title= Radio's Conquest of Space: The experimental rise of radio communication
| publisher= Murray Hill Books, Inc.
| date= 1946
| isbn= 9780405060526
| url= https://archive.org/details/radiosconquestof00mcnirich/page/53 
}}</ref>{{rp|p.54, 98}}<ref name="Hong" />{{rp|p.5-9,22}}<ref name="Sarkar" />{{rp|p.260, 263-265}}<ref name="Coe1">{{cite book
| last1= Coe
| first1= Lewis
| title= Wireless Radio: A History
| publisher= McFarland
| date= 2006
| pages= 4–6, 13
| url= https://books.google.com/books?id=W1JAeg1PiWIC&pg=PA4
| isbn= 978-0786426621
}}</ref>  Due to the influence of Maxwell's theory, their thinking was dominated by the similarity between radio waves and light waves; they thought of radio waves as an invisible form of light.<ref name="Hong" />{{rp|p.5-9,22}}<ref name="Sarkar" />{{rp|p.260, 263-265}} By analogy with light, they assumed that radio waves only traveled in straight lines, so they thought radio transmission was limited by the visual [[horizon]] like existing optical signalling methods such as [[semaphore]], and therefore was not capable of longer distance communication.<ref name="Lee1"/><ref name="Weightman">{{cite book
| last1= Weightman
| first1= Gavin
| title= Signor Marconi's Magic Box: The Most Remarkable Invention Of The 19th Century & The Amateur Inventor Whose Genius Sparked A Revolution
| publisher= Da Capo Press
| date= 2009
| pages= 52
| url= https://books.google.com/books?id=UCdvcmMTpGgC&pg=PT52
| isbn= 978-0786748549
}}</ref><ref name="Gregersen">{{cite book
| last1= Gregersen
| first1= Erik
| title= The Britannica Guide to Sound and Light
| publisher= The Rosen Publishing Group
| date= 2011
| pages= 159
| url= https://books.google.com/books?id=EMBelfRf6dkC&q=Marconi+%22ground+wave%22+%22Kennelly+heaviside+layer%22+%22straight+lines%22&pg=PA169
| isbn= 978-1615303007
}}</ref> As late as 1894 Oliver Lodge speculated that the maximum distance Hertzian waves could be transmitted was a half mile.<ref name="Hong" />{{rp|p.5-9,22}}

To investigate the similarity between radio waves and [[light wave]]s, these researchers concentrated on producing short [[wavelength]] high-frequency waves with which they could duplicate classic [[optics]] experiments with radio waves, using [[quasioptics|quasioptical]] components such as [[Prism (optics)|prism]]s and [[Lens (optics)|lens]]es made of [[paraffin wax]], [[sulfur]], and [[pitch (resin)|pitch]] and wire [[diffraction grating]]s.<ref name="Sarkar" />{{rp|p.476-484}} Their short antennas generated radio waves in the [[very high frequency|VHF]], [[ultra high frequency|UHF]], or [[microwave]] bands. In his various experiments, Hertz produced waves with frequencies from 50 to 450&nbsp;MHz, roughly the frequencies used today by broadcast [[television transmitter]]s. Hertz used them to perform historic experiments demonstrating [[standing wave]]s, [[refraction]], [[diffraction]], [[polarization (waves)|polarization]] and [[interference (wave propagation)|interference]] of radio waves.<ref name="Hertz2">Hertz, H., "On radiation", ''Wiedemann's Annalen'', Vol. 36, December 13, 1988, p. 769, reprinted in {{cite book
| last1= Hertz
| first1= Heinrich
| title= Electric Waves: Being Researches on the Propagation of Electric Action with Finite Velocity Through Space
| publisher= Dover Publications
| date= 1893
| pages= [https://archive.org/details/electricwavesbe00hertgoog/page/n191 172]–185
| url= https://archive.org/details/electricwavesbe00hertgoog
}} translated to English by D. E. Jones</ref><ref name="Sarkar" />{{rp|p.19, 260, 331-332}} He also measured the speed of radio waves, showing they traveled at the same speed as light. These experiments established that light and radio waves were both forms of Maxwell's [[electromagnetic wave]]s, differing only in frequency. [[Augusto Righi]] and [[Jagadish Chandra Bose]] around 1894 generated [[microwave]]s of 12 and 60&nbsp;GHz respectively, using small metal balls as resonator-antennas.<ref name="Bose">{{cite journal
| last1= Bose
| first1= Jagadish Chandra
| title= On a complete apparatus for the study of the properties of electric waves
| journal= The London, Edinburgh, and Dublin Philosophical Magazine
| volume= 43
| issue= 5
| pages= 55–88
| date= January 1897
| url= https://books.google.com/books?id=OLYvAQAAIAAJ&pg=PA55
| doi =10.1080/14786449708620959
| access-date= January 30, 2018}}</ref><ref name="Sarkar" />{{rp|p.291-308}}

The high frequencies produced by Hertzian oscillators could not travel beyond the horizon. The dipole resonators also had low capacitance and couldn't store much [[electric charge|charge]], limiting their power output.<ref name="Hong" />{{rp|p.5-9,22}} Therefore, these devices were not capable of long distance transmission; their reception range with the primitive receivers employed was typically limited to roughly 100 yards (100 meters).<ref name="Hong" />{{rp|p.5-9,22}}
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