Product Code Database
A crystal radio receiver, also called a crystal set, is a simple radio receiver, popular in the early days of radio. It uses only the power of the received radio wave to produce sound, needing no external power supply. It is named for its most important component, a crystal detector, originally made from a piece of crystalline mineral such as galena. This component is now called a diode.
Crystal radios are the simplest type of radio receiver and can be made with a few inexpensive parts, such as a wire for an antenna, a inductor of wire, a capacitor, a crystal detector, and crystal earpiece. However they are passive receivers, while other radios use an amplifier powered by current from a 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.
The Rectifier property of a contact between a mineral and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as a detector of radio waves in 1894 by Jagadish Chandra Bose,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 in his microwave optics experiments. They were first used as a demodulator for radio communication reception in 1902 by G. W. Pickard. archived: part1, part2, part3, part4 Crystal radios were the first widely used type of radio receiver, and the main type used during the wireless telegraphy era.crystal detectors were used in receivers in greater numbers than any other type of detector after about 1907. 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.
Around 1920, crystal sets were superseded by the first amplifying receivers, which used . With this technological advance, crystal sets became obsolete for commercial use but continued to be built by hobbyists, youth groups, and the Boy Scouts 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.Jack Bryant (2009) Birmingham Crystal Radio Group, Birmingham, Alabama, US. Retrieved 2010-01-18. The Xtal Set Society midnightscience.com . Retrieved 2010-01-18.Darryl Boyd (2006) Stay Tuned Crystal Radio website. Retrieved 2010-01-18.Al Klase Crystal Radios, Klase's SkyWaves website . Retrieved 2010-01-18.Mike Tuggle (2003) Designing a DX crystal set Antique Wireless Association journal. Retrieved 2010-01-18.
Crystal radios receive amplitude modulated (AM) signals, although FM designs have been built. They can be designed to receive almost any radio frequency band, but most receive the medium wave band. A few receive shortwave radio bands, but strong signals are required. The first crystal sets received wireless telegraphy signals broadcast by spark-gap transmitters at frequencies as low as 20 kHz.Long distance transoceanic stations of the era used wavelengths of 10,000 to 20,000 meters, corresponding to frequencies of 15 to 30 kHz.
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. The power available to a receiving antenna decreases with the square of its distance from the radio transmitter. Even for a powerful commercial Radio station, if it is more than a few miles from the receiver the power received by the antenna is very small, typically measured in or . In modern crystal sets, signals as weak as 50 at the antenna can be heard. Crystal radios can receive such weak signals without using Amplifier only due to the great sensitivity of human hearing, which can detect sounds with an intensity of only 10−16 W/cm2. 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 stations, although the radiotelegraphy signals used during the wireless telegraphy era could be received at hundreds of miles, and crystal receivers were even used for transoceanic communication during that period.Marconi used carborundum detectors for a time around 1907 in his first commercial transatlantic wireless link between Newfoundland, Canada and Clifton, Ireland.
Serious crystal radio hobbyists use "inverted L" and T-aerial, 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. 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 , , and barbed wire fences as antennas.
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. 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. 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 magnetic permeability (this eliminated the less reliable mechanical contact). on Crystal Radios and Plans, Stay Tuned website
The antenna is an integral part of the tuned circuit and its 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. Many early crystal sets did not have a tuning capacitor, and relied instead on the capacitance inherent in the wire antenna (in addition to significant parasitic capacitance in the coil) 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. 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 (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.
The crystal detector worsened the problem, because it has relatively low 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. 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. This reduced the impedance loading of the tuned circuit, as well as improving the impedance match with the detector.
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.Alley & Atwood (1973) Electronic Engineering, p. 318 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. The smaller coil was mounted on a 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 transformer, that allowed a better match of the antenna impedance to the rest of the circuit. One or both of the coils usually had several taps which could be selected with a switch, 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.
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. 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. Alternatively, some radios (circuit, right) used a battery-powered buzzer attached to the input circuit to adjust the detector. 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, but various other types of crystals were also used, the most common being iron pyrite (fool's gold, FeS2), silicon, molybdenite (MoS2), silicon carbide (carborundum, SiC), and a zincite-bornite (ZnO-Cu5FeS4) crystal-to-crystal junction trade-named Perikon. Crystal radios have also been improvised from a variety of common objects, such as blue steel razor blades and pencil, rusty needles, and pennies In these, a semiconductor layer of oxide or sulfide on the metal surface is usually responsible for the rectifying action.
In modern sets, a semiconductor diode is used for the detector, which is much more reliable than a crystal detector and requires no adjustments. Germanium diodes (or sometimes ) are used instead of silicon diodes, because their lower forward voltage drop (roughly 0.3 V compared to 0.6 V) makes them more sensitive.
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 forward bias voltage was applied across the detector by a battery and potentiometer." The sensitivity of the Perikon detector can be approximately doubled by connecting a battery across its terminals to give approximately 0.2 volt" " 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..." 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.
The early earphones used with wireless-era crystal sets had moving iron drivers that worked in a way similar to the horn of the period. Each earpiece contained a permanent magnet about which was a coil of wire which formed a second electromagnet. 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 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 Ω.
Modern crystal sets use Piezoelectricity , which are much more sensitive and also smaller. 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 of the receiver. The piezoelectric earphone's higher resistance, in parallel with its capacitance of around 9 pF, creates a filter that allows the passage of low frequencies, but blocks the higher frequencies. In that case a bypass capacitor is not needed (although in practice a small one of around 0.68 to 1 nF is often used to help improve quality), but instead a 10–100 kΩ resistor must be added in parallel with the earphone's input.
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.Walter B. Ford, " High Power Crystal Set", August 1960, Popular Electronics Similarly, modern low-impedance (8 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.
Therefore, the first 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" which were translated back to text by an operator who knew Morse code. The device which detected the radio signal was called a " detector". Since there were no amplifier 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.
The coherer was a very poor detector, motivating much research to find better detectors. 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. Researchers investigating the effect of radio waves on various types of "imperfect" contacts to develop better coherers, invented crystal detectors.
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. Some method was needed to allow the receiver to select which transmitter's signal to receive. In 1892, William Crookes gave an influential lecture on radio in which he suggested using resonance, then called syntony, to reduce the bandwidth of transmitters and receivers. Different transmitters could then be "tuned" to transmit on different frequencies so they did not interfere. 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.
Between 1897 and 1900 the advantages of tuned systems, also called "syntonic" systems, became clear, and wireless researchers incorporated , consisting of and connected together, into their transmitters and receivers. The resonant circuit acted like an electrical analog of a tuning fork. It had a high 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.
Oliver Lodge, who had been researching resonance for years archived patented the first tuned or "syntonic" transmitter and receiver on 10 May 1897British patent GB189711575 Lodge, O. J. Improvements in Syntonized Telegraphy without Line Wires filed: May 10, 1897, granted: August 10, 1898 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. In 1911 when the patent was renewed the Marconi Company was forced to buy it to protect its own syntonic system against infringement suits.
The solution which multiple researchers found was to use two resonant circuits in the transmitter and receiver, in the form of a double-tuned inductively-coupled circuit, or resonant transformer (oscillation transformer). 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. 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; 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. 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.
The first person to use resonant circuits in a radio application was Nikola Tesla, who invented the resonant transformer in 1891." 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" 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. 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. This system, patented by Tesla 2 September 1897,US Patent No. 645576, Nikola Tesla, System of transmission of electrical energy, filed: 2 September 1897; granted: 20 March 1900 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). However, Tesla was interested in wireless power and never developed a practical radio communication system. Other researchers applied the circuit to radio: inductively coupled radio systems were patented by Oliver Lodge in February 1898,US Patent no. 609,154 Oliver Joseph Lodge, Electric Telegraphy, filed: 1 February 1898, granted: 16 August 1898 Karl Ferdinand Braun in November 1899,British patent no. 189922020 Karl Ferdinand Braun, 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 and John Stone Stone in February 1900.US Patent no. 714,756, John Stone Stone Method of electric signaling, filed: 8 February 1900, granted: 2 December 1902
Marconi initially paid little attention to syntony, 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" patentBritish patent no. 7777, Guglielmo Marconi, Improvements in apparatus for wireless telegraphy, filed: 26 April 1900, granted: 13 April 1901. Corresponding US Patent no. 763,772, Guglielmo Marconi, Apparatus for wireless telegraphy, filed: 10 November 1900, granted: 28 June 1904. claimed rights to the inductively coupled "four circuit" transmitter and receiver. 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. This patent gave Marconi a near monopoly of syntonic wireless telegraphy in England and America. Morse (1925) Radio: Beam and Broadcast, p. 30 Tesla sued Marconi's company for patent infringement but didn't have the resources to pursue the action.
Ferdinand Braun discovered the importance 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 on grounds of the prior patents of Tesla, Lodge, and Stone, but the decision did not specify who had rights to the four circuit wireless system. This came long after spark transmitters had become obsolete.
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. 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 has argued that this detector functioned by the semiconductor's change in resistance with temperature, as a bolometer, not a rectifying detector. 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. Bose's semiconductor galena detector is considered the forerunner of the semiconductor diode, He patented the detector 30 September 1901 Jagadis Chunder Bose, Detector for Electrical Disturbances, filed: 30 September 1901, granted 29 March 1904 and this is often considered the first patent on a semiconductor device.
The generation of an audio signal without a DC bias battery made Pickard realize the device was acting as a rectifier. Pickard began to experiment and found an oxidized steel surface worked better, so he tried magnetite (Fe3O4). On 16 October 1902 he received a radio station using magnetite touched by a copper wire, the first crystal detector.
During the next seven years, Pickard conducted an exhaustive search to find which substances formed the most sensitive detecting contacts, eventually testing thousands of minerals, and discovered about 250 rectifying crystals. In 1906 he obtained a sample of fused silicon, an artificial product recently synthesized in electric furnaces, and it outperformed all other substances. He patented the silicon detector 30 August 1906. Greenleaf Whittier Pickard, Means for Receiving Intelligence Communicated by Electric Waves, filed: 30 August 1906, granted: 20 November 1906 In 1907 he formed a company to manufacture his detectors, Wireless Specialty Products Co., and the silicon detector was the first crystal detector to be sold commercially. 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 in 1908, Greenleaf Whittier Pickard, Oscillation receiver, filed: 15 September 1908, granted: 16 February 1909 which stood for " PERfect p Ic Kard c ONtact".
On 23 March 1906 Henry Harrison Chase Dunwoody, a retired general in the U.S. Army Signal Corps, patented the silicon carbide (carborundum) detector, Henry H. C. Dunwoody, Wireless Telegraph System, filed: 23 March 1906, granted: 4 December 1906 using another recent product of electric furnaces. The semiconductor silicon carbide has a wide band gap and Pickard discovered the detector's sensitivity could be increased by applying a forward bias, a DC potential of around a volt from a battery and potentiometer, to it.
Braun around 1899 began to experiment with crystals as radio detectors and in 1906 patented a galena cat whisker detector in Germany. German patent 178871 Karl Ferdinand Braun, Wellenempfindliche Kontaktstel, filed: 18 February 1906, granted: 22 October 1906 In 1906 L. W. Austin invented a silicon–tellurium detector, Louis W. Austin, "Receiver", filed: 27 October 1906, granted: 5 March 1907 in 1907 Pickard invented the molybdenite detector, Greenleaf Whittier Pickard, "Oscillation-detecting means for receiving intelligence communicated by electric waves", filed: 11 March 1907, granted: 17 November 1908 and in 1911 Thompson H. Lyon invented the cerussite detector. Thompson Harris Lyon, "Wave-detector", filed: 12 October 1911, granted: 14 November 1911 In 1908 Wichi Torikata at Tokyo Imperial University investigated 200 minerals and found cassiterite (tin oxide), pyrolusite (manganese dioxide), zincite, galena, and pyrite were sensitive, and subsequently tested all the mineral samples at the Mineral College and found 34 rectifying minerals.
Around 1907 crystal detectors replaced the coherer and electrolytic detector in receivers to become the most widely used form of radio detector.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 are practically obsolete, and comparatively few of the magnetic and Audion or valve triode detectors have been installed." Until the triode vacuum tube began to be used in World War I, crystals were the best radio reception technology, used in cutting-edge receivers in wireless telegraphy stations, as well as in homemade crystal radios.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."
Wireless telegraphy companies such as Marconi and Telefunken manufactured sophisticated inductively coupled crystal radios as communication receivers in ship radio rooms and shore stations. Rugged military versions were made for naval warships and military communication stations., p. 5-6, 30 Lightweight crystal radios such as the SCR-54 were part of portable radiotelegraphy stations carried by 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 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.
Galena (lead sulfide, PbS, sometimes sold under the names "Lenzite" and "Hertzite"), was the most widely used crystal detector since it was the most sensitive. Other common crystalline minerals used were iron pyrite (iron sulfide, FeS2, "fool's gold", also sold under the trade names "Pyron", "Ferron" and "Radiocite"), molybdenite (molybdenum disulfide, MoS2), and cerussite (lead carbonate, PbCO3). 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 before each use, the tip of the wire dragged across the surface of the crystal while the user listened for radio noise in the earphones, to find an active rectifying site. Another widely used type was Pickard's "Perikon" detector, consisting of zincite and chalcopyrite crystals touching.
Much research went into finding better detectors and many types of crystals were tried. The goal of researchers was to find rectifying crystals that were less fragile and sensitive to vibration than galena and the other cat-whisker detectors above. 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. Carborundum proved to be the best of these; it could rectify with a steel point pressed firmly against it with a spring, or even clamped between two flat contacts, 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. 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: the arc converter (Poulsen arc) and the Alexanderson alternator. 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). 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.
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.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 are practically obsolete... but the use of Audions...is increasing." However the popularity and sales of crystal radios continued to increase for a few years due to the sudden rise of radio broadcasting. After World War I, began experimenting with transmitting sound, voice and music, by amplitude modulation (AM), and a growing community of radio listeners built or bought crystal radios to listen to them. In 1922 the United States Bureau of Standards (now NIST), responding to consumer interest, released a publication entitled Construction and Operation of a Simple Homemade Radio Receiving Outfit., archived on archive.org website This article (see drawing) 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.
One solution was the "intensifier"; such as the version invented by S. G. Brown Co. and used by the Marconi Co. 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 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.
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.
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 , , and amplifying and regenerative , 25 years before the invention of the transistor.
and [https://books.google.com/books?id=2rQ1AQAAIAAJ&pg=PA294 "The Crystodyne Principle"], ''Radio News'', September 1924, pages 294-295, 431.
However his achievements were overlooked because of the success of vacuum tubes. His technology was dubbed "Crystodyne" by science publisher [[Hugo Gernsback]] 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.
So during the 1920s vacuum tube receivers replaced crystal radios in all except poor households.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" The temperamental, unreliable action of the crystal detector had always been a barrier to its acceptance as a standard component in commercial radio equipment and was one reason for its rapid replacement. Frederick Seitz, an early semiconductor researcher, wrote:
The crystal radio became a cheap alternative receiver used for emergency communication and by people who could not afford tube radios: teenagers, the poor, and those in developing countries. Building a crystal set remained a popular educational project to introduce people to radio, used by organizations like the Scouting. The galena detector, the most widely used type among amateurs, became virtually the only detector used in crystal radios from this point on. Crystal radios were kept as emergency backup radios on ships. During World War II in Nazi-occupied Europe the radio saw use as an easily constructed, easily concealed clandestine radio by Resistance groups.
In World War II, when Allied troops were halted near Anzio 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. 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. One type used a blue steel razor blade and a pencil lead for a detector. 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 "" by the popular press, and they became part of the folklore of World War II.
In some German-occupied countries during 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.
The transistor radio introduced in 1954 took over the crystal radio's market niche of a cheap portable radio. So during the 1950s to 1970s the only remaining market for crystal radios was as a scientific educational novelty toy for children.
In the late 1950s, a variety of cheap novelty crystal radios in plastic cases, typically imported from Japan, were sold as educational toys. One was the "rocket radio", shaped like a rocket (see picture) For a detector they used a sealed germanium diode which did not need adjustment like the cat's whisker detector. As an audio output device they used a piezoelectric crystal earpiece, which was far more efficient than dynamic earphones, and also did not load the tuned circuit, reducing the Q factor of the tuned circuit and thus the selectivity of the receiver, as dynamic earphones did. For a tuning coil they used a ferrite core loop antenna, which was more compact than the previous air core coils, also functioned as an antenna, and eliminated the need for a ground connection. The radio is tuned to different stations by moving the ferrite core in and out of the coil, changing the magnetic permeability and thus the inductance of the coil. The reception range of these simple radios was limited to strong local AM radio stations within 15–25 miles. They usually had an alligator clip which could be clipped to an external wire antenna, to increase the range.
The Scouting have continued to include the educational construction of a crystal radio in their program since the 1920s.
Recently, communities of have started building classically designed long-distance crystal receivers similar to those from the radiotelegraphy era. Much effort goes into the visual appearance of these sets as well as their performance. Annual crystal radio DXing (long distance reception) and building competition allow these set owners to compete with each other.
|
|
