Crystal radio

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An example of a modern set created by VE6AB
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An example of a modern set created by VE6AB

The crystal radio receiver (also known as a crystal set) is a very simple kind of radio receiver. It needs no battery or power source except the power received from radio waves by a long wire outside antenna. 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 from ordinary radios containing active powered amplifiers in many respects. 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 need only receive an electrical energy "signal" from the radio wave.

Crystal radio was invented by a long, vague chain of discoveries in the late 1800's that culminated into the first practical kind of radio receiver in the early 1900's; and constitutes the origin of the field of electronics. The first practical use of crystal radio was to receive dot and dash coded 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.

Today making and operating crystal radios is a popular hobby for many reasons, including:

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

Groups of enthusiasts[1] 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] modern solid state diodes, ultra-thin litz wire inductors, and low loss capacitors yield performance far beyond that of the original receivers.

Contents

[edit] How it works (simple version)

A crystal radio receives programs broadcast from radio stations. Radio stations convert sound into radio waves and send out the waves everywhere. Radio waves travel across the crystal radio antenna all the time. Radio waves make radio wave electricity flow between the antenna wire and the ground wire. This electricity is connected to the crystal radio. The crystal radio uses a tuner to tune the electricity to receive just one station. Then it uses a crystal detector to convert this radio wave electricity back to sound electricity. It uses an earphone to convert the sound electricity to sound you can hear.

[edit] Importance of grounding

The long wire type antennas often used with crystal radios are monopoles. 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 importance of this is easy to overlook by those familiar with amplified radios. Amplified radios use energy 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.

[edit] History

[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.

Around 1906, researchers discovered that certain metallic minerals such as galena also could be used to detect signals. These devices were called 'crystal detectors'. Greenleaf Whittier Pickard on August 30, 1906 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. 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.

In 1922 the (then named) U.S. Bureau of Standards publication: Construction and Operation of a Simple Homemade Radio Receiving Outfit showed how almost any family having a member handy with simple tools could make a radio, and became a best seller. More than any other system this design was responsible for bringing radio to the general public.

[edit] 1920s and 1930s

While there were a number of earlier experiments with radio broadcasts to the general public, most 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 U.S. 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. Many of them cost more than $2,000 USD (in year 2005 equivalent dollars), and less affluent families could not afford to have one. Newspapers and magazines in many countries urged readers interested in radio to acquire one of the inexpensive crystal sets or build their own. To minimize the cost, many of the plans suggested winding the tuning coil on an empty cylindrical oatmeal box. For years afterwards, home experimenters used oatmeal boxes as coil forms for homemade radios. Even the crystal itself could be made by mixing powdered sulfur into molten lead to form the lead sulfide "crystal". The crystal radio did not require batteries, but it did require the user to purchase a commercially made set of headphones (or telephone receivers as they were called in those days), since that accessory was not suitable for home construction.

[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. Some resourceful GIs found that a crude crystal set could be made from a coil made out of wire salvaged from broken equipment and 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.

[edit] Later years

"Rocket radio"
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"Rocket radio"

While it never regained the popularity and general use that it enjoyed at its beginnings, the circuit is still used.

The Scouting Movement (who emerged as the unofficial custodians of crystal radio lore) has kept construction of a set somewhere in the program since the 1920s. A large number of prefabricated novelty items and simple kits could be easily 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 highly sophisticated examples of the instruments. As much effort goes into the visual appearance of these sets as well as their performance, and some truly outstanding examples can be found. Annual crystal radio DX contests and building contests allow these sets to compete with each other and help form a community of interest in the subject.

[edit] Construction

A crystal set is the simplest radio receiver, consisting of a long-wire antenna, a variable inductor and a variable capacitor forming a tank circuit to select the desired radio signal frequency, and a detector consisting of a diode demodulator usually consisting of a sharp wire called a cat's whisker pressing against a sensitive point on a mineral crystal in a holder. A typical crystal set circuit diagram is shown at the following address:http://www.midnightscience.com/images/cigar/cig_circ.gif

A semiconducting mineral crystal, typically lead sulphide (galena) or cadmium sulphide is fixed inside a brass cup and the radio operator finds the loudest signal by touching the cat's whisker, to various points on the surface of the crystal. Alternately, a discrete semiconductor diode can replace a makeshift cat's whisker diode. The most expensive part can be the length of antenna wire.

The detector extracts the amplitude modulation from the radio signal by rectifying it, and provides an audio output in proportion to the strength of the signal coming from the antenna. The entire set is passive, requiring no external power. Because no electrical amplification is used, sensitive earphones are required (a crystal earpiece being the general choice in modern designs). These sets have no way to control the audio volume.

[edit] See also

[edit] External links

[edit] Patent

[edit] General

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