Crystal radio

A crystal radio receiver, also called a crystal set or cat's whisker receiver, is a very simple radio receiver, popular in the early days of radio(SONY VGP-BPS8 battery). 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 It gets its name from its most important component, known as a crystal detector(SONY VAIO VGN-FZ Battery), originally made with a piece of crystalline mineral such as galena. This component is now called a diode.

Overview

Crystal radios are the simplest type of radio receiver, and can be handmade with a few inexpensive parts, like an antenna wire, tuning coil of copper wire, crystal detector and earphones(SONY VAIO VGN-FZ11S Battery). They are distinct from ordinary radios because they are passive receivers, while other radios use a separate source of electric power such as abattery or the mains power to amplify the weak radio signal from the antenna so it is louder(SONY vgp-bps9 battery). 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(sony vgp-bpl9 battery), and crystal detectors were developed and applied to radio receivers between 1894 and 1906 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(SONY VAIO VGN-FZ32 Battery). 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(SONY VAIO VGN-FZ440N Battery).

After about 1920, crystal sets were superseded by the first amplifying receivers, which used vacuum tubes (Audions), and became obsolete for commercial use. However they continued to be built by hobbyists(SONY VAIO VGN-FZ280E Battery), 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(SONY VAIO VGN-FZ460E Battery).

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(SONY VAIO VGN-FZ190 Battery). The first crystal sets received wireless telegraphy signals broadcast by spark-gap transmitters at frequencies as low as 20 kHz.

How it works

A crystal radio can be thought of as a radio receiver reduced to its essentials. It consists at a minimum of these components(SONY VAIO VGN-FZ150E Battery):

An antenna to pick up the radio waves and convert them to electric currents.

A tuned circuit to select the signal of the radio station to be received, out of all the signals received by the antenna(SONY VAIO VGN-FZ260E Battery). This consists of a coil of wire called an inductor or tuning coil and a capacitor connected together, one or both of which is adjustable and can be used to tune in different stations. In some circuits a capacitor is not used, because the antenna also serves as the capacitor(SONY VAIO VGN-FZ140E Battery). The tuned circuit has a natural resonant frequency, and allows radio signals at this frequency to pass while rejecting signals at all other frequencies.

A semiconductor crystal detector which extracts the audio signal (modulation) from the radio frequency carrier wave(SONY VAIO VGN-FZ11l Battery). It does this by only allowing current to pass through it in one direction, blocking half of the oscillations of the radio wave. This rectifiesthe alternating currentradio wave to a pulsing direct current, whose strength varies with the audio signal(SONY VAIO VGN-FZ11z Battery). This current can be converted to sound by the earphone. Early sets used a cat's whisker detector, a fine wire touching the surface of a pebble of crystalline mineral such as galena. It was this component that gave crystal sets their name(SONY VAIO VGN-FZ11m Battery).

An earphone to convert the audio signal to sound waves so they can be heard. The low power produced by crystal radios is insufficient to power a loudspeaker so earphones are used(SONY VAIO VGN-FZ18m Battery).

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(SONY VAIO VGN-FZ21m Battery). 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(SONY VAIO VGN-FZ410 Battery). 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(SONY VAIO VGN-FZ21 Battery), which can detect sounds with an energy 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(SONY VAIO VGN-FZ160 Battery), within distances of about 25 miles forAM 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(SONY VAIO VGN-FZ210CE Battery).

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(SONY VAIO VGN-FZ38M Battery).

Antenna

The antenna converts the energy in the electromagnetic radio waves 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(SONY VAIO VGN-FZ31z Battery), 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(Sony Vaio VGN-FZ31S battery). 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 (SONY VAIO VGN-FZ31E Battery) (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 antennas or ferrite loopstick antennas used in modern radios(SONY VAIO VGN-FZ31J Battery).

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(SONY VAIO VGN-FZ31M Battery). 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 escapes, and barbed wire fences as antennas(SONY VAIO VGN-FZ31B Battery).

Ground

The wire antennas used with crystal receivers are monopole antennas 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(HP PAVILION DV6000 battery), 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(SONY VGP-BPS13 Battery), 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 (SONY VGP-BPS13B/B Battery) (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(Dell INSPIRON 1525 battery).

Tuned circuit

The tuned circuit, consisting of the coil and capacitor, acts as a resonator, analogous to a tuning fork for sound waves. Electric charge flows rapidly back and forth between the plates of the capacitor through the coil(Dell INSPIRON E1505 battery), 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(Dell INSPIRON 1420 battery), 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 capacitanceC of the capacitor and the inductance L of the coil(Dell INSPIRON 1520 battery):

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(Dell INSPIRON 1464 battery), 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(Dell INSPIRON 1564 battery), moving the core in and out of the coil, varying the inductance by changing the magnetic permeability.

The antenna is an integral part of the tuned circuit and its reactance contributes to determining the resonant frequency. The antenna usually acts as a capacitor because antennas shorter than a quarter-wavelength have capacitive reactance(Dell INSPIRON 1764 battery). Many early crystal sets did not have a tuning capacitor, due either to their cost or availability, and 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(Dell N3010 Battery). If available, the addition of a real capacitor does increase the selectivity of the receiver.

The earliest crystal receivers (fig. 2) did not have a tuned circuit at all, and just consisted of a crystal detector D1 connected between the antenna and ground, with an earphone E1 across it(SONY VGP-BPS13/B Battery). Since this circuit lacked any frequency-selective elements besides the broad resonance of the antenna, it had no way of rejecting unwanted stations, so all stations within a wide band of frequencies were heard in the earphone (in practice the most powerful usually drowns out the others) (Sony VAIO VGN-FZ15T Battery). It was used in the earliest days of radio, when only one or two stations were within a crystal set's limited range.

Impedance matching

An important principle used in crystal radio design to transfer maximum power to the earphone is impedance matching(Sony VAIO VGN-FZ15G Battery). 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 ohms) (SONY VGP-BPS13A/S Battery) 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(SONY VGP-BPS13AS Battery). 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(SONY VGP-BPS13S Battery). 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(SONY VGP-BPS13B/S Battery). 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(SONY VAIO PCG-5G2L battery). 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(SONY VAIO PCG-5G3L battery), 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) (SONY VAIO PCG-5J1L battery).

One of the drawbacks of crystal sets is that they are vulnerable to interference from stations near in frequency to the desired station; they have low selectivity. Often two or more stations are heard simultaneously(SONY VAIO PCG-5K2L battery). 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(SONY VAIO PCG-5L1L battery). This was a worse problem during the wireless era because the spark-gap transmitters of the era produced much wider bandwidth signals than modern transmitters, that spread interference over the frequencies of other stations(SONY VAIO PCG-5J2L battery). The tuned circuit had wide bandwidth because the crystal detector connected across it had relatively low resistancewhich "loaded" the tuned circuit, damping the oscillations, reducing its Q. In many circuits such as fig(SONY VAIO PCG-6S2L battery). 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) (SONY VAIO PCG-6S3L battery).

Inductively-coupled receivers

In more sophisticated crystal receivers (fig. 5) the tuning coil was replaced with an adjustable air core antenna couplingtransformer (L1, L2)which improved the selectivity by a technique called loose coupling(SONY VAIO PCG-6V1L battery). This consisted of two magnetically coupled coils of wire, one (theprimary, 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(SONY VAIO PCG-6W1L battery), 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(SONY VAIO PCG-7111L battery): 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(SONY VAIO PCG-6W3L battery). Reducing the couplingbetween 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(SONY VAIO PCG-7113L battery). 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(SONY VAIO PCG-7133L battery). 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(SONY VAIO PCG-7Z1L battery).

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(SONY VAIO PCG-7Z2L battery).

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(SONY VAIO PCG-8Y1L battery).

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 semiconducting mineral. This formed a crude unstable semiconductor diode (Schottky diode) (SONY VAIO PCG-8Y2L battery), which allowed current to flow in one direction but blocked current flowing in the opposite direction. Modern crystal sets use modern semiconductor diodes. The detector rectified the alternating current radio signal to a pulsing direct current(SONY VAIO PCG-8Z2L battery), 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(SONY VAIO PCG-8Z1L battery).

The rectified current from the detector still had radio frequency pulses from the carrier in it, which couldn't pass through the high inductance of the earphones. Therefore a smallcapacitor, called a blocking or bypass capacitor (C2, fig. 4 and C3, fig. 5) (SONY VAIO PCG-7112L battery), 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(SONY VAIO PCG-6W2L battery).

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(SONY VAIO PCG-5K1L battery). 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(SONY pcga-bp2ea battery). 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(SONY VGP-BPS21A/B Battery), 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(SONY VGP-BPS21B Battery), 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(SONY VGP-BPS21 Battery). 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(SONY VGP-BPS21/S Battery).

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 diodes) are used instead of silicon diodes(SONY VGP-BPS13A/Q Battery), 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 must overcome the diode's forward voltage drop of around one-half to one volt, and so is insufficient to drive the device far into its conduction region(SONY VGN NR11Z/T battery). Therefore it has a high AC resistance. To improve the sensitivity of some of the early crystal detectors, such as silicon carbide, a small forward bias voltage equal to the forward voltage drop was applied across the detector by a battery and potentiometer (B1, R1, fig. 7) (SONY VGN NR11Z/S battery). This improved sensitivity by moving the DC operating point to the "knee" of 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 fidelityreproduction of the sound(SONY VGN NR11M/S battery). 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 loudspeakers. Each earpiece contained a magnet wound with coils of wire to form an electromagnet(SONY VGN NR11S/S battery), 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(SONY VGP-BPS13B/Q Battery). 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(SONY VGP-BPS13Q Battery), 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 earpieces, 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(SONY Vaio VGN-FW21M Battery). 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(SONY Vaio VGN-FW21L Battery), 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(SONY Vaio VGN-FW21J Battery), 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 (SONY Vaio VGN-FW21E Battery).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(SONY Vaio VGN-FW11S Battery).

History

Crystal radio was invented by a long, partly obscure chain of discoveries in the late 19th century that gradually evolved into more and more practical radio receivers in the early 20th century; and constitutes the origin of the field of electronics(SONY Vaio VGN-FW11M Battery). The earliest practical use of crystal radio was to receive Morse code radio signals transmitted by early amateur radio experimenters using very powerful spark-gap transmitters(SONY Vaio VGN-FW11 Battery). 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(SONY VAIO VGN-FZ21J Battery).

Early years

Early radio telegraphy used spark gap and arc transmitters as well as high-frequency alternators running at radio frequencies. At first aBranley Coherer was used to indicate the presence of a radio signal. However, these lacked the sensitivity to convert weak signals(SONY VAIO VGN-FZ21Z Battery).

In the early 20th century, various researchers discovered that certain metallic minerals, such as galena, could be used to detect radio signals. In 1901, Sir Jagadish Chandra Bose filed for a US patent for "A Device for Detecting Electrical Disturbances" that mentioned the use of a galena crystal(SONY VAIO VGN-FZ21E Battery); 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(SONY Vaio VGN-FW31M Battery), 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(SONY Vaio VGN-FW465J Battery). 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(SONY Vaio VGN-FW139E/H Battery), and quite sufficient for urban signal strengths. Several other minerals also performed well as detectors. Another benefit of crystals was that they could demodulate amplitude modulated signals. This mode was used in radiotelephones and to broadcast voice and music for a public audience(SONY Vaio VGN-FW139E Battery). 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(SONY Vaio VGN-FW31E Battery).

In 1922 the (then named) U.S. Bureau of Standards released a publication entitled, Construction and Operation of a Simple Homemade Radio Receiving Outfit(SONY Vaio VGN-FW17W Battery). 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(SONY Vaio VGN-FW32J Battery). 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 [91] and is still frequently built by enthusiasts today(SONY Vaio VGN-FW31J Battery).

1920s and 1930s

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(SONY VGN-CR42E battery). 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(SONY VGN-CR42S battery), 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(SONY VGN-CR42Z battery). To minimize the cost, many of the plans suggested winding the tuning coil on empty pasteboard containers such as oatmeal boxes, which became a common foundation for homemade radios.

Valveless amplifier

A "Carbon amplifier" consisting of a carbon microphone and an electromagnetic earpiece sharing a common membrane and case(SONY VGN-CR42ZR battery). This was used in the telephone industry and in hearing aids nearly since the invention of both components and long before vacuum tubes. This could be readily bought or 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(SONY VGN-CR41SR battery).

Crystodyne

In the early 1920s Russia, devastated by civil war, Oleg Losev was experimenting with applying voltage biases to various kinds of crystals for manufacture of radio detectors. The result was astonishing - with a zincyte (zinc oxide) (SONY VGN-CR41E battery) crystal he gained amplification. This was negative resistance phenomenon, decades before thetunnel diode. After the first experiments, he built regenerative and superheterodyne receivers(SONY VGN-CR41S battery), 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 tubes and modern semiconductor devices(SONY VGN-CR41Z battery).

1940s

When Allied troops were halted near Anzio, Italy during the spring of 1944, personal portable radios were strictly prohibited as the Germans had radio detecting equipment that could detect the local oscillator signal of superheterodyne receivers. Crystal sets lack local oscillators(SONY VGN-CR31Z battery), 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(SONY VGN-CR31E battery), 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(SONY VGN-CR31S battery), 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(SONY VGN-CR31SR battery). 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(SONY VGN-CR21SR battery).

Later years

While it never regained the popularity and general use that it enjoyed at its beginnings, the circuit is still used. The Boy Scouts (who emerged as the unofficial custodians of crystal radio lore) kept construction of a set in their program since the 1920s. A large number of prefabricated novelty items and simple kits could be found through the 1950s and 1960s, and many children with an interest in electronics built one(SONY VGN-CR21Z battery).

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(SONY VGN-CR21S battery). 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 contests allow these set owners to compete with each other and form a community of interest in the subject(SONY VGN-CR21E battery).

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(SONY VGN-CR11E battery).

There is a long history of less successful attempts and unverified claims to recover the power in the carrier of the received signal itself. Traditional crystal sets use half-wave rectifiers. As AM signals have amodulation factor of only 30% by voltage at peaks(SONY VGN-CR11M battery), no more than 9% of received signal power (P = U2 / R) is actual audio information, and 91% is just rectified DC voltage. Given that the audio signal is unlikely to be at peak all the time, the ratio of energy is, in practice, even greater. Considerable effort was made to convert this DC voltage into sound energy(SONY VGN-CR11S battery). 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"(SONY VGN-CR11Z battery).

Construction and operation

Simple version

Illustrated here is a crystal radio circuit that is simple in design but ineffective in tuning to the medium wave AM broadcast band. It consists of a tuner made of a fixedparallel coil and variable capacitor tank circuit with the antenna and ground connected across it(SONY VGN-CR11SR battery). There are many practical crystal radio circuits, but connecting both the antenna and a variable capacitor across a fixed coil like this makes tuning the whole two octave AM Broadcast Band impractical(SONY VAIO VGN-NR110E/W battery).

The reason for this is that to be effective, crystal radio antennas are typically about 20 m long and 6 m high, and act something like a 250 to 300 pF capacitor. (Antennas in general have capacitance, inductance and resistance, but long-wire ones are substantially capacitive at AM radio frequencies.) (SONY VAIO VGN-NR110E/S battery) If a typical 250 pF antenna is connected to the top of a tank circuit which uses a coil of more than about 75 μH, the circuit cannot be tuned much above 1400 kHz. The size of the fixed coil must be less than 75 μH to have any chance of tuning the top of the band (around 1600 kHz or 1710 kHz). Even with a 70 μH coil(SONY VAIO VGN-NR110E/T battery), a 1000 pF variable capacitor is required to tune near the bottom of the band (around 540 kHz). However, the same variable capacitor must have a minimum value of about 4 pF to tune to the top of the band. This represents a capacitance ratio of 1:250(SONY VAIO VGN-NR110E battery), which is very high. Stray capacitance typically imposes a lower capacitance limit, and consequently restricts the ratio in practice to about 200:1. Other kinds of variable capacitors are seldom used for crystal radios because of their excessive losses(SONY VAIO VGN-NR11S battery). Consequently, experienced designers avoid this circuit. However, it does work adequately for receiving at a single frequency.

The tuning range limitation can be overcome by making the coil variable instead of the capacitor(SONY VAIO VGN-NR11Z battery), by providing a number of selectable taps along the coil winding, which makes available a fixed set of preset frequencies, or by a sliding contact.