Crystal radio
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 signal to produce sound, needing no external power. It is named for its most important component, a crystal detector, originally made from a piece of crystalline mineral such as galena.[1] This component is now called a diode.
This article is about unpowered radio receivers. For crystal-controlled oscillators (as used in radios), see Crystal oscillator.
Crystal radios are the simplest type of radio receiver[2] 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 (because a crystal set has insufficient power for a loudspeaker).[3] 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.[4]
The rectifying property of a contact between a mineral and a metal was discovered in 1874 by Karl Ferdinand Braun.[5][6][7] Crystals were first used as a detector of radio waves in 1894 by Jagadish Chandra Bose,[8][9] in his microwave optics experiments. They were first used as a demodulator for radio communication reception in 1902 by G. W. Pickard.[10] Crystal radios were the first widely used type of radio receiver,[11] and the main type used during the wireless telegraphy era.[12] 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.[13]
Around 1920, crystal sets were superseded by the first amplifying receivers, which used vacuum tubes.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[11] but continued to be built by hobbyists, youth groups, and the Boy Scouts[14] 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.[15][16][17][18][19]
Crystal radios receive amplitude modulated (AM) signals, although FM designs have been built.[20][21] They can be designed to receive almost any radio frequency band, but most receive the AM broadcast band.[22] 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.[23][24]
A crystal radio can be thought of as a radio receiver reduced to its essentials.[3][39] It consists of at least these components:[22][40][41]
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.[3] The power available to a receiving antenna decreases with the square of its distance from the radio transmitter.[46] 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.[3] In modern crystal sets, signals as weak as 50 picowatts at the antenna can be heard.[47] Crystal radios can receive such weak signals without using amplification only due to the great sensitivity of human hearing,[3][48] which can detect sounds with an intensity of only 10−16 W/cm2.[49] 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,[50][51] although the radiotelegraphy signals used during the wireless telegraphy era could be received at hundreds of miles,[51] and crystal receivers were even used for transoceanic communication during that period.[52]
Use as a power source[edit]
A crystal radio tuned to a strong local transmitter can be used as a power source for a second amplified receiver of a distant station that cannot be heard without amplification.[112]: 122–123
There is a long history of unsuccessful attempts and unverified claims to recover the power in the carrier of the received signal itself. Conventional crystal sets use half-wave rectifiers. As AM signals have a modulation factor of only 30% by voltage at peaks, no more than 9% of received signal power () is actual audio information, and 91% is just rectified DC voltage. <correction> The 30% figure is the standard used for radio testing, and is based on the average modulation factor for speech. Properly-designed and managed AM transmitters can be run to 100% modulation on peaks without causing distortion or "splatter" (excess sideband energy that radiates outside of the intended signal bandwidth). 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[113] amplifier in 1966. Sometimes efforts to recover this power are confused with other efforts to produce a more efficient detection.[114] This history continues now with designs as elaborate as "inverted two-wave switching power unit".[112]: 129
