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{{about|unpowered radio receivers|crystal-controlled oscillators (as used in radios)|Crystal oscillators}}
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{{Redirect|Crystal set|the Australian rock band|The Crystal Set}}
[[File:Crystal radio enthusiast.JPG|thumb|Boy listening to a modern crystal radio]]
 
A '''crystal radio receiver''', also called a '''crystal set''' or '''cat's whisker receiver''', is a very simple [[radio receiver]], popular in the early days of radio. It needs no [[battery (electricity)|battery]] or power source and runs on the power received from [[radio waves]] by a long wire [[antenna (radio)|antenna]]. It gets its name from its most important component, known as a [[crystal detector]], originally made with a piece of crystalline mineral such as [[galena]].<ref name="Carr">{{Cite book | last = Carr | first = Joseph J. | authorlink = | coauthors = | title = Old Time Radios! Restoration and Repair | publisher = McGraw-Hill Professional | year = 1990 | location = USA | pages = 7–9 | url = http://books.google.com/books?id=OVHYa_S2nKIC&pg=PA7 | doi = | isbn = 0-8306-3342-1 }}</ref> This component is now called a [[diode]].
 
Crystal radios are the simplest type of radio receiver<ref name="Petruzellis">{{Cite book | last = Petruzellis | first = Thomas | authorlink = | coauthors = | title = 22 Radio and Receiver Projects for the Evil Genius | publisher = McGraw-Hill Professional | year = 2007 | location = US | pages = 40, 44 | url = http://books.google.com/books?id=AJBBf5hCYqIC&pg=PA40 | doi = | isbn = 978-0-07-148929-4 }}</ref> and can be handmade with a few inexpensive parts, like an antenna wire, tuning [[inductor|coil]] of copper wire, capacitor, [[crystal detector]] and [[crystal earpiece|earphones]].<ref name="Gonzo">{{Cite book | last = Field | first = Simon Quellen | authorlink = | coauthors = | title = Gonzo gizmos: Projects and devices to channel your inner geek | publisher = Chicago Review Press | year = 2003 | location = USA | page = 85 | url = http://books.google.com/books?id=t-N1KdTb0FwC&pg=PT97 | doi = | isbn = 978-1-55652-520-9 }}</ref> They are distinct from ordinary radios because they are [[Passivity (engineering)|passive]] receivers, while other radios use a separate source of [[electric power]] such as a [[battery (electricity)|battery]] or the [[mains power]] to [[Amplifier|amplify]] the weak radio signal from the [[antenna (radio)|antenna]] so it is louder. Thus crystal sets produce rather weak sound and must be listened to with sensitive earphones, and can only pick up stations within a limited range.<ref name="Schaeffer">{{Cite book | last = Schaeffer | first = Derek K. | authorlink = | coauthors = Thomas H. Lee | title = The Design and Implementation of Low Power CMOS Receivers | publisher = Springer | year = 1999 | location = | pages = 3–4 | url = http://books.google.com/books?id=4IDLK8NMDBQC&pg=PA4 | doi = | isbn = 0-7923-8518-7 }}</ref>
 
The [[Rectifier|rectifying]] property of crystals was discovered in 1874 by [[Karl Ferdinand Braun]],<ref>{{Cite book | last = Braun | first = Ernest | authorlink = | coauthors = Stuart MacDonald | title = Revolution in Miniature: The history and impact of semiconductor electronics, 2nd Ed. | publisher = Cambridge Univ. Press | year = 1982 | location = UK | pages = 11–12 | url = http://books.google.com/books?id=03c4wldf-k4C&pg=PA11 | doi = | isbn = 978-0-521-28903-0 }}</ref><ref name="Riordan">{{Cite book | last = Riordan | first = Michael | authorlink = | coauthors = 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 = USA | pages = 19–21 | url = http://books.google.com/books?id=SZ6wm5ZSUmsC&pg=PA92 | doi = | isbn = 0-393-31851-6 }}</ref><ref name="Sarkar">{{Cite book | last = Sarkar | first = Tapan K. | authorlink = | coauthors = | title = History of wireless | publisher = John Wiley and Sons | year = 2006 | location = USA | page = 333 | url = http://books.google.com/books?id=NBLEAA6QKYkC&pg=PA333 | doi = | isbn = 0-471-71814-9 }}</ref> and crystal detectors were developed and applied to radio receivers in 1904 by [[Jagadish Chandra Bose]],<ref>Bose was first to use a crystal as a radio wave detector, using galena detectors to receive microwaves starting around 1894 and receiving a patent in 1904. {{Cite journal | last = Emerson | first = D. T. | authorlink = | coauthors = | title = The work of Jagadish Chandra Bose: 100 years of mm wave research | journal = IEEE Transactions on Microwave Theory and Techniques | volume = 45 | issue = 12 | pages = 2267–2273 | publisher = | date = Dec 1997 | url = http://books.google.com/books?id=09Zsv97IH1MC&pg=PA88 | issn = | doi =10.1109/22.643830 | accessdate = 2010-01-19 |bibcode = 1997ITMTT..45.2267E }}</ref><ref>[http://books.google.com/books?id=NBLEAA6QKYkC&pg=PA94 Sarkar (2006) ''History of wireless'', p.94, 291-308]</ref> [[Greenleaf Whittier Pickard|G. W. Pickard]]<ref name="Douglas">{{Cite journal | last = Douglas | first = Alan | authorlink = | coauthors = | title = The crystal detector | journal = IEEE Spectrum | volume = | issue = | page = 64 | publisher = Inst. of Electrical and Electronic Engineers | location = New York | date = April 1981 | url = http://www.crystalradio.net/crystalplans/xximages/thecrystaldetector1.jpg | issn = | doi = | accessdate = 2010-03-14}} on [http://www.crystalradio.net/crystalplans/ Stay Tuned website]</ref> and others. Crystal radios were the first widely used type of radio receiver,<ref name="Basalla">{{Cite book | last = Basalla | first = George | authorlink = | coauthors = | title = The Evolution of Technology | publisher = Cambridge University Press | year = 1988 | location = UK | page = 44 | url = http://books.google.com/books?id=EBtnG36-1WIC&pg=PA44 | doi = | isbn = 0-521-29681-1 }}</ref> and the main type used during the [[wireless telegraphy]] era.<ref>crystal detectors were used in receivers in greater numbers than any other type of detector after about 1907. {{Cite journal | last = Marriott | first = Robert H. | authorlink = | coauthors = | title = United States Radio Development | journal = Proc. of the Inst. of Radio Engineers | volume = 5 | issue = 3 | page = 184 | publisher = Institute of Radio Engineers | location = USA | date = September 17, 1915 | url = http://books.google.com/books?id=bh0B93CuXnkC&pg=PA184 | issn = | doi = | accessdate = 2010-01-19 }}</ref> Sold and homemade by the millions, the inexpensive and reliable crystal radio was a major driving force in the introduction of radio to the public, contributing to the development of radio as an entertainment medium around 1920.<ref>{{Cite book | last = Corbin | first = Alfred | authorlink = | coauthors = | title = The Third Element: A Brief History of Electronics | publisher = AuthorHouse | year = 2006 | location = | pages = 44–45 | url = http://books.google.com/books?id=-9lt4HL-AlwC&pg=PA45 | doi = | isbn = 1-4208-9084-0 }}</ref>
 
After about 1920, crystal sets were superseded by the first amplifying receivers, which used [[vacuum tube]]s ([[Audion]]s), and became obsolete for commercial use.<ref name="Basalla" /> However they continued to be built by hobbyists, youth groups and the [[Boy Scouts of America|Boy Scouts]]<ref name="Kent">{{Cite book | last = Kent | first = Herb | authorlink = | coauthors = David Smallwood, Richard M. Daley | title = The Cool Gent: The Nine Lives of Radio Legend Herb Kent | publisher = Chicago Review Press | year = 2009 | location = US | pages = 13–14 | url = http://books.google.com/books?id=f92pJ3JhaJYC&pg=PT33 | doi = | isbn = 1-55652-774-8 }}</ref> as a way of learning about the technology of radio. Today they are still sold as educational devices, and there are groups of enthusiasts devoted to their construction<ref>Jack Bryant (2009) [http://crystalradio.us/ Birmingham Crystal Radio Group], Birmingham, Alabama, USA . Retrieved 2010-01-18.</ref><ref>[http://www.midnightscience.com/ The Xtal Set Society] midnightscience.com . Retrieved 2010-01-18.</ref><ref>Darryl Boyd (2006) [http://crystalradio.net/ Stay Tuned Crystal Radio website] . Retrieved 2010-01-18.</ref><ref>Al Klase [http://www.skywaves.ar88.net/xtal/xtal.htm Crystal Radios], Klase's [http://www.skywaves.ar88.net/index.htm SkyWaves] website . Retrieved 2010-01-18.</ref><ref>Mike Tuggle (2003) [http://www.antiquewireless.org/otb/dxxtal.htm Designing a DX crystal set] [http://www.antiquewireless.org/otb/index.htm Antique Wireless Association] journal . Retrieved 2010-01-18.</ref><ref>Rainer Steinfuehr (2009) [http://www.oldradioworld.de/gollum/index.html Gollum´s Crystal Receiver World] ''[http://www.oldradioworld.de/ Wumpus's Old Radio World]'' website, Berlin, Germany. Retrieved 2010-01-18.</ref> who hold competitions comparing the performance of their home-built designs.<ref>[http://crystalradio.us/crystalcontests/index.htm Elmer Memorial Crystal Radio DX Contest], sponsored by [http://crystalradio.us/aboutus/index.htm Birmingham Crystal Radio Group], Birmingham, Alabama, USA . Retrieved 2010-01-18.</ref><ref>[http://www.crystalradio.net/contest/index.html Crystal Radio Building Contest], by [http://www.midnightscience.com/ The Xtal Set Society] midnightscience.com . Retrieved 2010-01-18.</ref>
 
Crystal radios can be designed to receive almost any [[radio frequency]] band, but most receive the [[medium wave|AM broadcast]] band.<ref name="Williams" >{{Cite book | last = Williams | first = Lyle R. |title = The New Radio Receiver Building Handbook | publisher = The Alternative Electronics Press | year = 2006 | pages = 20–23 | url = http://books.google.com/books?id=XiKgKdeBi6cC&pg=PT20 | isbn=978-1-84728-526-3}}</ref> A few receive the [[Shortwave bands|49-meter]] international [[shortwave]] band, but strong signals are required. By the nature of their operation, crystal radios can only demodulate [[amplitude modulation]] (AM) signals, and not [[frequency modulation]] (FM) or digital signals.  The first crystal sets received [[wireless telegraphy]] signals broadcast by [[spark-gap transmitter]]s at frequencies as low as 20&nbsp;kHz.<ref>{{Cite book | last = Lescarboura | first = Austin C. | title = Radio for Everybody | publisher = Scientific American Publishing Co. | year = 1922 | location = New York | pages = 4, 110, 268 | url = http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA110 }}</ref><ref>Long distance transoceanic stations of the era used wavelengths of 10,000 to 20,000 meters, correstponding to frequencies of 15 to 30 kHz.{{cite book | last = Morecroft | first = John H. | authorlink = | coauthors = A. Pinto, Walter A. Curry | title = Principles of Radio Communication | publisher = John Wiley & Sons. | year = 1921 | location = New York | page = 187 | url = http://books.google.com/books?id=jusOAAAAYAAJ&pg=PA187 }}</ref>
 
== History ==
[[File:Crystal radio advertisement.png|thumb|A family listening to a crystal radio in the 1920s]]
[[File:GWP-836531Patent.png|thumb|Greenleaf Whittier Pickard's U.S. Patent 836,531 "Means for receiving intelligence communicated by electric waves" diagram]]
[[File:Radio receiver 1914.jpg|thumb|Radio receiver, Basel, Switzerland, 1914]]
[[File:NBS 120 Set.jpg|thumb|NBS Circular 120 Home Crystal Radio Project]]
 
Crystal radio was invented by a long, partly obscure chain of [[history of radio|discoveries]] in the late 19th century that gradually evolved into more and more practical radio receivers in the early 20th century; and constitutes the origin of the field of [[electronics]]. The earliest practical use of crystal radio was to receive [[Morse code]] radio signals transmitted by early [[Amateur radio operator|amateur radio]] experimenters using very powerful [[spark-gap transmitter]]s. As electronics evolved, the ability to send voice signals by radio caused a technological explosion in the years around 1920 that evolved into today's radio [[broadcasting]] industry.
 
=== Early years ===
Early radio telegraphy used [[Spark-gap transmitter|spark gap]] and [[arc transmitter]]s as well as [[Alexanderson alternator|high-frequency alternators]] running at [[radio frequencies]]. At first a [[coherer|Branley Coherer]] was used to indicate the presence of a radio signal. However, these lacked the sensitivity to convert weak signals.
 
In the early 20th century, various researchers discovered that certain metallic [[mineral]]s, such as [[galena]], could be used to detect radio signals.<ref>In May 1901, [[Karl Ferdinand Braun]] of [[Strasbourg]] used [[psilomelane]], a manganese oxide ore, as an R.F. detector:  Ferdinand Braun (December 27, 1906) [http://babel.hathitrust.org/cgi/pt?id=njp.32101050985140#page/1199/mode/1up "Ein neuer Wellenanzeiger (Unipolar-Detektor)"] (A new R.F. detector (one-way detector)), ''Elektrotechnische Zeitschrift'', '''27''' (52) : 1199-1200.  From page 1119:<br>
"Im Mai 1901 habe ich einige Versuche im Laboratorium gemacht und dabei gefunden, daß in der Tat ein Fernhörer, der in einen aus Psilomelan und Elementen bestehenden Kreis eingeschaltet war, deutliche und scharfe Laute gab, wenn dem Kreise schwache schnelle Schwingungen zugeführt wurden.  Das Ergebnis wurde nachgeprüft, und zwar mit überraschend gutem Erfolg, an den Stationen für drahtlose Telegraphie, an welchen zu dieser Zeit auf den Straßburger Forts von der Königlichen Preußischen Luftschiffer-Abteilung unter Leitung des Hauptmannes von Sigsfeld gearbeitet wurde."<br>
(In May 1901, I did some experiments in the lab and thereby found that in fact an earphone, which was connected in a circuit consisting of psilomelane and batteries, produced clear and strong sounds when weak, rapid oscillations were introduced to the circuit.  The result was verified -- and indeed with surprising success -- at the stations for wireless telegraphy, which, at this time, were operated at the Strasbourg forts by the Royal Prussian Airship-Department under the direction of Capt. von Sigsfeld.)<br>Braun also states that he had been researching the conductive properties of semiconductors since 1874.  See:  Braun, F. (1874) [http://books.google.com/books?id=YBJbAAAAYAAJ&pg=PA556#v=onepage&q&f=false "Ueber die Stromleitung durch Schwefelmetalle"] (On current conduction through metal sulfides), ''Annalen der Physik und Chemie'', '''153''' (4) :  556-563.  In these experiments, Braun applied a cat's whisker to various semiconducting crystals and observed that current flowed in only one direction.<br>Braun patented an R.F. detector in 1906.  See:  (Ferdinand Braun), "Wellenempfindliche Kontaktstelle" (R.F. sensitive contact), Deutsches Reichspatent DE 178,871, (filed:  Feb. 18, 1906 ; issued:  Oct. 22, 1906).  Available on-line at:  [http://www.cdvandt.org/Braun%20DE178871A.pdf Foundation for German communication and related technologies].</ref><ref>Other inventors who patented crystal R.F. detectors:
*  In 1906, Henry Harrison Chase Dunwoody (1843-1933) of Washington, D.C., a retired general of the U.S. Army's Signal Corps, received a patent for a carborundum R.F. detector.  See:  Dunwoody, Henry H. C. [http://www.google.com/patents/US837616 "Wireless-telegraph system,"] U. S. patent 837,616 (filed:  March 23, 1906 ; issued:  December 4, 1906).
*  In 1907, [[Louis Winslow Austin]] received a patent for his R.F. detector consisting of tellurium and silicon.  See:  Louis W. Austin, [http://www.google.com/patents/US846081 "Receiver,"] U.S. patent 846,081 (filed:  Oct. 27, 1906 ; issued:  March 5, 1907).
*  In 1908, Wichi Torikata of the Imperial Japanese Electrotechnical Laboratory of the Ministry of Communications in Tokyo was granted Japanese patent 15,345 for the “Koseki” detector, consisting of crystals of zincite and bornite.</ref>
 
In 1901, Sir [[Jagadish Chandra Bose]] filed for a US patent for "A Device for Detecting Electrical Disturbances" that mentioned the use of a galena crystal; this was granted in 1904, #755840.<ref>Jagadis Chunder Bose, [http://www.google.com/patents/US755840 "Detector for electrical disturbances,"] U.S. patent no. 755,840 (filed: September 30, 1901 ; issued: March 29, 1904).</ref> The device depended on the large variation of a semiconductor's conductance with temperature; today we would call his invention a bolometer.{{citation needed|date=November 2013}}  Bose's patent is frequently, but erroneously, cited as a type of rectifying detector. On August 30, 1906, [[Greenleaf Whittier Pickard]] filed a patent for a silicon crystal detector, which was granted on November 20, 1906.<ref>Greenleaf Whittier Pickard, [http://www.google.com/patents/US836531 "Means for receiving intelligence communicated by electric waves,"] U.S. patent no. 836,531 (filed: August 30, 1906 ; issued: November 20, 1905).</ref> Pickard's detector was revolutionary in that he found that a fine pointed wire known as a "[[Cat's whisker diode|cat's whisker]]", in delicate contact with a mineral, produced the best semiconductor effect (that of rectification). A crystal detector includes a crystal, a special thin wire that contacts the crystal and the stand that holds the components in place. The most common crystal used is a small piece of [[galena]]; [[pyrite]] was also often used, as it was a more easily adjusted and stable mineral, and quite sufficient for urban signal strengths. Several other minerals also performed well as detectors. Another benefit of crystals was that they could [[demodulate]] [[amplitude modulated]] signals. This mode was used in [[radiotelephone]]s and to [[Broadcasting|broadcast]] [[voice message|voice]] and [[music]] for a public audience. Crystal sets represented an inexpensive and technologically simple method of receiving these signals at a time when the embryonic radio broadcasting industry was beginning to grow.
 
=== 1920s and 1930s ===
In 1922 the (then named) [[National Bureau of Standards|U.S. Bureau of Standards]] released a publication entitled ''Construction and Operation of a Simple Homemade Radio Receiving Outfit''.<ref>[http://www.crystalradio.net/crystalplans/xximages/nsb_120.pdf]</ref> This article showed how almost any family having a member who was handy with simple tools could make a radio and tune into weather, crop prices, time, news and the opera. This design was significant in bringing radio to the general public. NBS followed that with a more selective two-circuit version, ''Construction and Operation of a Two-Circuit Radio Receiving Equipment With Crystal Detector'', which was published the same year <ref>[http://www.crystalradio.net/crystalplans/xximages/nbs121.pdf]</ref> and is still frequently built by enthusiasts today.
 
In the beginning of the 20th century, radio had little commercial use and radio experimentation was a hobby for many people.<ref>Bondi, Victor."American Decades:1930-1939"</ref> Some historians consider the Autumn of 1920 to be the beginning of commercial radio broadcasting for entertainment purposes. Pittsburgh, PA, station [[KDKA (AM)|KDKA]], owned by [[Westinghouse Electric (1886)|Westinghouse]], received its license from the United States [[Department of Commerce]] just in time to broadcast the Harding-Cox presidential election returns. In addition to reporting on special events, broadcasts to farmers of crop price reports were an important public service in the early days of radio.
 
In 1921, factory-made radios were very expensive. Since less affluent families could not afford to own one, newspapers and magazines carried articles on how to build a crystal radio with common household items. To minimize the cost, many of the plans suggested winding the tuning coil on empty pasteboard containers such as oatmeal boxes, which became a common foundation for homemade radios.
 
=== Valveless amplifier ===
A "carbon amplifier" consisting of a [[carbon microphone]] and an electromagnetic earpiece sharing a common membrane and case.<ref>A carbon microphone can both convert sound into electrical signals and convert electrical signals into sound (with a gain of about 100).  By 1896, 27 patents had been issued for "carbon amplifiers" or "repeaters".
*  By 1904 Herbert E. Shreeve had developed a carbon amplifier for relaying telephone signals over long distances.  Also, the (U.S.) Western Electric company used a carbon amplifier in its No. 66B hearing aid.  See:  [http://www.aqpl43.dsl.pipex.com/MUSEUM/COMMS/mechamp/mechamp.htm Electro-mechanical amplifiers].
*  A mechanical telephone relay, which was invented by Herbert E. Shreeve, an engineer with the Western Electric Co., was patented in 1905.  See:  Herbert E. Shreeve, [http://www.google.com/patents/US791655 "Telephone-current reinforcer or relay,"] U.S. patent no. 791,655 (filed:  July 8, 1904 ; issued: June 6, 1905).  See also:  Herbert E. Shreeve, [http://www.google.com/patents/US791656 "Telephone-current reinforcer or relay,"] U.S. patent no. 791,656 (filed:  Feb. 28, 1904 ; issued: June 6, 1905).
*  The ''Acousticon'' hearing aid employed a carbon amplifier.  See:  [http://www.hearingaidmuseum.com/gallery/General_Info/GenInfoCarbon/info/generalinfo-carbon.htm The Hearing Aid Museum].
*  Other early hearing aids that were based on carbon amplifiers are mentioned here:  [http://beckerexhibits.wustl.edu/did/timeline/index.htm Washington University School of Medicine:  Timeline of hearing devices and early deaf education]
*  In 1924, the Siemens Corp. of Germany patented a carbon-amplifier hearing aid.  See:  [http://hearing.siemens.com/Resources/PressDatabase/_Global/media-database/business/PR_pic_Hist_BTE.html Siemens Hearing Instruments:  1924 carbon amplifier].</ref> This was used in the telephone industry and in [[hearing aid]]s nearly since the invention of both components and long before vacuum tubes. This could be readily bought or made from surplus telephone parts for use with a crystal radio. Unlike vacuum tubes, it could run with only a flashlight or car battery and had an indefinite lifetime.
 
=== Crystodyne ===
In the early 1920s [[Russia]], devastated by civil war, [[Oleg Losev]] was experimenting with applying voltage [[bias]]es to various kinds of crystals for manufacture of radio detectors. The result was astonishing: with a [[zincite]] ([[zinc oxide]]) crystal he gained amplification.<ref>Peter Robin Morris, ''A history of the world semiconductor industry'', IET, 1990, ISBN 0-86341-227-0, page 15</ref><ref>http://earlyradiohistory.us/1924cry.htm</ref><ref>In 1924, Losev's research was publicized in several French publications:
*  ''Radio Revue'', no. 28, p. 139 (1924)
*  I. Podliasky (May 25, 1924) (Crystal detectors as oscillators), ''Radio Électricité'', '''5''' : 196-197.
*  Vinogradsky (September 1924) ''L'Onde Electrique''
English-language publications noticed the French articles and also publicized Losev's work:
*  Pocock (June 11, 1924)''The Wireless World and Radio Review'', '''14''' :  299-300.
*  Victor Gabel (October 1 & 8, 1924) "The crystal as a generator and amplifier," ''The Wireless World and Radio Review'', '''15''' :  2ff , 47ff.
*  O. Lossev (October 1924) "Oscillating crystals," ''The Wireless World and Radio Review'', '''15''' :  93-96.
*  Round and Rust (August 19, 1925) ''The Wireless World and Radio Review'', pp. 217-218.
*  "The Crystodyne principle," ''Radio News'', pages 294, 295, 431 (September 1924).  See also the October 1924 issue of ''Radio News''.  (It was Hugo Gernsbach, publisher of ''Radio News'', who coined the term "crystodyne".)  This article is available on-line at:  [http://www.radiomuseum.org/forumdata/users/5944/file/Crystodyne.pdf Radio Museum.org] and [http://ebookbrowse.com/crystodyne-pdf-d183652518 E-Book Browse].</ref> This was [[negative resistance]] phenomenon, decades before the [[tunnel diode]]. After the first experiments, he built regenerative and [[superheterodyne]] receivers, and even transmitters. However, this discovery was not supported by authorities and soon forgotten and no device was produced in mass quantity beyond a few examples for research. Crystodyne was produced in primitive conditions; it can be made in a rural forge, unlike [[vacuum tube]]s and modern semiconductor devices.
 
=== 1940s ===
[[File:Homemade radio receiver with razorblade.JPG|thumb|left|"Foxhole radio" using a battery carbon touching a razor blade for a detector ''(left)'', Greece, 1935]]
 
When [[Allied]] troops were halted near [[Anzio|Anzio, Italy]] during the spring of 1944, personal [[portable radio]]s were strictly prohibited as the Germans had radio detecting equipment that could detect the [[local oscillator]] signal of [[superheterodyne]] receivers. Crystal sets lack local oscillators, so they cannot be detected in this way. Some resourceful GIs found that a crude crystal set could be made from a coil made of salvaged wire, a rusty razor blade and a pencil lead for a diode. By lightly touching the pencil lead to spots of blue on the blade, or to spots of rust, they formed what is called a point contact [[diode]] and the rectified signal could be heard on headphones or crystal ear pieces. The idea spread across the beachhead, to other parts of the war, and to popular culture. The sets were dubbed ''"foxhole receivers"'' by the popular press, and they became part of the [[folklore]] of [[Second World War|World War II]].
 
In some Nazi occupied countries there were widespread confiscations of radio sets from the civilian population. This led to particularly determined listeners building their own "clandestine receivers" which frequently amounted to little more than a basic crystal set. However anyone doing so risked imprisonment or even death if caught and in most parts of Europe the signals from the BBC (or other allied stations) were not strong enough to be received on such a set. However there were places such as the [[Occupation of the Channel Islands|Channel Islands]] and [[Netherlands]] where it was possible.
 
=== Later years ===
While it never regained the popularity and general use that it enjoyed at its beginnings, the [[Electronic circuit|circuit]] is still used. The [[Boy Scout]]s (who emerged as the unofficial custodians of crystal radio lore) kept construction of a set in their program since the 1920s. A large number of prefabricated novelty items and simple kits could be found through the 1950s and 1960s, and many children with an interest in electronics built one.
 
Building crystal radios was a [[Fad|craze]] in the 1920s, and again in the 1950s. Recently, [[hobbyist]]s have started designing and building sophisticated examples of the instruments. Much effort goes into the visual appearance of these sets as well as their performance, and some outstanding examples can be found. Annual crystal radio [[DXing|'DX' contests]] (long distance reception) and building [[competition|contest]]s allow these set owners to compete with each other and form a community of interest in the subject.
 
== Design ==
[[File:CrystalRadio.jpg|thumb|Pictorial diagram from 1922 showing the circuit of a crystal radio. This common circuit did not use a tuning [[capacitor]], but used the capacitance of the antenna to form the [[tuned circuit]] with the coil]]
 
A crystal radio can be thought of as a radio receiver reduced to its essentials.<ref name="Gonzo" /><ref>{{cite web | last = Purdie | first = Ian C. | authorlink = | coauthors = | title = Crystal Radio Set | work = electronics-tutorials.com | publisher = Ian Purdie | year = 2001 | url = http://www.electronics-tutorials.com/receivers/crystal-radio-set.htm | doi = | accessdate = 2009-12-05 }}</ref> It consists at a minimum of these components:<ref name="Williams" /><ref name="Lescarboura" >{{Cite book | last = Lescarboura | first = Austin C. | authorlink = | coauthors = | title = Radio for Everybody | publisher = Scientific American Publishing Co. | year = 1922 | location = New York | pages = 93–94 | url = http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA93 | doi = | isbn = }}</ref><ref>{{cite web | last = Kuhn | first = Kenneth A. | authorlink = | coauthors = | title = Introduction | work = Crystal Radio Engineering | publisher = Prof. Kenneth Kuhn website, Univ. of Alabama | date = Jan 6, 2008 | url = http://www.kennethkuhn.com/students/crystal_radios/introduction.pdf | doi = | accessdate = 2009-12-07 }}</ref>
 
* An [[Antenna (radio)|antenna]] to pick up the [[radio wave]]s and convert them to electric currents.
* A [[tuned circuit]] to select the signal of the [[radio station]] to be received, out of all the signals received by the antenna. This consists of a coil of wire called an [[inductor]] or ''tuning coil'' and a [[capacitor]] connected together, one or both of which is adjustable and can be used to tune in different stations. In some circuits a capacitor is not used, because the antenna also serves as the capacitor. The tuned circuit has a natural [[resonant frequency]], and allows radio signals at this [[frequency]] to pass while rejecting signals at all other frequencies.
* A [[semiconductor]] crystal [[detector (radio)|detector]] which extracts the [[audio signal]] ([[modulation]]) from the radio frequency [[carrier wave]]. It does this by only allowing current to pass through it in one direction, blocking half of the oscillations of the radio wave. This ''[[rectifier|rectifies]]'' the [[alternating current]] radio wave to a pulsing [[direct current]], whose strength varies with the audio signal. This current can be converted to sound by the earphone. Early sets used a [[cat's whisker detector]], a fine wire touching the surface of a pebble of crystalline mineral such as [[galena]]. It was this component that gave crystal sets their name.
* An [[earphone]] to convert the audio signal to sound waves so they can be heard. The low power produced by crystal radios is insufficient to power an unamplified [[loudspeaker]] so earphones are used.
 
The sound power produced by the earphone of a crystal set comes solely from the [[radio station]] being received, via the radio waves picked up by the antenna.<ref name="Gonzo" /> The power picked up by a receiving antenna decreases with the square of its distance from the [[radio transmitter]].<ref>{{cite web | last = Fette | first = Bruce A. | authorlink = | coauthors = | title = RF Basics: Radio Propagation | work = | publisher = [http://www.rfengineer.net/ RF Engineer Network] | date = Dec 27, 2008 | url = http://www.rfengineer.net/1170/rf-basics-radio-propagation/ | doi = | accessdate = 2010-01-18 }}</ref> Even for a powerful commercial [[Radio station|broadcasting station]], if it is more than a few miles from the receiver the power received by the antenna is very small, typically measured in [[microwatt]]s or [[nanowatt]]s.<ref name="Gonzo" /> In modern crystal sets, signals as weak as 50 [[picowatt]]s at the antenna can be heard.<ref name="Payor42">{{Cite journal | last = Payor | first = Steve | authorlink = | coauthors = | title = Build a Matchbox Crystal Radio | journal = Popular Electronics | volume = | issue = | page = 42 | publisher = | date = June 1989 | url = http://www.crystalradio.net/crystalplans/xximages/matchbox2.jpg | issn = | doi = | accessdate = 2010-05-28}} on [http://www.crystalradio.net/crystalplans/ Stay Tuned] website</ref> Crystal radios can receive such weak signals without using [[Amplifier|amplification]] only due to the great sensitivity of human [[hearing (sense)|hearing]],<ref name="Gonzo" /><ref name="Lee">{{Cite book | last = Lee | first = Thomas H. | authorlink = | coauthors = | title = Planar Microwave Engineering: A practical guide to theory, measurement, and circuits | publisher = Cambridge Univ. Press | year = 2004 | location = UK | pages = 297–304 | url = http://books.google.com/books?id=uoj3IWFxbVYC&pg=PA299 | doi = | isbn = 978-0-521-83526-8 }}</ref> which can detect sounds with a power of only 10<sup>−16</sup> [[Watt (unit)|W]]/cm<sup>2</sup>.<ref>{{cite web | last = Nave | first = C. Rod | authorlink = | coauthors = | title = Threshold of hearing | work = HyperPhysics | publisher = Dept. of Physics, Georgia State University | date = | url = http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html | doi = | accessdate = 2009-12-06 }}</ref> Therefore crystal receivers have to be designed to convert the energy from the radio waves into sound as efficiently as possible. Even so, they are usually only able to receive nearby stations, within distances of about 25 miles for [[AM broadcasting|AM broadcast]] stations,<ref>[http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA144 Lescarboura, 1922], p. 144</ref><ref name="Binns">{{Cite journal | last = Binns | first = Jack | authorlink = | coauthors = | title = Jack Binn's 10 commandments for the radio fan | journal = Popular Science | volume = 101 | issue = 5 | pages = 42–43 | publisher = Modern Publishing Co. | location = New York | date = November 1922 | url = http://books.google.com/books?id=pSkDAAAAMBAJ&pg=PA42 | issn = | doi = | accessdate = 2010-01-18 }}</ref> although the [[radiotelegraphy]] signals used during the [[wireless telegraphy]] era could be received at hundreds of miles,<ref name="Binns" /> and crystal receivers were even used for transoceanic communication during that period.<ref name="Beauchamp">Marconi used carborundum detectors for a time around 1907 in his first commercial transatlantic wireless link between Newfoundland, Canada and Clifton, Ireland. {{cite book 
  | last = Beauchamp
  | first = Ken
  | title = History of Telegraphy
  | publisher = Institution of Electrical Engineers
  | year = 2001
  | location =
  | pages = 191
  | url = http://books.google.com/books?id=k3XCkncd83AC&pg=PA191
  | doi =
  | id =
  | isbn = 0852967926}}</ref>
 
Passive receiver development was abandoned with the advent of reliable vacuum tubes around 1920, and subsequent crystal radio research was the work of [[radio amateur]]s and hobbyists.<ref name="Klase">{{cite web | last = Klase | first = Alan R. | authorlink = | coauthors = | title = Crystal Set Design 102 | work = Skywaves | publisher = Alan Klase personal website | year = 1998 | url = http://www.skywaves.ar88.net/xtalset102/xtalset102.htm | doi = | accessdate = 2010-02-07 }}</ref> Many different circuits have been used.<ref name="Petruzellis" /><ref>a list of circuits from the wireless era can be found in {{Cite book | last = Sleeper | first = Milton Blake | authorlink = | coauthors = | title = Radio hook-ups: a reference and record book of circuits used for connecting wireless instruments | publisher = The Norman W. Henley publishing co. | year = 1922 | location = USA | pages = 7–18 | url = http://books.google.com/books?id=k8Wpg2wxDLcC | doi = | isbn = }}</ref><ref>{{Cite book | last = May | first = Walter J. | authorlink = | coauthors = | title = The Boy's Book of Crystal Sets | publisher = Bernard's | year = 1954 | location = London | pages = | url = http://books.google.com/books?id=9Bs5PwAACAAJ&dq=Walter+May+Boy's+book+of+crystal+sets+intitle:Boy's+intitle:book+intitle:of+intitle:crystal+intitle:sets+inauthor:Walter+inauthor:May | doi = | isbn = }} is a collection of 12 circuits</ref> The following sections discuss the parts of a crystal radio in greater detail.
 
=== Antenna ===
The antenna converts the energy in the electromagnetic [[radio wave]]s striking it to an [[alternating current|alternating]] [[electric current]] in the antenna, which is connected to the tuning coil. Since in a crystal radio all the power comes from the antenna, it is important that the antenna collect as much power from the radio wave as possible. The larger an antenna, the more power it can intercept. In addition, antennas are most efficient when their length is close to a multiple of a quarter-[[wavelength]] of the radio waves they are receiving. Since the length of the waves used with crystal radios is very long ([[AM broadcast]] band waves are 182-566 [[meter (unit)|m]] or 597–1857&nbsp;ft. long)<ref>{{cite web | last = Purdie | first = Ian | authorlink = | coauthors = | title = A Basic Crystal Set | work = Ian Purdie's Amateur Radio Pages | publisher = personal website | year = 1999 | url = http://my.integritynet.com.au/purdic/crystal_set.htm | doi = | accessdate = 2010-02-27 }}</ref> the antenna is made as long as possible,<ref name="KuhnAntenna">{{cite web | last = Kuhn | first = Kenneth | authorlink = | coauthors = | title = Antenna and Ground System | work = Crystal Radio Engineering | publisher = Kenneth Kuhn website, Univ. of Alabama | date = Dec 9, 2007 | url = http://www.kennethkuhn.com/students/crystal_radios/antenna_and_ground_system.pdf | doi = | accessdate = 2009-12-07 }}</ref> out of a [[Random wire antenna|long wire]], in contrast to the [[whip antenna]]s or ferrite [[loop antenna|loopstick antennas]] used in modern radios.
 
Serious crystal radio hobbyists use "inverted L" and [[T-aerial|"T" type antennas]], consisting of hundreds of feet of wire suspended as high as possible between buildings or trees, with a feed wire attached in the center or at one end leading down to the receiver.<ref name="Marx">{{Cite book | last = Marx, | first = Harry J. | authorlink = | coauthors = Adrian Van Muffling | title = Radio Reception: A simple and complete explanation of the principles of radio telephony | publisher = G.P. Putnam's sons | year = 1922 | location = USA | pages = 130–131 | url = http://books.google.com/books?id=BgY9AAAAYAAJ&pg=PA130 | doi = | isbn = }}</ref><ref>{{Cite book | last = Williams | first = Henry Smith | authorlink = | coauthors = | title = Practical Radio | publisher = Funk and Wagnalls | year = 1922 | location = New York | page = 58 | url = http://books.google.com/books?id=-CwwAAAAYAAJ&pg=PA58 | doi = | isbn = }}</ref> However more often random lengths of wire dangling out windows are used. A popular practice in early days (particularly among apartment dwellers) was to use existing large metal objects, such as [[bedspring]]s,<ref name="Kent" /> [[fire escape]]s, and [[barbed wire]] fences as antennas.<ref name="Binns" /><ref>{{Cite journal | last = Putnam | first = Robert | authorlink = | coauthors = | title = Make the aerial a good one | journal = Tractor and Gas Engine Review | volume = 15 | issue = 10 | page = 9 | publisher = Clarke Publishing Co. | location = New York | date = October 1922 | url = http://books.google.com/books?id=XMTmAAAAMAAJ&pg=RA6-PA9 | issn = | doi = | accessdate = 2010-01-18 }}</ref><ref>[http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA100 Lescarboura 1922, p.100]</ref>
 
=== Ground ===
The wire antennas used with crystal receivers are [[monopole antenna]]s which develop their output voltage with respect to ground. They require a return circuit connected to [[ground (electricity)|ground]] (earth) so that the current from the antenna, after passing through the receiver, can flow into the ground. The ground wire is attached to a radiator, a water pipe, or a metal stake driven into the ground.<ref>{{Cite book | last = Collins | first = Archie Frederick | authorlink = | coauthors = | title = The Radio Amateur's Hand Book | publisher = Forgotten Books | year = 1922 | location = USA | pages = 18–22 | url = http://books.google.com/books?id=jpMi0V8qoKsC&pg=PA18 | doi = | isbn = 1-60680-119-8 }}</ref><ref>[http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA102 Lescarboura, 1922, p. 102-104]</ref> A good ground is more important for crystal sets than for powered receivers, because crystal sets have low [[input impedance]] to transfer power efficiently from the antenna, so significant current flows in the antenna/ground circuit. A low resistance ground connection (preferably below 25 Ω) is necessary because any resistance in the ground dissipates power from the antenna.<ref name="KuhnAntenna" /> In contrast, modern receivers are voltage-operated devices, with high input impedance, so little current flows in the antenna/ground circuit. Also, [[mains power]]ed receivers are grounded adequately through their power cords.
 
=== Tuned circuit ===
[[File:Simplest crystal radio circuit.svg|thumb|left|upright|The earliest crystal receiver circuit did not have a [[tuned circuit]]]]
 
The [[tuned circuit]], consisting of the coil and capacitor, acts as a [[resonator]], analogous to a [[tuning fork]] for sound waves.<ref name="Hausmann">{{Cite book | last = Hausmann | first = Erich, Ed. | authorlink = | coauthors = | title = Radio Phone Receiving; a Practical Book for Everybody | publisher = D. Van Nostrand | year = 1922 | location = New York | page = 48 | url = http://books.google.com/books?id=6qFRAAAAMAAJ&pg=PA48 | doi = | isbn = 1-110-37159-4 }}</ref> Electric charge flows rapidly back and forth between the plates of the capacitor through the coil, oscillating at the frequency of the radio signal. It has a high [[Electrical impedance|impedance]] at the desired radio signal's frequency, but a low impedance at all other frequencies,<ref>{{Cite book | last = Hayt | first = William H. | first2=Jack E. |last2=Kemmerly | title = Engineering Circuit Analysis, 2nd Ed. |publisher=McGraw-Hill |year=1971 |location=New York |pages=398–399 |isbn=978-0-07-027382-5}}</ref> so the desired signal is passed on to the detector which is connected across the tuned circuit, while the other signals are short-circuited to ground. The frequency of the station received is the [[resonant frequency]] ''f'' of the tuned circuit, determined by the [[capacitance]] ''C'' of the capacitor and the [[inductance]] ''L'' of the coil:<ref name="KuhnResonantCircuit">{{cite web | last = Kuhn | first = Kenneth A. | authorlink = | coauthors = | title = Resonant Circuit | work = Crystal Radio Engineering | publisher = Prof. Kenneth Kuhn website, Univ. of Alabama | date = Jan 6, 2008 | url = http://www.kennethkuhn.com/students/crystal_radios/resonant_circuit.pdf | doi = | accessdate = 2009-12-07 }}</ref>
 
:<math>f = \frac {1}{ 2 \pi \sqrt {LC}} \,</math>
 
In inexpensive sets the inductor had a sliding spring contact that pressed against the windings and could be slid up and down the coil, to allow a larger or smaller number of turns of the coil into the circuit, varying the [[inductance]], to tune in different stations.<ref name="Carr" /> Alternatively, a [[variable capacitor]] is used to tune the radio instead of the coil.<ref>{{Cite journal | last = Clifford | first = Martin | authorlink = | coauthors = | title = The early days of radio | journal = Radio Electronics | volume = | issue = | pages = 61–64 | publisher = | date = July 1986 | url = http://www.crystalradio.net/crystalplans/xximages/earlydays3.jpg | issn = | doi = | accessdate = 2010-07-19}} on [http://www.crystalradio.net/crystalplans/ Stay Tuned] website</ref> Some modern crystal sets use a [[ferrite core]] tuning coil, in which the core is mounted on a threaded shaft and a knob turns the shaft, moving the core in and out of the coil, varying the inductance by changing the [[permeability (electromagnetism)|magnetic permeability]].<ref name="Blanchard">{{Cite journal | last = Blanchard | first = T. A. | authorlink = | coauthors = | title = Vestpocket Crystal Radio | journal = Radio-electronics | volume = | issue = | page = 196 | publisher = | date = October 1962 | url = http://crystalradio.net/crystalplans/xximages/vestpocketradio1.jpg | issn = | doi = | accessdate = 2010-08-19}} on [http://crystalradio.net/crystalplans/ Crystal Radios and Plans, Stay Tuned website]</ref>
 
The antenna is an integral part of the tuned circuit and its [[reactance (electronics)|reactance]] contributes to determining the resonant frequency. The antenna usually acts as a capacitor, because antennas shorter than a quarter-wavelength have [[capacitive reactance]].<ref name="KuhnAntenna" /> Many early crystal sets did not have a tuning capacitor,<ref name="Principles">{{Cite book | last = | first = | authorlink = | coauthors = | title = The Principles Underlying Radio Communication, 2nd Ed., Radio pamphlet no. 40 | publisher = Prepared by US National Bureau of Standards, United States Army Signal Corps | year = 1922 | location = USA | pages = 421–425 | url = http://books.google.com/books?id=4JVEAAAAIAAJ&pg=PA422 | doi = | id = | isbn = }}</ref> and relied instead on the capacitance inherent in the wire antenna (in addition to significant [[parasitic capacitance]] in the coil itself<ref>[http://books.google.com/books?id=6qFRAAAAMAAJ&pg=PA57 Hausmann 1922, p. 57]</ref>) to form the tuned circuit with the coil.
 
The earliest crystal receivers did not have a tuned circuit at all, and just consisted of a crystal detector connected between the antenna and ground, with an earphone across it.<ref name="Carr" /><ref name="Principles" /> Since this circuit lacked any frequency-selective elements besides the broad [[resonance]] of the antenna, it had little ability to reject unwanted stations, so all stations within a wide band of frequencies were heard in the earphone<ref name="Klase" /> (in practice the most powerful usually drowns out the others). It was used in the earliest days of radio, when only one or two stations were within a crystal set's limited range.
 
==== Impedance matching ====
[[File:Two slider crystal radio circuit.svg|thumb|upright|"Two slider" circuit.<ref name="Klase" /> The two sliding contacts on the coil allowed the impedance of the radio to be adjusted to match the antenna as the radio was tuned, resulting in stronger reception]]
 
An important principle used in crystal radio design to transfer maximum power to the earphone is [[impedance matching]].<ref name="Klase" /><ref>{{Cite book | last = Paul J. Nahin | first = Paul J. | authorlink = | coauthors = | title = The science of radio: with MATLAB and Electronics Workbench demonstrations | publisher = Springer | year = 2001 | location = USA | pages = 60–62 | url = http://books.google.com/books?id=V1GBW6UD4CcC&pg=PA61 | doi = | isbn = 0-387-95150-4 }}</ref><ref>Technical discussions of impedance matching in crystal radios can be found in [http://www.bentongue.com/xtalset/0def_exp/0def_exp.html Ben H. Tongue (2007) ''Practical considerations, etc.'', Crystal Radio Set Systems: Design, Measurement, and Improvement; Ben Tongue personal website] and [http://www.oldradioworld.de/gollum/analysis.htm Berthold Bosch (2002) ''Crystal Set analysis'', Gollum's Crystal Receiver World]</ref> The maximum power is transferred from one part of a circuit to another when the [[wave impedance|impedance]] ([[Electrical resistance|resistance]]) of the two circuits is equal.<ref name="Carr" /><ref>{{Cite book | last = Smith | first = K. c. a. | authorlink = | coauthors = R. E. Alley | title = Electrical circuits: An introduction | publisher = Cambridge University Press | year = 1992 | location = UK | page = 218 | url = http://books.google.com/books?id=Aj9blWqYg2YC&pg=PA217 | doi = | isbn = 0-521-37769-2 }}</ref><ref name="Alley">{{Cite book | last = Alley | first = Charles L. | authorlink = | coauthors = Kenneth W. Atwood | title = Electronic Engineering, 3rd Ed. | publisher = John Wiley & Sons | year = 1973 | location = New York | page = 269 | url = | doi = | isbn = 0-471-02450-3 }}</ref> However in crystal sets, the impedance of the antenna-ground system (around 10-200 [[ohm]]s<ref name="KuhnAntenna" />) is usually lower than the impedance of the receiver's tuned circuit (thousands of ohms at resonance),<ref>{{cite web | last = Tongue | first = Ben H. | authorlink = | coauthors = | title = Practical considerations, helpful definitions of terms and useful explanations of some concepts used in this site | work = Crystal Radio Set Systems: Design, Measurement, and Improvement | publisher = [http://www.bentongue.com/ Ben Tongue personal website] | date = 2007-11-06 | url = http://www.bentongue.com/xtalset/0def_exp/0def_exp.html | doi = | accessdate = 2010-02-07 }}</ref> and also varies depending on the quality of the ground, length of the antenna, and what frequency the receiver is tuned to in the band.<ref name="Payor42" /> Therefore in better receiver circuits, to match the antenna impedance to the receiver's impedance, the antenna was connected across only a portion of the tuning coil's turns.<ref name="KuhnResonantCircuit" /><ref name="Principles" /> This made the coil act as an [[Impedance matching|impedance matching transformer]] (in an [[autotransformer]] connection) in addition to its tuning function. The tuned circuit's high impedance was transformed down by a factor equal to the square root of the turns ratio (the number of turns the antenna was connected across, to the total number of turns of the coil), to match the antenna impedance.<ref name="Alley" /> In the "two-slider" circuit, popular during the wireless era, both the antenna and the detector circuit were attached to the coil with sliding contacts, allowing (interactive)<ref>{{Cite book | last = Bucher | first = Elmer Eustace | authorlink = | coauthors = | title = Practical Wireless Telegraphy: A complete text book for students of radio communication, Revised Ed. | publisher = Wireless Press, Inc | year = 1921 | location = New York | page = 133 | url = http://books.google.com/books?id=DI9RAAAAMAAJ&pg=PA134 | doi = | isbn = }}</ref> adjustment of both the resonant frequency and the turns ratio.<ref>[http://books.google.com/books?id=BgY9AAAAYAAJ&pg=PA94 Marx & Van Muffling (1922) ''Radio Reception''], p.94</ref><ref>{{Cite book | last = Stanley | first = Rupert | authorlink = | coauthors = | title = Textbook on Wireless Telegraphy, Vol. 1 | publisher = Longman's Green & Co. | year = 1919 | location = London | pages = 280–281 | url = http://books.google.com/books?id=LLhRAAAAMAAJ&pg=PA284 | doi = | isbn = }}</ref><ref name="Collins">{{Cite book | last = Collins | first = Archie Frederick | authorlink = | coauthors = | title = The Radio Amateur's Hand Book | publisher = Forgotten Books | year = 1922 | location = USA | pages = 23–25 | url = http://books.google.com/books?id=jpMi0V8qoKsC&pg=PA25 | doi = | isbn = 1-60680-119-8 }}</ref> Alternatively a multiposition switch was used to select taps on the coil. These controls were adjusted until the station sounded loudest in the earphone.
 
The other place impedance matching was often used was between the tuning coil and the crystal detector/earphone circuit, to match the impedance of the detector.<ref name="Carr" /><ref name="Klase" /> To accomplish this the detector, like the antenna, was connected to a tap on the coil.<ref name="KuhnTappingInductors">{{cite web | last = Kuhn | first = Kenneth | authorlink = | coauthors = | title = Tapping Inductors | work = Crystal Radio Engineering | publisher = Kenneth Kuhn website, Univ. of Alabama | date = March 1, 2008 | url = http://www.kennethkuhn.com/students/crystal_radios/tapping_inductors.pdf | doi = | accessdate = 2009-12-07 }}</ref><ref name="Hadgraft">{{cite web | last = Hadgraft | first = Peter | authorlink = | coauthors = | title = The Crystal Set 5/6 | work = The Crystal Corner | publisher = Kev's Vintage Radio and Hi-Fi page | date = | url = http://oldkevspage.tripod.com/wr/xtal3.html | doi = | accessdate = 2010-05-28 }}</ref> This also improved the receiver's selectivity (see below).
 
==== Problem of selectivity ====
[[File:Crystal radio with impedance matching.svg|thumb|left|Direct-coupled circuit with impedance matching<ref name="Klase" />]]
 
One of the drawbacks of crystal sets is that they are vulnerable to interference from stations near in [[frequency]] to the desired station; they have low [[selectivity (electronic)|selectivity]].<ref name="Petruzellis" /><ref name="Schaeffer" /><ref name="Payor42" /> Often two or more stations are heard simultaneously. This is because the simple tuned circuit doesn't reject nearby signals well; it allows through a wide band of frequencies, that is, it has a large [[bandwidth (signal processing)|bandwidth]] (low [[Q factor]]) compared to modern receivers.<ref name="Schaeffer" /> This was a worse problem during the [[Wireless telegraphy|wireless]] era because the [[spark-gap transmitter]]s of the era produced much wider bandwidth signals than modern transmitters, that spread interference over the frequencies of other stations.<ref name="Schaeffer" /> The tuned circuit had wide bandwidth because the crystal detector connected across it had relatively low [[Electrical resistance|resistance]] which "loaded" the tuned circuit, damping the oscillations, reducing its [[Q factor|Q]].<ref name="Payor42" /><ref name="Wenzel">{{cite web | last = Wenzel | first = Charles | authorlink = | coauthors = | title = Simple crystal radio | work = Crystal radio circuits | publisher = [http://www.techlib.com/default.htm techlib.com] | year = 1995 | url = http://www.techlib.com/electronics/crystal.html | doi = | accessdate = 2009-12-07 }}</ref> In many circuits, the selectivity was improved by connecting the detector and earphone circuit to a tap across only a fraction of the coil's turns.<ref name="Klase" /> This reduced the impedance loading of the tuned circuit, as well as improving the impedance match with the detector.<ref name="Klase" />
 
==== Inductively coupled receivers ====
[[File:Inductively coupled crystal radio circuit.svg|thumb|Inductively-coupled circuit with impedance matching. This type was used in most quality crystal receivers]]
[[File:Crystal radio receiver from wireless era.png|thumb|Amateur-built crystal receiver with "loose coupler" antenna transformer, Belfast, around 1914]]
 
In more sophisticated crystal receivers, the tuning coil was replaced with an adjustable air core [[Antenna tuner|antenna coupling]] [[transformer]]<ref name="Carr" /><ref name="Klase" /> which improved the [[selectivity (electronic)|selectivity]] by a technique called ''loose coupling''.<ref name="Principles" /><ref name="Collins" /><ref>{{Cite journal | last = Hogan | first = John V. L. | authorlink = | coauthors = | title = The Selective Double-Circuit Receiver | journal = Radio Broadcast | volume = 1 | issue = 6 | pages = 480–483 | publisher = Doubleday Page & Co. | location = New York | date = October 1922 | url = http://books.google.com/books?id=VMcnAAAAYAAJ&pg=RA5-PA480 | issn = | doi = | accessdate = 2010-02-10 }}</ref> This consisted of two [[Magnetic coupling|magnetically coupled]] coils of wire, one (the ''primary'') attached to the antenna and ground and the other (the ''secondary'') attached to the rest of the circuit. The current from the antenna created an alternating magnetic field in the primary coil, which induced a voltage in the secondary coil which was rectified and powered the earphone. Each of the coils functioned as a tuned circuit that was tuned to the frequency of the station: the primary coil [[Resonant circuit|resonated]] with the capacitance of the antenna (or sometimes another capacitor), and the secondary resonated with the tuning capacitor. The two circuits interacted to form a [[Transformer#Resonant transformers|resonant transformer]]. Reducing the ''coupling'' between the coils, by physically separating them so less of the [[magnetic field]] of one intersects the other (reducing the [[mutual inductance]]), narrows the bandwidth, resulting in much sharper, more selective tuning than a single tuned circuit.<ref name="Principles" /><ref>Alley & Atwood (1973) ''Electronic Engineering'', p. 318</ref> However this involved a tradeoff; looser coupling also reduced the amount of signal getting through the transformer. So the transformer was made with adjustable coupling. One type common in early days, called a "loose coupler", consisted of a smaller coil inside a larger coil.<ref name="Klase" /><ref>[http://books.google.com/books?id=BgY9AAAAYAAJ&pg=PA94 Marx & Van Muffling (1922) ''Radio Reception''], p.96-101</ref> The smaller coil was mounted on a rack so it could be slid linearly in or out of the larger coil. If interference was encountered, the smaller coil would be slid further out of the larger, loosening the coupling and narrowing the bandwidth, to reject the interfering signal.
 
The antenna coupling transformer also functioned as an [[impedance matching|impedance matching transformer]], to match the antenna impedance to the rest of the circuit. One or both of the coils usually had several taps which could be selected with a switch, to adjust the turns ratio.
 
Coupling transformers were difficult to adjust, because the three adjustments, the tuning of the primary circuit, the tuning of the secondary circuit, and the coupling, were all interactive, and changing one affected the others.<ref>{{Cite book | last = U.S. Signal Corps | first = | authorlink = | coauthors = | title = Radiotelegraphy | publisher = Government Printing Office | date = October 1916 | location = USA | page = 70 | url = http://books.google.com/books?id=PbRBAAAAIAAJ&pg=PA70 | doi = | isbn = }}</ref>
 
=== Crystal detector ===
[[File:Kristallradio (3).jpg|thumb|left|Galena cat's whisker detector]]
[[File:Germanium Diode 1N60.jpg|thumb|left|[[Germanium diode]] used in modern crystal radios (about 3 mm long)]]
 
[[File:Amplitude modulation detection.png|thumb|How the crystal detector works.<ref>[http://books.google.com/books?id=BgY9AAAAYAAJ&pg=PA43 Marx & Van Muffling (1922) ''Radio Reception'', p.43, fig.22]</ref><ref name="Campbell">{{Cite journal | last = Campbell | first = John W. | authorlink = | coauthors = | title = Radio Detectors and How They Work | journal = Popular Science | volume = 145 | issue = 4 | pages = 206–209 | publisher = Popular Science Publishing Co. | location = New York | date = October 1944 | url = http://books.google.com/books?id=PyEDAAAAMBAJ&pg=PA206& | issn = | doi = | accessdate = 2010-03-06 }}</ref> ''<span style="color:red;">(A)</span>'' The [[Amplitude modulation|amplitude modulated]] radio signal from the tuned circuit. The rapid oscillations are the [[radio frequency]] [[carrier wave]]. The [[audio signal]] (the sound) is contained in the slow variations ([[modulation]]) of the size of the waves. This signal cannot be converted to sound by the earphone, because the audio excursions are the same on both sides of the axis, averaging out to zero, which would result in no net motion of the earphone's diaphragm. ''<span style="color:red;">(B)</span>'' The crystal conducts current better in one direction than the other, producing a signal whose amplitude does not average zero but varies with the audio signal. ''<span style="color:red;">(C)</span>''  A bypass capacitor is used to remove the radio frequency carrier pulses, leaving the audio signal]]
 
In early sets, this was a cat's whisker detector, a fine metal wire on an adjustable arm that touched the surface of a crystal of a [[semiconductor|semiconducting]] [[mineral]].<ref name="Carr" /><ref name="Riordan" /><ref>{{Cite book | last = Harte | first = Bernard | authorlink = | coauthors = | title = When Radio Was the Cat's Whiskers | publisher = Rosanberg | year = 2002 | location = | pages = 149–150 | url = http://books.google.com/books?id=IEnPtx0M2EEC&pg=PA149 | doi = | isbn = 1-877058-08-4 }}</ref> This formed a crude unstable [[semiconductor diode]] ([[Schottky diode]]), which allowed current to flow better in one direction than in the opposite direction.<ref>{{Cite book | last = Lee | first = Thomas H. | authorlink = | coauthors = | title = The Design of CMOS Radio-Frequency Integrated Circuits | publisher = Cambridge University Press | year = 2004 | location = UK | pages = 4–6 | url = http://books.google.com/books?id=DzcMK-2mFQUC&printsec=frontcover&dq=Lee+CMOS+radio+frequency+integrated+circuits&cd=1#v=onepage&q=crystal%20detector&f=false | doi = | isbn = 0-521-83539-9 }}</ref> Modern crystal sets use modern [[semiconductor diode]]s.<ref name="Wenzel" /> The detector [[rectifier|rectified]] the [[alternating current]] radio signal to a pulsing [[direct current]], which had the audio [[modulation]] signal impressed on it, so it could be converted to sound by the earphone, which was connected in series (or sometimes in parallel) with the detector.<ref name="Williams" /><ref name="Campbell" />
 
The rectified current from the detector still had [[radio frequency]] pulses from the carrier in it, which did not pass well through the high [[inductance]] of the earphones. A small [[capacitor]], called a blocking or [[bypass capacitor]], was often placed across the earphone terminals to bypass these pulses around the earphone to ground,<ref>[http://books.google.com/books?id=LLhRAAAAMAAJ&pg=PA282 Stanley (1919) ''Text-book on Wireless Telegraphy'', p.282]</ref> although the earphone cord usually had enough capacitance that this component could be omitted.<ref name="Lee" /><ref name="Principles" />
 
In a cat's whisker detector only certain sites on the crystal surface functioned as rectifying junctions, and the device was very sensitive to the pressure of the crystal-wire contact, which could be disrupted by the slightest vibration.<ref name="Riordan" /><ref name="Hausmann2">[http://books.google.com/books?id=m18r0frukVgC&pg=PA60 Hausmann (1922), p.60-61]</ref> Therefore a usable contact point had to be found by trial and error before each use. The operator dragged the wire across the crystal surface until a radio station or "static" sounds were heard in the earphones.<ref name="Lescarboura2">[http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA143 Lescarboura (1922), p.143-146]</ref> An alternative adjustment method was to use a battery-powered [[buzzer]] attached to the ground wire to provide a test signal.<ref name="Lescarboura2" /> The spark at the buzzer's electrical contacts served as a weak [[radio transmitter]], so when the detector began working the buzz could be heard in the earphones, and the buzzer was then turned off.
 
[[Galena]] (lead sulfide) was probably the most common crystal used in cat's whisker detectors,<ref name="Collins" /><ref name="Hausmann2" /> but various other types of crystals were also used, such as [[iron pyrite]] (Fool's gold, FeS<sub>2</sub>), [[silicon]], [[molybdenite]] (MoS<sub>2</sub>), [[silicon carbide]] (carborundum, SiC), and a [[zincite]]-[[bornite]] (ZnO-Cu<sub>5</sub>FeS<sub>4</sub>) crystal-to-crystal junction trade-named ''Perikon''.<ref name="Lee" /><ref>[http://books.google.com/books?id=LLhRAAAAMAAJ&pg=PA311 Stanley (1919), p. 311-318]</ref> Crystal radios have also been made with [[PN junction|rectifying junctions]] improvised from a variety of common objects, such as blue steel [[razor blade steel|razor blades]] and lead pencils,<ref name="Lee" /><ref>{{Cite journal | last = Gernsback | first = Hugo | authorlink = | coauthors = | title = Foxhole emergency radios | journal = Radio-Craft | volume = 16 | issue = 1 | page = 730 | publisher = Radcraft Publications | location = New York | date = September 1944 | url = http://www.crystalradio.net/crystalplans/xximages/foxholeemergencyradios1.jpg | issn = | doi = | accessdate = 2010-03-14}} on [http://www.crystalradio.net/crystalplans/ Crystal Plans and Circuits, Stay Tuned website]</ref> rusty needles,<ref>{{Cite journal | last = Douglas | first = Alan | authorlink = | coauthors = | title = The Crystal Detector | journal = IEEE Spectrum | volume = 18 | issue = 4 | pages = 64–65 | publisher = Inst. of Electrical and Electronic Engineers | date = April 1981 | url = http://www.crystalradio.net/crystalplans/xximages/thecrystaldetector1.jpg | issn = | doi = | accessdate = 2010-03-28 }}</ref> and pennies<ref name="Lee" /> In these, a [[semiconductor|semiconducting]] layer of oxide or sulfide on the metal surface is usually responsible for the rectifying action.<ref name="Lee" />
 
In modern sets a [[semiconductor diode]] is used, which is much more reliable than a cat's whisker detector and doesn't require any adjustments.<ref name="Lee" /><ref name="Wenzel" /><ref name="KuhnDiode">{{cite web | last = Kuhn | first = Kenneth A. | authorlink = | coauthors = | title = Diode Detectors | work = Crystal Radio Engineering | publisher = Prof. Kenneth Kuhn website, Univ. of Alabama | date = Jan 6, 2008 | url = http://www.kennethkuhn.com/students/crystal_radios/diode_detectors.pdf | doi = | accessdate = 2009-12-07 }}</ref> [[Germanium diode]]s (or sometimes [[Schottky diode]]s) are used instead of [[silicon diode]]s, because their lower forward voltage drop (roughly 0.3V compared to 0.6V<ref name="Hadgraft" />) makes them more sensitive.<ref name="Wenzel" /><ref>{{cite web | last = Kleijer | first = Dick | authorlink = | coauthors = | title = Diodes | work = | publisher = crystal-radio.eu | date = | url = http://www.crystal-radio.eu/endiodes.htm | doi = | accessdate = 2010-05-27 }}</ref>
 
All semiconductor detectors function rather inefficiently in crystal receivers, because the low voltage signal level is too low to result in much difference between forward better conduction and reverse weaker conduction.  To improve the sensitivity of some of the early crystal detectors, such as silicon carbide, a small [[Biasing (electronics)|forward bias]] voltage was applied across the detector by a battery and [[potentiometer]].<ref>[http://books.google.com/books?id=4JVEAAAAIAAJ&pg=PA434#v=onepage&q=&f=false ''The Principles Underlying Radio Communication'' (1922), p.439-440]</ref><ref>{{Cite book | last = Bucher | first = Elmer Eustace | authorlink = | coauthors = | title = Practical Wireless Telegraphy: A complete text book for students of radio communication, Revised Ed. | publisher = Wireless Press, Inc | year = 1921 | location = New York | pages = 134–135 | url = http://books.google.com/books?id=DI9RAAAAMAAJ&pg=PA134 | doi = | isbn = }}</ref>  Bias can move the diode's operating point higher on the detection curve to produce more signal voltage at the expense of less signal current (higher impedance). There is a limit to the benefit that this produces, depending on the other impedances of the radio.  This improved sensitivity by moving the DC operating point to a more desirable voltage-current operating point (impedance) on the junction's [[I-V curve]].
 
=== Earphones ===
[[File:Crystal radio circuit with bias and buzzer.svg|thumb|Circuit with detector bias battery to improve sensitivity and buzzer to adjust cat's whisker]]
[[File:Poste a diode 1.JPG|thumb|left|upright|Modern crystal radio with [[Crystal earpiece|piezoelectric earphone]]]]
 
The requirements for earphones used in crystal sets are different from earphones used with modern audio equipment. They have to be efficient at converting the electrical signal energy to sound waves, while most modern earphones are designed for [[high fidelity]] reproduction of the sound.<ref name="FieldEarphone">[http://books.google.com/books?id=t-N1KdTb0FwC&pg=PT93 Field 2003, p.93-94]</ref> In early homebuilt sets, the earphones were the most costly component.<ref>[http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA285 Lescarboura (1922), p.285]</ref>
 
The early earphones used with wireless-era crystal sets had [[moving iron speaker|moving iron drivers]] that worked similarly to [[loudspeaker]]s. Each earpiece contained a [[magnet]] wound with coils of wire to form an [[electromagnet]], with poles close to a steel diaphragm. When the [[audio signal]] from the radio was passed through the electromagnet's windings, it created a varying [[magnetic field]] that pulled on the diaphragm, causing it to vibrate. The vibrations of the diaphragm pushed and pulled on the air in front of it, creating sound waves. Standard headphones used in telephone work had a low [[Electrical impedance|impedance]], often 75 Ω, and required more current than a crystal radio could supply, so the type used with radios was wound with more turns of finer wire and had an impedance of 2000-8000 Ω.<ref>[http://books.google.com/books?id=jpMi0V8qoKsC&pg=PA27 Collins (1922), p. 27-28]</ref><ref>[http://books.google.com/books?id=-CwwAAAAYAAJ&pg=PA79 Williams (1922), p. 79]</ref><ref>[http://books.google.com/books?id=4JVEAAAAIAAJ&pg=PA441 ''The Principles Underlying Radio Communication'' (1922), p. 441]</ref>
 
Modern crystal sets use [[Piezoelectricity|piezoelectric]] [[crystal earpiece]]s, which are much more sensitive and also smaller.<ref name="FieldEarphone" /> They consist of a [[piezoelectric]] crystal with electrodes attached to each side, glued to a light diaphragm. When the audio signal from the radio set is applied to the electrodes, it causes the crystal to vibrate, vibrating the diaphragm. Crystal earphones are designed as [[ear buds]] that plug directly into the ear canal of the wearer, coupling the sound more efficiently to the eardrum. Their resistance is much higher, typically megohms, so they don't "load" the tuned circuit; increasing the [[selectivity (electronic)|selectivity]] of the receiver. However the earphone's higher resistance, in parallel with its capacitance of around 9 pF, creates a [[low pass filter]] which removes the higher audio frequencies, distorting or eliminating the sound.<ref>{{Cite journal | last = Payor | first = Steve | authorlink = | coauthors = | title = Build a Matchbox Crystal Radio | journal = Popular Electronics | volume = | issue = | page = 45 | publisher = | date = June 1989 | url = http://www.crystalradio.net/crystalplans/xximages/matchbox5.jpg | issn = | doi = | accessdate = 2010-05-28 }}</ref> So a bypass capacitor is not needed (although in practice a small one of around 0.68 to 1 nF is often used to help improve quality), and instead a 10-100 kΩ resistor must be added across the earphone's input.<ref>[http://books.google.com/books?id=t-N1KdTb0FwC&pg=PT97#v=onepage&q&f=false Field (2003), p. 94]</ref>
 
Modern low impedance (8 Ω) earphones cannot be used unmodified in crystal sets because the receiver doesn't produce enough current to drive them. They are sometimes used by adding an audio transformer to match their impedance with the higher impedance of the circuit.
 
== Use as a power source ==
A crystal radio tuned to a strong local transmitter can be used just as a power source for a second amplified receiver for distant stations that cannot be heard with a plain crystal radio.<ref name="simple_AM_rx">{{cite book |last=Polyakov |first=V.&nbsp;T. |authorlink= |title=Техника радиоприёма. Простые приёмники АМ сигналов |trans_title=Receiving techniques. Simple receivers for AM signals |url= |chapter=3.3.2&nbsp;Питание полем мощных станций |trans_chapter= |accessdate= |year=2001 |publisher= |location=Moscow |language=Russian |isbn=5-94074-056-1 |page= |pages=256}}</ref>{{rp|122–123}}
 
There is a long history of unsuccessful attempts and unverified claims to recover the power in the carrier of the received signal itself. Traditional crystal sets use half-wave [[rectifier]]s. As [[AM broadcasting|AM]] signals have a [[modulation]] factor of only 30% by voltage at peaks{{Citation needed|date=June 2007}}, no more than 9% of received signal power (<math>P = U^2/R</math>) is actual audio information, and 91% is just rectified DC voltage. Given that the audio signal is unlikely to be at peak all the time, the ratio of energy is, in practice, even greater. Considerable effort was made to convert this DC voltage into sound energy. Some earlier attempts include a one-[[transistor]]<ref>Radio-Electronics, 1966, №2</ref> amplifier in 1966. Sometimes efforts to recover this power are confused with other efforts to produce a more efficient detection.<ref>QST [Amateur Radio Magazine] January 2007, "High Sensitivity Crystal Set" <http://www.arrl.org/files/file/Technology/tis/info/pdf/culter.pdf></ref> This history continues now with designs as elaborate as "inverted two-wave switching power unit".<ref name="simple_AM_rx" />{{rp|129}}
 
==Example Images==
{{multiple image
| align    = center
| footer    = During the [[wireless telegraphy]] era before 1920, crystal receivers were "state of the art", and sophisticated models were produced. After 1920 crystal sets became the cheap alternative to [[vacuum tube]] radios, used in emergencies and by youth and the poor.
| width = 85
| image1    = RX 1914 tranchée .JPG
| caption1  = Soldier listening to a crystal radio during World War I, 1914
| image2    = Australian signallers 1916.jpg
| caption2  = Australian signallers using a Marconi Mk III crystal receiver, 1916.
| image3    = Marconi crystal radio receiver.jpg
| caption3  = Marconi Type 103 crystal set.
| image4    = Signal Corps SCR-54A crystal radio.jpg
| caption4  = SCR-54A crystal set used by US Signal Corps in World War I
| image5    = Marconi Type 106 crystal radio receiver.jpg
| caption5  = Marconi Type 106 crystal receiver used for transatlantic communication, ca. 1921
| image6    = PensacolaAug081920sCrystalRadioKodak.jpg
| caption6  = Homemade "loose coupler" set ''(top)'', Florida, ca. 1920
| image7    = D 1925 Jahnke Schiebspulendetektor.jpg
| caption7  = Crystal radio, Germany, ca. 1924
| image8    = Kristallmottagare.JPG
| caption8  = Swedish "box" crystal radio with earphones, ca. 1925
| image9    = Heliogen medium wave galena radio.JPG
| caption9  = German Heliogen brand radio showing "basket-weave" coil, 1935
| image10  = Radio detefon 1.jpg
| caption10 = Polish Detefon brand radio, 1930-1939, using a "cartridge" type crystal ''(top)''
}}
 
== See also ==
{{Portal|Radio}}
 
* [[Batteryless radio]]
* [[Camille Papin Tissot]]
* [[Cat's-whisker detector]]
* [[Coherer]]
* [[Demodulator]]
* [[Detector (radio)]]
* [[Electrolytic detector]]
* [[Foxhole radio]]
* [[History of radio]]
 
== References ==
{{reflist|30em}}
 
== Further reading ==
* Ellery W. Stone (1919). [http://books.google.com/books?id=MZTaMdOH7VIC Elements of Radiotelegraphy]. D. Van Nostrand company. 267 pages.
* Elmer Eustice Bucher (1920). [http://books.google.com/books?id=TK-7a_BjUCwC The Wireless Experimenter's Manual: Incorporating how to Conduct a Radio Club].
* Milton Blake Sleeper (1922). [http://books.google.com/books?id=k8Wpg2wxDLcC Radio Hook-ups: A Reference and Record Book of Circuits Used for Connecting Wireless Instruments]. The Norman W. Henley publishing co.; 67 pages.
* JL Preston and HA Wheeler (1922) "Construction and operation of a simple homemade radio receiving outfit", Bu. of Standards, C-120: Apr. 24, 1922.
* PA Kinzie (1996). Crystal Radio: History, Fundamentals, and Design. Xtal Set Society.
* Thomas H. Lee, [http://books.google.com/books?id=DzcMK-2mFQUC The Design of CMOS Radio-Frequency Integrated Circuits]
* Derek K. Shaeffer and Thomas H. Lee, [http://books.google.com/books?id=4IDLK8NMDBQC The Design and Implementation of Low-Power CMOS Radio Receivers]
* Ian L. Sanders. Tickling the Crystal&nbsp;— Domestic British Crystal Sets of the 1920s; Volumes 1-5. BVWS Books (2000–2010).
 
== External links ==
{{Commons category|Crystal radios}}
{{Wikisource|QST/March 1916/An Interview with the Inventor of the Crystaloi Detector|An Interview with the Inventor of the Crystaloi Detector}}
 
* ''[http://www.midnightscience.com/ The Xtal Set Society]'', Dedicated to once again building and experimenting with radio electronics.
* ''[http://scitoys.com/scitoys/scitoys/radio/radio.html#crystal Building a simple crystal radio]''.Field, Simon Quellen, Scitoys.
* ''[http://www.crystalradio.net/ Stay Tuned]''. Crystal radio plans and projects.
* ''[http://www.clarion.org.au/crystalset/mystery.html Build the Mystery Crystal set]'' A simple and surprisingly effective and sensitive design.
* ''[http://www.qsl.net/kc4iwt/xtal/SWMystery.htm Shortwave Mystery Crystal Radio]'' A shortwave version of the Mystery Crystal Set by KC4IWT.
* ''[http://www.vcomp.co.uk A website with lots of information on early radio and crystal sets]
* ''[http://www.hobbytech.com/crystalradio/crystalradio.htm Hobbydyne Crystal Radios]'' History and Technical Information on Crystal Radios
* ''[http://www.bentongue.com/ Ben Tongue's Technical Talk]'' Section 1 links to "Crystal Radio Set Systems: Design, Measurements and Improvement".
* "''[http://www.a-reny.com/iexplorer/cristadyne.html Semiconductor archeology or tribute to unknown precusors]''". earthlink.net/~lenyr.
* Nyle Steiner K7NS, [http://www.sparkbangbuzz.com/els/znrfamp2-el.htm Zinc Negative Resistance RF Amplifier for Crystal Sets and Regenerative Receivers Uses No Tubes or Transistors]. November 20, 2002.
* [http://homepage.ntlworld.com/lapthorn/xtal.htm Crystal Set DX?] Roger Lapthorn G3XBM
* [http://www.open2.net/sciencetechnologynature/worldaroundus/buildcrystalradio.html Building a crystal radio set] A guide to building a simple crystal radio receiver.
* [http://www.makearadio.com/index.php Website which has a large selection of homebuilt crystal and tube radios built by Dave Schmarder]
* [http://www.thelen.us/1crystal.php Documentation of a high performance crystal radio set by Wayne Thelen]
* [http://www.crystal-sets.com/ British Crystal Set Definitive Information]
 
{{Telecommunications}}
 
[[Category:History of radio]]
[[Category:Radio electronics]]
[[Category:Types of radios]]
[[Category:Amateur radio receivers]]
[[Category:Receiver (radio)]]
[[Category:Electronic design]]
 
{{Link GA|de}}

Latest revision as of 12:07, 2 November 2014

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