Crystal radio

thumb|Swedish crystal radio from 1922 made by [[Radiotechnique|Radiola, with earphones. The device at top is the radio's cat's whisker detector. A second pair of earphone jacks is provided.]]

thumb|1970s-era [[Arrow Electronics|Arrow crystal radio marketed to children. The earphone is on left. The antenna wire, right, has a clip to attach to metal objects such as a bedspring, which serve as an additional antenna to improve reception.]]

A crystal radio receiver, also called a crystal set, is a simple radio receiver, popular in the early days of radio. It uses only the power of the received radio wave to produce sound, needing no external power supply. It is named for its most important component, a crystal detector, originally made from a piece of crystalline mineral such as galena. This component is now called a diode.

Crystal radios are the simplest type of radio receiver and can be made with a few inexpensive parts, such as a wire for an antenna, a coil of wire, a capacitor, a crystal detector, and earphones. However they are passive receivers, while other radios use an amplifier powered by current from a battery or wall outlet to make the radio signal louder. Thus, crystal sets produce rather weak sound and must be listened to with sensitive earphones, and can receive stations only within a limited range of the transmitter.

The rectifying property of a contact between a mineral and a metal was discovered in 1874 by Karl Ferdinand Braun. 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 with the beginning of radio broadcasting around 1920. Manufacturers such as Crosley Radio Corporation marketed inexpensive crystal receivers during the early broadcasting boom.

Around 1920, crystal sets were superseded by the first amplifying receivers, which used vacuum tubes. With this technological advance, crystal sets became obsolete for commercial use mainly as a way of learning about the technology of radio. They are still sold as educational devices, and there are groups of enthusiasts devoted to their construction.

Crystal radios receive amplitude modulated (AM) signals, although FM designs have been built. They can be designed to receive almost any radio frequency band, but most receive the AM broadcast band. A few receive shortwave bands, but strong signals are required. The first crystal sets received wireless telegraphy signals broadcast by spark-gap transmitters at frequencies as low as 20 kHz.

Basic principles

thumb|Block diagram of a crystal radio receiver

thumb|Circuit diagram of a simple crystal radio

A crystal radio can be thought of as a radio receiver reduced to its essentials. It consists of at least these components:

An antenna in which the radio wave induces electric currents.
A resonant circuit (tuned circuit) which selects the frequency of the desired radio station from all the radio signals received by the antenna. The tuned circuit consists of a coil of wire (called an inductor) and a capacitor connected together. The circuit has a resonant frequency, and allows radio waves at that frequency to pass through to the detector while largely blocking waves at other frequencies. One or both of the coil or capacitor is adjustable, allowing the circuit to be tuned to different frequencies to select the station to receive. In some circuits a capacitor is not used and the antenna serves this function, as an antenna that is shorter than a quarter-wavelength of the radio waves it is meant to receive is capacitive.
A semiconductor crystal detector that demodulates the radio signal to extract the audio signal (modulation). The crystal detector functions as a square law detector, demodulating the radio frequency alternating current to its audio frequency modulation. The detector's audio frequency output is converted to sound by the earphone. Early sets used a "cat whisker detector" consisting of a small piece of crystalline mineral such as galena with a fine wire touching its surface. The crystal detector was the component that gave crystal radios their name. Modern sets use modern semiconductor diodes, although some hobbyists still experiment with crystal or other detectors.
An earphone to convert the audio signal to sound waves so they can be heard. The low power produced by a crystal receiver is insufficient to power a loudspeaker, hence earphones are used.

thumb|300px|Pictorial diagram from 1922 showing the circuit of a crystal radio. This common circuit did not use a tuning [[capacitor, but used the capacitance of the antenna to form the tuned circuit with the coil. The detector was a cat whisker detector, consisting of a piece of galena with a thin wire in contact with it on a part of the crystal, making a diode contact]]

As a crystal radio has no power supply, the sound power produced by the earphone comes solely from the transmitter of the radio station being received, via the radio waves captured by the antenna. 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. Crystal radios can receive such weak signals without using amplification only due to the great sensitivity of human hearing, which can detect sounds with an intensity of only 10−16 W/cm2. Therefore, crystal receivers have to be designed to convert the energy from the radio waves into sound waves as efficiently as possible. Even so, they are usually only able to receive stations within distances of about 25 miles for AM broadcast stations, although the radiotelegraphy signals used during the wireless telegraphy era could be received at hundreds of miles,

Design

Commercial passive receiver development was abandoned with the advent of reliable vacuum tubes around 1920, and subsequent crystal radio research was primarily done by radio amateurs and hobbyists. Many different circuits have been used. The following sections discuss the parts of a crystal radio in greater detail.

Antenna

thumb|Diagram of an [[inverted-L antenna, a common wire antenna used with crystal radios]]

The antenna converts the energy in the electromagnetic radio waves 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. Antennas of the type commonly used with crystal sets are most effective when their length is close to a multiple of a quarter-wavelength of the radio waves they are receiving. Since the length of the waves used with crystal radios is very long (AM broadcast band waves are long) the antenna is made as long as possible, from a long wire, in contrast to the whip antennas or ferrite loopstick antennas used in modern radios.

Serious crystal radio hobbyists use "inverted L" and "T" type antennas, A good ground is more important for crystal sets than it is for powered receivers, as crystal sets are designed to have a low input impedance needed to transfer power efficiently from the antenna. A low resistance ground connection (preferably below 25 Ω) is necessary because any resistance in the ground reduces available power from the antenna. Hence, signals at undesired frequencies pass through the tuned circuit to ground, while the desired frequency is instead passed on to the detector (diode) and stimulates the earpiece and is heard. The frequency of the station received is the resonant frequency f of the tuned circuit, determined by the capacitance C of the capacitor and the inductance L of the coil:

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

The circuit can be adjusted to different frequencies by varying the inductance (L), the capacitance (C), or both, "tuning" the circuit to the frequencies of different radio stations. Some modern crystal sets use a ferrite core tuning coil, in which a ferrite magnetic core is moved into and out of the coil, thereby varying the inductance by changing the magnetic permeability (this eliminated the less reliable mechanical contact).

The antenna is an integral part of the tuned circuit and its reactance contributes to determining the circuit's resonant frequency. Antennas usually act as a capacitance, as antennas shorter than a quarter-wavelength have capacitive reactance. According to the maximum power transfer theorem, the maximum power is transferred from one part of a circuit to another when the impedance of one circuit is the complex conjugate of that of the other; this implies that the two circuits should have equal resistance. However, in crystal sets, the impedance of the antenna-ground system (around 10–200 ohms and also varies depending on the quality of the ground attachment, length of the antenna, and the frequency to which the receiver is tuned. adjustment of both the resonant frequency and the turns ratio. In many circuits, the selectivity was improved by connecting the detector and earphone circuit to a tap across only a fraction of the coil's turns. This

consists of two magnetically coupled coils of wire, one (the primary) attached to the antenna and ground and the other (the secondary) attached to the rest of the circuit. However, the looser coupling also reduced the power of the signal passed to the second circuit. The transformer was made with adjustable coupling, to allow the listener to experiment with various settings to gain the best reception.

One design common in early days, called a "loose coupler", consisted of a smaller secondary coil inside a larger primary coil.

Crystal detector

thumb|left|Galena crystal detector

thumb|left|[[Germanium diode used in modern crystal radios (about 3 mm long)]]

thumb|How the crystal detector works. (A) The [[Amplitude modulation|amplitude modulated radio signal from the tuned circuit. The rapid oscillations are the radio frequency carrier wave. The audio signal (the sound) is contained in the slow variations (modulation) of the amplitude (hence the term amplitude modulation, AM) of the waves. This signal cannot be converted to sound by the earphone, because the audio excursions are the same on both sides of the axis, averaging out to zero, which would result in no net motion of the earphone's diaphragm. (B) The crystal conducts current better in one direction than the other, producing a signal whose amplitude does not average to zero but varies with the audio signal. (C) A bypass capacitor is used to remove the radio frequency carrier pulses, leaving the audio signal]]

thumb|Circuit with detector bias battery (B1) to improve sensitivity and buzzer (BZ) to aid in adjustment of the cat whisker

The crystal detector demodulates the radio frequency signal, extracting the modulation (the audio signal which represents the sound waves) from the radio frequency carrier wave. In early receivers, a type of crystal detector often used was a "cat whisker detector". The point of contact between the wire and the crystal acted as a semiconductor diode. The cat whisker detector constituted a crude Schottky diode that allowed current to flow better in one direction than in the opposite direction. Modern crystal sets use modern semiconductor diodes. but various other types of crystals were also used, the most common being iron pyrite (fool's gold, FeS2), silicon, molybdenite (MoS2), silicon carbide (carborundum, SiC), and a zincite-bornite (ZnO-Cu5FeS4) crystal-to-crystal junction trade-named Perikon. Germanium diodes (or sometimes Schottky diodes) are used instead of silicon diodes, because their lower forward voltage drop (roughly 0.3 V compared to 0.6 V) makes them more sensitive.

All semiconductor detectors function rather inefficiently in crystal receivers, because the low voltage input to the detector is too low to result in much difference between forward better conduction direction, and the reverse weaker conduction. To improve the sensitivity of some of the early crystal detectors, such as silicon carbide, a small forward bias voltage was applied across the detector by a battery and potentiometer. The bias moves the diode's operating point higher on the detection curve producing more signal voltage at the expense of less signal current (higher impedance). There is a limit to the benefit that this produces, depending on the other impedances of the radio. This improved sensitivity was caused by moving the DC operating point to a more desirable voltage-current operating point (impedance) on the junction's I-V curve. The battery did not power the radio, but only provided the biasing voltage which required little power.

Earphones

thumb|left|upright|Modern crystal radio with [[Crystal earpiece|piezoelectric earphone]]

The requirements for earphones used in crystal sets are different from earphones used with modern audio equipment. They have to be efficient at converting the electrical signal energy to sound waves, while most modern earphones sacrifice efficiency in order to gain high fidelity reproduction of the sound. Similarly, modern low-impedance (8 Ω) earphones cannot be used unmodified in crystal sets because the receiver does not produce enough current to drive them. They are sometimes used by adding an audio transformer to match their impedance with the higher impedance of the driving antenna circuit.

History

The first radio transmitters, used during the initial three decades of radio from 1887 to 1917, a period called the wireless telegraphy or radiotelegraphy era, were primitive spark transmitters which generated radio waves by discharging a capacitance through an electric spark. Each spark produced a transient pulse of radio waves which decreased rapidly to zero. The transmitter was switched on and off rapidly by the operator using a telegraph key, creating different length pulses of damped radio waves ("dots" and "dashes") to spell out text messages in Morse code. was a primitive device called a coherer, developed in 1890 by Édouard Branly and improved by Marconi and Oliver Lodge. It worked by complicated thin film surface effects, so scientists of the time didn't understand how it worked, except for a vague idea that radio wave detection depended on some mysterious property of "imperfect" electrical contacts. To receive enough energy from this wideband signal the receiver had to have a wide bandwidth also.

When more than one spark transmitter was transmitting in a given area, their frequencies overlapped, so their signals interfered with each other, resulting in garbled reception. Some method was needed to allow the receiver to select which transmitter's signal to receive. on radio in which he suggested using resonance, then called syntony, to reduce the bandwidth of transmitters and receivers. The receiver would also have a resonant circuit, and could receive a particular transmission by "tuning" its resonant circuit to the same frequency as the transmitter, analogously to tuning a musical instrument to resonance with another. This is the system used in all modern radio. patented the first tuned or "syntonic" transmitter and receiver on 10 May 1897 A radio communication system with two inductively coupled tuned circuits in the transmitter and two in the receiver, all four tuned to the same frequency, was called a "four circuit" system, and proved to be the key to practical radio communication. At a March 1893 St. Louis lecture he had demonstrated a wireless system that, although it was intended for wireless power transmission, had many of the elements of later radio communication systems. This system, patented by Tesla 2 September 1897, 4 months after Lodge's "syntonic" patent, was in effect an inductively coupled radio transmitter and receiver, the first use of the "four circuit" system claimed by Marconi in his 1900 patent (below). claimed rights to the inductively coupled "four circuit" transmitter and receiver. on grounds of the prior patents of Tesla, Lodge, and Stone, This came long after spark transmitters had become obsolete.

Invention of crystal detector

Braun's experiments

The "unilateral conduction" of crystals was discovered by Karl Ferdinand Braun, a German physicist, in 1874 at the University of Würzburg. He studied copper pyrite (Cu5FeS4), iron pyrite (iron sulfide, FeS2), galena (PbS) and copper antimony sulfide (Cu3SbS4).

This was before radio waves had been discovered, and Braun did not apply these devices practically but was interested in the nonlinear current–voltage characteristic that these sulfides exhibited. Braun's method of making contact with the crystal may have been crucial: he placed the sample on a circle of wire, then touched it with the end of a slender silver wire, a "cat's whisker" contact.

Like other scientists since Hertz, Bose was investigating the similarity between radio waves and light by duplicating classic optics experiments with radio waves.

He experimented with many substances as contact detectors, focusing on galena.

His detectors consisted of a small galena crystal with a metal point contact pressed against it with a thumbscrew, mounted inside a closed waveguide ending in a horn antenna to collect the microwaves. He patented the detector 30 September 1901 discovering rectification of radio waves in 1902 while experimenting with a coherer detector consisting of a steel needle resting across two carbon blocks. In 1907 he formed a company to manufacture his detectors, Wireless Specialty Products Co., Pickard went on to produce other detectors using the crystals he had discovered; the more popular being the iron pyrite "Pyron" detector and the zincite–chalcopyrite crystal-to-crystal "Perikon" detector In 1906 L. W. Austin invented a silicon–tellurium detector, and in 1911 Thompson H. Lyon invented the cerussite detector. In 1908 Wichi Torikata at Tokyo Imperial University investigated 200 minerals and found cassiterite (tin oxide), pyrolusite (manganese dioxide), zincite, galena, and pyrite were sensitive, and subsequently tested all the mineral samples at the Mineral College and found 34 rectifying minerals. Until the triode vacuum tube began to be used in World War I, crystals were the best radio reception technology,

Wireless telegraphy companies such as Marconi and Telefunken manufactured sophisticated inductively coupled crystal radios as communication receivers in ship radio rooms and shore stations. Lightweight crystal radios such as the SCR-54 were part of portable radiotelegraphy stations carried by army troops in World War 1 to communicate with their commanders behind the lines. After the war, electronics firms produced inexpensive "box" crystal radios for consumers. Many radio amateurs worldwide built their own crystal sets, following instructions in radio magazines.

Galena (lead sulfide, PbS, sometimes sold under the names "Lenzite" was the most widely used crystal detector since it was the most sensitive. Other common crystalline minerals used molybdenite (molybdenum disulfide, MoS2), The goal of researchers was to find rectifying crystals that were less fragile and sensitive to vibration than galena and the other cat-whisker detectors above.]]

Between about 1904 and 1915 the first types of radio transmitters were developed which produced continuous sinusoidal waves: the arc converter (Poulsen arc) and the Alexanderson alternator. However the popularity and sales of crystal radios continued to increase for a few years due to the sudden rise of radio broadcasting. In 1922 the United States Bureau of Standards (now NIST), responding to consumer interest, released a publication entitled Construction and Operation of a Simple Homemade Radio Receiving Outfit. This article (see drawing) showed how anyone who was handy with simple tools could make a crystal radio and tune into weather, crop prices, time, news and the opera.

Use of crystal radios continued to grow until the 1920s when vacuum tube radios replaced them. This distance was at the extreme edge of a crystal receiver's reception range. Before amplifying vacuum tubes became available, wireless companies tried to develop technology to make the received signal stronger.

One solution was the "intensifier"; such as the version invented by S. G. Brown Co. and used by the Marconi Co. The output current of the crystal receiver was passed through a winding on the pole pieces of a permanent magnet. Mounted close to the magnet poles was a steel resonant reed. The reed was adjusted to resonate at the audio spark frequency of the transmitter. When the reed vibrated, switch contacts on the reed periodically closed a battery circuit with an earphone, creating a buzzing sound in the earphone. Due to resonance, signals that were too weak to be heard directly excited large vibrations in the reed, allowing them to be detected.

Crystodyne

Some semiconductor junctions have a property called negative resistance which means the current through them decreases as the voltage increases over a part of their I–V curve. This allows a diode, normally a passive device, to function as an amplifier or oscillator. For example, when connected to a resonant circuit and biased with a DC voltage, the negative resistance of the diode can cancel the positive resistance of the circuit, creating a circuit with zero AC resistance, in which spontaneous oscillating currents arise. This property was first observed in crystal detectors around 1909 by William Henry Eccles and Pickard. They noticed that when their detectors were biased with a DC voltage to improve their sensitivity, they would sometimes break into spontaneous oscillations.

He realized that amplifying crystals could be an alternative to the fragile, expensive, energy-wasting vacuum tube. He used biased negative resistance crystal junctions to build solid-state amplifiers, oscillators, and amplifying and regenerative radio receivers, 25 years before the invention of the transistor. However his achievements were overlooked because of the success of vacuum tubes. His technology was dubbed "Crystodyne" by science publisher Hugo Gernsback

The temperamental, unreliable action of the crystal detector had always been a barrier to its acceptance as a standard component in commercial radio equipment One type used a blue steel razor blade and a pencil lead for a detector. The lead point touching the semiconducting oxide coating (magnetite) on the blade formed a crude point-contact diode. By carefully adjusting the pencil lead on the surface of the blade, they could find spots capable of rectification. The sets were dubbed "foxhole radios" by the popular press, and they became part of the folklore of World War II.

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

Post World War II to present

After World War II, the development of modern semiconductor diodes finally made the galena cat whisker detector obsolete. For a detector they used a sealed germanium diode which did not need adjustment like the cat's whisker detector. As an audio output device they used a piezoelectric crystal earpiece, which was far more efficient than dynamic earphones, and also did not load the tuned circuit, reducing the Q factor of the tuned circuit and thus the selectivity of the receiver, as dynamic earphones did. For a tuning coil they used a ferrite core loop antenna, which was more compact than the previous air core coils, also functioned as an antenna, and eliminated the need for a ground connection. The radio is tuned to different stations by moving the ferrite core in and out of the coil, changing the magnetic permeability and thus the inductance of the coil. The reception range of these simple radios was limited to strong local AM radio stations within 15–25 miles. They usually had an alligator clip which could be clipped to an external wire antenna, to increase the range.

The Boy Scouts have continued to include the educational construction of a crystal radio in their program since the 1920s.

Recently, communities of hobbyists have started building classically designed long-distance crystal receivers similar to those from the radiotelegraphy era. Much effort goes into the visual appearance of these sets as well as their performance. Annual crystal radio 'DX' contests (long distance reception) and building contests allow these set owners to compete with each other.

Use as a power source

There is a history of experimental designs of transistorized "free-power" crystal radios which use power harvested from the received signal, or ambient radio noise, to amplify the output. With a strong local radio station and a good antenna, a transistor amplifier can be powered by the DC current rectified by a diode in a crystal receiver, to amplify the audio output enough to operate a loudspeaker. A crystal radio tuned to a strong local transmitter can also be used as a power source for a second amplified receiver of a distant station that cannot be heard without amplification.

References

External links

A website with 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".
Crystal Radio Explained How to Build Your Own No Power Radio
"'". 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
Details of crystals used in crystal sets
http://www.crystal-radio.eu/endiodes.htm Diodes
http://www.crystal-radio.eu/engev.htm How to build a sensitive crystal receiver?
http://uv201.com/Radio_Pages/Pre-1921/crystal_detectors.htm Crystal Detectors
http://www.sparkmuseum.com/DETECTOR.HTM Radio Detectors
The Crystal Set Perfected