Crystal radio

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"Crystal Set" redirects here. For the Australian rock band, see The Crystal Set.
A modern crystal set

A crystal radio receiver, also called a crystal set, is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source and runs on the power received from radio waves by a long wire antenna. It gets its name from its most important component, known as a crystal detector, originally made with a piece of crystalline mineral such as galena. This component is now called a diode.

Crystal radios are the simplest type of radio receiver,[1] and can be handmade with a few inexpensive parts, like an antenna wire, tuning coil of copper wire, crystal detector and earphones.[2] They are technically distinct in many respects from ordinary radios because they are passive receivers, while other radios use a separate source of electric power such as a battery or the mains power to amplify (magnify) the weak radio signal from the antenna so it is louder. Thus crystal sets produce rather weak sound and must be listened to with earphones, and can only pick up stations within a limited range.[3] Developed in experiments with crystals from about 1895 to 1906 by pioneer radio researchers Karl Ferdinand Braun[4][5][6][7], Jagadish Chandra Bose[8][9] and others, crystal radios were the first widely used type of radio receiver,[10] and the main type used during the wireless telegraphy era.[11] At the end of that era, around 1920, they were superseded by the first amplifying receivers, which used vacuum tubes (Audions), and became obsolete for commercial use.[10] However they continued to be built by hobbyists and youth groups such as the Boy Scouts[12] as a way of learning about the technology of radio. Today they are still sold as educational devices, and there are groups of enthusiasts devoted to their construction[13][14][15][16][17][18] who hold competitions comparing the performance of their home-built designs.[19][20]

Crystal radios can be designed to receive almost any radio frequency band, but most receive the AM broadcast band.[21] A few receive the 49-meter international short wave band, partly because the radio waves are stronger in those bands. The first crystal sets received wireless telegraphy signals broadcast by spark-gap transmitters at frequencies as low as 25 kHz.[22] Crystal radios are designed to receive AM signals, although they can be made to detect FM signals by a technique called slope detection.[1]


Contents

[edit] How it works

A crystal radio can be thought of as a radio receiver reduced to its essentials.[23][2] It consists at a minimum of these components:[21][24][25]

  • An antenna to pick up the radio waves and convert them to electric currents.
  • A tuned circuit to select the signal of the radio station to be received, out of all the signals received by the antenna. This consists of an inductor or tuning coil and a capacitor connected together, one of which is adjustable and used to tune in different stations. The tuned circuit has a natural resonant frequency, and allows radio signals at this frequency to pass while rejecting signals at all other frequencies. The Special Type of Capacitor used for tuning typically is a Variable Capacitor.
  • A semiconductor crystal detector which extracts the audio signal (modulation) from the radio frequency carrier wave. It does this by only allowing current to pass through it in one direction, blocking half of the oscillations of the radio wave. This rectifies the alternating current radio wave to a pulsing direct current, whose strength varies with the audio signal. This current can be converted to sound by the earphone. It was this component that gave crystal sets their name.
  • An earphone to convert the audio signal to sound waves so they can be heard. The low power produced by crystal radios is insufficient to power a loudspeaker so earphones are used.

The sound power produced by the earphone of a crystal set comes solely from the radio station being received, via the radio waves picked up by the antenna.[2] The power picked up by a receiving antenna decreases with the square of its distance from the radio transmitter.[26] Even for a powerful commercial 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 microwatts or nanowatts.[2] Early crystal sets could receive signals as weak as 2.5 nanowatts at the antenna.[27] Crystal radios can receive such weak signals without using amplification only due to the great sensitivity of human hearing,[2][28] which can detect sounds with an energy of only 10-16 W/cm2.[29] Therefore crystal receivers have to be designed to convert the energy from the radio waves into sound as efficiently as possible. Even so, they are usually only able to receive nearby stations, within distances of about a hundred miles for the telegraphy stations of the wireless era and about 25 miles for AM broadcast stations.[30][31]

[edit] History

Crystal radio was invented by a long, partly obscure chain of discoveries in the late 1800s that gradually evolved into more and more practical radio receivers in the early 1900s; and constitutes the origin of the field of electronics. The earliest practical use of crystal radio was to receive Morse code radio signals transmitted by early amateur radio experimenters using very powerful spark-gap transmitters. As electronics evolved, the ability to send voice signals by radio caused a technological explosion in the years around 1920 that evolved into today's radio broadcasting industry.

[edit] Early years

Early radio telegraphy used spark gap and arc transmitters as well as high-frequency alternators running at radio frequencies. At first a primitive detector called a Branley Coherer was used to indicate the presence (or absence) of a radio signal. However, these lacked the sensitivity to convert weak signals.

Greenleaf Whittier Pickard's U.S. Patent 836,531 "Means for receiving intelligence communicated by electric waves" diagram.

In the early 1900s, various researchers discovered that certain metallic minerals, such as galena, could be used to detect radio signals. In 1901, Sir Jagadish Chandra Bose filed for a US patent for "A Device for Detecting Electrical Disturbances" that mentioned the use of a galena crystal; this was granted in 1904, #755840. However, his work, and the patent, went somewhat unnoticed in the western scientific world, as on August 30, 1906, Greenleaf Whittier Pickard filed a patent for a silicon crystal detector, which was granted on November 20, 1906. Pickard's detector was revolutionary in that he found that a fine pointed wire known as a "cat's whisker", in delicate contact with a mineral produced the best semiconductor effect. A crystal detector includes a crystal, a special thin wire that contacts the crystal and the stand that holds the components in place. The most common crystal used is a small piece of galena; pyrite was also often used, as it was a more easily adjusted and stable mineral, and quite sufficient for urban signal strengths. Several other minerals also performed well as detectors. Another benefit of crystals was that they could demodulate amplitude modulated signals. This mode was used in radiotelephones and to broadcast voice and music for a public audience. Crystal sets represented an inexpensive and technologically simple method of receiving these signals at a time when the embryonic radio broadcasting industry was beginning to grow.

NBS Circular 120 Home Crystal Radio Project.

In 1922 the (then named) U.S. Bureau of Standards released a publication entitled, Construction and Operation of a Simple Homemade Radio Receiving Outfit[1]. This article showed how almost any family having a member handy with simple tools could make a radio and tune in to weather, crop prices, time, news and the opera. This design was significant in bringing radio to the general public. NBS followed that with more selective two-circuit version Construction and Operation of a Two-Circuit Radio Receiving Equipment With Crystal Detector that was published the same year [2]and is still frequently built by enthusiasts today.

[edit] 1920s and 1930s

Sophisticated crystal receiver from around 1914

In the beginning of the 20th century, radios were only for some people who considered it as a hobby. Radios were not accessible to the public, so they built their own radios by themselves with wires wrapped around baseball bats, boxes to form receivers, the transmitters were made from glass and iron, and the speakers they built were from newspapers wrapped in a cone shape.[32]

Still, some historians consider the Autumn of 1920 to be the beginning of radio broadcasting for entertainment purposes. Pittsburgh, PA, station KDKA, owned by Westinghouse, received its license from the United States Department of Commerce just in time to broadcast the Harding-Cox presidential election returns. In addition to reporting on special events, broadcasts to farmers of crop price reports were an important public service, in the early days of radio.

In 1921, factory-made radios were very expensive. When compared to the dollar value of today, some would have cost around $2,000 USD[citation needed] . Since less affluent families could not afford to own one, newspapers and magazines carried articles on how to build a crystal radio with common household items. To minimize the cost, many of the plans suggested winding the tuning coil on empty pasteboard containers such as oatmeal boxes, which became a common foundation for homemade radios.

[edit] Non-electric amplification

  • As gas lighting and kerosene lamps were widely used before the adoption of electric power, their flame was used for sound amplification. A ceramic cone with a pinhole on its tip was inserted in the middle of the flame, and an earphone unit was attached to the cone's open bottom and sealed air-tight. This acted like a little pump, modulating the fire by periodically sucking away the combustible mixture at negative half-wave, and injecting it back on positive.
  • Air pump amplification was first used in pathephones, where a pump was driven by the same spring motor as a turntable. A needle-sized air pipe was placed near a sound membrane, which acted as an air valve and modulated the air flow, amplifying the sound. This method was easily converted for crystal radio, either in a dedicated device or just by putting a "pumped" pathephone's needle on an earpiece's membrane instead of a gramophone record.

[edit] Valveless amplifier

"Carbon amplifier" consisting of a carbon microphone and an electromagnetic earpiece sharing a common membrane and case. This was used in the telephone industry and in hearing aids nearly since the invention of both components and long before vacuum tubes. This could be readily bought or handcrafted from surplus telephone parts for use with a crystal radio. Unlike vacuum tubes, it could run with only a flashlight or car battery and had an almost infinite lifetime.

[edit] Cristadyne

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In the early 1920s Russia, devastated by civil war, the young scientist Oleg Losev was experimenting with applying voltage biases to various kinds of crystals, with purpose to refine the reception. The result was astonishing - with a zincyte (zinc oxide) crystal he gained amplification.[33][34] This was negative resistance phenomenon, decades before the tunnel diode. After the first experiments, he built regenerative and superheterodyne receivers, and even transmitters. However, this discovery was not supported by authorities and soon forgotten and no device was produced in mass quantity beyond a few examples for research.

The USSR registered all radio receivers until 1962, and typewriters and copy machines until its demise. Crystadine was produced in primitive conditions; it can be made in a rural forge - unlike vacuum tubes and modern semiconductor devices. Oleg Losev died 1943 in Leningrad.

[edit] 1940s

When Allied troops were halted near Anzio, Italy during the spring of 1944, personal portable radios were strictly prohibited, as the Germans had radio detecting equipment that could detect the local oscillator signal of superheterodyne receivers. Crystal sets lack local oscillators, so they cannot be detected in this way. Some resourceful GIs found that a crude crystal set could be made from a coil made of salvaged wire, a rusty razor blade and a pencil lead for a diode. By lightly touching the pencil lead to spots of blue on the blade, or to spots of rust, they formed what is called a point contact diode and the rectified signal could be heard on headphones or crystal ear pieces. The idea spread across the beachhead, to other parts of the war, and to popular civilian culture. The sets were dubbed "foxhole receivers" by the popular press, and they became part of the folklore of World War II.

In some Nazi occupied countries there were widespread confiscations of radio sets from the civilian population. This led to particularly determined listeners building their own "clandestine receivers" which frequently amounted to little more than a basic crystal set. However anyone doing so risked imprisonment or even death if caught and in most parts of Europe the signals from the BBC (or other allied stations) were not strong enough to be received on such a set.[citation needed] However there were places such as the Channel Islands and Netherlands where it was possible.

[edit] Later years

While it never regained the popularity and general use that it enjoyed at its beginnings, the circuit is still used. The Boy Scouts (who emerged as the unofficial custodians of crystal radio lore) kept construction of a set in their program since the 1920s. A large number of prefabricated novelty items and simple kits could be found through the 1950s and 1960s, and many children with an interest in electronics built one.

Building crystal radios was a craze in the 1920s, and again in the 1950s. Recently, hobbyists have started designing and building sophisticated examples of the instruments. Much effort goes into the visual appearance of these sets as well as their performance, and some outstanding examples can be found. Annual crystal radio DX contests and building contests allow these set owners to compete with each other and form a community of interest in the subject.

[edit] Attempts at recovering RF carrier power

A crystal radio tuned to a strong local transmitter can be used just as a power source for a second amplified (often a power-efficient regenerative) receiver for distant stations that cannot be heard with a plain crystal radio.[35]

There is a long history of less successful attempts and unverified claims to recover the power in the carrier of the received signal itself. Traditional crystal sets use half-wave rectifiers. As AM signals have a modulation factor of only 30% by voltage at peaks[citation needed], no more than 9% of received signal power (P = U2 / R) is actual audio information, and 91% is just rectified DC voltage. Given that the audio signal is unlikely to be at peak all the time, the ratio of energy is, in practice, even greater. Considerable effort was made to convert this DC voltage into sound energy. Some earlier attempts include a one-transistor[36] amplifier in 1966. Sometimes efforts to recover this power are confused with other efforts to produce a more efficient detection.[37]. This history continues now with designs as elaborate as "inverted two-wave switching power unit"[35] and bridge amplifiers[citation needed].

At least one cellular phone manufacturer is researching harvesting stray radio signals as a power source to charge their product's batteries. Prototypes are already able to absorb 5 to 10 milliwatts from the air. [38]

[edit] Construction and operation

[edit] Importance of grounding

The long wire type antennas often used with crystal radios are Monopole antennas. To receive signals from this type of antenna, a ground reference is needed to provide a place for the antenna signal electricity to flow into and out of. Because crystal radios have no other source of power than the electrical power they receive from the antenna, the grounds for crystal radios must be much better than those used by amplified radios. The ground provides a good electrical conductor to complete the circuit for the electrical signal induced by the radio wave between the antenna and ground. The importance of this is easy to overlook by those familiar with amplified radios. Amplified radios use energy (voltage) detectors and as such do not need to take much raw power from the antenna and need little or no physical ground. Crystal radios rely on power detection and need to encourage as much antenna current as possible to flow. This requires effective grounding for this to be able to happen.

[edit] The naive circuit

This 'naive circuit' is not practical for the AM Broadcast Band.

The impractical crystal radio circuit illustrated here is often naively proposed to tune the medium wave AM broadcast band with a tuner made of a fixed parallel coil and variable capacitor tank circuit with the antenna and ground connected across it. There are many practical crystal radio circuits, but connecting both the antenna and a variable capacitor across a fixed coil like this makes tuning the whole two octave AM Broadcast Band impractical.

The reason for this is that to be effective, crystal radio antennas are typically about 20 m long and 6 m high, and act something like a 250 to 300 pF capacitor. (Antennas in general have capacitance, inductance and resistance, but long-wire ones are substantially capacitive at AM radio frequencies.) If a typical 250 pF antenna is connected to the top of a tank circuit which uses a coil of more than about 75 μH, the circuit cannot be tuned much above 1400 kHz. The size of the fixed coil must be less than 75 μH to have any chance of tuning the top of the band (around 1600 kHz or 1710 kHz). Even with a 70 μH coil, a 1000 pF variable capacitor is required to tune near the bottom of the band (around 540 kHz). However, the same variable capacitor must have a minimum value of about 4 pF to tune to the top of the band. This represents a capacitance ratio of 1:250, which is very high. Stray capacitance typically imposes a lower capacitance limit, and consequently restricts the ratio in practice to about 200:1. Other kinds of variable capacitors are seldom used for crystal radios because of their excessive losses. Consequently, experienced designers avoid this circuit. However, it does work adequately for receiving at a single frequency.

The tuning range limitation can be overcome by making the coil variable instead of the capacitor, e.g. by providing a number of selectable taps along the coil winding, which makes available a fixed set of preset frequencies.

[edit] See also

Radio portal


Radio
Batteryless radio, Cat's whisker detector, Coherer, Detector (radio), Demodulator, Electrolytic detector, History of radio, Hot wire barretter, Magnetic detector, Radio receiver, Transistor radio, Wireless Telegraphy
People
Alfred Powell Morgan - Author of books on early electronics
Lists
List of obsolete technological nomenclature
Other
Wireless energy transfer, Energy efficiency, Numbers station (related to espionage)

[edit] Notes

  1. ^ a b Petruzellis, Thomas (2007). 22 Radio and Receiver Projects for the Evil Genius. US: McGraw-Hill Professional. pp. 40,45. ISBN 9780071489294. http://books.google.com/books?id=AJBBf5hCYqIC&pg=PT56. 
  2. ^ a b c d e Field, Simon Quellen (2003). Gonzo gizmos: Projects and devices to channel your inner geek. USA: Chicago Review Press. pp. 85. ISBN 9781556525209. http://books.google.com/books?id=t-N1KdTb0FwC&pg=PT97. 
  3. ^ Schaeffer, Derek K.; Thomas H. Lee (1999). The Design and Implementation of Low Power CMOS Receivers. Springer. pp. 3-4. ISBN 0792385187. http://books.google.com/books?id=4IDLK8NMDBQC&pg=PA4. 
  4. ^ Steinfuehr, Rainer (1996). "Short history of crystal receivers". Gollum's Crystal Receiver World. Rainer Steinfuehr website. http://www.oldradioworld.de/gollum/dhistor.htm. 
  5. ^ Braun discovered the rectifying abilities of crystal junctions in 1874, and invented the tuned circuit around 1900 Braun, Ernest; Stuart MacDonald (1982). Revolution in Miniature: The history and impact of semiconductor electronics, 2nd Ed.. UK: Cambridge Univ. Press. pp. 11-12. ISBN 9780521289030. http://books.google.com/books?id=03c4wldf-k4C&pg=PA11. 
  6. ^ Riordan, Michael; Lillian Hoddeson (1988). Crystal fire: the invention of the transistor and the birth of the information age. USA: W. W. Norton & Company. pp. 19-21. ISBN 0393318516. http://books.google.com/books?id=SZ6wm5ZSUmsC&pg=PA92. 
  7. ^ Sarkar, Tapan K. (2006). History of wireless. USA: John Wiley and Sons. pp. 333. ISBN 0471718149,. http://books.google.com/books?id=NBLEAA6QKYkC&pg=PA333. 
  8. ^ Bose was first to use a crystal as a radio wave detector, using galena detectors to receive microwaves starting around 1895 and receiving a patent in 1904. Emerson, D. T. (Dec. 1997). "The work of Jagadish Chandra Bose: 100 years of mm wave research". IEEE Transactions on Microwave Theory and Techniques 45 (12): 2267-2273. http://books.google.com/books?id=09Zsv97IH1MC&pg=PA88. Retrieved 2010-01-19. 
  9. ^ Sarkar (2006) History of wireless, p.94, 291-308
  10. ^ a b Basalla, George (1988). The Evolution of Technology. UK: Cambridge University Press. pp. 44. ISBN 0521296811. http://books.google.com/books?id=EBtnG36-1WIC&pg=PA44. 
  11. ^ crystal detectors were used in receivers in greater numbers than any other type of detector after about 1907. Marriott, Robert H. (Sept. 17, 1915). "United States Radio Development". Proc. of the Inst. of Radio Engineers (USA: Institute of Radio Engineers) 5 (3): 184. http://books.google.com/books?id=bh0B93CuXnkC&pg=PA184. Retrieved 2010-01-19. 
  12. ^ Kent, Herb; David Smallwood, Richard M. Daley (2009). The Cool Gent: The Nine Lives of Radio Legend Herb Kent. US: Chicago Review Press. pp. 13-14. ISBN 1556527748. http://books.google.com/books?id=f92pJ3JhaJYC&pg=PT33. 
  13. ^ Jack Bryant (2009) Birmingham Crystal Radio Group, Birmingham, Alabama, USA (retrieved 2010-01-18)
  14. ^ The Xtal Set Society midnightscience.com (retrieved 2010-01-18)
  15. ^ Darryl Boyd (2006) Stay Tuned Crystal Radio website (retrieved 2010-01-18)
  16. ^ Al Klase Crystal Radios, Klase's SkyWaves website (retrieved 2010-01-18)
  17. ^ Mike Tuggle (2003) Designing a DX crystal set Antique Wireless Association journal (retrieved 2010-01-18)
  18. ^ Rainer Steinfuehr (2009) Gollum´s Crystal Receiver World Wumpus's Old Radio World website, Berlin, Germany(retrieved 2010-01-18)
  19. ^ Elmer Memorial Crystal Radio DX Contest, sponsored by Birmingham Crystal Radio Group, Birmingham, Alabama, USA (retrieved 2010-01-18)
  20. ^ Crystal Radio Building Contest, by The Xtal Set Society midnightscience.com (retrieved 2010-01-18)
  21. ^ a b Williams, Lyle R. (2006). The New Radio Receiver Building Handbook. The Alternative Electronics Press. pp. 20-23. ISBN 184728526. http://books.google.com/books?id=XiKgKdeBi6cC&pg=PT20. 
  22. ^ Sarkar (2006) History of wireless, p.102
  23. ^ Purdie, Ian C. (2001). "Crystal Radio Set". electronics-tutorials.com. Ian Purdie. http://www.electronics-tutorials.com/receivers/crystal-radio-set.htm. Retrieved 2009-12-05. 
  24. ^ Lescarboura, Austin C. (1922). Radio for Everybody. New York: Scientific American Publishing Co.. pp. 93-94. http://books.google.com/books?id=pf0WAAAAYAAJ&pg=PA93. 
  25. ^ Kuhn, Kenneth A. (Jan. 6, 2008). "Introduction". Crystal Radio Engineering. Prof. Kenneth Kuhn website, Univ. of Alabama. http://www.kennethkuhn.com/students/crystal_radios/introduction.pdf. Retrieved 2009-12-07. 
  26. ^ Fette, Bruce A. (Dec. 27, 2008). "RF Basics: Radio Propagation". RF Engineer Network. http://www.rfengineer.net/1170/rf-basics-radio-propagation/. Retrieved 2010-01-18. 
  27. ^ wireless-era crystal receivers required an antenna signal of 10 μA through a 25 Ω antenna impedance, or 2.5 nW, to be audible; a good signal was 40 μA or 40 nW. Secor, Henry Winfield (1920). The How and Why of Radio Apparatus. New York: Experimenter Publishing Co.. pp. 60. http://books.google.com/books?id=wxZLAAAAMAAJ&pg=PA60. 
  28. ^ Lee, Thomas H. (2004). Planar Microwave Engineering: A practical guide to theory, measurement, and circuits. UK: Cambridge Univ. Press. pp. 299. ISBN 9780521835268. http://books.google.com/books?id=uoj3IWFxbVYC&pg=PA299. 
  29. ^ Nave, C. Rod. "Threshold of hearing". HyperPhysics. Dept. of Physics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/HFrame.html. Retrieved 2009-12-06. 
  30. ^ Lescarboura, 1922, p. 144
  31. ^ Binns, Jack (November 1922). "Jack Binn's 10 commandments for the radio fan". Popular Science (New York: Modern Publishing Co.) 101 (5): 42-43. http://books.google.com/books?id=pSkDAAAAMBAJ&pg=PA42. Retrieved 2010-01-18. 
  32. ^ Bondi, Victor."American Decades:1930-1939"
  33. ^ http://www.a-reny.com/iexplorer/cristadyne.html
  34. ^ http://earlyradiohistory.us/1924cry.htm
  35. ^ a b Polyakov V.T. "Simple receivers for AM signals", ISBN 5-94074-056-1 (in Russian)
  36. ^ Radio-Electronics, 1966, №2
  37. ^ QST [Amateur Radio Magazine] January 2007, "High Sensitivity Crystal Set" (http://www.arrl.org/qst/2007/01/culter.pdf )
  38. ^ Christopher Null. "Prototype Nokia phone recharges without wires". Yahoo! Canada Co.. http://ca.tech.yahoo.com/blogs/the_working_guy/rss/article/3638. Retrieved June 20, 2009. 

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