Crystal radio

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

A crystal radio receiver, also called a crystal set, is a very simple radio receiver, popular in the early days of radio. It needs no battery or power source and runs on the power received from radio waves by a long outdoor wire antenna.

Contents

[edit] Overview

Simple crystal radios are often made with a few hand made parts, like an antenna wire, tuning coil of copper wire, crystal detector and earphones. Because crystal radios are passive radio receivers, they are technically distinct in many respects from ordinary radios containing active powered amplifiers. This is because they must receive and preserve as much electrical power as possible from the antenna and convert it to sound power whereas ordinary radios amplify the weak electrical energy "signal" from the radio wave. Today making and operating crystal radios is a popular hobby for many reasons, including:

  • Historical and nostalgic significance
  • The astonishing results one can get despite its utter simplicity
  • The challenge of receiving weak distant signals without amplification

Crystal radios can be designed to receive almost any radio frequency band, but most are designed for the AM broadcast band. A few receive the 49-meter international short wave band, partly because the radio waves are stronger in those bands. The first crystal sets received wireless telegraphy signals broadcast by spark-gap transmitters at frequencies as low as 20 kHz and below. Although crystal radios are designed to detect AM, they can be made to detect FM broadcasts in the 100 MHz range by a technique called slope detection.

Groups of enthusiasts[1][dead link] and a number of web sites[2] are devoted to their construction. Regular contests are held comparing the performance of various designs with each other. Reportedly,[3][dead link] modern diodes, ultra-thin litz wire inductors, and low loss capacitors yield performance far beyond that of traditional receiver designs.

[edit] How it works

Pictorial diagram showing parts and connections for a crystal radio.

A crystal radio receives audio broadcasts from AM radio stations. At each station sound is used to modulate the amplitude of a radio wave that is fed to a transmitting antenna. The radio waves from many transmitters simultaneously induce electric currents between the receiver antenna wire and ground. The receiver comprises a tuner circuit that selects the radio wavelength of the desired broadcast, followed by the crystal detector (such as a Cat's whisker detector or diode) that converts the radio wave back to the original audio waveform for reproduction typically by earphones.

[edit] History

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

[edit] Early years

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

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

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

NBS Circular 120 Home Crystal Radio Project.

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

[edit] 1920s and 1930s

Sophisticated crystal receiver from around 1914

In the beginning of the 20th century, radios were only for some people who considered it as a hobby. Radios were not accessible to the public, so they built their own radios by themselves with wires wrapped around baseball bats, boxes to form receivers, the transmitters were made from glass and iron, and the speakers they built were from newspapers wrapped in a cone shape.[4]

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

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

[edit] Non-electric amplification

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

[edit] Valveless amplifier

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

[edit] Cristadyne

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In the early 1920s Russia, devastated by civil war, the young scientist Oleg Losev was experimenting with applying voltage biases to various kinds of crystals, with purpose to refine the reception. The result was astonishing - with a zincyte (zinc oxide) crystal he gained amplification.[5][6] This was negative resistance phenomenon, decades before the tunnel diode. After the first experiments, he built regenerative and superheterodyne receivers, and even transmitters. However, this discovery was not supported by authorities and soon forgotten and no device was produced in mass quantity beyond a few examples for research.

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

[edit] 1940s

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

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

[edit] Later years

While it never regained the popularity and general use that it enjoyed at its beginnings, the circuit is still used. The Boy Scouts (who emerged as the unofficial custodians of crystal radio lore) kept construction of a set in their program since the 1920s. A large number of prefabricated novelty items and simple kits could be found through the '50s and '60s, and many children with an interest in electronics built one.

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

[edit] Attempts at recovering RF carrier power

A crystal radio tuned to a strong local transmitter can be used just as a power source for a second amplified (often a power-efficient regenerative) receiver for distant stations that cannot be heard with a plain crystal radio.[7]

There is long history of less successful attempts and unverified claims to recover the power in the carrier of the received signal itself. Traditional crystal sets use half-wave rectifiers. As AM signals have a modulation factor of only 30% by voltage at peaks[citation needed], no more than 9% of received signal power (P = U2 / R) is actual audio information, and 91% is just rectified DC voltage. Given that the audio signal is unlikely to be at peak all the time, the ratio of energy is, in practice, even greater. Considerable effort was made to convert this DC voltage into sound energy. Some earlier attempts include a one-transistor[8] amplifier in 1966. Sometimes efforts to recover this power are confused with other efforts to produce a more efficient detection.[9]. This history continues now with designs as elaborate as "inverted two-wave switching power unit"[7] and bridge amplifiers[citation needed].

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

[edit] Construction and operation

[edit] Importance of grounding

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

[edit] The naive circuit

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

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

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

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

[edit] See also

Radio portal


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

[edit] Notes

  1. ^ "Bellsouth Net"
  2. ^ "Rap'n Tap" Discussion Group
  3. ^ "Bellsouth Dxlog"]
  4. ^ Bondi, Victor."American Decades:1930-1939"
  5. ^ http://www.a-reny.com/iexplorer/cristadyne.html
  6. ^ http://earlyradiohistory.us/1924cry.htm
  7. ^ a b Polyakov V.T. "Simple receivers for AM signals", ISBN 5-94074-056-1 (in Russian)
  8. ^ Radio-Electronics, 1966, №2
  9. ^ QST [Amateur Radio Magazine] January 2007, "High Sensitivity Crystal Set" (http://www.arrl.org/qst/2007/01/culter.pdf )
  10. ^ Christopher Null. "Prototype Nokia phone recharges without wires". Yahoo! Canada Co.. http://ca.tech.yahoo.com/blogs/the_working_guy/rss/article/3638. Retrieved June 20, 2009. 

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