Editing Crystal radio

{{short description|Simple radio receiver circuit for AM reception}}
{{about|unpowered radio receivers|crystal-controlled oscillators (as used in radios)|Crystal oscillator}}
{{Redirect|Crystal set|the music band|The Crystal Set}}
[[File:Kristallradio.JPG|thumb|Swedish crystal radio from 1922 made by [[Radiotechnique|Radiola]], with earphones. The device at top is the radio's [[cat's whisker detector]]. A second pair of earphone jacks is provided.]]
[[File:Vintage Arrow Germanium Crystal Radio (23708349181).jpg|thumb|1970s-era [[Arrow Electronics|Arrow]] crystal radio marketed to children. The earphone is on left.  The antenna wire, right, has a clip to attach to metal objects such as a bedspring, which serve as an additional antenna to improve reception.]]

A '''crystal radio receiver''', also called a '''crystal set''', is a simple [[Receiver (radio)|radio receiver]], popular in the early days of radio. It uses only the power of the received radio signal to produce sound, needing no external power. It is named for its most important component, a [[crystal detector]], originally made from a piece of crystalline mineral such as [[galena]].<ref name="CarrJ">{{Cite book | last = Carr | first = Joseph J. | title = Old Time Radios! Restoration and Repair | publisher = McGraw-Hill Professional | year = 1990 | location = US  | url = https://books.google.com/books?id=OVHYa_S2nKIC&pg=PA7 | isbn = 0-8306-3342-1 }}</ref>{{rp|p=7-9}} 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 | title = 22 Radio and Receiver Projects for the Evil Genius | publisher = McGraw-Hill Professional | year = 2007 | location = US | pages = 40, 44 | url = https://books.google.com/books?id=AJBBf5hCYqIC&pg=PA40 | isbn = 978-0-07-148929-4 }}</ref> and can be made with a few inexpensive parts, such as a wire for an antenna,  a [[inductor|coil]] of wire, a capacitor, a crystal detector, and [[crystal earpiece|earphones]].<ref name="Gonzo">{{Cite book | last = Field | first = Simon Quellen | title = Gonzo gizmos: Projects and devices to channel your inner geek | publisher = Chicago Review Press | year = 2003 | location = US | page = 85 | url = https://books.google.com/books?id=t-N1KdTb0FwC&pg=PT97 | isbn = 978-1-55652-520-9 }}</ref>  However they are [[Passivity (engineering)|passive]] receivers, while other radios use an [[amplifier]] powered by current from a [[Battery (electricity)|battery]] or wall outlet to make the radio signal louder. Thus, crystal sets produce rather weak sound and must be listened to with sensitive earphones, and can receive stations only within a limited range of the transmitter.<ref name="Schaeffer">{{Cite book | last = Schaeffer | first = Derek K. |author2=Thomas H. Lee | title = The Design and Implementation of Low Power CMOS Receivers | publisher = Springer | year = 1999 | pages = 3–4 | url = https://books.google.com/books?id=4IDLK8NMDBQC&pg=PA4 | isbn = 0-7923-8518-7 }}</ref>

The [[Rectifier|rectifying]] property of a contact between a [[mineral]] and a [[metal]] was discovered in 1874 by [[Karl Ferdinand Braun]].<ref>{{Cite book | last = Braun | first = Ernest |author2=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 = https://books.google.com/books?id=03c4wldf-k4C&pg=PA11 | isbn = 978-0-521-28903-0 }}</ref><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 = 19–21 | url = https://books.google.com/books?id=SZ6wm5ZSUmsC&pg=PA92 | isbn = 0-393-31851-6 }}</ref><ref name="SarkarT" />{{rp|p=333}} Crystals were first used as a [[Detector (radio)|detector]] of radio waves in 1894 by [[Jagadish Chandra Bose]],<ref>Bose was first to use crystals for electromagnetic wave detection, using galena detectors to receive microwaves starting around 1894 and receiving a patent in 1904 {{Cite journal | last = Emerson | first = D. T. | title = The work of Jagadis Chandra Bose: 100 years of millimeter-wave research | journal = IEEE Transactions on Microwave Theory and Techniques | volume = 45 | issue = 12 | pages = 2267–2273 | date = Dec 1997 | url = https://books.google.com/books?id=09Zsv97IH1MC&pg=PA88 | doi =10.1109/22.643830 | access-date = 2010-01-19 |bibcode = 1997ITMTT..45.2267E | isbn = 9780986488511 | url-access = subscription }}</ref><ref name="SarkarT" />{{rp|p=291-308}} in his microwave optics experiments. They were first used as a [[demodulator]] for radio communication reception in 1902 by [[Greenleaf Whittier Pickard|G. W. Pickard]].<ref name="Douglas">{{Cite journal | last = Douglas | first = Alan | title = The crystal detector | journal = IEEE Spectrum | page = 64 | publisher = Inst. of Electrical and Electronic Engineers | location = New York | date = April 1981 | url = http://www.crystalradio.net/crystalplans/xximages/thecrystaldetector1.jpg | doi = 10.1109/MSPEC.1981.6369482| s2cid = 44288637 | access-date = 2010-03-14| hdl = 10366/158938 | hdl-access = free }} on [http://www.crystalradio.net/crystalplans/ Stay Tuned website]</ref> Crystal radios were the first widely used type of radio receiver,<ref name="Basalla">{{Cite book | last = Basalla | first = George | title = The Evolution of Technology | publisher = Cambridge University Press | year = 1988 | location = UK | page = 44 | url = https://books.google.com/books?id=EBtnG36-1WIC&pg=PA44 | isbn = 0-521-29681-1 }}</ref> and the main type used during the [[wireless telegraphy]] era.<ref name="Marriott" >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. | 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 = US | date = September 17, 1915 | url = https://books.google.com/books?id=bh0B93CuXnkC&pg=PA184 | doi = 10.1109/jrproc.1917.217311| s2cid = 51644366 | access-date = 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 with the beginning of [[radio broadcasting]] around 1920.<ref>{{Cite book | last = Corbin | first = Alfred | title = The Third Element: A Brief History of Electronics | publisher = AuthorHouse | year = 2006 | pages = 44–45 | url = https://books.google.com/books?id=-9lt4HL-AlwC&pg=PA45 | isbn = 1-4208-9084-0 }}</ref>

Around 1920, crystal sets were superseded by the first amplifying receivers, which used [[vacuum tube]]s.  With this technological advance, crystal sets became obsolete for commercial use<ref name="Basalla" /> but 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 |author2=David Smallwood |author3=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 = https://books.google.com/books?id=f92pJ3JhaJYC&pg=PT33 | isbn = 978-1-55652-774-6 }}</ref> mainly as a way of learning about the technology of radio. 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, US. Retrieved 2010-01-18.</ref><ref>[http://www.midnightscience.com/ The Xtal Set Society] {{Webarchive|url=https://web.archive.org/web/20060114052628/http://www.midnightscience.com/ |date=2006-01-14 }} 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] {{Webarchive|url=https://web.archive.org/web/20100124053252/http://www.antiquewireless.org/otb/dxxtal.htm |date=2010-01-24 }} [http://www.antiquewireless.org/otb/index.htm Antique Wireless Association] {{Webarchive|url=https://web.archive.org/web/20100523142803/http://www.antiquewireless.org/otb/index.htm |date=2010-05-23 }} journal. Retrieved 2010-01-18.</ref>

Crystal radios receive [[amplitude modulation|amplitude modulated]] (AM) signals, although [[frequency modulation|FM]] designs have been built.<ref name="Solomon">{{Cite web|date=2007-12-30|last = Solomon|first = Larry J.|title = FM Crystal Radios|url=http://solomonsmusic.net/FM_CrystalRadio.html|access-date=2022-02-20|archive-url = https://web.archive.org/web/20071230044543/http://solomonsmusic.net/FM_CrystalRadio.html|archive-date = 2007-12-30}}</ref><ref name="Petruzellis1">{{Cite book | last = Petruzellis | first = Thomas | title = 22 Radio and Receiver Projects for the Evil Genius | publisher = McGraw-Hill Professional | year = 2007 | location = US | page = 39 | url = https://books.google.com/books?id=AJBBf5hCYqIC&pg=PA39 | isbn = 978-0-07-148929-4 }}</ref>  They 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 = https://books.google.com/books?id=XiKgKdeBi6cC&pg=PT20 | isbn=978-1-84728-526-3}}</ref> A few receive [[shortwave radio|shortwave]] bands, but strong signals are required.  The first crystal sets received [[wireless telegraphy]] signals broadcast by [[spark-gap transmitter]]s at frequencies as low as 20&nbsp;kHz.<ref name="Lescarboura" />{{rp|p=110,268}}<ref>Long distance transoceanic stations of the era used wavelengths of 10,000 to 20,000 meters, corresponding to frequencies of 15 to 30 kHz.{{cite book | last = Morecroft | first = John H. |author2=A. Pinto |author3=Walter A. Curry | title = Principles of Radio Communication | publisher = John Wiley & Sons| year = 1921 | location = New York | page = [https://archive.org/details/principlesradio02currgoog/page/n201 187] | url = https://archive.org/details/principlesradio02currgoog }}</ref>

==Basic principles==
[[File:Crystal radio receiver block diagram.svg|thumb|Block diagram of a crystal radio receiver]]
[[File:Common crystal radio circuit.svg|thumb|Circuit diagram of a simple crystal radio]]

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. | title = Crystal Radio Set | work = electronics-tutorials.com | publisher = Ian Purdie | year = 2001 | url = http://www.electronics-tutorials.com/receivers/crystal-radio-set.htm | access-date = 2009-12-05 }}</ref> It consists of at least these components:<ref name="Williams" /><ref name="Lescarboura" >{{Cite book | last = Lescarboura | first = Austin C. | title = Radio for Everybody | publisher = Scientific American Publishing Co. | year = 1922 | location = New York | url = https://archive.org/details/RadioForEverybody }}</ref>{{rp|p=94}}<ref>{{cite web | last = Kuhn | first = Kenneth A. | 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 | access-date = 2009-12-07 }}</ref><ref name="Pillai">{{cite book | last = Pillai | first = Devarajan | title = Retro-tech Reimagined  | date = 2025 | pages = Chapter 4 | url = https://www.google.com/books/edition/Retro_Tech_Reimagined/j0BaEQAAQBAJ?hl=en&gbpv=1&dq=%22crystal+radio%22+germanium+silicon+diode&pg=PT29 }}</ref>
* An [[Antenna (radio)|antenna]] in which the [[radio wave]] induces [[electric current]]s.
* A [[resonant circuit]] (tuned circuit) which selects the [[frequency]] of the desired [[radio station]] from all the radio signals received by the antenna. The tuned circuit consists of a coil of wire (called an [[inductor]]) and a [[capacitor]] connected together. The circuit has a [[resonant frequency]], and allows radio waves at that frequency to pass through to the detector while largely blocking waves at other frequencies. One or both of the coil or capacitor is adjustable, allowing the circuit to be tuned to different frequencies to select the station to receive. In some circuits a capacitor is not used and the antenna serves this function, as an antenna that is shorter than a quarter-wavelength of the radio waves it is meant to receive is capacitive.
* A [[semiconductor]] crystal [[detector (radio)|detector]] that [[demodulate]]s the radio signal to extract the [[audio signal]] ([[modulation]]).  The crystal detector functions as a [[square law detector]],<ref>H. C. Torrey, C. A. Whitmer, ''Crystal Rectifiers'', New York: McGraw-Hill, 1948, pp. 3–4</ref> demodulating the radio frequency [[alternating current]] to its audio frequency modulation.  The detector's audio frequency output is converted to sound by the earphone. Early sets used a "[[cat's whisker detector|cat whisker detector]]"<ref name="Jensen">{{cite book | last1 = Jensen | first1 = Peter R. | title = Wireless at War | publisher = Rosenberg Publishing| date = 2003| pages = 103| url = https://books.google.com/books?id=9YFRAQAAQBAJ&q=%22cat%27s+whisker+detector%22&pg=PT103| isbn = 1922013846}}</ref><ref name="Morgan">{{cite book| last1 = Morgan| first1 = Alfred Powell | title = Wireless Telegraph Construction for Amateurs, 3rd Ed.| publisher = D. Van Nostrand Co.| date = 1914| pages = 199| url = https://books.google.com/books?id=cBc9AAAAYAAJ&q=cat+whisker+detector&pg=PA199}}</ref><ref name="Braun">{{cite book| last1 = Braun| first1 = Agnès| last2 = Braun| first2 = Ernest| last3 = MacDonald| first3 = Stuart| title = Revolution in Miniature: The History and Impact of Semiconductor Electronics| publisher = Cambridge University Press| date = 1982| pages = 11–12| url = https://books.google.com/books?id=03c4wldf-k4C&q=%22cat%27s+whisker+detector%22&pg=PA11| isbn = 0521289033}}</ref> consisting of a small piece of crystalline mineral such as [[galena]] with a fine wire touching its surface. The [[crystal detector]] was the component that gave crystal radios their name.  Modern sets use modern [[semiconductor diode]]s, although some hobbyists still experiment with crystal or other detectors.
* An [[earphone]] to convert the audio signal to sound waves so they can be heard.  The low power produced by a crystal receiver is insufficient to power a [[loudspeaker]], hence earphones are used.

[[File:CrystalRadio.jpg|thumb|300px|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. The detector was a [[cat whisker detector]], consisting of a piece of galena with a thin wire in contact with it on a part of the crystal, making a diode contact]]
As a crystal radio has no power supply, the sound power produced by the earphone comes solely from the [[transmitter]] of the radio station being received, via the radio waves captured by the antenna.<ref name="Gonzo" /> The power available to a receiving antenna decreases with the square of its distance from the [[radio transmitter]].<ref>{{cite web |last=Fette |first=Bruce A. |title=RF Basics: Radio Propagation |publisher=RF Engineer Network |date=Dec 27, 2008 |url=http://www.rfengineer.net/1170/rf-basics-radio-propagation/ |access-date=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="Payor">{{Cite journal | last = Payor | first = Steve | title = Build a Matchbox Crystal Radio | journal = Popular Electronics  | date = June 1989 | url = http://www.crystalradio.net/crystalplans/xximages/matchbox2.jpg | access-date = 2010-05-28}} on [http://www.crystalradio.net/crystalplans/ Stay Tuned] website</ref>{{rp|p=42}} 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="LeeT">{{Cite book | last = Lee | first = Thomas H. | title = Planar Microwave Engineering: A practical guide to theory, measurement, and circuits | publisher = Cambridge Univ. Press | year = 2004 | location = UK | pages = 297–304 | url = https://books.google.com/books?id=uoj3IWFxbVYC&pg=PA299 | isbn = 978-0-521-83526-8 }}</ref>{{rp|p=297-304}} which can detect sounds with an intensity of only 10<sup>−16</sup> [[Watt (unit)|W]]/cm<sup>2</sup>.<ref>{{cite web | last = Nave | first = C. Rod | title = Threshold of hearing | work = HyperPhysics | publisher = Dept. of Physics, Georgia State University | url = http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html | access-date = 2009-12-06 }}</ref> Therefore, crystal receivers have to be designed to convert the energy from the radio waves into sound waves as efficiently as possible. Even so, they are usually only able to receive stations within distances of about 25 miles for [[AM broadcasting|AM broadcast]] stations,<ref name="Lescarboura" />{{rp|p=144}}<ref name="Binns">{{Cite journal | last = Binns | first = Jack | 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 = https://books.google.com/books?id=pSkDAAAAMBAJ&pg=PA42 | access-date = 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 | page = 191 | url = https://books.google.com/books?id=k3XCkncd83AC&pg=PA191 | isbn = 0852967926}}</ref>

==Design==
Commercial passive receiver development was abandoned with the advent of reliable vacuum tubes around 1920, and subsequent crystal radio research was primarily done by [[radio amateur]]s and hobbyists.<ref name="Klase">{{cite web | last = Klase | first = Alan R. | title = Crystal Set Design 102 | work = Skywaves | publisher = Alan Klase personal website | year = 1998 | url = http://www.skywaves.ar88.net/xtalset102/xtalset102.htm | access-date = 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 | 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 = US | pages = [https://archive.org/details/radiohookupsare00sleegoog/page/n17 7]–18 | url = https://archive.org/details/radiohookupsare00sleegoog }}</ref><ref>{{Cite book | last = May | first = Walter J. | title = The Boy's Book of Crystal Sets | publisher = Bernard's | year = 1954 | location = London | url = https://books.google.com/books?id=9Bs5PwAACAAJ&q=Walter+May+Boy's+book+of+crystal+sets+intitle:Boy's+intitle:book+intitle:of+intitle:crystal+intitle:sets+inauthor:Walter+inauthor:May }} is a collection of 12 circuits</ref> The following sections discuss the parts of a crystal radio in greater detail.

=== Antenna ===
[[Image:Wire inverted L antenna construction.png|thumb|Diagram of an [[inverted-L antenna]], a common wire antenna used with crystal radios]]
The antenna converts the energy in the electromagnetic [[radio wave]]s 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. Antennas of the type commonly used with crystal sets are most effective 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 {{convert|182|-|566|m|ft|lk=in|disp=or}} long)<ref>{{cite web | last = Purdie | first = Ian | 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 | access-date = 2010-02-27 | archive-url = https://web.archive.org/web/20091029201804/http://my.integritynet.com.au/purdic/crystal_set.htm | archive-date = 2009-10-29 | url-status = dead }}</ref> the antenna is made as long as possible,<ref name="KuhnAntenna">{{cite web | last = Kuhn | first = Kenneth | 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 | access-date = 2009-12-07 }}</ref> from 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]],<ref name="Kinzie" />{{rp|p=48-51}} 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. |author2=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 = US  | url = https://archive.org/details/RadioReception }}</ref>{{rp|p=131}}<ref>{{cite book | last = Williams | first = Henry Smith | title = Practical Radio | publisher = Funk and Wagnalls | year = 1922 | location = New York | page = [https://archive.org/details/PracticalRadio1922/page/n81 58] | url = https://archive.org/details/PracticalRadio1922 }}</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 | 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 = https://books.google.com/books?id=XMTmAAAAMAAJ&pg=RA6-PA9 | access-date = 2010-01-18 }}</ref><ref name="Lescarboura" />{{rp|p=100}}

=== Ground ===
The wire antennas used with crystal receivers are [[monopole antenna]]s which develop their output voltage with respect to ground. The receiver thus requires a connection to [[ground (electricity)|ground]] (the earth) as a return circuit for the current.  The ground wire was attached to a radiator, water pipe, or a metal stake driven into the ground.<ref>{{Cite book | last = Collins | first = Archie Frederick | title = The Radio Amateur's Hand Book | publisher = Forgotten Books | year = 1922 | location = US | pages = 18–22 | url = https://books.google.com/books?id=jpMi0V8qoKsC&pg=PA18 | isbn = 1-60680-119-8 }}</ref><ref name="Lescarboura" />{{rp|p=102-104}}<ref name="Kinzie" />{{rp|p=48-51}}  In early days if an adequate ground connection could not be made a [[counterpoise (ground system)|counterpoise]] was sometimes used.<ref name="NBS40">{{cite book| last = US Bureau of Standards | title =  The Principles Underlying Radio Communication (Radio communication pamphlet no.40, 2nd Ed.)| publisher = published by US Army Signal Corps| date = May 24, 1921 | url = https://books.google.com/books?id=4JVEAAAAIAAJ&pg=PA434}}</ref>{{rp|p=309-311}}<ref name="Hausmann">{{cite book |last1=Hausmann |first1=Erich |last2=Goldsmith |first2=Alfred Norton |last3=Hazeltine |first3=Louis Alan |title=Radio Phone Receiving: A Practical Book for Everybody |publisher=D. Van Nostrand Company |date=1922  |url=https://archive.org/details/radiophonerecei01hoergoog |isbn=1-110-37159-4 }}</ref>{{rp|p=45}}  A good ground is more important for crystal sets than it is for powered receivers, as crystal sets are designed to have a low [[input impedance]] needed to transfer power efficiently from the antenna. A low resistance ground connection (preferably below 25&nbsp;Ω) is necessary because any resistance in the ground reduces available power from the antenna.<ref name="KuhnAntenna" /> In contrast, modern receivers are voltage-driven devices, with high input impedance, hence little current flows in the antenna/ground circuit. Also, [[mains power]]ed receivers are grounded adequately through their power cords, which are in turn attached to the earth through the building wiring.

=== Tuned circuit ===
[[File:Simplest crystal radio circuit.svg|thumb|left|upright=0.5|The earliest crystal receiver circuit did not have a [[tuned circuit]].]]

The [[tuned circuit]], consisting of a coil and a [[capacitor]] connected together, acts as a [[resonator]], similar to a tuning fork.<ref name="Hausmann" />{{rp|p=48}} Electric charge, induced in the antenna by the radio waves, flows rapidly back and forth between the plates of the capacitor through the coil. The circuit has a high [[Electrical impedance|impedance]] at the desired radio signal's frequency, but a low impedance at all other frequencies.<ref>{{Cite book |last1=Hayt |first1=William H. |first2=Jack E. |last2=Kemmerly |title=Engineering Circuit Analysis, 2nd Ed. |publisher=McGraw-Hill |year=1971 |location=New York |pages=[https://archive.org/details/engineeringcircu0000hayt_u3z4/page/398 398–399] |isbn=978-0-07-027382-5 |url=https://archive.org/details/engineeringcircu0000hayt_u3z4/page/398 }}</ref>  Hence, signals at undesired frequencies pass through the tuned circuit to ground, while the desired frequency is instead passed on to the detector (diode) and stimulates the earpiece and is heard. 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. | 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 | access-date = 2009-12-07 }}</ref>

:<math>f = \frac {1}{ 2 \pi \sqrt {LC}} \,</math>

The circuit can be adjusted to different frequencies by varying the inductance (L), the capacitance (C), or both, "tuning" the circuit to the frequencies of different radio stations.<ref name="CarrJ" />{{rp|p=7-9}}  In the lowest-cost sets, the inductor was made variable via a spring contact pressing against the windings that could slide along the coil, thereby introducing a larger or smaller number of turns of the coil into the circuit, varying the [[inductance]].  Alternatively, a [[variable capacitor]] is used to tune the circuit.<ref>{{Cite journal | last = Clifford | first = Martin | title = The early days of radio | journal = Radio Electronics | pages = 61–64 | date = July 1986 | url = http://www.crystalradio.net/crystalplans/xximages/earlydays3.jpg | access-date = 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 a ferrite [[magnetic core]] is moved into and out of the coil, thereby varying the inductance by changing the [[permeability (electromagnetism)|magnetic permeability]] (this eliminated the less reliable mechanical contact).<ref name="Blanchard">{{Cite journal | last = Blanchard | first = T. A. | title = Vestpocket Crystal Radio | journal = Radio-Electronics | page = 196 | date = October 1962 | url = http://crystalradio.net/crystalplans/xximages/vestpocketradio1.jpg | access-date = 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 circuit's resonant frequency.  Antennas usually act as a [[capacitance]], as antennas shorter than a quarter-wavelength have [[capacitive reactance]].<ref name="KuhnAntenna" /> Many early crystal sets did not have a tuning capacitor,<ref name="NBS40" />{{rp|p=421-425}} and relied instead on the capacitance inherent in the wire antenna (in addition to significant [[parasitic capacitance]] in the coil<ref name="Hausmann" />{{rp|p=57}}) 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="CarrJ" />{{rp|p=7-9}}<ref name="NBS40" />{{rp|p=421-422}} 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 ====
{{multiple image
| align             = right
| direction         = horizontal
| header            = 
| image1            = Two slider crystal radio circuit.svg
| width1            = 150
| image2            = Crystal radio.agr.jpg
| width2            = 200
| footer            = "Two slider" crystal radio circuit.<ref name="Klase" /> and example from 1920s. 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. The capacitance which created the tuned circuit was provided by the parasitic capacitance of the coil, and the antenna.
}}

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 = Nahin | first = Paul J. | author-link = Paul J. Nahin | title = The science of radio: with MATLAB and Electronics Workbench demonstrations | publisher = Springer | year = 2001 | location = US | pages = 60–62 | url = https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA61 | isbn = 0-387-95150-4 }}</ref> The maximum power is transferred from one part of a circuit to another when the [[wave impedance|impedance]] of one circuit is the complex conjugate of that of the other; this implies that the two circuits should have equal resistance.<ref name="CarrJ" />{{rp|p=7-9}}<ref>{{Cite book | last = Smith | first = K. c. a. |author2=R. E. Alley | title = Electrical circuits: An introduction | publisher = Cambridge University Press | year = 1992 | location = UK | page = 218 | url = https://books.google.com/books?id=Aj9blWqYg2YC&pg=PA217 | isbn = 0-521-37769-2 }}</ref><ref name="Alley">{{Cite book | last = Alley | first = Charles L. |author2=Kenneth W. Atwood | title = Electronic Engineering, 3rd Ed. | publisher = John Wiley & Sons | year = 1973 | location = New York | page = 269 | 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. |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=Ben Tongue |date=2007-11-06 |url=http://www.bentongue.com/xtalset/0def_exp/0def_exp.html |access-date=2010-02-07 |archive-date=2016-06-04 |archive-url=https://web.archive.org/web/20160604112023/http://www.bentongue.com/xtalset/0def_exp/0def_exp.html |url-status=dead }}</ref> and also varies depending on the quality of the ground attachment, length of the antenna, and the frequency to which the receiver is tuned.<ref name="Payor" />{{rp|p=42}}
Therefore, in improved receiver circuits, in order 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="NBS40" />{{rp|p=422}} This made the tuning coil act as an [[Impedance matching|impedance matching transformer]] (in an [[autotransformer]] connection) in addition to providing the tuning function. The antenna's low resistance was increased (transformed) by a factor equal to the square of the turns ratio (the ratio of the number of turns the antenna was connected to, to the total number of turns of the coil), to match the resistance across the tuned circuit.<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 name="BucherE">{{Cite book |last=Bucher |first=Elmer Eustace |title=Practical Wireless Telegraphy: A complete text book for students of radio communication |edition=Revised |publisher=Wireless Press, Inc |year=1921 |location=New York |url=https://archive.org/details/practicalwirele02buchgoog}}</ref>{{rp|p=133}} adjustment of both the resonant frequency and the turns ratio.<ref name="Marx" />{{rp|p=94}}<ref name="Stanley">{{cite book |last=Stanley |first=Rupert |title=Textbook on Wireless Telegraphy, Vol. 1 |publisher=Longman's Green & Co. |year=1919 |location=London |url=https://archive.org/details/textbookonwirel02stangoog}}</ref>{{rp|p=281}}<ref name="Collins">{{Cite book |last=Collins |first=Archie Frederick |title=The Radio Amateur's Hand Book |publisher=Forgotten Books |year=1922 |pages=23–25 |url=https://books.google.com/books?id=jpMi0V8qoKsC&pg=PA25 |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.
[[File:Crystal radio with impedance matching.svg|thumb|left|Direct-coupled circuit with taps for impedance matching<ref name="Klase" />]]

==== Problem of selectivity ====
One of the drawbacks of crystal sets is that they are vulnerable to interference from stations near in [[frequency]] to the desired station.<ref name="Petruzellis" /><ref name="Schaeffer" /><ref name="Payor" />{{rp|p=42}} Often two or more stations are heard simultaneously. This is because the simple tuned circuit does not reject nearby signals well; it allows a wide band of frequencies to pass through, that is, it has a large [[bandwidth (signal processing)|bandwidth]] (low [[Q factor]]) compared to modern receivers, giving the receiver low [[selectivity (radio)|selectivity]].<ref name="Schaeffer" />

The crystal detector worsened the problem, because it has relatively low [[Electrical resistance|resistance]], thus it "loaded" the tuned circuit, drawing significant current and thus damping the oscillations, reducing its Q factor so it allowed through a broader band of frequencies.<ref name="Payor" />{{rp|p=42}}<ref name="Wenzel">{{cite web |last=Wenzel |first=Charles |title=Simple crystal radio |work=Crystal radio circuits |publisher=techlib.com |year=1995 |url=http://www.techlib.com/electronics/crystal.html |access-date=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" /><ref name="Kinzie" />{{rp|p=63}} This reduced the impedance loading of the tuned circuit, as well as improving the impedance match with the detector.<ref name="Klase" />
{{clear}}

==== Inductive coupling ====
[[File:Inductively coupled crystal radio circuit.svg|thumb|Inductively-coupled circuit with impedance matching. This type was used in most quality crystal receivers in the early 20th century.]]
[[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 is replaced with an adjustable air core [[Antenna tuner|antenna coupling]] [[transformer]]<ref name="CarrJ" />{{rp|p=7-9}}<ref name="Klase" /> which improves the [[selectivity (radio)|selectivity]] by a technique called ''loose coupling''.<ref name="NBS40" />{{rp|p=423-425}}<ref name="Collins" /><ref>{{Cite journal
 | last = Hogan | first = John V. L. | 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 = https://books.google.com/books?id=VMcnAAAAYAAJ&pg=RA5-PA480
 | access-date = 2010-02-10 }}</ref> This
consists 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 creates an alternating magnetic field in the primary coil, which induced a current in the secondary coil which was then rectified and powered the earphone. Each of the coils functions as a [[tuned circuit]]; the primary coil [[Resonant circuit|resonated]] with the capacitance of the antenna (or sometimes another capacitor), and the secondary coil resonated with the tuning capacitor. Both the primary and secondary were tuned to the frequency of the station.  The two circuits interacted to form a [[Transformer types#Resonant transformer|resonant transformer]].

Reducing the ''coupling'' between the coils, by physically separating them so that less of the [[magnetic field]] of one intersects the other, reduces the [[mutual inductance]], narrows the bandwidth, and results in much sharper, more selective tuning than that produced by a single tuned circuit.<ref name="NBS40" />{{rp|p=424-425}}<ref>Alley & Atwood (1973) ''Electronic Engineering'', p. 318</ref> However, the looser coupling also reduced the power of the signal passed to the second circuit. The transformer was made with adjustable coupling, to allow the listener to experiment with various settings to gain the best reception.

One design common in early days, called a "loose coupler", consisted of a smaller secondary coil inside a larger primary coil.<ref name="Klase" /><ref name="Marx" />{{rp|p=96-101}} The smaller coil was mounted on a [[Rack and pinion|rack]] so it could be slid linearly in or out of the larger coil. If radio interference was encountered, the smaller coil would be slid further out of the larger, loosening the coupling, narrowing the bandwidth, and thereby rejecting the interfering signal.

The antenna coupling transformer also functioned as an [[impedance matching|impedance matching transformer]], that allowed a better match of the antenna impedance to the rest of the circuit.<ref name="Kinzie" />{{rp|p=64}} One or both of the coils usually had several taps which could be selected with a switch, allowing adjustment of the number of turns of that transformer and hence 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 of the coils, were all interactive, and changing one affected the others.<ref>{{Cite book | last = US Signal Corps | title = Radiotelegraphy | publisher = Government Printing Office | date = October 1916 | location = US | page = [https://archive.org/details/radiotelegraphy02corpgoog/page/n76 70] | url = https://archive.org/details/radiotelegraphy02corpgoog }}</ref>

=== Crystal detector ===
{{main|Crystal detector (radio)}}
[[File:Kristallradio (3).jpg|thumb|left|Galena crystal 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 name="Kinzie" />{{rp|p=72-73}}<ref name="Marx" />{{rp|p.43,fig.22}}<ref name="Campbell">{{Cite journal
 | last = Campbell | first = John W. | 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 = https://books.google.com/books?id=PyEDAAAAMBAJ&pg=PA206
 | access-date = 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 amplitude (hence the term amplitude modulation, AM) 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 to 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]]
[[File:Crystal radio circuit with bias and buzzer.svg|thumb|Circuit with detector bias battery to improve sensitivity and buzzer to aid in adjustment of the cat whisker]]

The crystal [[detector (radio)|detector]] [[demodulation|demodulates]] the radio frequency signal, extracting the [[modulation]] (the [[audio signal]] which represents the sound waves) from the radio frequency [[carrier wave]]. In early receivers, a type of crystal detector often used was a "[[cat's whisker detector|cat whisker detector]]".<ref name="Morgan" /><ref>H. V. Johnson, A Vacation Radio Pocket Set. ''Electrical Experimenter'', vol. II, no. 3, p. 42, Jul. 1914</ref> The point of contact between the wire and the crystal acted as a semiconductor [[diode]].  The cat whisker detector constituted a crude [[Schottky diode]] that allowed current to flow better in one direction than in the opposite direction.<ref>"The cat's-whisker detector is a primitive point-contact diode. A point-contact junction is the simplest implementation of a Schottky diode, which is a majority-carrier device formed by a metal-semiconductor junction." {{cite web| last = Shaw| first = Riley| title = The cat's-whisker detector| publisher = Riley Shaw's personal blog| date = April 2015| url = https://rileyjshaw.com/blog/the-cat%27s-whisker-detector/| access-date = 1 May 2018| archive-date = 25 September 2017| archive-url = https://web.archive.org/web/20170925151632/http://rileyjshaw.com/blog/the-cat%27s-whisker-detector/| url-status = dead}}</ref><ref>{{Cite book | last = Lee | first = Thomas H. | title = The Design of CMOS Radio-Frequency Integrated Circuits | publisher = Cambridge University Press | year = 2004 | location = UK | pages = 4–6 | url = https://books.google.com/books?id=DzcMK-2mFQUC&q=crystal+detector | isbn = 0-521-83539-9 }}</ref> Modern crystal sets use modern [[semiconductor diode]]s.<ref name="Wenzel" />  The crystal functions as an [[envelope detector]], [[rectifier|rectifying]] the [[alternating current]] radio signal to a pulsing [[direct current]], the peaks of which trace out the audio signal, so it can be converted to sound by the earphone, which is connected to the detector.<ref name="Williams" />{{failed verification|reason=Williams p 22 assumes the voltage is an absurd 7 V.|date=May 2016}}<ref name="Campbell" />{{failed verification|reason=Campbell only says demodulation requires a nonlinear element; Campbell talkes about the "envelope" of a vacuum tube.|date=May 2016}}
The rectified current from the detector has [[radio frequency]] pulses from the carrier frequency in it, which are blocked by the high inductive reactance and do not pass well through the coils of early date earphones. Hence, a small [[capacitor]] called a [[bypass capacitor]] is often placed across the earphone terminals; its low reactance at radio frequency bypasses these pulses around the earphone to ground.<ref name="Stanley" />{{rp|p=282}}  In some sets the earphone cord had enough capacitance that this component could be omitted.<ref name="NBS40" />{{rp|p=424}}

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="Hausmann" />{{rp|p=60-61}} 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="Lescarboura" />{{rp|p=143-146}}  Alternatively, some radios ''(circuit, right)'' used a battery-powered [[Spark-gap transmitter|buzzer]] attached to the input circuit to adjust the detector.<ref name="Lescarboura" />{{rp|p=143-146}} The spark at the buzzer's electrical contacts served as a weak source of static, so when the detector began working, the buzzing could be heard in the earphones.  The buzzer was then turned off, and the radio tuned to the desired station.

[[Galena]] (lead sulfide) was the most common crystal used,<ref name="Collins" /><ref name="Hausmann" />{{rp|p=60-61}}<ref name="Hirsch">{{cite journal
  | last1  = Hirsch
  | first1 = William Crawford
  | title = Radio Apparatus – What is it made of?
  | journal = The Electrical Record
  | volume = 31
  | issue = 6
  | pages = 393–394
  | publisher = The Gage Publishing Co.
  | location = New York
  | date = June 1922
  | url = https://books.google.com/books?id=cm42AQAAMAAJ&q=%22widest+use%22+galena+cerrussite+silicon+pyrite+molybdenite&pg=PA393
  | access-date = 10 July 2018}}</ref> but various other types of crystals were also used, the most common being [[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 name="Stanley" />{{rp|p=311-318}} Crystal radios have also been improvised from a variety of common objects, such as blue steel [[razor blade steel|razor blades]] and [[pencil|lead pencils]],<ref name="Lee" /><ref name="Gernsback1944" /> rusty needles,<ref>{{Cite journal | last = Douglas | first = Alan | 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 | doi = 10.1109/mspec.1981.6369482| s2cid = 44288637 | access-date = 2010-03-28 | hdl = 10366/158938 | hdl-access = free }}</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 for the detector, which is much more reliable than a crystal detector and requires no adjustments.<ref name="Lee" /><ref name="Wenzel" /><ref name="KuhnDiode">{{cite web | last = Kuhn | first = Kenneth A. | 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 | access-date = 2009-12-07 }}</ref> Germanium diodes (or sometimes [[Schottky diode]]s) are used instead of silicon diodes, because their lower forward voltage drop (roughly 0.3&nbsp;V compared to 0.6&nbsp;V<ref name="Hadgraft">{{cite web | last = Hadgraft | first = Peter | title = The Crystal Set 5/6 | work = The Crystal Corner | publisher = Kev's Vintage Radio and Hi-Fi page | url = http://oldkevspage.tripod.com/wr/xtal3.html | access-date = 2010-05-28 | archive-url = https://web.archive.org/web/20100720233222/http://oldkevspage.tripod.com/wr/xtal3.html | archive-date = 2010-07-20 | url-status = dead }}</ref>) makes them more sensitive.<ref name="Wenzel" /><ref>{{cite web | last = Kleijer | first = Dick | title = Diodes | publisher = crystal-radio.eu | url = http://www.crystal-radio.eu/endiodes.htm | access-date = 2010-05-27 }}</ref>

All semiconductor detectors function rather inefficiently in crystal receivers, because the low voltage input to the detector is too low to result in much difference between forward better conduction direction, and the 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 name="NBS40" />{{rp|p=439}}<ref name="Robison">"''The sensitivity of the Perikon ''[detector]'' can be approximately doubled by connecting a battery across its terminals to give approximately 0.2 volt''"  {{cite book| last1 = Robison| first1 = Samuel Shelburne | title = Manual of Wireless Telegraphy for the Use of Naval Electricians, Vol. 2| publisher = US Naval Institute| date = 1911| location = Washington DC| pages = 131 | url = https://books.google.com/books?id=7gQ9AAAAYAAJ&q=Perikon+crystal+battery&pg=PA131}}</ref><ref name="Bucher" >"''Certain crystals if this combination ''[zincite-bornite]'' respond better with a local battery while others do not require it...but with practically any crystal it aids in obtaining the sensitive adjustment to employ a local battery...''"{{Cite book | last = Bucher | first = Elmer Eustace | 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, 140 | url = https://books.google.com/books?id=DI9RAAAAMAAJ&pg=PA134 }}</ref>  The bias moves the diode's operating point higher on the detection curve producing 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 was caused by moving the DC operating point to a more desirable voltage-current operating point (impedance) on the junction's [[I-V curve]]. The battery did not power the radio, but only provided the biasing voltage which required little power.

=== Earphones ===
[[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  sacrifice efficiency in order to gain [[high fidelity]] reproduction of the sound.<ref name="FieldEarphone">[https://books.google.com/books?id=t-N1KdTb0FwC&pg=PT93 Field 2003, pp. 93–94]</ref> In early homebuilt sets, the earphones were the most costly component.<ref name="Lescarboura" />{{rp|p=285}}

[[File:High impedance headset.jpg|thumb|249x249px|1600 ohm magnetic headset]]
The early earphones used with wireless-era crystal sets had [[moving iron speaker|moving iron drivers]] that worked in a way similar to the horn [[loudspeaker]]s of the period. Each earpiece contained a permanent [[magnet]] about which was a coil of wire which formed a second  [[electromagnet]].<ref name="Kinzie" />{{rp|p=79}} Both magnetic poles were close to a steel diaphragm of the speaker. When the [[audio signal]] from the radio was passed through the electromagnet's windings, current was caused to flow in the coil which created a varying [[magnetic field]] that augmented or diminished that due to the permanent magnet. This varied the force of attraction on the diaphragm, causing it to vibrate. The vibrations of the diaphragm push and pull 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. Therefore, the type used with crystal set radios (and other sensitive equipment) was wound with more turns of finer wire giving it a high impedance of 2000–8000&nbsp;Ω.<ref>[https://books.google.com/books?id=jpMi0V8qoKsC&pg=PA27 Collins (1922), pp. 27–28]</ref><ref>[https://archive.org/details/PracticalRadio1922/page/n100 <!-- pg=79 --> Williams (1922), p. 79]</ref><ref name="NBS40" />{{rp|p=441}}

Modern crystal sets use [[Piezoelectricity|piezoelectric]] [[crystal earpiece]]s, which are much more sensitive and also smaller.<ref name="FieldEarphone" /><ref name="Kinzie" />{{rp|p=79-80}} 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 do not greatly "load" the tuned circuit, allowing increased [[selectivity (radio)|selectivity]] of the receiver. The piezoelectric earphone's higher resistance, in parallel with its capacitance of around 9&nbsp;pF, creates a  [[low pass filter|filter]] that allows the passage of low frequencies, but blocks the higher frequencies.<ref name="Payor" />{{rp|p=45}} In that case a bypass capacitor is not needed (although in practice a small one of around 0.68 to 1&nbsp;nF is often used to help improve quality), but instead a 10–100&nbsp;kΩ resistor must be added in parallel with the earphone's input.<ref name="Gonzo" />{{rp|p=94}}<ref name="Kinzie" />{{rp|p=80}}

Although the low power produced by crystal radios is typically insufficient to drive a [[loudspeaker]], some homemade 1960s sets have used one, with an audio [[transformer]] to match the low impedance of the speaker to the circuit.<ref name="Kinzie" />{{rp|p=80-81}}<ref>Walter B. Ford, "[http://www.rfcafe.com/references/popular-electronics/high-power-crystal-set-august-1960-popular-electronics.htm High Power Crystal Set]", August 1960, ''[[Popular Electronics]]''</ref>  Similarly, modern low-impedance (8&nbsp;Ω) earphones cannot be used unmodified in crystal sets because the receiver does not 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 driving antenna circuit.

== History ==
The first [[radio transmitter]]s, used during the initial three decades of radio from 1887 to 1917, a period called the ''wireless telegraphy era'', were primitive [[spark gap transmitter|spark transmitters]] which generated radio waves by discharging a [[capacitance]] through an [[electric spark]].<ref name="NahinP">{{cite book
 | last1  = Nahin
 | first1 = Paul J.
 | title  = The Science of Radio: With Matlab and Electronics Workbench Demonstration, 2nd Ed.
 | publisher = Springer Science & Business Media 
 | date   = 2001
 | url    = https://books.google.com/books?id=V1GBW6UD4CcC&pg=PA45
 | isbn   = 978-0387951508
 }}</ref>{{rp|45-48}}<ref name="CoeL">{{cite book
 | last1  = Coe
 | first1 = Lewis 
 | title  = Wireless Radio: A History
 | publisher = McFarland
 | date   = 2006
 | url    = https://books.google.com/books?id=W1JAeg1PiWIC&pg=PA3
 | isbn   = 978-0786426621
 }}</ref>{{rp|3-8}}<ref name="McNicolD">{{cite book
 | last1  = McNicol
 | first1 = Donald
 | title  = Radio's Conquest of Space
 | publisher = Murray Hill Books
 | date   = 1946
 | url    = https://archive.org/stream/radiosconquestof00mcnirich#page/57/mode/2up
 }}</ref>{{rp|57-68}}  Each spark produced a transient pulse of radio waves which decreased rapidly to zero.<ref name="LeeT" />{{rp|4-9,297-300}}<ref name="PhillipsV" />{{rp|6-8}} These [[damped wave]]s could not be modulated to carry sound, as in modern [[amplitude modulation|AM]] and [[frequency modulation|FM]] transmission. So spark transmitters could not transmit sound, and instead transmitted information by [[radiotelegraphy]].<ref name="CodellaBeginnings">{{cite web
| last= Codella
| first= Christopher F.
| title= Beginnings
| work= Ham Radio History
| publisher= C. F. Codella's private website
| date= 2016
| url= http://w2pa.net/HRH/beginnings/
| access-date= 22 May 2018}}</ref>  The transmitter was switched on and off rapidly by the operator using a [[telegraph key]], creating different length pulses of damped radio waves ("dots" and "dashes") to spell out text messages in [[Morse code]].<ref name="CoeL" />{{rp|3-8}}

Therefore, the first radio receivers did not have to extract an audio signal from the radio wave like modern receivers, but just detected the presence of the radio wave, and produced a sound during the "dots" and "dashes"<ref name="PhillipsV" />{{rp|8}} which were translated back to text by an operator who knew Morse code.  The device which detected the radio signal was called a "''[[detector (radio)|detector]]''". Since there were no [[amplifier|amplifying]] devices at this time, the sensitivity of the receiver mostly depended on the detector and the antenna.  The crystal detector was the most successful of many  detector devices invented during this period. 

===Coherer receiver===
The crystal receiver developed from an earlier device, the first primitive radio receiver, called the coherer receiver.  [[Guglielmo Marconi]] invented the first practical radiotelegraphy receivers and transmitters in 1894.   Radio began to be used commercially around 1900.  The detector used in the first receivers<ref name="PhillipsV">
{{cite book
 | last1  = Phillips
 | first1  = Vivian J.
 | title  = Early Radio Wave Detectors
 | publisher  = Inst. of Electrical Engineers
 | date  = 1980
 | location  = London
 | language  = 
 | url  = https://archive.org/details/EarlyRadioWaveDetectors/page/205
 | doi  = 
 | id  = 
 | isbn  = 978-0906048245
 }}</ref>{{rp|18-22, 30-35}} was a primitive device called a ''[[coherer]]'', developed in 1890 by [[Édouard Branly]] and improved by Marconi and [[Oliver Lodge]].<ref name="LeeT" />{{rp|4-9,297-300}}<ref name="PhillipsV" />{{rp|30-35}}  Made in many forms, the most well known form consisted of a glass tube with electrodes at each end, containing loose metal filings in contact with the electrodes.<ref name="BraunA">{{cite book
 | last1  = Braun
 | first1 = Agnès
 | last2  = Braun
 | first2 = Ernest
 | last3  = MacDonald
 | first3 = Stuart
 | title  = Revolution in Miniature: The History and Impact of Semiconductor Electronics
 | publisher = Cambridge University Press
 | date   = 1982
 | url    = https://books.google.com/books?id=03c4wldf-k4C&q=%22crystal+detector%22&pg=PA11
 | isbn   = 978-0521289030
 }}</ref>{{rp|11-12}}<ref name="LeeT" />{{rp|3-5}}  Before a radio wave was applied, this device had a high [[electrical resistance]], in the megohm range.  When a radio wave from the antenna was applied across the electrodes it caused the filings to "cohere" or clump together and the coherer's resistance fell, causing a DC current from a battery to pass through it, which rang a bell or produced a mark on a paper tape representing the "dots" and "dashes" of Morse code.  Most coherers had to be tapped mechanically between each pulse of radio waves to return them to a nonconductive state.<ref name="SterlingC">{{cite book | last1 = Sterling| first1 = Christopher H.| last2 = O'Del| first2 = Cary| title = The Concise Encyclopedia of American Radio| publisher = Routledge| pages = 199-200 | date = 2010| url = https://books.google.com/books?id=dmmLAgAAQBAJ&q=%22crystal+radio%22+cat+whisker&pg=PA200| isbn = 978-1135176846}}</ref><ref name="PhillipsV" />{{rp|18-21}}

The coherer was a very poor detector,<ref name="SterlingC" /> motivating much research to find better detectors.<ref name="Kinzie">{{cite book
  | last = Kinzie
  | first = P. A.
  | title = Crystal Radio: History, Fundamentals, and Design
  | publisher = The Xtal Set Society
  | date = 1996
  | location = 
  | language = 
  | url = https://commons.princeton.edu/josephhenry/wp-content/uploads/sites/71/2021/11/Kinzie-Crystal-radio-fundamentals-and-design.pdf
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 1887736050
  | mr = 
  | zbl = 
  | jfm =}}</ref>{{rp|p=15}}  It worked by complicated thin film surface effects, so scientists of the time didn't understand how it worked, except for a vague idea that radio wave detection depended on some mysterious property of "imperfect" electrical contacts.<ref name="LeeT" />{{rp|5}}  Researchers investigating the effect of radio waves on various types of "imperfect" contacts to develop better coherers, invented crystal detectors.<ref name="PhillipsV" />{{rp|205-206}}<ref name="LeeT" />{{rp|5}}

===Tuning===
[[Image:Braun wireless receiving transformer 1905.jpg|thumb|upright=0.6|Braun receiving transformer from 1904]]

"Tuning" means adjusting the [[frequency]] of the receiver to the frequency of the desired radio transmission. The first receivers had no [[tuned circuit]] (resonant circuit), the detector was connected directly between the antenna and ground. Due to the lack of any [[Selectivity (radio)|frequency selective]] components besides the antenna, the [[bandwidth (signal processing)|bandwidth]] of the receiver, the band of frequencies it received, was equal to the broad bandwidth of the antenna.<ref name="McNicolD" />{{rp|57-68}}<ref name="CarrJ" />{{rp|p=9-10}}<ref name="StoneE">{{cite book
 | last1  = Stone
 | first1 = Ellery W. 
 | title  = Elements of Radiotelegraphy
 | publisher = D. Van Nostrand Co.
 | date   = 1919
 | url    = https://archive.org/details/elementsradiote01stongoog
 }}</ref>{{rp|203}}<ref name="HongS">{{cite book
 | last1  = Hong
 | first1 = Sungook 
 | title  = Wireless: From Marconi's Black-box to the Audion
 | publisher = MIT Press
 | date   = 2001
 | url    = https://books.google.com/books?id=UjXGQSPXvIcC&pg=PA89
 | isbn   = 978-0262082983
 }}</ref>{{rp|89-100}}<ref name="AitkenH" /> This was acceptable and even necessary because the first Hertzian spark transmitters also lacked a resonant circuit. Due to the impulsive nature of the spark, the energy of the radio waves was spread over a very wide band of frequencies.<ref name="AitkenH">{{cite book
 | last1  = Aitken
 | first1 = Hugh G.J. 
 | title  = Syntony and Spark: The Origins of Radio
 | publisher = Princeton Univ. Press
 | date   = 2014
 | url    = https://books.google.com/books?id=Mez_AwAAQBAJ&pg=PA254
 | isbn   = 978-1400857883
 }}</ref>{{rp|126-129}}<ref name="BeauchampK">{{cite book
 | last1  = Beauchamp
 | first1 = Ken 
 | title  = History of Telegraphy
 | publisher = IET 
 | date   = 2001
 | url    = https://books.google.com/books?id=k3XCkncd83AC&pg=PA185
 | isbn   = 978-0852967928
 }}</ref>{{rp|184-190}} To receive enough energy from this wideband signal the receiver had to have a wide bandwidth also.

When more than one spark transmitter was transmitting in a given area, their frequencies overlapped, so their signals interfered with each other, resulting in garbled reception.<ref name="McNicolD" />{{rp|57-68}}<ref name="HongS" />{{rp|89-100}}<ref name="KennellyA">{{cite book
 | last1  = Kennelly
 | first1 = Arthur E.
 | title  = Wireless Telegraphy: An Elementary Treatise
 | publisher = Moffatt, Yard and Co.
 | date   = 1906
 | location = New York
 | url    = https://archive.org/details/wirelesstelegra01kenngoog
 | quote  = selective signaling.
 }}</ref>{{rp|183}} Some method was needed to allow the receiver to select which transmitter's signal to receive.<ref name="KennellyA" />{{rp|183}}<ref name="AitkenH" />{{rp|128}} In 1892, [[William Crookes]] gave an influential lecture<ref name="CrookesW">{{cite journal
  | last1 = Crookes
  | first1 = William
  | title = Some Possibilities of Electricity
  | journal = The Fortnightly Review
  | volume = 51
  | date = February 1, 1892
  | url = http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
  | access-date = August 19, 2015
  | archive-date = September 29, 2018
  | archive-url = https://web.archive.org/web/20180929160315/http://www.tfo.upm.es/ImperialismoWeb/ArtCrookes.htm
  | url-status = dead
  }}</ref>{{rp|174-176}} on radio in which he suggested using [[resonance]], then called ''syntony'', to reduce the bandwidth of transmitters and receivers.<ref name="AitkenH" />{{rp|112-114}} Different transmitters could then be "tuned" to transmit on different frequencies so they did not interfere.<ref name="SarkarT">{{cite book
 | last1  = Sarkar
 | first1 = T. K.
 | last2  = Mailloux
 | first2 = Robert
 | last3  = Oliner
 | first3 = Arthur A.
 | title  = History of Wireless
 | publisher = John Wiley and Sons
 | date   = 2006
 | url    = https://archive.org/stream/HistoryOfWireless#page/n323/mode/2up
 | isbn   = 978-0471783015
 | author-link1=
 | author-link3=
 }}</ref><ref name="RockmanH">{{cite book
 | last1  = Rockman
 | first1 = Howard B. 
 | title  = Intellectual Property Law for Engineers and Scientists
 | publisher = John Wiley and Sons
 | date   = 2004
 | pages  = 196–199
 | url    = https://books.google.com/books?id=6B88P-32IIQC&pg=PA196
 | isbn   = 978-0471697398
 }}</ref>  The receiver would also have a [[resonant circuit]], and could receive a particular transmission by "tuning" its resonant circuit to the same frequency as the transmitter, analogously to tuning a musical instrument to resonance with another. This is the system used in all modern radio.<ref name="AitkenH" />{{rp|36}}

Between 1897 and 1900 the advantages of tuned systems, also called "syntonic"<ref name="AitkenH" />{{rp|254}} systems, became clear, and wireless researchers incorporated [[resonant circuit]]s, consisting of [[capacitor]]s and [[inductor]]s connected together, into their transmitters and receivers.<ref name="McNicolD" />{{rp|57-68}}<ref name="KloosterJ">{{cite book
 | last1  = Klooster
 | first1 = John W.
 | title  = Icons of Invention
 | publisher = ABC-CLIO
 | date   = 2007
 | url    = https://books.google.com/books?id=WKuG-VIwID8C&q=Tesla&pg=PA161
 | isbn   = 978-0313347436
 }}</ref>{{rp|159–161}}<ref name="HongS" />{{rp|89-100}}<ref name="AitkenH" />{{rp|254-255}} The resonant circuit acted like an electrical analog of a [[tuning fork]]. It had a high [[impedance (electrical)|impedance]] at its [[resonant frequency]], but a low impedance at all other frequencies. Connected between the antenna and the detector it served as a [[bandpass filter]], passing the signal of the desired station to the detector, but routing all other signals to ground.<ref name="CarrJ" />{{rp|9-10}}

[[Oliver Lodge]], who had been researching resonance for years<ref name="AitkenH" />{{rp|p.108-109}}<ref name="ThrowerK">{{cite conference
| last1= Thrower
| first1= K. R.
| title= History of tuning
| conference= Proceedings of the 1995 International Conference on 100 Years of Radio
| publisher= Institute of Engineering Technology
| date= 5 September 1995
| pages= 107-108
| location= London
| url= https://ieeexplore.ieee.org/document/491801
| doi= 10.1049/cp:19950799
| isbn= 0-85296-649-0
| access-date= 20 June 2018}} [https://www.nonstopsystems.com/radio/pdf-hell/article-IC-1995-Thrower.pdf archived]</ref> patented the first tuned or "syntonic" transmitter and receiver on 10 May 1897<ref name="Patent11575">British patent GB189711575 Lodge, O. J. ''[https://worldwide.espacenet.com/publicationDetails/biblio?CC=GB&NR=189711575A&KC=A&FT=D# Improvements in Syntonized Telegraphy without Line Wires]'' filed: May 10, 1897, granted: August 10, 1898</ref><ref name="Lee">{{cite book
| last= Lee
| first= Thomas H.
| title= The Design of CMOS Radio-Frequency Integrated Circuits
| edition= 2nd
| publisher= Cambridge University Press
| date= 2004
| location= UK
| url= https://books.google.to/books?id=mlYhAwAAQBAJ&printsec=copyright#v=onepage&q&f=false
| isbn= 978-0521835398}}</ref>{{rp|34-36}}<ref name="AitkenH" />{{rp|131–138}}
<ref name="HongS" />{{rp|90-93}}  Although his circuit did not see much practical use, Lodge's "syntonic" patent was important because it was the first to propose a radio transmitter and receiver containing resonant circuits which were tuned to resonance with each other.<ref name="BeauchampK" />{{rp|189-190}}<ref name="ThrowerK"/><ref name="AitkenH"/>{{rp|141}} In 1911 when the patent was renewed the Marconi Company was forced to buy it to protect its own syntonic system against infringement suits.<ref name="HongS" />{{rp|48, 99}}<ref name="AitkenH"/>{{rp|253, 259}}

====Inductive coupling and court case====
Wireless researchers found that a single resonant circuit used in transmitters and receivers did not have a narrow enough [[bandwidth (signal processing)|bandwidth]] to reduce interference between different stations adequately.<ref name="HongS" />{{rp|p=91-93}}<ref name="AitkenH"/>{{rp|p=245-246}}<ref name="CarrJ" />{{rp|p=10}}  

The solution which multiple researchers found was to use two resonant circuits in the transmitter and receiver, in the form of a double-tuned [[inductive coupling|inductively-coupled]] circuit, or [[resonant transformer]] ([[oscillation transformer]]).<ref name="SarkarT" />{{rp|p=349-358}}<ref name="AitkenH"/>{{rp|254}}<ref name="McNicolD" />{{rp|p=59}} In a receiver, the antenna and ground were connected to a coil of wire, which was magnetically coupled to a second coil with a capacitor across it, which was connected to the detector.<ref name="CarrJ" />{{rp|p=10}}<ref name="BucherE" />{{rp|p=132-133}} The alternating current from the antenna through the primary coil created a [[magnetic field]] which induced a current in the secondary coil which fed the detector. Both primary and secondary were tuned circuits;<ref name="HongS" />{{rp|p=98}} the primary coil resonated with the capacitance of the antenna, while the secondary coil resonated with the capacitor across it. Both were adjusted to the same [[resonant frequency]].  

Similarly, two coupled resonant circuits were used in the spark transmitter.<ref name="CodellaSparkRadio" >{{cite web
| last= Codella
| first= Christopher F.
| title= Spark Radio
| work= Ham Radio History
| publisher= C. F. Codella's private website
| date= 2016
| url= http://w2pa.net/HRH/spark-radio/
| access-date= 22 May 2018}}</ref>  A radio communication system with two inductively-coupled tuned circuits in the transmitter and two in the receiver, all four tuned to the same frequency, was called a "four circuit" system, and proved to be the key to practical radio communication.<ref name="BucherE" />{{rp|p=135}}<ref name="AitkenH"/>{{rp|p=250}}

The first person to use resonant circuits in a radio application was [[Nikola Tesla]], who invented the [[resonant transformer]] in 1891.<ref name="SarkarT" />{{rp|p=356}}<ref name="Wheeler">"''Tesla is entitled to either distinct priority or independent discovery of''" three concepts in wireless theory: "''(1) the idea of inductive coupling between the driving and the working circuits (2) the importance of tuning both circuits, i.e. the idea of an 'oscillation transformer' (3) the idea of a capacitance loaded open secondary circuit''" {{cite journal
| last= Wheeler
| first= L. P.
| title= Tesla's contribution to high frequency
| journal= Electrical Engineering
| volume= 62
| issue= 8
| pages= 355–357
| date= August 1943
| issn= 0095-9197
| doi= 10.1109/EE.1943.6435874
| s2cid= 51671246
}}</ref> At a March 1893 St. Louis lecture he had demonstrated a wireless system that, although it was intended for [[wireless power transmission]], had many of the elements of later radio communication systems.<ref name="Sterling2013">{{cite book
| last1= Sterling
| first1= Christopher H.
| title= Biographical Encyclopedia of American Radio
| publisher= Routledge
| date= 2013
| pages= 382–383
| url= https://books.google.com/books?id=mdQq-663faAC&q=tesla&pg=PA383
| isbn= 978-1136993756
}}</ref><ref name="Uth">{{cite book
| last1= Uth
| first1= Robert
| title= Tesla, Master of Lightning
| publisher= Barnes and Noble Publishing
| date= 1999
| pages= 65–70
| url= https://books.google.com/books?id=3W6_h6XG6VAC&q=tesla&pg=PA66
| isbn= 978-0760710050
}}</ref><ref name="RockmanH" /> A grounded capacitance-loaded spark-excited [[resonant transformer]] (his ''[[Tesla coil]]'') attached to an elevated wire monopole antenna transmitted radio waves, which were received across the room by a similar wire antenna attached to a receiver consisting of a second grounded resonant transformer tuned to the transmitter's frequency, which lighted a [[Geissler tube]].<ref name="Regal">{{cite book
| last1= Regal
| first1= Brian
| title= Radio: The Life Story of a Technology
| publisher= Greenwood Publishing Group
| date= 2005
| pages= 21–23
| url= https://books.google.com/books?id=N2rNO6FX8o4C&q=Tesla&pg=PA22
| isbn= 978-0313331671
}}</ref><ref name="CheneyM">{{cite book
  | last = Cheney
  | first = Margaret
  | title = Tesla: Man out of time
  | publisher = Simon and Schuster
  | date = 2001
  | location = 
  | pages = 
  | language = 
  | url = https://books.google.com/books?id=HIuK7iLO9zgC&pg=PA96
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 9780743215367
  | mr = 
  | zbl = 
  | jfm =}}</ref>{{rp|p=96-97}}  This system, patented by Tesla 2 September 1897,<ref name="Patent645576">US Patent No. 645576, Nikola Tesla, ''[https://patents.google.com/patent/US645576 System of transmission of electrical energy]'', filed: 2 September 1897; granted: 20 March 1900</ref> 4 months after Lodge's "syntonic" patent, was in effect an inductively coupled radio transmitter and receiver, the first use of the "four circuit" system claimed by Marconi in his 1900 patent ''(below)''.<ref name="AitkenH"/>{{rp|255}}<ref name="Wunsch">{{cite journal
| last1= Wunsch
| first1= A. David
| title= Misreading the Supreme Court: A Puzzling Chapter in the History of Radio
| journal= Antenna
| volume= 11
| issue= 1
| date= November 1998
| url= http://www.mercurians.org/1998_Fall/misreading.htm
| access-date= 3 December 2018}}</ref><ref name="SarkarT" />{{rp|p=353}}<ref name="RockmanH" /><ref name="Sterling2013"/> However, Tesla was interested in [[wireless power]] and never developed a practical radio ''communication'' system.<ref name="Coe2">{{cite book
| last1= Coe
| first1= Lewis
| title= Wireless Radio: A History
| publisher= McFarland
| date= 2006
| pages= 111–113
| url= https://books.google.com/books?id=W1JAeg1PiWIC&pg=PA112
| isbn= 978-0786426621
}}</ref><ref name="Smith">{{cite book |last=Smith |first=Craig B. |title=Lightning: Fire from the Sky|publisher=Dockside Consultants Inc. |date=2008 |url=https://books.google.com/books?id=iGZw0qPDk8YC&q=%22tesla+coil%22&pg=PT11 |isbn=978-0-615-24869-1}}</ref><ref name="Regal"/><ref name="SarkarT" />{{rp|p.352-353, 355-358}}  Other researchers applied the circuit to radio: inductively coupled radio systems were patented by [[Oliver Lodge]] in February 1898,<ref name="Patent609154">US Patent no. 609,154 Oliver Joseph Lodge, [https://patents.google.com/patent/US609154 Electric Telegraphy], filed: 1 February 1898, granted: 16 August 1898</ref><ref name="AitkenH"/>{{rp|254}}<ref name="White1"/> [[Karl Ferdinand Braun]] in November 1899,<ref name="PatentGB189922020">British patent no. 189922020 Karl Ferdinand Braun, [https://worldwide.espacenet.com/publicationDetails/biblio?DB=en.worldwide.espacenet.com&II=0&ND=3&adjacent=true&locale=en_EP&FT=D&date=19000922&CC=GB&NR=189922020A&KC=A# ''Improvements in or related to telegraphy without the use of continuous wires''], applied: 3 November 1899, complete specification: 30 June 1900, granted: 22 September 1900</ref><ref name="AitkenH"/>{{rp|255-256}}<ref name="HongS" />{{rp|p.98-100}}<ref name="SarkarT" />{{rp|p.352-353, 355-358}}  and [[John Stone Stone]] in February 1900.<ref name="Patent714756">US Patent no. 714,756, John Stone Stone [https://patents.google.com/patent/US714756 Method of electric signaling], filed: 8 February 1900, granted: 2 December 1902</ref><ref name="AitkenH"/>{{rp|256-257}}<ref name="White1"/>

Marconi initially paid little attention to syntony,<ref name="HongS" />{{rp|p=91}} but later developed a radio system incorporating these improvements, calling his resonant transformer a "jigger".  In spite of the above prior patents, Marconi in his 26 April 1900 "7777" patent<ref name="Patent763772">British patent no. 7777, Guglielmo Marconi, [https://worldwide.espacenet.com/publicationDetails/biblio?CC=GB&NR=190007777&KC=&FT=E&locale=en_EP Improvements in apparatus for wireless telegraphy], filed: 26 April 1900, granted: 13 April 1901. Corresponding US Patent no. 763,772, Guglielmo Marconi, [https://patents.google.com/patent/US763772 Apparatus for wireless telegraphy], filed: 10 November 1900, granted: 28 June 1904.</ref> claimed rights to the inductively coupled "four circuit" transmitter and receiver.<ref name="SarkarT" />{{rp|p.352-353, 355-358}}<ref name="White1"/><ref name="RockmanH"/> Granted a British patent, the US patent office twice rejected Marconi's claim as lacking originality, but in a 1904 appeal a new patent commissioner reversed the decision and granted the patent.<ref name="White1"/><ref name="RockmanH"/>  This patent gave Marconi a near monopoly of syntonic wireless telegraphy in England and America.<ref name="Morse3">[https://archive.org/stream/radiobeamandbroa029214mbp#page/n35/mode/2up/ Morse (1925) ''Radio: Beam and Broadcast'', p. 30]</ref><ref name="BeauchampK"/> Tesla sued Marconi's company for patent infringement but didn't have the resources to pursue the action.

Braun discovered the value of loose coupling between the transformer coils in reducing the bandwidth.  He and Marconi shared the 1909 [[Nobel prize in physics]] for "contributions to the development of wireless telegraphy". 

In 1943 the [[US Supreme Court]] invalidated Marconi's patent<ref name="Findlaw">{{cite web
| title= No. 369 (1943) Marconi Wireless Co. of America v. United States
| work= United States Supreme Court decision
| publisher= Findlaw.com website
| date= June 21, 1943
| url= https://caselaw.findlaw.com/us-supreme-court/320/1.html
| access-date= March 14, 2017}}</ref> on grounds of the prior patents of Tesla, Lodge, and Stone,<ref name="Sterling2013" /><ref name="RockmanH"/> but the decision did not specify who had rights to the four circuit wireless system.<ref name="White1">{{cite web
| last= White
| first= Thomas H.
| title= Nikola Tesla: The Guy Who DIDN'T "Invent Radio"
| work= United States Early Radio History
| publisher= T. H. White's personal website
| date= 1 November 2012
| url= https://earlyradiohistory.us/tesla.htm
| access-date= 20 June 2018}}</ref>  This came long after spark transmitters had become obsolete.

===Invention of crystal detector===

====Braun's experiments====
The "unilateral conduction" of crystals was discovered by [[Karl Ferdinand Braun]], a German physicist, in 1874 at the [[University of Würzburg]].<ref name="SeitzF">{{cite conference
 | last1  = Seitz
 | first1 = Frederick
 | last2  = Einspruch
 | first2 = Norman
 | title = The Tangled History of Silicon in Electronics
 | conference = Silicon Materials Science and Technology: Proceedings of the Eighth International Symposium on Silicon Materials Science and Technology, Vol. 1
 | pages = 73
 | publisher = The Electrochemical Society
 | date = 4 May 1998
 | location = San Diego
 | url = https://books.google.com/books?id=SnQfAQAAIAAJ&q=%22crystal+rectifier%22+bose+braun&pg=PA73
 | isbn = 9781566771931
 | access-date = 27 June 2018}}</ref>{{rp|73–74}}<ref name="BraunF">{{Citation |last=Braun |first=F. |author-link=Ferdinand Braun |year=1874 |url=https://books.google.com/books?id=YBJbAAAAYAAJ&pg=PA556 |title=Ueber die Stromleitung durch Schwefelmetalle |trans-title=On current conduction through metal sulfides |language=de |journal=Annalen der Physik und Chemie |volume=153 |issue=4 |doi=10.1002/andp.18752291207|bibcode=1875AnP...229..556B }}</ref>{{rp|556-563}}  He studied [[copper pyrite]] (Cu<sub>5</sub>FeS<sub>4</sub>), [[iron pyrite]] (iron sulfide, FeS<sub>2</sub>), galena (PbS) and copper antimony sulfide (Cu<sub>3</sub>SbS<sub>4</sub>).<ref name="Pierce1">
{{cite journal
 | last1  = Pierce
 | first1 = George W.
 | title = Crystal rectifiers for electric currents and electric oscillations, Part 1: Carborundum
 | journal = Physical Review
 | volume = 25
 | issue = 1
 | pages = 31–60
 | date = July 1907
 | url = https://books.google.com/books?id=Uq32C_83yr4C&q=galena++Ferdinand+braun&pg=PA58
 | doi = 10.1103/physrevseriesi.25.31
 | access-date = 25 July 2018
 | bibcode = 1907PhRvI..25...31P}}</ref>
This was before radio waves had been discovered, and Braun did not apply these devices practically but was interested in the [[linear circuit|nonlinear]] [[current–voltage characteristic]] that these sulfides exhibited.  Graphing the current as a function of voltage across a contact made by a piece of mineral touched by a wire cat whisker, he found the result was a line that was flat for current in one direction but curved upward for current in the other direction, instead of a straight line, showing that these substances did not obey [[Ohm's law]].<ref name="Kinzie" />{{rp|p=8-9}}  They conducted current much better in one direction than the other.

====Bose's experiments====
{{multiple image
| align = right
| direction = horizontal
| header   = 
| image1   = US Patent 755840-Jagadish Chandra Bose-Detector for electrical disturbances fig 1-3.png
| caption1 = Bose's galena detector from his 1901 patent.<ref name="Patent755840" /> This version was deliberately made to look and function like a human eyeball, with a lens focusing millimeter waves on the galena contact.
| width1   = 180
| image2   = Jagadish Chandra Bose microwave apparatus.png
| caption2 = Bose's millimeter wave spectrometer, 1897. The spark transmitter is inside the box ''(R)''. The galena detector is inside the horn antenna ''(F)'' with a thumbscrew ''(t)'' which adjusted the pressure of a metal point against the crystal.  The battery ''(V)'' creates a current through the detector measured by the galvanometer ''(G)'' 
| width2   = 350
| footer   =
}}

[[Jagadish Chandra Bose]] first used crystals for radio wave detection, in his experiments with [[microwave]]s at the [[University of Calcutta]] from 1894 to 1900.<ref name="Emerson">{{cite journal
 | last1 = Emerson
 | first1 = D. T.
 | 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
 | date = December 1997
 | url = https://www.cv.nrao.edu/~demerson/bose/bose.html
 | doi = 10.1109/22.643830
 | access-date = 29 July 2018
 | bibcode = 1997ITMTT..45.2267E
 }} also reprinted on [http://indiandefence.com/threads/the-work-of-jagadish-chandra-bose-100-years-of-mm-wave-research.26471/ IndianDefense] {{Webarchive|url=https://web.archive.org/web/20180809215426/http://indiandefence.com/threads/the-work-of-jagadish-chandra-bose-100-years-of-mm-wave-research.26471/ |date=2018-08-09 }}</ref><ref name="SarkarSengupta">Sarkar, Tapan K.; Sengupta, Dipak L. "An appreciation of J. C. Bose's pioneering work in millimeter and microwaves" in {{cite book
 | last1  = Sarkar
 | first1 = T. K.
 | last2  = Mailloux
 | first2 = Robert
 | last3  = Oliner
 | first3 = Arthur A.
 | title  = History of Wireless
 | publisher = John Wiley and Sons
 | date   = 2006
 | url    = https://archive.org/stream/HistoryOfWireless#page/n323/mode/2up
 | isbn   = 978-0471783015
 | author-link1=Tapan Sarkar
 | author-link3=Arthur A. Oliner
 }}</ref>{{rp|295–296, 301–305}}
Like other scientists since Hertz, Bose was investigating the similarity between radio waves and light by duplicating classic [[optics]] experiments with radio waves.<ref name="SarkarT" />{{rp|477–483}}  For a receiver he first used a [[coherer]] consisting of a steel spring pressing against a metal surface with a current passing through it.  Dissatisfied with this detector, around 1897 Bose measured the change in resistivity of dozens of metals and metal compounds exposed to microwaves.<ref name="SarkarSengupta" /><ref name="BoseJ">
{{cite journal
 | last1  = Bose
 | first1 = Jagadish Chandra
 | title = On electric touch and the molecular changes produced in matter by electric waves
 | journal = Proceedings of the Royal Society of London
 | volume = 66
 | issue = 424–433
 | date = January 1899
 | doi = 10.1098/rspl.1899.0124
 | bibcode = 1899RSPS...66..452C
 | s2cid = 121203904
 }}</ref>{{rp|452–474}}
He experimented with many substances as contact detectors, focusing on [[galena]].

His detectors consisted of a small galena crystal with a metal point contact pressed against it with a thumbscrew, mounted inside a closed [[waveguide]] ending in a [[horn antenna]] to collect the microwaves.<ref name="SarkarT" />{{rp|295–296, 301–305}}<ref name="Kinzie" />{{rp|p=12}}  Bose passed a current from a battery through the crystal, and used a [[galvanometer]] to measure it.  When microwaves struck the crystal the galvanometer registered a drop in resistance of the detector.  Thomas Lee notes that this detector functioned by the semiconductor's high temperature coefficient of resistance, as a [[bolometer]], not a rectifying detector.<ref name="LeeT" />{{rp|p=5-6}}  At the time scientists thought that radio wave detectors functioned by some mechanism analogous to the way the eye detected light, and Bose found his detector was also sensitive to visible light and ultraviolet, leading him to call it an ''artificial retina''.  He patented the detector 30 September 1901.<ref name="SeitzF" />{{rp|73-74}}<ref name="Patent755840">{{US patent|755840}} Jagadis Chunder Bose, ''Detector for Electrical Disturbances'', filed: 30 September 1901, granted 29 March 1904</ref>  This is often considered the first patent on a semiconductor device.

====Pickard: discovery of rectification====
{{multiple image
| align = right
| direction = horizontal
| header   =
| image1   = Microphone wireless detector 1909.png
| caption1 = "Microphone" coherer detector from 1909 similar to one Pickard discovered rectification with, widely used in the first receivers.  It consists of a steel needle resting on two carbon blocks. A semiconducting layer of corrosion on the steel may have been responsible for the rectification.
| width1   = 220
| image2   = GWP-836531Patent.png
| caption2 = Pickard's circuit diagram of a crystal receiver in his {{US patent|836531}}, ''Means for Receiving Intelligence Communicated by Electric Waves'', filed: 30 August 1906, granted: 20 November 1906
| width2   = 178
| footer   =
}}

[[Greenleaf Whittier Pickard]], an engineer with the American Wireless Telephone and Telegraph Co. invented the rectifying contact detector,<ref name="Britannica">
{{cite encyclopedia
 | title = Greenleaf Whittier Pickard
 | encyclopedia = Encyclopaedia Britannica online
 | publisher = Encyclopaedia Britannica Inc.
 | date = 2018
 | url = https://www.britannica.com/biography/Greenleaf-Whittier-Pickard
 | access-date = 31 July 2018}}</ref> discovering [[Rectifier|rectification]] of radio waves in 1902 while experimenting with a [[coherer]] detector consisting of a steel needle resting across two carbon blocks.<ref name="DouglasA">{{cite journal| last1 = Douglas| first1 = Alan| title = The Crystal Detector| journal = IEEE Spectrum| volume = 18| issue = 4| date = April 1981| issn = 0018-9235| doi = 10.1109/MSPEC.1981.6369482| s2cid = 44288637| hdl = 10366/158938| hdl-access = free}} archived: [http://www.crystalradio.net/crystalplans/xximages/thecrystaldetector1.jpg part1], [http://www.crystalradio.net/crystalplans/xximages/thecrystaldetector2.jpg part2], [http://www.crystalradio.net/crystalplans/xximages/thecrystaldetector3.jpg part3], [http://www.crystalradio.net/crystalplans/xximages/thecrystaldetector4.jpg part4]</ref>{{rp|64–69}}<ref name="PickardG">{{cite journal| last1 = Pickard| first1 = Greenleaf Whittier | title = How I Invented the Crystal Detector| journal = Electrical Experimenter| volume = 7| issue = 4| date = August 1919| url = https://www.americanradiohistory.com/Archive-Electrical-Experimenter/EE-1919-08.pdf| access-date = 13 June 2016}}</ref>{{rp|325–330, 360}}<ref name="Kinzie" />{{rp|p=17-18}}  On 29 May 1902 he was operating this device, listening to a radiotelegraphy station.  Coherers required an external current source to operate, so he had the coherer and telephone earphone connected in series with a 3 cell [[Electric battery|battery]] to provide power to operate the earphone. Annoyed by background "frying" noise caused by the current through the carbon, he reached over to cut two of the battery cells out of the circuit to reduce the current<ref name="DouglasA" />{{rp|64–69}}<ref name="PickardG" />{{rp|325–330, 360}}
{{Quote
|text=The frying ceased, and the signals, though much weakened, became materially clearer through being freed of their background of microphonic noise.  Glancing over at my circuit, I discovered to my great surprise that instead of cutting out two of the cells I had cut out all three; so, therefore, the telephone diaphragm was being operated solely by the energy of the receiver signals.  A contact detector operating without local battery seemed so contrary to all my previous experience that ... I resolved at once to thoroughly investigate the phenomenon.
|author=
|source=
}}
The generation of an audio signal without a DC bias battery made Pickard realize the device was acting as a rectifier.  

During the next four years, Pickard conducted an exhaustive search to find which substances formed the most sensitive detecting contacts, eventually testing thousands of minerals,<ref name="SeitzF" /> and discovered about 250 rectifying crystals.<ref name="LeeT" />{{rp|4-9,297-300}}<ref name="DouglasA" />{{rp|64–69}}<ref name="PickardG"/>{{rp|325–330, 360}}  In 1906 he obtained a sample of fused [[silicon]], an artificial product recently synthesized in electric furnaces, and it outperformed all other substances.<ref name="DouglasA" />{{rp|64–69}}<ref name="PickardG" />{{rp|325–330, 360}}  He patented the silicon detector 30 August 1906.<ref name="SeitzF" /><ref name="Patent836531" >{{US patent|836531}} Greenleaf Whittier Pickard, ''Means for Receiving Intelligence Communicated by Electric Waves'', filed: 30 August 1906, granted: 20 November 1906</ref>  In 1907 he formed a company to manufacture his detectors, Wireless Specialty Products Co.,<ref name="Kinzie" />{{rp|p=25-26}} and the silicon detector was the first crystal detector to be sold commercially.<ref name="DouglasA" />{{rp|64–69}}<ref name="SwinyardW">{{cite journal
  | last = Swinyard
  | first = William O.
  | title = The development of the art of radio receiving from the early 1920s to the present
  | journal = Proceedings of the I.R.E.
  | volume = 50
  | issue = 5
  | pages = 793-794
  | publisher = Institute of Radio Engineers
  | location = 
  | date = May 1962
  | language = 
  | url = https://www.google.com/books/edition/Fiftieth_Anniversary_1912_1962/cNYUNnI2-0EC?hl=en&gbpv=1&pg=PA682
  | jstor = 
  | issn = 
  | doi = 
  | id = 
  | mr = 
  | zbl = 
  | jfm = 
  | access-date = 8 May 2025}}</ref>  Pickard went on to produce other detectors using the crystals he had discovered; the more popular being the [[iron pyrite]] "Pyron" detector and the [[zincite]]–[[chalcopyrite]] crystal-to-crystal "Perikon" detector<ref name="Kinzie" />{{rp|p=24}} in 1908,<ref name="Patent912726">{{US patent|912726}} Greenleaf Whittier Pickard, ''Oscillation receiver'', filed: 15 September 1908, granted: 16 February 1909</ref> which stood for "'''PER'''fect p'''I'''c'''K'''ard c'''ON'''tact".<ref name="LeeT" />{{rp|4-9,297-300}}

Around 1906 it was recognised that mineral crystals could be a better detector than the coherer, crystal radios began to be made, and many new crystal detectors were invented.<ref name="Kinzie" />{{rp|p=20-21, 23}}  In 1906 [[Henry Harrison Chase Dunwoody]],<ref name="Kinzie" />{{rp|p=19}} a retired general in the U.S. Army Signal Corps, patented the [[silicon carbide]] ([[carborundum]]) detector,<ref name="Patent837616">{{US patent|837616}} Henry H. C. Dunwoody, ''Wireless Telegraph System'', filed: 23 March 1906, granted: 4 December 1906</ref><ref name="SwinyardW" /><ref name="Collins" /> using another recent product of electric furnaces.  Braun began to experiment with crystals as radio detectors and patented a galena cat whisker detector in Germany.<ref name="Patent178871">
German patent 178871 Karl Ferdinand Braun, ''[https://worldwide.espacenet.com/publicationDetails/originalDocument?CC=DE&NR=178871C&KC=C&FT=D&ND=3&DB=&locale=en_EP#  Wellenempfindliche Kontaktstel]'', filed: 18 February 1906, granted: 22 October 1906</ref>  [[Louis Winslow Austin|L. W. Austin]] invented a silicon–tellurium detector.<ref name="Kinzie" />{{rp|p=13}}

===Use during the radiotelegraphy era===
During the [[radiotelegraphy]] era, from the beginning of radio in 1894 to 1920, there was virtually no [[broadcasting]]; radio was used as a person-to-person text messaging service.<ref name="CarrJ" />{{rp|p=4}}  For the first 10 years [[coherer]]s and [[electrolytic detector]]s were used in receivers.   Long distance radio communication depended on high power transmitters (up to 1 MW), huge wire antennas, and a receiver with a sensitive detector.<ref name="DouglasA" />{{rp|64–69}}

Around 1907 crystal detectors replaced the [[coherer]] and [[electrolytic detector]] in receivers to become the most widely used form of radio detector.<ref name="Marriott" /><ref name="Robison1">The 1911 edition of the US Navy's manual of radio stated: "''There are but two types of detectors now in use: crystal or rectifying detectors and the electrolytic. Coherers and microphones ''[another type of coherer detector]'' are practically obsolete, and comparatively few of the magnetic and Audion or valve ''[triode]'' detectors have been installed.''"{{cite book
 | last1  = Robison
 | first1 = Samuel Shelburne 
 | title  = Manual of Wireless Telegraphy for the Use of Naval Electricians, 2nd Ed.
 | publisher = United States Naval Institute
 | date   = 1911
 | location = Washington DC
 | pages  = 128
 | url    = https://books.google.com/books?id=7gQ9AAAAYAAJ&pg=PA128
 }}</ref>  Until the [[triode]] [[vacuum tube]] began to be used in World War I, crystals were the best radio reception technology,<ref name="Kinzie" />{{rp|p=17}} used in cutting-edge receivers in wireless telegraphy stations, as well as in homemade crystal radios.<ref>The 1913 edition of the US Navy's manual of radio stated: "''Only one type of detector is now in use: the crystal. Coherers and microphones are practically obsolete, and comparatively few magnetic and Audion or valve ''[triode]'' detectors have been installed.''"{{cite book
 | last1  = Phillips
 | first1  = Vivian J.
 | title  = Early Radio Wave Detectors
 | publisher  = Inst. of Electrical Engineers
 | date  = 1980
 | location  = London
 | language  = 
 | url  = https://archive.org/details/EarlyRadioWaveDetectors/page/205
 | doi  = 
 | id  = 
 | pages = 212
 | isbn  = 978-0906048245
 }}</ref>    
{{gallery |height=130 |width= |mode=packed |align=center |header= |footer=
|Image:Marconi Type 106 crystal radio receiver.jpg |Marconi Type 106 crystal receiver used for transatlantic communication, {{circa|1917}} |alt5=
|Image:Wireless Specialty Products IP-501 military crystal radio receiver.jpg|Wireless Specialty Products type IP-501 naval crystal/audion receiver widely used in World War 1  
|Image:Marconi crystal radio receiver.jpg |Marconi Type 103 shipboard crystal set with parts labeled |alt3=
|Image:Signal Corps SCR-54A crystal radio.jpg |[[SCR-54]]-A portable crystal set used by US Signal Corps in World War I |alt4=
|Image:RX 1914 tranchée.jpg|Soldier listening to a crystal radio during World War I, 1914|alt1=World War I soldier in a dugout with headphones listening to a radio
|Image:Australian signallers 1916.jpg |Australian signallers using a Marconi Mk III crystal receiver, 1916 |alt2=
|Image:Crystal receiver used in Telefunken radiotelegraphy stations 1915.jpg|A crystal receiver used in long distance Telefunken stations
}}
Wireless telegraphy companies such as Marconi and Telefunken manufactured sophisticated inductively-coupled crystal radios as communication receivers in ship radio rooms and shore stations.<ref name="BucherE" />{{rp|p=144-157}}  Rugged military versions were made for naval warships and military communication stations.<ref name="WSA">{{cite web
  | title = Radio Telegraph and Telephone Equipment
  | work = 
  | website = vacuumtubearchive.com
  | publisher = Wireless Specialty Apparatus Co.
  | date = 1917
  | url = https://vacuumtubearchive.com/wp-content/uploads/2023/12/WSA.pdf
  | format = PDF
  | doi = 
  | accessdate = 23 May 2025}}, p. 5-6, 30</ref>  Portable military radios such as the [[SCR-54]] were provided to army troops in World War 1 to communicate with their commanders behind the lines.  After the war electronics firms produced inexpensive "box" crystal radios for consumers.   And thousands of [[radio amateur]]s worldwide, many of them teenage boys, built their own crystal sets, following instructions in radio magazines, to get in on the exciting new hobby of radio.<ref name="Kinzie" />{{rp|p=26}}

[[File:NBS 120 Set.jpg|thumb|US [[National Bureau of Standards|Bureau of Standards]] 1922 Circular 120 "''Construction and Operation of a Simple Homemade Radio Receiving Outfit''" taught Americans how to build a crystal radio.<ref>{{cite web|url=https://archive.org/details/circularofbureau120unse| last = US Bureau of Standards |title = Construction and Operation of a Simple Homemade Radio Receiving Outfit, Bureau of Standards Circular 120|date=April 24, 1922|publisher =U.S. Government Printing Office}}</ref>]]

[[Galena]] ([[lead(II) sulfide|lead sulfide]], PbS, sometimes sold under the names "Lenzite"<ref name="NBS40" />{{rp|p=433-439}} and "Hertzite"),<ref name="LeeT" /><ref name="Hirsch" /><ref name="Cockaday">{{cite book| last1 = Cockaday| first1 = Laurence M.| title = Radio-telephony for Everyone| publisher = Frederick A. Stokes Co. | date = 1922| location = New York| pages = 94| url = https://books.google.com/books?id=8x5IAAAAIAAJ&q=%22almost+universally%22+galena&pg=PA94}}</ref> was the most widely used crystal detector since it was the most sensitive.  Other common crystalline minerals used<ref name="Loomis" />{{rp|p=298}}  were [[iron pyrite]] (iron sulfide, FeS<sub>2</sub>, "fool's gold", also sold under the trade names "Pyron"<ref name="Morgan" /> and "Ferron"<ref name="NBS40" />),<ref name="Sievers">{{cite book | last1  = Sievers | first1 = Maurice L. | title  = Crystal Clear: Vintage American Crystal Sets, Crystal Detectors, and Crystals, Vol. 1 | publisher = Sonoran Publishing | date   = 1995 | pages  = 3–5 | url    = https://books.google.com/books?id=2huyl5tOpWAC&pg=PA3 | isbn   = 978-1886606012}}</ref><ref name="Hirsch" /><ref name="Stanley" />{{rp|p=311-318}} [[molybdenite]] ([[molybdenum disulfide]], MoS<sub>2</sub>),<ref name="NBS40" /><ref name="Hirsch" /><ref name="Stanley" />{{rp|p=311-318}} and [[cerussite]] ([[lead carbonate]], PbCO<sub>3</sub>).<ref name="Hirsch" /> A disadvantage of these detectors was they required a delicate wire "cat whisker" contact, which could be disrupted by the slightest vibration.  So they had to be readjusted constantly.     

Much research went into finding better detectors and many types of crystals were tried.<ref name="Edelman">
{{cite book
 | last1  = Edelman
 | first1 = Phillip E.
 | title  = Experimental Wireless Stations
 | publisher = Norman W. Henly Publishing Co.
 | date   = 1920
 | location = New York
 | pages  = 258–259
 | url    = https://books.google.com/books?id=TfhAAQAAIAAJ&q=258&pg=PA258
 }}</ref>  The goal of researchers was to find rectifying crystals that were less fragile and sensitive to vibration than galena and the other detectors above.<ref name="NBS40" />{{rp|p=434-435}}  Another desired property was tolerance of high currents; many crystals would become insensitive when subjected to discharges of atmospheric electricity from the outdoor wire antenna, or current from the powerful spark transmitter leaking into the receiver.<ref name="Kinzie" />{{rp|p=27}}  Carborundum proved to be the best of these;<ref name="BucherE" />{{rp|p=135}} it could rectify with a steel point pressed firmly against it, or even clamped between two flat contacts,<ref name="Kinzie" />{{rp|p=24}} so carborundum contacts didn't need to be adjusted before each use like the delicate cat whisker types. Therefore, carborundum detectors were used in shipboard wireless stations where waves caused the floor to rock, and military stations where gunfire was expected.<ref name="LeeT" />{{rp|4-9,297-300}}<ref name="NBS40" />{{rp|p=435}}  Silicon detectors, although less sturdy than carborundum, also used a spring-loaded point contact which could not be jarred loose, so they were also used in professional and military stations.   

Between about 1904 and 1915 the first types of radio transmitters were developed which produced [[continuous wave|continuous sinusoidal waves]]: the [[arc converter]] (Poulsen arc) and the [[Alexanderson alternator]].<ref name="NBS40" />{{rp|p=396-415}}  These slowly replaced the old [[damped wave]] spark transmitters.  Besides having a longer transmission range, these transmitters could be [[modulated]] with an [[audio signal]] to transmit sound by [[amplitude modulation]] (AM) [[radiotelephony]].  Unlike the coherer, the rectifying action of the crystal detector allowed it to [[demodulate]] an AM radio signal, producing audio (sound).<ref name="SterlingC" />  Although other detectors used at the time, the [[electrolytic detector]], [[Fleming valve]] and the triode could also rectify AM signals, crystals were the simplest, cheapest AM detector.<ref name="SterlingC" /><ref name="Kinzie" />{{rp|p=30-31}} 

During World War I the [[triode]] vacuum tube, the first practical [[amplifier]], was developed into a reliable component, and commercial and military wireless stations switched from crystal receivers to more sensitive vacuum tube receivers.<ref name="Kinzie" />{{rp|p=31}}<ref name="Robison2">The 1918 edition of the US Navy's manual of radio stated: "''There are two types of detectors now in use: the Audion ''[triode]'' and the crystal or rectifying detector. Coherers and microphones ''[another type of coherer detector]'' are practically obsolete... but the use of Audions...is increasing.''"{{cite book
 | last1  = Robison
 | first1 = Samuel Shelburne 
 | title  = Manual of Wireless Telegraphy for the Use of Naval Electricians, 4th Ed.
 | publisher = United States Naval Institute
 | date   = 1918
 | location = Washington DC
 | pages  = 156
 | url    = https://books.google.com/books?id=_rNRAAAAMAAJ&q=audion+crystal&pg=PA156
 }}</ref>  However the popularity and sales of crystal radios continued to increase for a few years due to the sudden rise of [[radio broadcasting]].<ref name="Kinzie" />{{rp|p=32}}   After World War I, radio stations began experimenting with transmitting sound, voice and music, by [[amplitude modulation]], and a growing community of radio listeners built or bought crystal radios to listen to them.<ref name="SwinyardW" /><ref name="SterlingC" /><ref name="Craddock">
{{cite web
 | last1  = Craddock
 | first1 = Christine D.
 | title = Crystal Radio: An historical survey
 | version = Honors Thesis
 | publisher = Ball State University, Muncie, Indiana
 | date = 24 March 1987
 | url = http://cardinalscholar.bsu.edu/bitstream/handle/handle/189950/C73_1987CraddockChristineD.pdf
 | access-date = 2 August 2018}}</ref> In 1922 the (then named) [[United States Bureau of Standards]], responding to consumer interest, released a publication entitled ''Construction and Operation of a Simple Homemade Radio Receiving Outfit''.<ref name="NBS120">{{cite web
  | title = Construction and operation of a simple homemade radio receiving outfit
  | work = Circular of the Bureau of Standards no. 120
  | website = 
  | publisher = Bureau of Standards, US Commerce Dept., Washington D.C.
  | date = 24 April 1922
  | url = https://archive.org/details/circularofbureau120unse
  | format = 
  | doi = 
  | accessdate = 28 May 2025}}, archived on archive.org website</ref> This article showed how anyone who was handy with simple tools could make a crystal radio and tune into weather, crop prices, time, news and the opera.  

Use of crystal radios continued to grow until the 1920s when vacuum tube radios replaced them.<ref name="SterlingC" /><ref name="Craddock" />

====Intensifiers====
Crystal receivers were even used for transatlantic communication. In transoceanic radiotelegraphy stations specialized sensitive inductively coupled crystal receivers fed by mile long wire antennas were used to receive Morse code [[telegram]] traffic and record it on paper tape.<ref>Marconi used carborundum detectors beginning 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 | pages = 191 | url = https://books.google.com/books?id=k3XCkncd83AC&pg=PA191 | isbn = 978-0852967928}}</ref>  This distance was at the extreme edge of a crystal receiver's reception range.  Before amplifying vacuum tubes became available, wireless companies tried to develop technology to make the received signal stronger. 

One solution was the "intensifier"; such as the version invented by S. G. Brown Co. and used by the Marconi Co.<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 = 
  | language = 
  | url = https://books.google.com/books?id=B82nsBBEMCYC&pg=PA310
  | archive-url= 
  | archive-date=
  | doi = 
  | id = 
  | isbn = 
  | mr = 
  | zbl = 
  | jfm =}}</ref>{{rp|p=310-311}} The output current of the crystal receiver was passed through a winding on the pole pieces of a permanent magnet.  Mounted close to the magnet poles was a steel resonant reed.  The reed was adjusted to [[resonant frequency|resonate]] at the audio spark frequency of the transmitter.  When the reed vibrated, switch contacts on the reed periodically closed a battery circuit with an earphone, creating a buzzing sound in the earphone.  Due to resonance, signals that were too weak to be heard directly excited large vibrations in the reed, allowing them to be detected.

====Crystodyne====
Some semiconductor junctions have a property called ''[[negative resistance]]'' which means the current through them decreases as the voltage increases over a part of their [[current–voltage characteristic|I–V curve]].  This allows a diode, normally a [[passivity (engineering)|passive]] device, to function as an [[amplifier]] or [[electronic oscillator|oscillator]].  For example, when connected to a [[resonant circuit]] and biased with a DC voltage, the negative resistance of the diode can cancel the positive resistance of the circuit, creating a circuit with zero AC resistance, in which spontaneous oscillating currents arise. This property was first observed in crystal detectors around 1909 by [[William Henry Eccles]]<ref name="Grebennikov">
{{cite book
 |last        = Grebennikov
 |first       = Andrei
 |title       = RF and Microwave Transmitter Design
 |publisher   = John Wiley & Sons
 |date        = 2011
 |page        = 4
 |url         = https://books.google.com/books?id=nGLdHfULzhYC&q=%22negative+resistance%22++%22crystal+detector%22&pg=PA4
 |isbn        = 978-0470520994
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20160917100859/https://books.google.com/books?id=nGLdHfULzhYC&pg=PA4&dq=%22negative+resistance%22++%22crystal+detector%22&hl=en#v=onepage&q=%22negative%20resistance%22%20%20%22crystal%20detector%22&f=false
 |archive-date = 2016-09-17
}}</ref><ref name="Pickard1">
{{cite journal
 | last = Pickard
 | first = Greenleaf W.
 | title = The Discovery of the Oscillating Crystal
 | journal = Radio News
 | volume = 6
 | issue = 7
 | page = 1166
 |date=January 1925
 | url = http://www.americanradiohistory.com/Archive-Radio-News/20s/Radio-News-1925-01-R.pdf
 | access-date = July 15, 2014}}</ref>
and Pickard.<ref name="White">
{{cite web
 |last        = White
 |first       = Thomas H.
 |title       = Section 14 – Expanded Audio and Vacuum Tube Development (1917–1924)
 |work        = United States Early Radio History
 |publisher   = earlyradiohistory.us
 |date        = 2003
 |url         = https://earlyradiohistory.us/sec014.htm
 |access-date  = September 23, 2012
 }}</ref>
They noticed that when their detectors were biased with a DC voltage to improve their sensitivity, they would sometimes break into spontaneous oscillations.<ref name="White" /> However these researchers just published brief accounts and did not pursue the effect.

The first person to exploit negative resistance practically was self-taught Russian physicist [[Oleg Losev]], who devoted his career to the study of crystal detectors.  In 1922 working at the new [[Nizhny Novgorod Radio Laboratory]] he discovered negative resistance in biased [[zincite]] ([[zinc oxide]]) point contact junctions.<ref name="White" /><ref name="Losev">
{{cite journal
 | last = Losev
 | first = O. V.
 | title = Oscillating Crystals
 | journal = Radio News
 | volume = 6
 | issue = 7
 | pages = 1167, 1287
 |date=January 1925
 | url = http://www.americanradiohistory.com/Archive-Radio-News/20s/Radio-News-1925-01-R.pdf
 | access-date = July 15, 2014}}</ref><ref name="Gabel">
{{cite journal
 |last        = Gabel
 |first       = Victor
 |title       = The Crystal as a Generator and Amplifier
 |journal     = The Wireless World and Radio Review
 |volume      = 15
 |pages       = 2–5
 |date        = October 1, 1924
 |url         = http://www.hpfriedrichs.com/downloads-lib/xtalgen.pdf
 |access-date  = March 20, 2014
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20141023072450/http://www.hpfriedrichs.com/downloads-lib/xtalgen.pdf
 |archive-date = October 23, 2014
}}</ref><ref name="Ben-Menahem">
{{cite book
 |last        = Ben-Menahem
 |first       = Ari
 |title       = Historical Encyclopedia of Natural and Mathematical Sciences, Vol. 1
 |publisher   = Springer
 |date        = 2009
 |page        = 3588
 |url         = https://books.google.com/books?id=9tUrarQYhKMC&q=losev+%22negative+resistance%22&pg=PA3588
 |isbn        = 978-3540688310
 |url-status     = live
 |archive-url  = https://web.archive.org/web/20171123190123/https://books.google.com/books?id=9tUrarQYhKMC&pg=PA3588&dq=losev+%22negative+resistance%22&hl=en&sa=X&ei=EKa8T4LxL8fiiAKm4IHEDQ&ved=0CEAQ6AEwAg#v=onepage&q=losev%20%22negative%20resistance%22&f=false
 |archive-date = 2017-11-23
}}</ref><ref name="LeeT" />{{rp|20}} 
He realized that amplifying crystals could be an alternative to the fragile, expensive, energy-wasting vacuum tube.  He used biased negative resistance crystal junctions to build solid-state [[amplifier]]s, [[oscillator]]s, and amplifying and regenerative [[radio receiver]]s, 25 years before the invention of the transistor.<ref name="Grebennikov" /><ref name="Gabel" /><ref name="LeeT" />{{rp|4-9}}<ref name="Gernsback1924">
{{cite journal
 |last       = Gernsback
 |first      = Hugo
 |title      = A Sensational Radio Invention
 |journal    = Radio News
 |page       = 291
 |date       = September 1924
 |url        = https://books.google.com/books?id=2rQ1AQAAIAAJ&pg=PA291
}} and [https://books.google.com/books?id=2rQ1AQAAIAAJ&pg=PA294 "The Crystodyne Principle"], ''Radio News'', September 1924, pages 294-295, 431.</ref>  However his achievements were overlooked because of the success of vacuum tubes.  His technology was dubbed "Crystodyne" by science publisher [[Hugo Gernsback]]<ref name="Gernsback1924" /> one of the few people in the West who paid attention to it.  After ten years he abandoned research into this technology and it was forgotten.<ref name="LeeT" />{{rp|20}}

===Use during the broadcast era===
{{gallery |height=135 |width= |mode=packed |align=center |header="Crystal radios after 1920" |footer=
|Image:Kristallmottagare.JPG |Swedish "box" crystal radio with earphones, {{circa|1925}} |alt8=
|Image:PensacolaAug081920sCrystalRadioKodak.jpg |Homemade "loose coupler" set ''(top)'', museum in Florida, {{circa|1920}} |alt6=
|Image:D 1925 Jahnke Schiebspulendetektor.jpg |Crystal radio, Germany, {{circa|1924}} |alt7=
|Image:Breadboard crystal radio 1925.jpg|DIY crystal radio from a 1925 article in an electronics magazine
|Image:Heliogen medium wave galena radio.JPG |German Heliogen brand radio using a "basket-weave" coil, 1935 |alt9=
|Image:Radio detefon 1.jpg |Polish Detefon brand radio, 1930–1939, using a "cartridge" type crystal ''(top)'' |alt10=
}}
[[Image:Crystal radio advertisement.png|thumb|Family listening to the first radio broadcasts on a crystal radio in 1922. Since crystal radios cannot drive loudspeakers they must share earphones.]] 

In the 1920s, the amplifying [[triode]] [[vacuum tube]], invented in 1907 by [[Lee De Forest]], replaced earlier technology in both radio transmitters and receivers.
AM [[radio broadcasting]] spontaneously arose around 1920, and radio listening exploded from a solitary hobby to become a hugely popular  public pastime.<ref name="SwinyardW" />  The initial listening audience for the new broadcasting stations was largely owners of crystal radios, as many people could not afford a tube radio.<ref name="Kinzie" />{{rp|p=32}}<ref name="SwinyardW" />  But lacking amplification, crystal radios had to be listened to with earphones, and could only receive nearby stations within about 25 - 50 miles.<ref name="SterlingC" /><ref name="SwinyardW" />  The amplifying vacuum tube radios which began to be mass-produced in 1921 had greater reception range, did not require the fussy adjustment of a cat whisker, and produced enough audio output power to drive [[loudspeaker]]s, allowing the entire family to listen comfortably together, or dance to Jazz Age music.<ref name="Kinzie" />{{rp|p=33}}<ref name="SterlingC" />

So during the 1920s vacuum tube receivers replaced crystal radios in all except poor households.<ref name="SeitzF" /><ref name="SterlingC" /><ref name="Phillips4">The 1920 "British Admiralty Handbook of Wireless Telegraphy" stated that: "''Crystal detectors are being replaced by ''[triode]'' valve detectors which are more stable, easier to adjust, and generally more satisfactory''".  The 1925 edition said valves were "''replacing the crystal for all ordinary purposes''" {{cite book
 | last1  = Phillips
 | first1  = Vivian J.
 | title  = Early Radio Wave Detectors
 | publisher  = Institute of Electrical Engineers
 | date  = 1980
 | location  = London
 | pages  = [https://archive.org/details/EarlyRadioWaveDetectors/page/212 212]
 | url  = https://archive.org/details/EarlyRadioWaveDetectors/page/212
 | isbn  = 978-0906048245
 }}</ref>
The temperamental, unreliable action of the crystal detector had always been a barrier to its acceptance as a standard component in commercial radio equipment<ref name="BraunA" />{{rp|11-12}} and was one reason for its rapid replacement.  Frederick Seitz, an early semiconductor researcher, wrote:<ref name="Riordan" />
{{Quote
|text=Such variability, bordering on what seemed the mystical, plagued the early history of crystal detectors and caused many of the vacuum tube experts of a later generation to regard the art of crystal rectification as being close to disreputable.
|author=
|source=
}}  
[[File:Crystal radio backup on SS Jeremiah O'Brien.agr.jpg|thumb|upright=0.6|Crystal radio used as a backup receiver on a World War II [[Liberty ship]]]]
The crystal radio became a cheap alternative receiver used for emergency communication and by people who could not afford tube radios:<ref name="SeitzF" /> teenagers, the poor, and those in developing countries.<ref name="Craddock" />  Building a crystal set remained a popular educational project to introduce people to radio,<ref name="Kinzie" />{{rp|p=34}}<ref name="SterlingC" /> used by organizations like the [[Scouting|Boy Scouts]].  The galena detector, the most widely used type among amateurs,<ref name="LeeT" />{{rp|4-9,297-300}} became virtually the only detector used in crystal radios from this point on.<ref name="Hirsch" /><ref name="Cockaday" />  Crystal radios were kept as emergency backup radios on ships.<ref name="Kinzie" />{{rp|p=32}}  During [[World War II]] in Nazi-occupied Europe the radio saw use as an easily constructed, easily concealed clandestine radio by Resistance groups.<ref name="Craddock" />

==== "Foxhole radios" ====
[[File:Foxhole radio from WW2.jpg|thumb|left|"Foxhole radio" used on the Italian Front in World War 2. It uses a pencil lead attached to a safety pin pressing against a razor blade for a detector.]]

In addition to mineral crystals, the oxide coatings of many metal surfaces act as [[semiconductor]]s (detectors) capable of rectification. Crystal radios have been improvised using detectors made from rusty nails, corroded pennies, and many other common objects.

In World War II, when [[Allies of World War II|Allied]] troops were halted near [[Anzio|Anzio, Italy]] during the spring of 1944, powered personal radio receivers were strictly prohibited as the Germans had equipment that could detect the [[local oscillator]] signal of [[superheterodyne]] receivers.{{Citation needed|reason=Significant claim about German capabilities which warrants direct substantiation.|date=February 2025}}  Crystal sets lack power driven local oscillators, hence they could not be detected. Some resourceful soldiers constructed "crystal" sets from discarded materials to listen to news and music.<ref name="Carusella">{{Cite book|title=Foxhole Radio: the ubiquitous razor blade radio of WWII|last=Carusella|first=Brian|publisher=Canyon Wren Press|year=2019|isbn=978-0578536583|pages=179–182|url = https://books.google.com/books?id=ymt6xwEACAAJ}}</ref><ref name="Gernsback1944">{{cite journal
  | last1  = Gernsback 
  | first1 = Hugo
  | title = Foxhole Emergency Radios
  | journal = Radio Craft
  | volume = 15
  | issue = 12
  | pages = 730
  | publisher = Radcraft Publications
  | date = September 1944
  | url = https://worldradiohistory.com/Archive-Radio-Craft/1940s/Radio-Craft-1944-09.pdf
  | access-date = 29 July 2020}}</ref> One type used a blue steel [[razor blade]] and a [[pencil lead]] for a detector.<ref>{{Cite news|title = G.I. Uses Razor as Radio|date = 29 April 1944|work = New York Times|url = https://www.nytimes.com/1944/04/29/archives/gi-uses-razor-as-radio-sets-up-own-receiver-for-rome-and-naples-on.html}}</ref>  The lead point touching the semiconducting oxide coating (magnetite) on the blade formed a crude point-contact diode.  By carefully adjusting the pencil lead on the surface of the blade, they could find spots capable of rectification.  The sets were dubbed "[[foxhole radio]]s" by the popular press, and they became part of the [[folklore]] of [[Second World War|World War II]].

In some German-occupied countries during [[World War II|WW2]] there were widespread confiscations of radio sets from the civilian population. This led determined listeners to build their own clandestine receivers which often amounted to little more than a basic crystal set. Anyone doing so risked imprisonment or even death if caught, and in most of Europe the signals from the [[BBC]] (or other allied stations) were not strong enough to be received on such a set.
{{clear}}

=== Modern day ===
After World War II, the development of modern semiconductor diodes finally made the galena cat whisker detector obsolete.<ref name="Kinzie" />{{rp|p=39}}<ref name="Craddock" />  In the few crystal radios still being made, the cat whisker detector was replaced by a [[germanium diode]], which didn't require adjustment.  The germanium diode was used because it was a more sensitive detector than the [[silicon diode]] due to it's lower forward voltage (0.3V versus 0.7V). 

While it never regained the popularity and general use that it enjoyed at its beginnings, the crystal radio circuit is still used. The [[Boy Scout]]s have kept the construction of a radio set in their program since the 1920s. A large number of prefabricated novelty radios 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 examples of the early instruments. Much effort goes into the visual appearance of these sets as well as their performance. 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.

==== "Rocket Radio" ====
In the late 1950s, the compact "rocket radio", shaped like a rocket, typically imported from Japan, was introduced, and gained moderate popularity.  It used a piezoelectric crystal earpiece (described later in this article), a ferrite core to reduce the size of the tuning coil (also described later), and a small germanium fixed diode, which did not require adjustment. To tune in stations, the user moved the rocket nosepiece, which, in turn, moved a ferrite core inside a coil, changing the inductance in a tuned circuit. Earlier crystal radios suffered from severely reduced Q, and resulting selectivity, from the electrical load of the earphone or earpiece. Furthermore, with its efficient earpiece, the "rocket radio" did not require a large antenna to gather enough signal. With much higher Q, it could typically tune in several strong local stations, while an earlier radio might only receive one station, possibly with other stations heard in the background.

For listening in areas where an electric outlet was not available, the "rocket radio" served as an alternative to the vacuum tube portable radios of the day, which required expensive and heavy batteries. Children could hide "rocket radios" under the covers, to listen to radio when their parents thought they were sleeping. Children could take the radios to public swimming pools and listen to radio when they got out of the water, clipping the ground wire to a chain link fence surrounding the pool. The rocket radio was also used as an emergency radio, because it did not require batteries or an AC outlet.

The rocket radio was available in several rocket styles, as well as other styles that featured the same basic circuit.<ref>[http://www.esnarf.com/rs.htm 1950s Crystal Radios]</ref>

[[Transistor radio]]s had become available at the time, but were expensive. Once those radios dropped in price, the rocket radio declined in popularity.

== Use as a power source ==
A crystal radio tuned to a strong local transmitter can be used as a power source for a second amplified receiver of a distant station that cannot be heard without amplification.<ref name="simple_AM_rx">{{cite book |last = Polyakov |first = V.&nbsp;T. |script-title = ru:Техника радиоприёма. Простые приёмники АМ сигналов |trans-title = Receiving techniques. Simple receivers for AM signals |chapter = 3.3.2&nbsp;Питание полем мощных станций |year = 2001 |location = Moscow |publisher= Knizhnai͡a palata|language = ru |isbn = 5-94074-056-1 |pages = 256 }}</ref>{{rp|122–123}}   There is a history of attempts and unverified claims of crystal radio designs which use the power in the received signal to amplify the output.  Some earlier attempts include a one-[[transistor]]<ref>Radio-Electronics, 1966, №2</ref> amplifier in 1966.

== See also ==
{{Portal|Radio}}

* [[Batteryless radio]]
* [[Coherer]]
* [[Demodulator]]
* [[Detector (radio)]]
* [[Electrolytic detector]]
* [[History of radio]]

== References ==
{{reflist|30em}}

== Further reading ==
* Ellery W. Stone (1919). [https://archive.org/details/elementsradiote00stongoog Elements of Radiotelegraphy]. D. Van Nostrand company. 267 pages.
* Elmer Eustice Bucher (1920). [https://archive.org/details/wirelessexperim00buchgoog The Wireless Experimenter's Manual: Incorporating how to Conduct a Radio Club].
* Milton Blake Sleeper (1922). [https://archive.org/details/radiohookupsare00sleegoog 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) "[https://archive.org/details/circularofbureau120unse Construction and operation of a simple homemade radio receiving outfit]", Bureau of Standards, C-120: Apr. 24, 1922.
* PA Kinzie (1996). Crystal Radio: History, Fundamentals, and Design. Xtal Set Society.
* Thomas H. Lee (2004). [https://books.google.com/books?id=DzcMK-2mFQUC The Design of CMOS Radio-Frequency Integrated Circuits]
* Derek K. Shaeffer and Thomas H. Lee (1999). [https://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.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".
* "''{{usurped|1=[https://web.archive.org/web/20130317230753/http://www.a-reny.com/iexplorer/cristadyne.html Semiconductor archeology or tribute to unknown precursors]}}''". 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.
* [https://web.archive.org/web/20070528195207/http://homepage.ntlworld.com/lapthorn/xtal.htm Crystal Set DX?] Roger Lapthorn G3XBM
* [https://www.electronics-notes.com/articles/history/radio-receivers/cats-whisker-crystal-types.php Details of crystals used in crystal sets]
* {{cite web |last1=Asquin |first1=Don |last2=Rabjohn |first2=Gord |date=April 2012 |publisher=Ottawa Electronics Club |url=http://www.ottawaelectronicsclub.org/presentations/2012/High_Performance_Crystal_Radios_for_electronics_club_v2_hires.pdf |title=High Performance Crystal Radios |access-date=2016-09-27}}
* http://www.crystal-radio.eu/endiodes.htm ''Diodes''
* http://www.crystal-radio.eu/engev.htm ''How to build a sensitive crystal receiver?''
* http://uv201.com/Radio_Pages/Pre-1921/crystal_detectors.htm ''Crystal Detectors''
* http://www.sparkmuseum.com/DETECTOR.HTM ''Radio Detectors''
* [https://jademann4.wixsite.com/crystalset-perfected ''The Crystal Set Perfected'']

{{Telecommunications}}

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[[Category:History of radio technology]]
[[Category:Radio electronics]]
[[Category:Types of radios]]
[[Category:Receiver (radio)]]