Crystal radio receiver
Encyclopedia
A crystal radio receiver, also called a crystal set or cat's whisker receiver, is a very simple radio receiver, popular in the early days of radio. It needs no battery
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 It gets its name from its most important component, known as a crystal detector, originally made with a piece of crystalline mineral such as galena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
.
of copper wire, crystal detector and earphones
. They are distinct 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
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.
The rectifying property of crystals was discovered in 1874 by Karl Ferdinand Braun
, and crystal detectors were developed and applied to radio receivers in 1904 by Jagadish Chandra Bose, G. W. Pickard
and others. Crystal radios were the first widely used type of radio receiver, and the main type used during the wireless telegraphy
era. Sold and homemade by the millions, the inexpensive and reliable crystal radio was a major driving force in the introduction of radio to the public, contributing to the development of radio as an entertainment medium around 1920.
After about 1920, crystal sets were superseded by the first amplifying receivers, which used vacuum tube
s (Audion
s), and became obsolete for commercial use. However they continued to be built by hobbyists, youth groups and the Boy Scouts
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 who hold competitions comparing the performance of their home-built designs.
Crystal radios can be designed to receive almost any radio frequency
band, but most receive the AM broadcast band. A few receive the 49-meter
international shortwave
band, 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 kHz.
A crystal radio can be thought of as a radio receiver reduced to its essentials. It consists at a minimum of these components:
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. The power picked up by a receiving antenna decreases with the square of its distance from the radio transmitter. 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. In modern crystal sets, signals as weak as 50 picowatts at the antenna can be heard. Crystal radios can receive such weak signals without using amplification
only due to the great sensitivity of human hearing
, which can detect sounds with a power of only 10−16 W/cm2. Therefore crystal receivers have to be designed to convert the energy from the radio waves into sound as efficiently as possible. Even so, they are usually only able to receive nearby stations, within distances of about 25 miles for AM broadcast
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.
Passive receiver development was abandoned with the advent of reliable vacuum tubes around 1920, and subsequent crystal radio research was the work of radio amateurs and hobbyists. Many different circuits have been used. The following sections discuss the parts of a crystal radio in greater detail.
s striking it to an alternating
electric current
in the antenna, which is connected to the tuning coil. Since in a crystal radio all the power comes from the antenna, it is important that the antenna collect as much power from the radio wave as possible. The larger an antenna, the more power it can intercept. In addition, antennas are most efficient when their length is close to a multiple of a quarter-wavelength
of the radio waves they are receiving. Since the length of the waves used with crystal radios is very long (AM broadcast band waves are 182-566 m or 597–1857 ft. long) the antenna is made as long as possible, out of a long wire
, in contrast to the whip antenna
s or ferrite loopstick antennas
used in modern radios.
Serious crystal radio hobbyists use "inverted L" and "T" type antennas, consisting of hundreds of feet of wire suspended as high as possible between buildings or trees, with a feed wire attached in the center or at one end leading down to the receiver. 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 bedsprings, fire escape
s, and barbed wire
fences as antennas.
s which develop their output voltage with respect to ground. They require a return circuit connected to ground
(earth) so that the current from the antenna, after passing through the receiver, can flow into the ground. The ground wire is attached to a radiator, a water pipe, or a metal stake driven into the ground. A good ground is more important for crystal sets than for powered receivers, because crystal sets have low input impedance
to transfer power efficiently from the antenna, so significant current flows in the antenna/ground circuit. A low resistance ground connection (preferably below 25 Ω) is necessary because any resistance in the ground dissipates power from the antenna. In contrast, modern receivers are voltage-operated devices, with high input impedance, so little current flows in the antenna/ground circuit. Also, mains powered receivers are grounded adequately through their power cords.
, analogous to a tuning fork
for sound waves. Electric charge flows rapidly back and forth between the plates of the capacitor through the coil, oscillating at the frequency of the radio signal. It has a high impedance
at the desired radio signal's frequency, but a low impedance at all other frequencies, so the desired signal is passed on to the detector which is connected across the tuned circuit, while the other signals are short-circuited to ground. The frequency of the station received is the resonant frequency f of the tuned circuit, determined by the capacitance
C of the capacitor and the inductance
L of the coil:
In inexpensive sets the inductor had a sliding spring contact that pressed against the windings and could be slid up and down the coil, to allow a larger or smaller number of turns of the coil into the circuit, varying the inductance
, to tune in different stations. Alternatively, a variable capacitor
is used to tune the radio instead of the coil. Some modern crystal sets use a ferrite core
tuning coil, in which the core is mounted on a threaded shaft and a knob turns the shaft, moving the core in and out of the coil, varying the inductance by changing the magnetic permeability
.
The antenna is itself a resonant circuit and its frequency can be shifted (tuned) with the reactance of inductors and/or capacitors in the radio. For the broadcast band, the antenna usually acts as a capacitor because antennas shorter than a quarter-wavelength have capacitive reactance to it can be tuned with a variable inductor so many early crystal sets did not use a tuning capacitor. They relied instead on the capacitance inherent in the wire antenna (in addition to significant parasitic capacitance
in the coil itself) to form the tuned circuit with the coil.
Some of the earliest crystal receivers (fig. 2) depended entirely on antenna resonance for tuning, and just consisted of a crystal detector D1 connected between the antenna and ground, with an earphone E1 across it. When transmitting stations were rare, there was little need for frequency-selective elements besides the broad resonance
of the antenna, so any station within a wide band of frequencies were heard in the earphone. It was used in the earliest days of radio, transmitting stations were separated by large distances and wide frequency differences.
. The maximum power is transferred from one part of a circuit to another when the impedance
(resistance
) of the two circuits is equal. However in crystal sets, the impedance of the antenna-ground system (around 10-200 ohm
s) is usually lower than the impedance of the receiver's tuned circuit (thousands of ohms at resonance), and also varies depending on the quality of the ground, length of the antenna, and what frequency the receiver is tuned to in the band. Therefore in better receiver circuits (fig. 4), to match the antenna impedance to the receiver's impedance, the antenna was connected across only a portion of the tuning coil's turns. This made the coil L1 act as an impedance matching transformer
(in an autotransformer
connection) in addition to its tuning function. The tuned circuit's high impedance was transformed down by a factor equal to the square root of the turns ratio (the number of turns the antenna was connected across, to the total number of turns of the coil), to match the antenna impedance. In the "two-slider" circuit (fig. 3), popular during the wireless era, both the antenna and the detector circuit were attached to the coil with sliding contacts, allowing (interactive) adjustment of both the resonant frequency and the turns ratio. Alternatively a multiposition switch ( S1, fig. 4) was used to select taps on the coil. These controls were adjusted until the station sounded loudest in the earphone.
The other place impedance matching was often used was between the tuning coil and the crystal detector/earphone circuit, to match the impedance of the detector. To accomplish this the detector D1 , like the antenna, was connected to a tap on the coil. This also improved the receiver's selectivity (see below).
to the desired station; they have low selectivity. Often two or more stations are heard simultaneously. This is because the simple tuned circuit doesn't reject nearby signals well; it allows through a wide band of frequencies, that is, it has a large bandwidth (low Q factor
) compared to modern receivers. This was a worse problem during the wireless
era because the spark-gap transmitter
s of the era produced much wider bandwidth signals than modern transmitters, that spread interference over the frequencies of other stations. The tuned circuit had wide bandwidth because the crystal detector connected across it had relatively low resistance
which "loaded" the tuned circuit, damping the oscillations, reducing its Q
. In many circuits such as fig. 4 and 5, 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 (see above).
transformer
(L1, L2) which improved the selectivity by a technique called loose coupling. This consisted of two magnetically coupled coils of wire, one (the primary, L1) attached to the antenna and ground and the other (the secondary, L2) attached to the rest of the circuit. The current from the antenna created an alternating magnetic field in the primary coil, which induced a voltage in the secondary coil which was rectified and powered the earphone. Each of the coils functioned as a tuned circuit that was tuned to the frequency of the station: the primary coil resonated with the capacitance of the antenna (or sometimes another capacitor, C1), and the secondary resonated with the tuning capacitor C2. The two circuits interacted to form a resonant transformer. Reducing the coupling between the coils, by physically separating them so less of the magnetic field
of one intersects the other (reducing the mutual inductance), narrows the bandwidth, resulting in much sharper, more selective tuning than a single tuned circuit. However this involved a tradeoff; looser coupling also reduced the amount of signal getting through the transformer. So the transformer was made with adjustable coupling. One type common in early days, called a "loose coupler", consisted of a smaller coil inside a larger coil. The smaller coil was mounted on a rack so it could be slid linearly in or out of the larger coil. If interference was encountered, the smaller coil would be slid further out of the larger, loosening the coupling and narrowing the bandwidth, to reject the interfering signal.
The antenna coupling transformer also functioned as an impedance matching transformer
, to match the antenna impedance to the rest of the circuit. One or both of the coils usually had several taps which could be selected with a switch (S1), to adjust the turns ratio.
Coupling transformers were difficult to adjust, because the three adjustments, the tuning of the primary circuit, the tuning of the secondary circuit, and the coupling, were all interactive, and changing one affected the others.
mineral
. This formed a crude unstable semiconductor diode (Schottky diode
), which allowed current to flow better in one direction than in the opposite direction. Modern crystal sets use modern semiconductor diodes. The detector rectified
the alternating current
radio signal to a pulsing direct current
, which had the audio modulation
signal impressed on it, so it could be converted to sound by the earphone, which was connected in series (or sometimes in parallel) with the detector.
The rectified current from the detector still had radio frequency
pulses from the carrier in it, which for some wavelengths did not pass well through the high inductance
of the earphones. In those cases, a small capacitor
, called a blocking or bypass capacitor (C2, fig. 4 and C3, fig. 5), was usually placed across the earphone terminals to bypass these pulses around the earphone to ground, although the earphone cord usually had enough capacitance that this component could be omitted.
In a cat's whisker detector only certain sites on the crystal surface functioned as rectifying junctions, and the device was very sensitive to the pressure of the crystal-wire contact, which could be disrupted by the slightest vibration. 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. An alternative adjustment method was to use a battery-powered buzzer
(BZ, fig. 7) attached to the ground wire to provide a test signal. The spark at the buzzer's electrical contacts served as a weak radio transmitter, so when the detector began working the buzz could be heard in the earphones, and the buzzer was then turned off.
Galena
(lead sulfide) was probably the most common crystal used in cat's whisker detectors, but various other types of crystals were also used, such as iron pyrite (Fool's gold, Fe2S), 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 made with rectifying junctions improvised from a variety of common objects, such as blue steel razor blades
and lead pencils, rusty needles, and pennies In these, a semiconducting
layer of oxide or sulfide on the metal surface is usually responsible for the rectifying action.
In modern sets a semiconductor diode is used, which is much more reliable than a cat's whisker detector and doesn't require any adjustments. Germanium diodes (or sometimes Schottky diode
s) are used instead of silicon diodes, because their lower forward voltage drop (roughly 0.3V compared to 0.6V) makes them more sensitive.
All semiconductor detectors function rather inefficiently in crystal receivers, because the low voltage signal level is too low to result in much difference between forward better conduction and reverse weaker conduction. Bias can move the diode's operating point higher on the detection curve to produce more signal voltage at the expense of less signal current (higher impedance). There is a limit to the benefit that this produces, depending on the other impedances of the radio. 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
(B1, R1, fig. 7) . This improved sensitivity by moving the DC operating point to a more desirable voltage-current operating point (impedance) on the junction's I-V curve.
reproduction of the sound. In early homebuilt sets, the earphones were the most costly component.
The early earphones used with wireless-era crystal sets had moving iron drivers
that worked similarly to loudspeaker
s. Each earpiece contained a magnet
wound with coils of wire to form an electromagnet
, with poles close to a steel diaphragm. When the audio signal
from the radio was passed through the electromagnet's windings, it created a varying magnetic field
that pulled on the diaphragm, causing it to vibrate. The vibrations of the diaphragm pushed and pulled on the air in front of it, creating sound waves. Standard headphones used in telephone work had a low impedance
, often 75 Ω, and required more current than a crystal radio could supply, so the type used with radios was wound with more turns of finer wire and had an impedance of 2000-8000 Ω.
Modern crystal sets use piezoelectric
crystal earpiece
s, 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 don't "load" the tuned circuit; increasing the selectivity of the receiver. However the earphone's higher resistance, in parallel with its capacitance of around 9 pF, creates a low pass filter which removes the higher audio frequencies, distorting or eliminating the sound. So a bypass capacitor is not needed (although in practice a small one of around 680pFd to .001 uFd is often used to help improve quality), and instead a 10-100 KΩ resistor must be added across the earphone's input.
Modern low impedance (8 Ω) earphones cannot be used unmodified in crystal sets because the receiver doesn't produce enough current to drive them. They are sometimes used by adding an audio transformer to match their impedance with the higher impedance of the circuit.
in the late 19th century that gradually evolved into more and more practical radio receivers in the early 20th century; and constitutes the origin of the field of electronics
. The earliest practical use of crystal radio was to receive Morse code
radio signals transmitted by early amateur radio
experimenters using very powerful spark-gap transmitter
s. As electronics evolved, the ability to send voice signals by radio caused a technological explosion in the years around 1920 that evolved into today's radio broadcasting
industry.
and arc transmitters as well as high-frequency alternators
running at radio frequencies. At first a Branley Coherer
was used to indicate the presence of a radio signal. However, these lacked the sensitivity to convert weak signals.
In the early 20th century, various researchers discovered that certain metallic mineral
s, such as galena
, could be used to detect radio signals.
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 radiotelephone
s 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.
In 1922 the (then named) U.S. Bureau of Standards released a publication entitled, Construction and Operation of a Simple Homemade Radio Receiving Outfit. 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 and is still frequently built by enthusiasts today.
In the beginning of the 20th century, radio had little commercial use and radio experimentation was a hobby for many people.
. Some historians consider the Autumn of 1920 to be the beginning of commercial 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. 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.
and an electromagnetic earpiece sharing a common membrane and case. This was used in the telephone industry and in hearing aid
s nearly since the invention of both components and long before vacuum tubes. This could be readily bought or made from surplus telephone parts for use with a crystal radio. Unlike vacuum tubes, it could run with only a flashlight or car battery and had an indefinite lifetime.
, devastated by civil war, Oleg Losev
was experimenting with applying voltage bias
es to various kinds of crystals for manufacture of radio detectors. The result was astonishing - with a zincyte (zinc oxide
) crystal he gained amplification. This was negative resistance
phenomenon, decades before the tunnel diode
. After the first experiments, he built regenerative and superheterodyne receivers, and even transmitters. However, this discovery was not supported by authorities and soon forgotten and no device was produced in mass quantity beyond a few examples for research.
Crystodyne was produced in primitive conditions; it can be made in a rural forge - unlike vacuum tube
s and modern semiconductor devices.
signal of superheterodyne receivers. Crystal sets lack local oscillators, so they cannot be
detected in this way. Some resourceful GIs found that a crude crystal set could be made from a coil made of salvaged wire, a rusty razor blade and a pencil lead for a diode. By lightly touching the pencil lead to spots of blue on the blade, or to spots of rust, they formed what is called a point contact diode
and the rectified signal could be heard on headphones or crystal ear pieces. The idea spread across the beachhead, to other parts of the war, and to popular culture. The sets were dubbed "foxhole receivers" by the popular press, and they became part of the folklore
of World War II.
In some Nazi occupied countries there were widespread confiscations of radio sets from the civilian population. This led to particularly determined listeners building their own "clandestine receivers" which frequently amounted to little more than a basic crystal set. However anyone doing so risked imprisonment or even death if caught and in most parts of Europe the signals from the BBC (or other allied stations) were not strong enough to be received on such a set. However there were places such as the Channel Islands
and Netherlands
where it was possible.
is still used. The Boy Scout
s (who emerged as the unofficial custodians of crystal radio lore) kept construction of a set in their program since the 1920s. A large number of prefabricated novelty items and simple kits could be found through the 1950s and 1960s, and many children with an interest in electronics built one.
Building crystal radios was a 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
(long distance reception) and building contest
s allow these set owners to compete with each other and form a community of interest in the subject.
There is a long history of unsuccessful attempts and unverified claims to recover the power in the carrier of the received signal itself. Traditional crystal sets use half-wave rectifier
s. As AM
signals have a modulation
factor of only 30% by voltage at peaks, no more than 9% of received signal power (
) 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
amplifier in 1966. Sometimes efforts to recover this power are confused with other efforts to produce a more efficient detection. This history continues now with designs as elaborate as "inverted two-wave switching power unit".
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
or power source and runs on the power received from radio waves
Radio waves
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies from 300 GHz to as low as 3 kHz, and corresponding wavelengths from 1 millimeter to 100 kilometers. Like all other electromagnetic waves,...
by a long wire antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
. 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
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
. This component is now called a diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
.
Overview
Crystal radios are the simplest type of radio receiver and can be handmade with a few inexpensive parts, like an antenna wire, tuning coilInductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
of copper wire, crystal detector and earphones
Crystal earpiece
A crystal earpiece, more properly called a piezoelectric earphone, is an earphone that produces sound by using a piezoelectric crystal, a material that changes its shape when electricity is applied to it...
. They are distinct from ordinary radios because they are passive receivers, while other radios use a separate source of electric power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
such as a battery
Battery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
or the mains power to amplify
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
the weak radio signal from the antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
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.
The rectifying property of crystals was discovered in 1874 by Karl Ferdinand Braun
Karl Ferdinand Braun
Karl Ferdinand Braun was a German inventor, physicist and Nobel laureate in physics. Braun contributed significantly to the development of the radio and television technology: he shared with Guglielmo Marconi the 1909 Nobel Prize in Physics.-Biography:Braun was born in Fulda, Germany, and...
, and crystal detectors were developed and applied to radio receivers in 1904 by Jagadish Chandra Bose, G. W. Pickard
Greenleaf Whittier Pickard
Greenleaf Whittier Pickard was a United States radio pioneer. Pickard was a researcher in the early days of wireless. He experimented with crystal detectors, antennas, wave propagation, and noise suppression...
and others. Crystal radios were the first widely used type of radio receiver, and the main type used during the wireless telegraphy
Wireless telegraphy
Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....
era. Sold and homemade by the millions, the inexpensive and reliable crystal radio was a major driving force in the introduction of radio to the public, contributing to the development of radio as an entertainment medium around 1920.
After about 1920, crystal sets were superseded by the first amplifying receivers, which used vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s (Audion
Audion
An Audion is a wireless signal detector device invented by Lee De Forest in 1906.Audion may also refer to:* Audion , an electronic music album by Larry Fast* Audion , a media player for Apple Macintosh created by Panic...
s), and became obsolete for commercial use. However they continued to be built by hobbyists, youth groups and the Boy Scouts
Boy Scouts of America
The Boy Scouts of America is one of the largest youth organizations in the United States, with over 4.5 million youth members in its age-related divisions...
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 who hold competitions comparing the performance of their home-built designs.
Crystal radios can be designed to receive almost any radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
band, but most receive the AM broadcast band. A few receive the 49-meter
Shortwave bands
Shortwave bands are frequency allocations for use within the shortwave radio spectrum . They are the primary medium for applications such as maritime communications, international broadcasting and worldwide amateur radio activity because they take advantage of ionospheric skip propagation to send...
international shortwave
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
band, but strong signals are required. The first crystal sets received wireless telegraphy
Wireless telegraphy
Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....
signals broadcast by spark-gap transmitter
Spark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...
s at frequencies as low as 20 kHz.
How it works
- An antennaAntenna (radio)An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
to pick up the radio waveRadio WaveRadio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s and convert them to electric currents. - A tuned circuit to select the signal of the radio stationRadio stationRadio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
to be received, out of all the signals received by the antenna. This consists of a coil of wire called an inductorInductorAn inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
or tuning coil and a capacitorCapacitorA capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
connected together, one or both of which is adjustable and can be used to tune in different stations. In some circuits a capacitor is not used, because the antenna also serves as the capacitor. The tuned circuit has a natural resonant frequency, and allows radio signals at this frequencyFrequencyFrequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
to pass while rejecting signals at all other frequencies. - A semiconductorSemiconductorA semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
crystal detectorDetector (radio)A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
which extracts the audio signalAudio signalAn audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
(modulationModulationIn electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
) from the radio frequency carrier waveCarrier waveIn telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
. It does this by only allowing current to pass through it in one direction, blocking half of the oscillations of the radio wave. This rectifiesRectifierA rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
the alternating currentAlternating currentIn alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
radio wave to a pulsing direct currentDirect currentDirect current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
, whose strength varies with the audio signal. This current can be converted to sound by the earphone. Early sets used a cat's whisker detector, a fine wire touching the surface of a pebble of crystalline mineral such as galenaGalenaGalena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
. It was this component that gave crystal sets their name. - An earphone to convert the audio signal to sound waves so they can be heard. The low power produced by crystal radios is insufficient to power an unamplified loudspeakerLoudspeakerA loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
so earphones are used.
The sound power produced by the earphone of a crystal set comes solely from the radio station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
being received, via the radio waves picked up by the antenna. The power picked up by a receiving antenna decreases with the square of its distance from the radio transmitter. Even for a powerful commercial broadcasting station
Radio station
Radio broadcasting is a one-way wireless transmission over radio waves intended to reach a wide audience. Stations can be linked in radio networks to broadcast a common radio format, either in broadcast syndication or simulcast or both...
, 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. In modern crystal sets, signals as weak as 50 picowatts at the antenna can be heard. Crystal radios can receive such weak signals without using amplification
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
only due to the great sensitivity of human hearing
Hearing (sense)
Hearing is the ability to perceive sound by detecting vibrations through an organ such as the ear. It is one of the traditional five senses...
, which can detect sounds with a power of only 10−16 W/cm2. Therefore crystal receivers have to be designed to convert the energy from the radio waves into sound as efficiently as possible. Even so, they are usually only able to receive nearby stations, within distances of about 25 miles for AM broadcast
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
stations, although the radiotelegraphy signals used during the wireless telegraphy
Wireless telegraphy
Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....
era could be received at hundreds of miles, and crystal receivers were even used for transoceanic communication during that period.
Passive receiver development was abandoned with the advent of reliable vacuum tubes around 1920, and subsequent crystal radio research was the work of radio amateurs and hobbyists. Many different circuits have been used. The following sections discuss the parts of a crystal radio in greater detail.
Antenna
The antenna converts the energy in the electromagnetic radio waveRadio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
s striking it to an alternating
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
in the antenna, which is connected to the tuning coil. Since in a crystal radio all the power comes from the antenna, it is important that the antenna collect as much power from the radio wave as possible. The larger an antenna, the more power it can intercept. In addition, antennas are most efficient when their length is close to a multiple of a quarter-wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of the radio waves they are receiving. Since the length of the waves used with crystal radios is very long (AM broadcast band waves are 182-566 m or 597–1857 ft. long) the antenna is made as long as possible, out of a long wire
Random wire antenna
A random wire antenna is a radio frequency antenna consisting of a wire whose length does not bear a relation to the wavelength of the radio waves used, but is typically chosen more for convenience. This type of antenna sometimes is called the zig-zag antenna, as it may be strung back and forth...
, in contrast to the whip antenna
Whip antenna
A whip antenna is an antenna consisting of a single straight flexible wire or rod, often mounted above some type of conducting surface called a ground plane. The bottom end of the whip is connected to the radio receiver or transmitter. They are designed to be flexible so that they won't break...
s or ferrite loopstick antennas
Loop antenna
A loop antenna is a radio antenna consisting of a loop of wire, tubing, or other electrical conductor with its ends connected to a balanced transmission line...
used in modern radios.
Serious crystal radio hobbyists use "inverted L" and "T" type antennas, consisting of hundreds of feet of wire suspended as high as possible between buildings or trees, with a feed wire attached in the center or at one end leading down to the receiver. 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 bedsprings, fire escape
Fire escape
A fire escape is a special kind of emergency exit, usually mounted to the outside of a building or occasionally inside but separate from the main areas of the building. It provides a method of escape in the event of a fire or other emergency that makes the stairwells inside a building inaccessible...
s, and barbed wire
Barbed wire
Barbed wire, also known as barb wire , is a type of fencing wire constructed with sharp edges or points arranged at intervals along the strand. It is used to construct inexpensive fences and is used atop walls surrounding secured property...
fences as antennas.
Ground
The wire antennas used with crystal receivers are monopole antennaMonopole antenna
A monopole antenna is a class of radio antenna consisting of a straight rod-shaped conductor, often mounted perpendicularly over some type of conductive surface, called a ground plane. The driving signal from the transmitter is applied, or for receiving antennas the output voltage is taken,...
s which develop their output voltage with respect to ground. They require a return circuit connected to ground
Ground (electricity)
In electrical engineering, ground or earth may be the reference point in an electrical circuit from which other voltages are measured, or a common return path for electric current, or a direct physical connection to the Earth....
(earth) so that the current from the antenna, after passing through the receiver, can flow into the ground. The ground wire is attached to a radiator, a water pipe, or a metal stake driven into the ground. A good ground is more important for crystal sets than for powered receivers, because crystal sets have low input impedance
Input impedance
The input impedance of an electrical network is the equivalent impedance "seen" by a power source connected to that network. If the source provides known voltage and current, such impedance can be calculated using Ohm's Law...
to transfer power efficiently from the antenna, so significant current flows in the antenna/ground circuit. A low resistance ground connection (preferably below 25 Ω) is necessary because any resistance in the ground dissipates power from the antenna. In contrast, modern receivers are voltage-operated devices, with high input impedance, so little current flows in the antenna/ground circuit. Also, mains powered receivers are grounded adequately through their power cords.
Tuned circuit
The tuned circuit, consisting of the coil and capacitor, acts as a resonatorResonator
A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical...
, analogous to a tuning fork
Tuning fork
A tuning fork is an acoustic resonator in the form of a two-pronged fork with the prongs formed from a U-shaped bar of elastic metal . It resonates at a specific constant pitch when set vibrating by striking it against a surface or with an object, and emits a pure musical tone after waiting a...
for sound waves. Electric charge flows rapidly back and forth between the plates of the capacitor through the coil, oscillating at the frequency of the radio signal. It has a high impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
at the desired radio signal's frequency, but a low impedance at all other frequencies, so the desired signal is passed on to the detector which is connected across the tuned circuit, while the other signals are short-circuited to ground. The frequency of the station received is the resonant frequency f of the tuned circuit, determined by the capacitance
Capacitance
In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...
C of the capacitor and the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
L of the coil:
In inexpensive sets the inductor had a sliding spring contact that pressed against the windings and could be slid up and down the coil, to allow a larger or smaller number of turns of the coil into the circuit, varying the inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
, to tune in different stations. Alternatively, a variable capacitor
Variable capacitor
A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically. Variable capacitors are often used in L/C circuits to set the resonance frequency, e.g. to tune a radio , or as a variable reactance, e.g...
is used to tune the radio instead of the coil. Some modern crystal sets use a ferrite core
Ferrite core
A ferrite core is a structure on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity .There are two broad applications for ferrite cores which...
tuning coil, in which the core is mounted on a threaded shaft and a knob turns the shaft, moving the core in and out of the coil, varying the inductance by changing the magnetic permeability
Permeability (electromagnetism)
In electromagnetism, permeability is the measure of the ability of a material to support the formation of a magnetic field within itself. In other words, it is the degree of magnetization that a material obtains in response to an applied magnetic field. Magnetic permeability is typically...
.
The antenna is itself a resonant circuit and its frequency can be shifted (tuned) with the reactance of inductors and/or capacitors in the radio. For the broadcast band, the antenna usually acts as a capacitor because antennas shorter than a quarter-wavelength have capacitive reactance to it can be tuned with a variable inductor so many early crystal sets did not use a tuning capacitor. They relied instead on the capacitance inherent in the wire antenna (in addition to significant parasitic capacitance
Parasitic capacitance
In electrical circuits, parasitic capacitance, stray capacitance or, when relevant, self-capacitance , is an unavoidable and usually unwanted capacitance that exists between the parts of an electronic component or circuit simply because of their proximity to each other...
in the coil itself) to form the tuned circuit with the coil.
Some of the earliest crystal receivers (fig. 2) depended entirely on antenna resonance for tuning, and just consisted of a crystal detector D1 connected between the antenna and ground, with an earphone E1 across it. When transmitting stations were rare, there was little need for frequency-selective elements besides the broad resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
of the antenna, so any station within a wide band of frequencies were heard in the earphone. It was used in the earliest days of radio, transmitting stations were separated by large distances and wide frequency differences.
Impedance matching
An important principle used in crystal radio design to transfer maximum power to the earphone is impedance matchingImpedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
. The maximum power is transferred from one part of a circuit to another when the impedance
Wave impedance
The wave impedance of an electromagnetic wave is the ratio of the transverse components of the electric and magnetic fields . For a transverse-electric-magnetic plane wave traveling through a homogeneous medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium...
(resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
) of the two circuits is equal. However in crystal sets, the impedance of the antenna-ground system (around 10-200 ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
s) is usually lower than the impedance of the receiver's tuned circuit (thousands of ohms at resonance), and also varies depending on the quality of the ground, length of the antenna, and what frequency the receiver is tuned to in the band. Therefore in better receiver circuits (fig. 4), to match the antenna impedance to the receiver's impedance, the antenna was connected across only a portion of the tuning coil's turns. This made the coil L1 act as an impedance matching transformer
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
(in an autotransformer
Autotransformer
An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where...
connection) in addition to its tuning function. The tuned circuit's high impedance was transformed down by a factor equal to the square root of the turns ratio (the number of turns the antenna was connected across, to the total number of turns of the coil), to match the antenna impedance. In the "two-slider" circuit (fig. 3), popular during the wireless era, both the antenna and the detector circuit were attached to the coil with sliding contacts, allowing (interactive) adjustment of both the resonant frequency and the turns ratio. Alternatively a multiposition switch ( S1, fig. 4) was used to select taps on the coil. These controls were adjusted until the station sounded loudest in the earphone.
The other place impedance matching was often used was between the tuning coil and the crystal detector/earphone circuit, to match the impedance of the detector. To accomplish this the detector D1 , like the antenna, was connected to a tap on the coil. This also improved the receiver's selectivity (see below).
Problem of selectivity
One of the drawbacks of crystal sets is that they are vulnerable to interference from stations near in frequencyFrequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
to the desired station; they have low selectivity. Often two or more stations are heard simultaneously. This is because the simple tuned circuit doesn't reject nearby signals well; it allows through a wide band of frequencies, that is, it has a large bandwidth (low Q factor
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
) compared to modern receivers. This was a worse problem during the wireless
Wireless telegraphy
Wireless telegraphy is a historical term used today to apply to early radio telegraph communications techniques and practices, particularly those used during the first three decades of radio before the term radio came into use....
era because the spark-gap transmitter
Spark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...
s of the era produced much wider bandwidth signals than modern transmitters, that spread interference over the frequencies of other stations. The tuned circuit had wide bandwidth because the crystal detector connected across it had relatively low resistance
Electrical resistance
The electrical resistance of an electrical element is the opposition to the passage of an electric current through that element; the inverse quantity is electrical conductance, the ease at which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical...
which "loaded" the tuned circuit, damping the oscillations, reducing its Q
Q factor
In physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
. In many circuits such as fig. 4 and 5, 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 (see above).
Inductively coupled receivers
In more sophisticated crystal receivers, (fig. 5) the tuning coil was replaced with an adjustable air core antenna couplingAntenna tuner
An antenna tuner, transmatch or antenna tuning unit is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching the impedance of the equipment to the antenna...
transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
(L1, L2) which improved the selectivity by a technique called loose coupling. This consisted of two magnetically coupled coils of wire, one (the primary, L1) attached to the antenna and ground and the other (the secondary, L2) attached to the rest of the circuit. The current from the antenna created an alternating magnetic field in the primary coil, which induced a voltage in the secondary coil which was rectified and powered the earphone. Each of the coils functioned as a tuned circuit that was tuned to the frequency of the station: the primary coil resonated with the capacitance of the antenna (or sometimes another capacitor, C1), and the secondary resonated with the tuning capacitor C2. The two circuits interacted to form a resonant transformer. Reducing the coupling between the coils, by physically separating them so less of the magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
of one intersects the other (reducing the mutual inductance), narrows the bandwidth, resulting in much sharper, more selective tuning than a single tuned circuit. However this involved a tradeoff; looser coupling also reduced the amount of signal getting through the transformer. So the transformer was made with adjustable coupling. One type common in early days, called a "loose coupler", consisted of a smaller coil inside a larger coil. The smaller coil was mounted on a rack so it could be slid linearly in or out of the larger coil. If interference was encountered, the smaller coil would be slid further out of the larger, loosening the coupling and narrowing the bandwidth, to reject the interfering signal.
The antenna coupling transformer also functioned as an impedance matching transformer
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....
, to match the antenna impedance to the rest of the circuit. One or both of the coils usually had several taps which could be selected with a switch (S1), to adjust the turns ratio.
Coupling transformers were difficult to adjust, because the three adjustments, the tuning of the primary circuit, the tuning of the secondary circuit, and the coupling, were all interactive, and changing one affected the others.
Crystal detector
In early sets, this was a cat's whisker detector, a fine metal wire on an adjustable arm that touched the surface of a crystal of a semiconductingSemiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
. This formed a crude unstable semiconductor diode (Schottky diode
Schottky diode
The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...
), which allowed current to flow better in one direction than in the opposite direction. Modern crystal sets use modern semiconductor diodes. The detector rectified
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
the alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
radio signal to a pulsing direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
, which had the audio modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
signal impressed on it, so it could be converted to sound by the earphone, which was connected in series (or sometimes in parallel) with the detector.
The rectified current from the detector still had radio frequency
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
pulses from the carrier in it, which for some wavelengths did not pass well through the high inductance
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...
of the earphones. In those cases, a small capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
, called a blocking or bypass capacitor (C2, fig. 4 and C3, fig. 5), was usually placed across the earphone terminals to bypass these pulses around the earphone to ground, although the earphone cord usually had enough capacitance that this component could be omitted.
In a cat's whisker detector only certain sites on the crystal surface functioned as rectifying junctions, and the device was very sensitive to the pressure of the crystal-wire contact, which could be disrupted by the slightest vibration. 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. An alternative adjustment method was to use a battery-powered buzzer
Buzzer
A buzzer or beeper is an audio signaling device, which may be mechanical, electromechanical, or piezoelectric. Typical uses of buzzers and beepers include alarm devices, timers and confirmation of user input such as a mouse click or keystroke....
(BZ, fig. 7) attached to the ground wire to provide a test signal. The spark at the buzzer's electrical contacts served as a weak radio transmitter, so when the detector began working the buzz could be heard in the earphones, and the buzzer was then turned off.
Galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
(lead sulfide) was probably the most common crystal used in cat's whisker detectors, but various other types of crystals were also used, such as iron pyrite (Fool's gold, Fe2S), silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, molybdenite
Molybdenite
Molybdenite is a mineral of molybdenum disulfide, MoS2. Similar in appearance and feel to graphite, molybdenite has a lubricating effect that is a consequence of its layered structure. The atomic structure consists of a sheet of molybdenum atoms sandwiched between sheets of sulfur atoms...
(MoS2), silicon carbide
Silicon carbide
Silicon carbide , also known as carborundum, is a compound of silicon and carbon with chemical formula SiC. It occurs in nature as the extremely rare mineral moissanite. Silicon carbide powder has been mass-produced since 1893 for use as an abrasive...
(carborundum, SiC), and a zincite
Zincite
Zincite is the mineral form of zinc oxide . Its crystal form is rare in nature; a notable exception to this is at the Franklin and Sterling Hill Mines in New Jersey, an area also famed for its many fluorescent minerals. It has a hexagonal crystal structure and a color that depends on the presence...
-bornite
Bornite
Bornite is a sulfide mineral with chemical composition Cu5FeS4 that crystallizes in the orthorhombic system .-Appearance:Bornite has a brown to copper-red color on fresh surfaces that tarnishes to various iridescent shades of blue to purple in places...
(ZnO-Cu5FeS4) crystal-to-crystal junction trade-named Perikon. Crystal radios have also been made with rectifying junctions improvised from a variety of common objects, such as blue steel razor blades
Razor blade steel
Razor blade steel, also known as razor steel, is special type of stainless steel designed specifically to be used as a razor blade. Its defining characteristics are its chemical composition and shape.-Chemical composition:...
and lead pencils, rusty needles, and pennies In these, a semiconducting
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
layer of oxide or sulfide on the metal surface is usually responsible for the rectifying action.
In modern sets a semiconductor diode is used, which is much more reliable than a cat's whisker detector and doesn't require any adjustments. Germanium diodes (or sometimes Schottky diode
Schottky diode
The Schottky diode is a semiconductor diode with a low forward voltage drop and a very fast switching action...
s) are used instead of silicon diodes, because their lower forward voltage drop (roughly 0.3V compared to 0.6V) makes them more sensitive.
All semiconductor detectors function rather inefficiently in crystal receivers, because the low voltage signal level is too low to result in much difference between forward better conduction and reverse weaker conduction. Bias can move the diode's operating point higher on the detection curve to produce more signal voltage at the expense of less signal current (higher impedance). There is a limit to the benefit that this produces, depending on the other impedances of the radio. To improve the sensitivity of some of the early crystal detectors, such as silicon carbide, a small forward bias
Biasing (electronics)
Biasing in electronics is the method of establishing predetermined voltages and/or currents at various points of an electronic circuit to set an appropriate operating point...
voltage was applied across the detector by a battery and potentiometer
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
(B1, R1, fig. 7) . This improved sensitivity by moving the DC operating point to a more desirable voltage-current operating point (impedance) on the junction's I-V curve.
Earphones
The requirements for earphones used in crystal sets are different from earphones used with modern audio equipment. They have to be efficient at converting the electrical signal energy to sound waves, while most modern earphones are designed for high fidelityHigh fidelity
High fidelity—or hi-fi—reproduction is a term used by home stereo listeners and home audio enthusiasts to refer to high-quality reproduction of sound or images, to distinguish it from the poorer quality sound produced by inexpensive audio equipment...
reproduction of the sound. In early homebuilt sets, the earphones were the most costly component.
The early earphones used with wireless-era crystal sets had moving iron drivers
Moving iron speaker
The earliest loudspeakers for speech and music were moving iron speakers. These are still used today in some miniature speakers where small size and low cost count, and sound quality is unimportant. A moving iron speaker consists of a ferrous metal diaphragm or reed, and a permanent magnet. The...
that worked similarly to loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s. Each earpiece contained a magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
wound with coils of wire to form an electromagnet
Electromagnet
An electromagnet is a type of magnet in which the magnetic field is produced by the flow of electric current. The magnetic field disappears when the current is turned off...
, with poles close to a steel diaphragm. When the audio signal
Audio signal
An audio signal is an analog representation of sound, typically as an electrical voltage. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical...
from the radio was passed through the electromagnet's windings, it created a varying magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
that pulled on the diaphragm, causing it to vibrate. The vibrations of the diaphragm pushed and pulled on the air in front of it, creating sound waves. Standard headphones used in telephone work had a low impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
, often 75 Ω, and required more current than a crystal radio could supply, so the type used with radios was wound with more turns of finer wire and had an impedance of 2000-8000 Ω.
Modern crystal sets use piezoelectric
Piezoelectricity
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...
crystal earpiece
Crystal earpiece
A crystal earpiece, more properly called a piezoelectric earphone, is an earphone that produces sound by using a piezoelectric crystal, a material that changes its shape when electricity is applied to it...
s, 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 don't "load" the tuned circuit; increasing the selectivity of the receiver. However the earphone's higher resistance, in parallel with its capacitance of around 9 pF, creates a low pass filter which removes the higher audio frequencies, distorting or eliminating the sound. So a bypass capacitor is not needed (although in practice a small one of around 680pFd to .001 uFd is often used to help improve quality), and instead a 10-100 KΩ resistor must be added across the earphone's input.
Modern low impedance (8 Ω) earphones cannot be used unmodified in crystal sets because the receiver doesn't produce enough current to drive them. They are sometimes used by adding an audio transformer to match their impedance with the higher impedance of the circuit.
History
Crystal radio was invented by a long, partly obscure chain of discoveriesHistory of radio
The early history of radio is the history of technology that produced radio instruments that use radio waves. Within the timeline of radio, many people contributed theory and inventions in what became radio. Radio development began as "wireless telegraphy"...
in the late 19th century that gradually evolved into more and more practical radio receivers in the early 20th century; and constitutes the origin of the field of electronics
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
. The earliest practical use of crystal radio was to receive Morse code
Morse code
Morse code is a method of transmitting textual information as a series of on-off tones, lights, or clicks that can be directly understood by a skilled listener or observer without special equipment...
radio signals transmitted by early amateur radio
Amateur radio operator
An amateur radio operator is an individual who typically uses equipment at an amateur radio station to engage in two-way personal communications with other similar individuals on radio frequencies assigned to the amateur radio service. Amateur radio operators have been granted an amateur radio...
experimenters using very powerful spark-gap transmitter
Spark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...
s. As electronics evolved, the ability to send voice signals by radio caused a technological explosion in the years around 1920 that evolved into today's radio broadcasting
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
industry.
Early years
Early radio telegraphy used spark gapSpark-gap transmitter
A spark-gap transmitter is a device for generating radio frequency electromagnetic waves using a spark gap.These devices served as the transmitters for most wireless telegraphy systems for the first three decades of radio and the first demonstrations of practical radio were carried out using them...
and arc transmitters as well as high-frequency alternators
Alexanderson alternator
An Alexanderson alternator is a rotating machine invented by Ernst Alexanderson in 1904 for the generation of high frequency alternating current up to 100 kHz, for use as a radio transmitter...
running at radio frequencies. At first a Branley Coherer
Coherer
The coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
was used to indicate the presence of a radio signal. However, these lacked the sensitivity to convert weak signals.
In the early 20th century, various researchers discovered that certain metallic mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s, such as galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
, 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
Greenleaf Whittier Pickard
Greenleaf Whittier Pickard was a United States radio pioneer. Pickard was a researcher in the early days of wireless. He experimented with crystal detectors, antennas, wave propagation, and noise suppression...
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
Cat's whisker diode
A cat’s whisker detector is an antique electronic component consisting of a thin wire that lightly touches a crystal of semiconducting mineral to make a crude point-contact rectifier. Developed by early radio researchers Jagadish Chandra Bose, G. W...
", 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
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
; pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
was also often used, as it was a more easily adjusted and stable mineral, and quite sufficient for urban signal strengths. Several other minerals also performed well as detectors. Another benefit of crystals was that they could demodulate amplitude modulated signals. This mode was used in radiotelephone
Radiotelephone
A radiotelephone is a communications system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephone" is often used to describe the usage of radio spectrum where it is important to distinguish the...
s and to broadcast
Broadcasting
Broadcasting is the distribution of audio and video content to a dispersed audience via any audio visual medium. Receiving parties may include the general public or a relatively large subset of thereof...
voice
Voice message
Voice message refers to a message that could be sent to a destination using voice media. Voice itself could be 'packaged' and sent through the IP backbone so that it reaches its marked 'address'. In a technical sense, the process of sending 'voice packets' is a semi passive way of communication...
and music
Music
Music is an art form whose medium is sound and silence. Its common elements are pitch , rhythm , dynamics, and the sonic qualities of timbre and texture...
for a public audience. Crystal sets represented an inexpensive and technologically simple method of receiving these signals at a time when the embryonic radio broadcasting industry was beginning to grow.
1920s and 1930s
. Some historians consider the Autumn of 1920 to be the beginning of commercial radio broadcasting for entertainment purposes. Pittsburgh, PA, station KDKA
KDKA (AM)
KDKA is a radio station licensed to Pittsburgh, Pennsylvania, USA. Created by the Westinghouse Electric Corporation on November 2, 1920, it is one of the world's first modern radio stations , a distinction that has also been challenged by other stations, although it has claimed to be the first in...
, owned by Westinghouse
Westinghouse Electric (1886)
Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...
, received its license from the United States Department of Commerce just in time to broadcast the Harding-Cox presidential election returns. In addition to reporting on special events, broadcasts to farmers of crop price reports were an important public service, in the early days of radio.
In 1921, factory-made radios were very expensive. Since less affluent families could not afford to own one, newspapers and magazines carried articles on how to build a crystal radio with common household items. To minimize the cost, many of the plans suggested winding the tuning coil on empty pasteboard containers such as oatmeal boxes, which became a common foundation for homemade radios.
Valveless amplifier
A "Carbon amplifier" consisting of a carbon microphoneCarbon microphone
The carbon microphone, also known as a carbon button microphone or a carbon transmitter, is a sound-to-electrical signal transducer consisting of two metal plates separated by granules of carbon. One plate faces outward and acts as a diaphragm...
and an electromagnetic earpiece sharing a common membrane and case. This was used in the telephone industry and in hearing aid
Hearing aid
A hearing aid is an electroacoustic device which typically fits in or behind the wearer's ear, and is designed to amplify and modulate sound for the wearer. Earlier devices, known as "ear trumpets" or "ear horns", were passive funnel-like amplification cones designed to gather sound energy and...
s nearly since the invention of both components and long before vacuum tubes. This could be readily bought or made from surplus telephone parts for use with a crystal radio. Unlike vacuum tubes, it could run with only a flashlight or car battery and had an indefinite lifetime.
Crystodyne
In the early 1920s RussiaRussia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
, devastated by civil war, Oleg Losev
Oleg Losev
Oleg Vladimirovich Losev was a scientist and inventor. He was born to a high-ranking family in Imperial Russia. He published a number of papers and patents during his short career. His observations of LEDs languished for half a century before being recognized in the late 20th and early 21st...
was experimenting with applying voltage bias
Bias
Bias is an inclination to present or hold a partial perspective at the expense of alternatives. Bias can come in many forms.-In judgement and decision making:...
es to various kinds of crystals for manufacture of radio detectors. The result was astonishing - with a zincyte (zinc oxide
Zinc oxide
Zinc oxide is an inorganic compound with the formula ZnO. It is a white powder that is insoluble in water. The powder is widely used as an additive into numerous materials and products including plastics, ceramics, glass, cement, rubber , lubricants, paints, ointments, adhesives, sealants,...
) crystal he gained amplification. This was negative resistance
Negative resistance
Negative resistance is a property of some electric circuits where an increase in the current entering a port results in a decreased voltage across the same port. This is in contrast to a simple ohmic resistor, which exhibits an increase in voltage under the same conditions. Negative resistors are...
phenomenon, decades before the tunnel diode
Tunnel diode
A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects....
. After the first experiments, he built regenerative and superheterodyne receivers, and even transmitters. However, this discovery was not supported by authorities and soon forgotten and no device was produced in mass quantity beyond a few examples for research.
Crystodyne was produced in primitive conditions; it can be made in a rural forge - unlike vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s and modern semiconductor devices.
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 oscillatorLocal oscillator
A local oscillator is an electronic device used to generate a signal normally for the purpose of converting a signal of interest to a different frequency using a mixer. This process of frequency conversion, also referred to as heterodyning, produces the sum and difference frequencies of the...
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
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
and the rectified signal could be heard on headphones or crystal ear pieces. The idea spread across the beachhead, to other parts of the war, and to popular culture. The sets were dubbed "foxhole receivers" by the popular press, and they became part of the folklore
Folklore
Folklore consists of legends, music, oral history, proverbs, jokes, popular beliefs, fairy tales and customs that are the traditions of a culture, subculture, or group. It is also the set of practices through which those expressive genres are shared. The study of folklore is sometimes called...
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. However there were places such as the Channel Islands
Occupation of the Channel Islands
The Channel Islands were occupied by Nazi Germany for much of World War II, from 30 June 1940 until the liberation on 9 May 1945. The Channel Islands are two British Crown dependencies and include the bailiwicks of Guernsey and Jersey as well as the smaller islands of Alderney and Sark...
and Netherlands
Netherlands
The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...
where it was possible.
Later years
While it never regained the popularity and general use that it enjoyed at its beginnings, the circuitElectronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
is still used. The Boy Scout
Boy Scout
A Scout is a boy or a girl, usually 11 to 18 years of age, participating in the worldwide Scouting movement. Because of the large age and development span, many Scouting associations have split this age group into a junior and a senior section...
s (who emerged as the unofficial custodians of crystal radio lore) kept construction of a set in their program since the 1920s. A large number of prefabricated novelty items and simple kits could be found through the 1950s and 1960s, and many children with an interest in electronics built one.
Building crystal radios was a craze
FAD
In biochemistry, flavin adenine dinucleotide is a redox cofactor involved in several important reactions in metabolism. FAD can exist in two different redox states, which it converts between by accepting or donating electrons. The molecule consists of a riboflavin moiety bound to the phosphate...
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
DXing
DXing is the hobby of tuning in and identifying distant radio or television signals, or making two way radio contact with distant stations in amateur radio, citizens' band radio or other two way radio communications. Many DXers also attempt to receive written verifications of reception from the...
(long distance reception) and building contest
Competition
Competition is a contest between individuals, groups, animals, etc. for territory, a niche, or a location of resources. It arises whenever two and only two strive for a goal which cannot be shared. Competition occurs naturally between living organisms which co-exist in the same environment. For...
s allow these set owners to compete with each other and form a community of interest in the subject.
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 receiver for distant stations that cannot be heard with a plain crystal radio.There is a long history of unsuccessful attempts and unverified claims to recover the power in the carrier of the received signal itself. Traditional crystal sets use half-wave rectifier
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
s. As AM
AM broadcasting
AM broadcasting is the process of radio broadcasting using amplitude modulation. AM was the first method of impressing sound on a radio signal and is still widely used today. Commercial and public AM broadcasting is carried out in the medium wave band world wide, and on long wave and short wave...
signals have a modulation
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
factor of only 30% by voltage at peaks, no more than 9% of received signal power (
) 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-transistorTransistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
amplifier in 1966. Sometimes efforts to recover this power are confused with other efforts to produce a more efficient detection. This history continues now with designs as elaborate as "inverted two-wave switching power unit".
See also
- RadioRadioRadio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
- Batteryless radioBatteryless radioRadio receivers were originally operated by battery. The term batteryless radio was initially used for the radio receivers which could be used directly by AC mains supply ....
- CohererCohererThe coherer was a primitive form of radio signal detector used in the first radio receivers during the wireless telegraphy era at the beginning of the twentieth century. Invented around 1890 by French scientist Édouard Branly, it consisted of a tube or capsule containing two electrodes spaced a...
- Detector (radio)Detector (radio)A detector is a device that recovers information of interest contained in a modulated wave. The term dates from the early days of radio when all transmissions were in Morse code, and it was only necessary to detect the presence of a radio wave using a device such as a coherer without necessarily...
- Demodulator
- Electrolytic detectorElectrolytic detectorThe electrolytic detector, or the bare-point electrolytic detector as it was also called, was a type of wet demodulator used in early radio receivers. This form of detector was in extensive use between the years 1902 and 1913, after which the superior vacuum tube diode became available...
- History of radioHistory of radioThe early history of radio is the history of technology that produced radio instruments that use radio waves. Within the timeline of radio, many people contributed theory and inventions in what became radio. Radio development began as "wireless telegraphy"...
- Camille Papin TissotCamille Papin TissotCamille Papin Tissot , was a pioneer of wireless telegraphy and established the first French operational radio connections at sea.- Origins :...
- Batteryless radio
External links
- The Xtal Set Society, Dedicated to once again building and experimenting with radio electronics.
- Building a simple crystal radio.Field, Simon Quellen, Scitoys.
- Stay Tuned. Crystal radio plans and projects.
- Build the Mystery Crystal set A simple and surprisingly effective and sensitive design.
- Shortwave Mystery Crystal Radio A shortwave version of the Mystery Crystal Set by KC4IWT.
- A website that lots of information on early radio and crystal sets
- Hobbydyne Crystal Radios History and Technical Information on Crystal Radios
- Ben Tongue's Technical Talk Section 1 links to "Crystal Radio Set Systems: Design, Measurements and Improvement".
- "Semiconductor archeology or tribute to unknown precusors". earthlink.net/~lenyr.
- Nyle Steiner K7NS, Zinc Negative Resistance RF Amplifier for Crystal Sets and Regenerative Receivers Uses No Tubes or Transistors. November 20, 2002.
- Crystal Set DX? Roger Lapthorn G3XBM
- Building a crystal radio set A guide to building a simple crystal radio receiver.
- Website which has a large selection of homebuilt crystal and tube radios built by Dave Schmarder.
- The Bose Institute
- Varun Aggarwal of MIT's page on Bose
- British Crystal Set Definitive Information.
Patents
"Means for receiving intelligence communicated by electric waves", 1906. G. W. Pickard. "Intelligence intercommunication by magnetic wave components", 1908. G. W. Pickard., "Space communication", 1910. G. W. Pickard., "System of radio communication", 1916. G. W. Pickard., "Radio telegraphy and telephony receiver", 1917. G. W. Pickard., "Radio telegraphy and telephony receiver", 1917. G. W. Pickard., "Radio telegraphy and telephony receiver", 1917. G. W. Pickard., "Crystal detector for radio communication", 1924. Hugo H. Pickron. (ed., uses "crystal radio" term in the patent.),"Functioning parts of mineral type detectors", 1926. L. B. Lambert., "Radio receiving set", 1927. A. Wikstrom., "Crystal radio apparatus", 1930. H. Adams., "Radio receiving set", 1931. W. J. Kayser., "Subminiature portable crystal radio", 1957. Keith L. Bell.Further reading
- Ellery W. Stone (1919). Elements of Radiotelegraphy. D. Van Nostrand company. 267 pages.
- Elmer Eustice Bucher (1920). The Wireless Experimenter's Manual: Incorporating how to Conduct a Radio Club.
- Milton Blake Sleeper (1922). Radio Hook-ups: A Reference and Record Book of Circuits Used for Connecting Wireless Instruments. The Norman W. Henley publishing co.; 67 pages.
- Robert Andrews Millikan, Henry Gordon Gale, Willard R. Pyle (1922). Practical physics. Ginn. 472 pages.
- JL Preston and HA Wheeler (1922) "Construction and operation of a simple homemade radio receiving outfit", Bu. of Standards, C-120: Apr. 24, 1922.
- PA Kinzie (1996). Crystal Radio: History, Fundamentals, and Design. Xtal Set Society.
- Thomas H. Lee, The Design of CMOS Radio-Frequency Integrated Circuits
- Derek K. Shaeffer and Thomas H. Lee, The Design and Implementation of Low-Power CMOS Radio Receivers
- Ian L. Sanders. Tickling the Crystal - Domestic British Crystal Sets of the 1920s; Volumes 1-5. BVWS Books (2000–2010).