Crystal radio
The crystal radio receiver (also known as a crystal set) was first built circa 1900 by Greenleaf Whittier Pickard, who used crystalline minerals to detect radio signals.
History
Early years
People first built and used simple and inexpensive crystal radio sets without batteries or electrical power. Even though vacuum tube radios were common following World War I, crystal radios remained popular, especially among beginning amateur radio enthusiasts, Boy Scouts and school children, who built crystal radios to learn basic electronics and communication.
Early wireless telegraphy used spark gap plasma arc transmitters powered by alternating current generators at frequencies of 400–1000 hertz. When these telegraph keyed Morse code signals were received, the tone or pitch of the generator was clearly heard varying the strength of the radio wave. Varying the strength in this way is called amplitude modulation. As wireless telegraphy became more widely used, the newer radio transmitters employed amplifiers that produced higher power and a tuned signal that required less bandwidth, thus reducing interference.
The first radio telephone transmitters also used amplitude modulation to carry speech and music on a radio wave. The crystal set received these signals almost as well as wireless telegraph signals. AM radio stations today still use amplitude modulation at power levels up to 50 kilowatts on the U.S. broadcast band, and 1 megawatt on shortwave bands. Crystal sets with long wire antennas can still be used today, but their poor selectivity causes signal overlap on crowded broadcast bands.
1920s and 1930s
When radio broadcasting became popular in the 1920s, many amateur experimenters bought or constructed crystal sets, often with the tuner inductor coil wound on a tubular oatmeal box or a drinking glass. This led to a series of adventure novels, the Radio Boys books, similar to the Hardy Boys books.
When electronic amplifiers and oscillators were invented, they were almost immediately put into service, rendering crystal sets obsolete first by the much more sensitive vacuum tube regenerative circuit designs and then the highly selective superheterodyne receivers.
During the Great Depression a crystal radio detector could be constructed from an inexpensive or homemade galena crystal and a safety pin. After this detector was connected to steel bedsprings (which doubled as an antenna) and grounded to household cold-water pipes, only inexpensive headphones were needed to listen to the world.
1940s
GIs during World War II constructed similar radios from rusty razor blades and pencil lead, the iron oxide crystals of the rust replacing the galena crystal and the graphite of the pencil lead substituting for the safety-pin wire. These crude, but functional, radios were nicknamed foxhole radios.
Later years
Crystal sets were the most common form of radio listening device for non-professionals in the early 1920s. A century after their first use, hobbyists still build and tinker with— and listen to— crystal radios constructed from just a few parts.
The most common modern design uses a coil for a tuner, and a semiconductor diode instead of a crystal. The output is usually from an earphone speaker. A lengthy antenna wire (15m (40ft) or more) is still helpful to get good sound. A modern design for a "trash radio" is constructed from a tin can and some wire.
In the amateur radio world, particularly amongst QRP operators, crystal recievers 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 reciever attached to a small audio amplifier.
Composition
Receiver
A crystal set is the simplest radio receiver, consisting of a long-wire antenna, a tuner to select the desired radio signal by frequency, and a detector consisting of a diode demodulator usually consisting of a sharp wire or pin pressing against a sensitive point on a galena mineral crystal in a holder.
A semiconductive mineral crystal, usually lead sulfide (galena) or cadmium sulfide is fixed inside a brass cup and the radio operator finds the loudest signal by touching the wire, called a "cat's whisker", to various points on the surface of the crystal; some commercial kits substitute a discrete silicon or germanium diode for the cat's whisker assembly, and it is claimed during World War II that GIs built sets with rusty razor blades (modern razor blades do not rust sufficiently to do the same). The radio is entirely powered from the radio waves it receives. The most expensive part can be the length of antenna wire.
The detector extracts the amplitude modulation from the radio signal 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. 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. Crystal sets have no oscillator so a counter-espionage organisation cannot determine the frequency being listened to.
See also
- Radio
- Radio receiver
- Transistor radio
- Demodulator
- Numbers station (related to espionage)
External links
- Toy Crystal Radios Volumes One and Two Color photos of commercially-made crystal radios from 1920 to the present, including rockets, tiny table models, pocket radios, and numerous radio novelties.
- G. W. Pickard's U.S. patent 836,531
- G. W. Pickard's U.S. patent 876,996
- 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.
- Field, Simon Quellen, Building a simple crystal radio. Scitoys.
- Stay Tuned. Crystal radio plans and projects.
- History of radio components.