Crystal radio receiver

The crystal radio receiver (also known as a crystal set) is a passive radio receiver consisting of a variable LC tuned circuit, a diode detector, and audio transducer. This is the original, and simplest type of radio receiver in existence. Such apparatus was in very wide use during the early history of radio and is still in limited use today.

Crystal sets with long wire antennas are still of interest to a small group of enthusiasts and a number of web sites are devoted to their construction. Regular contests are held pitting the performance of various designs against each other. Reportedly, modern solid state diodes, ultra-thin litz wire inductors, and low loss capacitors yield performance far in excess of the original receivers.

History

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, (sometimes known as a cat's whisker), 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 family member handy with simple tools could make a radio became a best seller. More than any other system this design was responsible for bringing radio to the general public.

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.

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.

Later years

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 programme 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.

In the amateur radio world, particularly amongst QRP operators, crystal receivers are sometimes used in homebrew projects. The most notable is the Pixie2 CW transceiver, an inexpensive kit radio which combines a small crystal-powered transmitter with a crystal receiver attached to a small audio amplifier.

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 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.

Espionage

It has been suggested that crystal radios may still be in use by spies. This may be because crystal sets have no local oscillator so a counter-espionage organisation cannot determine that any receiver is being used by picking up the local oscillator frequency.

External links

Patents

G. W. Pickard's "Means for receiving intelligence communicated by electric waves", 1906
G. W. Pickard's "Intelligence intercommunication by magnetic wave components", 1908
L. B. Lambert, ,"Functioning parts of mineral type detectors", 1926

General

Using and Modifying the Radio Shack '99 Xtal Set A project for beginning experimenters (Aug 13 2000)
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.

History of radio | Radio electronics | Types of radios

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