Editing Microwave (section)

== History ==
===Hertzian optics===
Microwaves were first generated in the 1890s in some of the earliest [[radio wave]] experiments by physicists who thought of them as a form of "invisible light".<ref name="Hong1">{{cite book
 | last1  = Hong
 | first1 = Sungook
 | title  = Wireless: From Marconi's Black-box to the Audion
 | publisher = MIT Press
 | date   = 2001
 | pages  = 5–9, 22
 | isbn   = 978-0262082983
 }}</ref> [[James Clerk Maxwell]] in his 1873 theory of [[electromagnetism]], now called [[Maxwell's equations]], had predicted that a coupled [[electric field]] and [[magnetic field]] could travel through space as an [[electromagnetic wave]], and proposed that light consisted of electromagnetic waves of short wavelength.  In 1888, German physicist [[Heinrich Hertz]] was the first to demonstrate the existence of electromagnetic waves, generating [[radio wave]]s using a primitive [[spark gap transmitter|spark gap radio transmitter]].<ref name="Roer">{{cite book
 | last1  = Roer
 | first1 = T.G.
 | title  = Microwave Electronic Devices
 | publisher = Springer Science and Business Media
 | date   = 2012
 | pages  = 1–12
 | url    = https://books.google.com/books?id=deDvBwAAQBAJ&pg=PA1
 | isbn   = 978-1461525004
 }}</ref>

Hertz and the other early radio researchers were interested in exploring the similarities between radio waves and light waves, to test Maxwell's theory. They concentrated on producing short wavelength radio waves in the [[ultrahigh frequency|UHF]] and microwave ranges, with which they could duplicate classic [[optics]] experiments in their laboratories, using [[quasioptics|quasioptical]] components such as [[Prism (optics)|prism]]s and [[lens (optics)|lens]]es made of [[paraffin wax|paraffin]], [[sulfur]] and [[pitch (resin)|pitch]] and wire [[diffraction grating]]s, to refract and diffract radio waves like light rays.<ref name="Sarkar1">{{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
 | pages  = 474–486
 | url    = https://archive.org/stream/HistoryOfWireless#page/n496/mode/2up
 | isbn   = 978-0471783015
 | author-link1=Tapan Sarkar
 | author-link3=Arthur A. Oliner
 }}</ref> Hertz produced waves up to 450&nbsp;MHz; his directional 450&nbsp;MHz transmitter consisted of a 26&nbsp;cm brass rod [[dipole antenna]] with a spark gap between the ends, suspended at the focal line of a [[parabolic antenna]] made of a curved zinc sheet, powered by high voltage pulses from an [[induction coil]].<ref name="Roer" /> His historic experiments demonstrated that radio waves like light exhibited [[refraction]], [[diffraction]], [[polarization (waves)|polarization]], [[interference (wave motion)|interference]] and [[standing wave]]s,<ref name="Sarkar1" /> proving that radio waves and light waves were both forms of Maxwell's [[electromagnetic wave]]s.

<gallery mode="packed" heights="150">
File:Drawing of Heinrich Hertz spark radio transmitter and parabolic antenna 1888.jpg|[[Heinrich Hertz]]'s 450&nbsp;MHz spark transmitter, 1888, consisting of 23&nbsp;cm dipole and spark gap at the focus of a 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:Bose 60 GHz microwave spark oscillator 1894.png|3&nbsp;mm spark ball oscillator Bose used to generate 60&nbsp;GHz waves
File:Refraction of Hertzian waves by paraffin prism.png|Microwave spectroscopy experiment by [[John Ambrose Fleming]] in 1897 showing refraction of 1.4&nbsp;GHz microwaves by paraffin prism, duplicating earlier experiments by Bose and Righi.
File:Oscillatore di Righi con riflettore parabolico - Museo scienza tecnologia Milano 08757 1.jpg|[[Augusto Righi]]'s 12&nbsp;GHz spark oscillator and receiver, 1895
File:Lodge spark oscillator ball 1894.jpg|Oliver Lodge's 5 inch oscillator ball he used to generate 1.2&nbsp;GHz microwaves in 1894
File:Marconi parabolic xmtr and rcvr 1895.jpg|1.2&nbsp;GHz microwave spark transmitter ''(left)'' and [[coherer]] receiver ''(right)'' used by [[Guglielmo Marconi]] during his 1895 experiments had a range of {{convert|6.5|km|mi|abbr=on|sigfig=2}}
</gallery>
Beginning in 1894 Indian physicist [[Jagadish Chandra Bose]] performed the first experiments with microwaves. He was the first person to produce [[millimeter wave]]s, generating frequencies up to 60&nbsp;GHz (5&nbsp;millimeter) using a 3&nbsp;mm metal ball spark oscillator.<ref name="Emerson">{{cite web |url=http://www.tuc.nrao.edu/~demerson/bose/bose.html |title=The work of Jagdish Chandra Bose: 100 years of MM-wave research |publisher=National Radio Astronomy Observatory |date=February 1998 |author=Emerson, D.T.}}</ref><ref name="Sarkar1" /> Bose also invented [[waveguide (electromagnetism)|waveguide]], [[horn antenna]]s, and [[semiconductor]] [[crystal detector]]s for use in his experiments.  Independently in 1894, [[Oliver Lodge]] and [[Augusto Righi]] experimented with 1.5 and 12&nbsp;GHz microwaves respectively, generated by small metal ball spark resonators.<ref name="Sarkar1" /> Russian physicist [[Pyotr Lebedev]] in 1895 generated 50&nbsp;GHz millimeter waves.<ref name="Sarkar1" /> In 1897 [[Lord Rayleigh]] solved the mathematical [[boundary-value problem]] of electromagnetic waves propagating through conducting tubes and dielectric rods of arbitrary shape<ref name="Packard" /><ref name="Rayleigh">{{cite journal
  | last1  = Strutt
  | first1 = William (Lord Rayleigh)
  | title =  On the passage of electric waves through tubes, or the vibrations of dielectric cylinders
  | journal = Philosophical Magazine
  | volume = 43
  | issue = 261
  | pages = 125–132
  | date = February 1897
  | doi = 10.1080/14786449708620969
  | url = https://zenodo.org/record/1431225
  }}</ref><ref name="Kizer">{{cite book
 | last1  = Kizer
 | first1 = George
 | title  = Digital Microwave Communication: Engineering Point-to-Point Microwave Systems
 | publisher = John Wiley and Sons
 | date   = 2013
 | pages  = 7
 | url    = https://books.google.com/books?id=JVhGmjQ8TyoC&q=southworth+bose+lodge+waveguide
 | isbn   = 978-1118636800
 }}</ref><ref name="Lee3">{{cite book
 | last1  = Lee
 | first1 = Thomas H.
 | title  = Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits, Vol. 1
 | publisher = Cambridge University Press
 | date   = 2004
 | pages  = 18, 118
 | url    = https://books.google.com/books?id=uoj3IWFxbVYC&pg=PA18
 | isbn   = 978-0521835268
 }}</ref> which gave the modes and [[cutoff frequency]] of microwaves propagating through a [[waveguide (electromagnetism)|waveguide]].<ref name="Roer" />

However, since microwaves were limited to [[line-of-sight propagation|line-of-sight]] paths, they could not communicate beyond the visual horizon, and the low power of the spark transmitters then in use limited their practical range to a few miles. The subsequent development of radio communication after 1896 employed lower frequencies, which could travel beyond the horizon as [[ground wave]]s and by reflecting off the [[ionosphere]] as [[skywave]]s, and microwave frequencies were not further explored at this time.

===First microwave communication experiments===
Practical use of microwave frequencies did not occur until the 1940s and 1950s due to a lack of adequate sources, since the [[triode]] [[vacuum tube]] (valve) [[electronic oscillator]] used in radio transmitters could not produce frequencies above a few hundred [[megahertz]] due to excessive electron transit time and interelectrode capacitance.<ref name="Roer" /> By the 1930s, the first low-power microwave vacuum tubes had been developed using new principles; the [[Barkhausen–Kurz tube]] and the [[split-anode magnetron]].<ref name="Karplus" /><ref name="Roer" /> These could generate a few watts of power at frequencies up to a few gigahertz and were used in the first experiments in communication with microwaves.

<gallery mode="packed" heights="150">
File:English Channel microwave relay antennas 1931.jpg|Antennas of 1931 experimental 1.7&nbsp;GHz microwave relay link across the English Channel
File:Experimental Westinghouse 3GHz microwave transmitter 1933.jpg|Experimental 3.3&nbsp;GHz (9&nbsp;cm) transmitter 1933 at Westinghouse labs <ref name="Mouromtseff"/> transmits voice over a mile.
File:Southworth demonstrating waveguide.jpg|Southworth ''(at left)'' demonstrating waveguide at [[Institute of Radio Engineers|IRE]] meeting in 1938, showing 1.5&nbsp;GHz microwaves passing through the 7.5 m flexible metal hose registering on a diode detector
File:Wilmer Barrow & horn antenna 1938.jpg|The first modern horn antenna in 1938 with inventor [[Wilmer L. Barrow]]
</gallery>
In 1931 an Anglo-French consortium headed by [[Andre C. Clavier]] demonstrated the first experimental [[microwave relay]] link, across the [[English Channel]] {{convert|40|mi|km}} between [[Dover]], UK and [[Calais]], France.<ref name="EC">{{cite magazine
  | title = Microwaves span the English Channel
  | magazine = Short Wave Craft
  | volume = 6
  | issue = 5
  | pages = 262, 310
  | publisher = Popular Book Co
  | location = New York
  | date = September 1935
  | url = http://www.americanradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1935-09.pdf
  | access-date = March 24, 2015}}</ref><ref name="Free">{{cite magazine
  | last1  = Free
  | first1 = E.E.
  | title = Searchlight radio with the new 7 inch waves
  | magazine = Radio News
  | volume = 8
  | issue = 2
  | pages = 107–109
  | publisher = Radio Science Publications
  | location = New York
  | date = August 1931
  | url = http://www.americanradiohistory.com/Archive-Radio-News/30s/Radio-News-1931-08-R.pdf
  | access-date = March 24, 2015}}</ref> The system transmitted telephony, telegraph and [[facsimile]] data over bidirectional 1.7&nbsp;GHz beams with a power of one-half watt, produced by miniature [[Barkhausen–Kurz tube]]s at the focus of {{convert|10|ft|m|0|adj=on}} metal dishes.

A word was needed to distinguish these new shorter wavelengths, which had previously been lumped into the "[[short wave]]" band, which meant all waves shorter than 200 meters. The terms ''quasi-optical waves'' and ''ultrashort waves'' were used briefly<ref name="Karplus">{{cite journal
  | last = Eduard
  | first = Karplus
  | title = Communication with quasi-optical waves
  | journal = Standards Yearbook, 1932
  | pages = 15
  | publisher = Bureau of Standards, US Dept. of Commerce
  | location = Washington D.C.
  | date = 1932
  | language = 
  | url = https://books.google.com/books?id=N-KSwF4ahGUC&pg=PA15
  | jstor = 
  | issn = 
  | doi = 
  | id = 
  | mr = 
  | zbl = 
  | jfm = 
  | access-date = 27 February 2025}}</ref><ref name="Loomis">{{cite book
  | last = Loomis
  | first = Mary Texanna 
  | title = Radio Theory and Operating: For the Radio Student and Practical Operator, 4th Ed.
  | publisher = Loomis Publishing Co.
  | date = 1928
  | location = 
  | pages = 603
  | language = 
  | url = https://books.google.com/books?id=B82nsBBEMCYC&pg=PA310
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 
  | mr = 
  | zbl = 
  | jfm =}}</ref><ref name="Mouromtseff">{{cite journal
  | last = Mouromtseff
  | first = Ilia A.
  | title = 3 1/2 Inch Waves Now Practical
  | journal = Short Wave Craft
  | volume = 4
  | issue = 5
  | pages = 266–267
  | publisher = Popular Book Corp.
  | location = New York
  | date = September 1933
  | language = 
  | url = https://www.worldradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-09.pdf#search=%22mouromtseff%20ultra-short%20waves%22
  | jstor = 
  | issn = 
  | doi = 
  | id = 
  | mr = 
  | zbl = 
  | jfm = 
  }}</ref> but did not catch on. The first usage of the word ''micro-wave'' apparently occurred in 1931.<ref name="Free" /><ref name="Ayto">{{cite book
 | last1  = Ayto
 | first1 = John
 | title  = 20th century words
 | date   = 2002
 | pages  = 269
 | publisher = Foreign Language Teaching and Research Press
 | url    = https://books.google.com/books?id=p0h5AAAAIAAJ&q=%22When+trials+with+wavelengths+as+low+as+18+cm+were+made+known,+there+was+undisguised+surprise+that+the+problem+of+the+micro-wave+had+been+solved+so+soon.%22
 | isbn   = 978-7560028743
 }}</ref>

===Radar development===
The development of [[radar]], mainly in secrecy, before and during [[World War II]], resulted in the technological advances which made microwaves practical.<ref name="Roer" /> Microwave wavelengths in the centimeter range were required to give the small radar antennas which were compact enough to fit on aircraft a narrow enough [[beamwidth]] to localize enemy aircraft. It was found that conventional [[transmission line]]s used to carry radio waves had excessive power losses at microwave frequencies, and [[George Southworth]] at [[Bell Labs]] and [[Wilmer Barrow]] at [[Massachusetts Institute of Technology|MIT]] independently invented [[waveguide (electromagnetism)|waveguide]] in 1936.<ref name="Packard">{{cite journal
  | last1  = Packard
  | first1 = Karle S.
  | title = The Origin of Waveguides: A Case of Multiple Rediscovery
  | journal = IEEE Transactions on Microwave Theory and Techniques
  | volume = MTT-32
  | issue = 9
  | pages = 961–969
  | date = September 1984
  | url = http://www.ieeeghn.org/wiki/images/8/86/MTT_Waveguide_History.pdf
  | doi = 10.1109/tmtt.1984.1132809
  | access-date = March 24, 2015|bibcode = 1984ITMTT..32..961P | citeseerx = 10.1.1.532.8921
  }}</ref> Barrow invented the [[horn antenna]] in 1938 as a means to efficiently radiate microwaves into or out of a waveguide. In a microwave [[radio receiver|receiver]], a [[linear circuit|nonlinear]] component was needed that would act as a [[detector (radio)|detector]] and [[frequency mixer|mixer]] at these frequencies, as vacuum tubes had too much capacitance. To fill this need researchers resurrected an obsolete technology, the [[point contact diode|point contact]] [[crystal detector]] (cat whisker detector) which was used as a [[demodulator]] in [[crystal radio]]s around the turn of the century before vacuum tube receivers.<ref name="Roer" /><ref name="Riordan">{{Cite book|author1-link=Michael Riordan (physicist) | last = Riordan | first = Michael |author2=Lillian Hoddeson |author2-link=Lillian Hoddeson| title = Crystal fire: the invention of the transistor and the birth of the information age | publisher = W. W. Norton & Company | year = 1988 | location = US | pages = 89–92 | url = https://books.google.com/books?id=SZ6wm5ZSUmsC&pg=PA89 | isbn = 978-0-393-31851-7 }}</ref> The low capacitance of [[semiconductor junction]]s allowed them to function at microwave frequencies. The first modern [[silicon]] and [[germanium]] [[diode]]s were developed as microwave detectors in the 1930s, and the principles of [[semiconductor physics]] learned during their development led to [[semiconductor electronics]] after the war.<ref name="Roer" />

<gallery mode="packed" heights="150">
File:R&B Magnetron.jpg|[[John Randall (physicist)|Randall]] and [[Harry Boot|Boot]]'s prototype cavity magnetron tube at the [[University of Birmingham]], 1940. In use the tube was installed between the poles of an electromagnet
File:Prototype klystron cutaway.jpg|First commercial klystron tube, by General Electric, 1940, sectioned to show internal construction
File:AI Mk. VIIIA radar in Bristol Beaufighter VIF CH16665.jpg|[[AI Mk. VIII radar|British Mk. VIII]], the first microwave air intercept radar, in nose of British fighter
File:US Army Signal Corps AN-TRC-1, 5, 6, & 8 microwave relay station 1945.jpg|Mobile US Army microwave relay station 1945 demonstrating relay systems using frequencies from 100&nbsp;MHz to 4.9&nbsp;GHz which could transmit up to 8 phone calls on a beam
</gallery>

The first powerful sources of microwaves were invented at the beginning of World War II: the [[klystron]] tube by [[Russell and Sigurd Varian]] at [[Stanford University]] in 1937, and the [[cavity magnetron]] tube by [[John Randall (physicist)|John Randall]] and [[Harry Boot]] at Birmingham University, UK in 1940.<ref name="Roer" /> Ten centimeter (3&nbsp;GHz) microwave radar powered by the magnetron tube was in use on British warplanes in late 1941 and proved to be a game changer.  Britain's 1940 decision to share its microwave technology with its US ally (the [[Tizard Mission]]) significantly shortened the war. The [[MIT Radiation Laboratory]] established secretly at [[Massachusetts Institute of Technology]] in 1940 to research radar, produced much of the theoretical knowledge necessary to use microwaves. The first microwave relay systems were developed by the Allied military near the end of the war and used for secure battlefield communication networks in the European theater.

===Post World War II exploitation===
After World War II, microwaves were rapidly exploited commercially.<ref name="Roer" /> Due to their high frequency they had a very large information-carrying capacity ([[bandwidth (signal processing)|bandwidth]]); a single microwave beam could carry tens of thousands of phone calls. In the 1950s and 60s transcontinental [[microwave transmission|microwave relay]] networks were built in the US and Europe to exchange telephone calls between cities and distribute television programs. In the new [[television broadcasting]] industry, from the 1940s microwave dishes were used to transmit [[backhaul (broadcasting)|backhaul]] video feeds from mobile [[production truck]]s back to the studio, allowing the first [[remote broadcast|remote TV broadcasts]]. The first [[communications satellite]]s were launched in the 1960s, which relayed telephone calls and television between widely separated points on Earth using microwave beams. In 1964, [[Arno Penzias]] and [[Robert Woodrow Wilson]] while investigating noise in a satellite horn antenna at [[Bell Labs]], Holmdel, New Jersey discovered [[cosmic microwave background radiation]].
{{multiple image
| align = center
| direction = horizontal
| header =
| image1 = Hogg horn antennas.jpg
| caption1 = C-band [[horn antenna]]s at a telephone switching center in Seattle, belonging to AT&T's Long Lines microwave relay network built in the 1960s.
| width1 = 149
| image2 = NIKE AJAX Anti-Aircraft Missile Radar3.jpg
| caption2 = Microwave lens antenna used in the radar for the 1954 [[Nike Ajax]] anti-aircraft missile
| width2 = 270
| image3 = NS Savannah microwave oven MD8.jpg
| caption3 = The first commercial microwave oven, Amana's [[Microwave ovens|Radarange]], installed in the kitchen of US merchant ship {{ship|NS|Savannah}} in 1961
| width3 = 119
| image4 = Telstar 1 replica.jpg
| caption4 = [[Telstar 1]] [[communications satellite]] launched July 10, 1962 the first satellite to relay television signals. The ring of [[microwave cavity]] antennas received the 6.39&nbsp;GHz [[uplink]], and transmitted the 4.17&nbsp;GHz [[downlink]] signal.
| width4 = 181
}}
Microwave radar became the central technology used in [[air traffic control]], maritime [[navigation]], [[anti-aircraft defense]], [[ballistic missile]] detection, and later many other uses.  Radar and satellite communication motivated the development of modern microwave antennas; the [[parabolic antenna]] (the most common type), [[cassegrain antenna]], [[lens antenna]], [[slot antenna]], and [[phased array]].

The ability of [[short wave]]s to quickly heat materials and cook food had been investigated in the 1930s by Ilia E. Mouromtseff at Westinghouse, and at the [[1933 Chicago World's Fair]] demonstrated cooking meals with a 60&nbsp;MHz radio transmitter.<ref name="SWC">{{cite journal| title = Cooking with Short Waves| journal = Short Wave Craft| volume = 4| issue = 7| page = 394| date = November 1933| url = http://www.americanradiohistory.com/Archive-Short-Wave-Television/30s/SW-TV-1933-11.pdf| access-date = 23 March 2015}}</ref> In 1945 [[Percy Spencer]], an engineer working on radar at [[Raytheon]], noticed that microwave radiation from a magnetron oscillator melted a candy bar in his pocket. He investigated cooking with microwaves and invented the [[microwave oven]], consisting of a magnetron feeding microwaves into a closed metal cavity containing food, which was patented by Raytheon on 8 October 1945. Due to their expense microwave ovens were initially used in institutional kitchens, but by 1986 roughly 25% of households in the U.S. owned one.   Microwave heating became widely used as an industrial process in industries such as plastics fabrication, and as a medical therapy to kill cancer cells in [[hyperthermy|microwave hyperthermy]].

The [[traveling wave tube]] (TWT) developed in 1943 by [[Rudolph Kompfner]] and [[John R. Pierce|John Pierce]] provided a high-power tunable source of microwaves up to 50&nbsp;GHz and became the most widely used microwave tube (besides the ubiquitous magnetron used in microwave ovens). The [[gyrotron]] tube family developed in Russia could produce megawatts of power up into [[millimeter wave]] frequencies and is used in industrial heating and [[plasma (physics)|plasma]] research, and to power [[particle accelerator]]s and nuclear [[fusion reactor]]s.

===Solid state microwave devices===
{{multiple image
| align = right
| direction = horizontal
| image1 = Atomic Clock-Louis Essen.jpg
| caption1 = First caesium atomic clock, and inventor Louis Essen ''(left)'', 1955
| width1 = 205
| image2 = Ruby maser amplifier 1961.jpg
| caption2 = Experimental ruby maser ''(lower end of rod)'', 1961
| width2 = 80
| image3 = Ganna gjenerators M31102-1.jpg
| caption3 = Microwave oscillator consisting of a [[Gunn diode]] inside a [[cavity resonator]], 1970s
| width3 = 120
| image4 = Radar Gun Electronics.jpg
| caption4 = Modern [[radar speed gun]].  At the right end of the copper [[horn antenna]] is the [[Gunn diode]] ''(grey assembly)'' which generates the microwaves.
| width4 = 180
}}

The development of [[semiconductor electronics]] in the 1950s led to the first [[solid state electronics|solid state]] microwave devices which worked by a new principle; [[negative resistance]] (some of the prewar microwave tubes had also used negative resistance).<ref name="Roer" /> The [[electronic oscillator|feedback oscillator]] and [[two-port]] amplifiers which were used at lower frequencies became unstable at microwave frequencies, and [[negative resistance]] oscillators and amplifiers based on [[one-port]] devices like [[diode]]s worked better.

The [[tunnel diode]] invented in 1957 by Japanese physicist [[Leo Esaki]] could produce a few milliwatts of microwave power. Its invention set off a search for better negative resistance semiconductor devices for use as microwave oscillators, resulting in the invention of the [[IMPATT diode]] in 1956 by [[W.T. Read]] and Ralph L. Johnston and the [[Gunn diode]] in 1962 by [[J. B. Gunn]].<ref name="Roer" /> Diodes are the most widely used microwave sources today.

Two low-noise [[Solid-state electronics|solid state]] negative resistance microwave [[amplifier]]s were developed; the [[maser]] invented in 1953 by [[Charles H. Townes]], [[James P. Gordon]], and [[H. J. Zeiger]], and the [[varactor]] [[parametric amplifier]] developed in 1956 by Marion Hines.<ref name="Roer" /> The parametric amplifier and the [[maser|ruby maser]], invented in 1958 by a team at [[Bell Labs]] headed by [[H.E.D. Scovil]] were used for low noise microwave receivers in radio telescopes and [[satellite ground station]]s.  The maser led to the development of [[atomic clock]]s, which keep time using a precise microwave frequency emitted by atoms undergoing an [[electron transition]] between two energy levels. Negative resistance amplifier circuits required the invention of new [[Reciprocity (electrical networks)|nonreciprocal]] waveguide components, such as [[circulator]]s, [[isolator (microwave)|isolator]]s, and [[directional coupler]]s. In 1969 Kaneyuki Kurokawa derived mathematical conditions for stability in negative resistance circuits which formed the basis of microwave oscillator design.<ref name="Kurokawa">{{cite journal
  | last = Kurokawa
  | first = Kaneyuki
  | title = Some Basic Characteristics of Broadband Negative Resistance Oscillator Circuits
  | journal = Bell System Tech. J.
  | volume = 48
  | issue = 6
  | pages = 1937–1955
  | date = July 1969
  | url = https://archive.org/details/bstj48-6-1937
  | doi = 10.1002/j.1538-7305.1969.tb01158.x
  | access-date = December 8, 2012}}</ref>

===Microwave integrated circuits===
[[File:LNB dissassembled.JPG|thumb|upright=0.7|[[ku band|k<sub>u</sub> band]] [[microstrip]] circuit used in [[satellite television]] dish]]
Prior to the 1970s microwave devices and circuits were bulky and expensive, so microwave frequencies were generally limited to the output stage of transmitters and the [[RF front end]] of receivers, and signals were [[heterodyning|heterodyned]] to a lower [[intermediate frequency]] for processing. The period from the 1970s to the present has seen the development of tiny inexpensive active solid-state microwave components which can be mounted on circuit boards, allowing circuits to perform significant [[signal processing]] at microwave frequencies. This has made possible [[satellite television]], [[cable television]], [[GPS]] devices, and modern wireless devices, such as [[smartphone]]s, [[Wi-Fi]], and [[Bluetooth]] which connect to networks using microwaves.

[[Microstrip]], a type of [[transmission line]] usable at microwave frequencies, was invented with [[printed circuit]]s in the 1950s.<ref name="Roer" /> The ability to cheaply fabricate a wide range of shapes on [[printed circuit board]]s allowed microstrip versions of [[capacitor]]s, [[inductor]]s, [[Stub (electronics)|resonant stubs]], [[Power dividers and directional couplers|splitters]], [[directional coupler]]s, [[diplexer]]s, [[electronic filter|filters]] and antennas to be made, thus allowing compact microwave circuits to be constructed.<ref name="Roer" />

[[Transistor]]s that operated at microwave frequencies were developed in the 1970s.  The semiconductor [[gallium arsenide]] (GaAs) has a much higher [[electron mobility]] than silicon,<ref name="Roer" /> so devices fabricated with this material can operate at 4 times the frequency of similar devices of silicon. Beginning in the 1970s GaAs was used to make the first microwave transistors,<ref name="Roer" /> and it has dominated microwave semiconductors ever since. MESFETs ([[metal-semiconductor field-effect transistor]]s), fast GaAs [[field effect transistor]]s using [[Schottky diode|Schottky junctions]] for the gate, were developed starting in 1968 and have reached cutoff frequencies of 100&nbsp;GHz, and are now the most widely used active microwave devices.<ref name="Roer" /> Another family of transistors with a higher frequency limit is the HEMT ([[high electron mobility transistor]]), a [[field effect transistor]] made with two different semiconductors, AlGaAs and GaAs, using [[heterojunction]] technology, and the similar HBT ([[heterojunction bipolar transistor]]).<ref name="Roer" />

GaAs can be made semi-insulating, allowing it to be used as a [[wafer (electronics)|substrate]] on which circuits containing [[passive component]]s, as well as transistors, can be fabricated by lithography.<ref name="Roer" /> By 1976 this led to the first [[integrated circuit]]s (ICs) which functioned at microwave frequencies, called [[monolithic microwave integrated circuit]]s (MMIC).<ref name="Roer" /> The word "monolithic" was added to distinguish these from microstrip PCB circuits, which were called "microwave integrated circuits" (MIC).   Since then, silicon MMICs have also been developed. Today MMICs have become the workhorses of both analog and digital high-frequency electronics, enabling the production of single-chip microwave receivers, broadband [[amplifier]]s, [[modem]]s, and [[microprocessor]]s.