A crystal radio receiver can be defined as a radio receiver that uses a “crystal” detector (a crystalline mineral such as galena), and it operates by only using signal power delivered from the antenna.
The crystal radio set was one of the first forms of radio used. It became the radio of choice for many households when radio broadcasts started. Although the first radio broadcast was made in 1906 by Reginald Fessenden in the USA, it was not until after 1920 that broadcasting started to expand and be open for many more people to enjoy. To be able to receive the broadcasts, people needed radio receivers that were affordable. At the time vacuum tube radios were expensive and therefore crystal radio receivers became the popular type of radio. But into the 1930’s when vacuum tube production rose and the costs fell, the crystal radio began to lose popularity. Although the era of the crystal radio set was relatively short lived, it enabled many companies and people to be able to receive radio stations.
In the early days of crystal sets, a Cat’s whisker was used. This used a small wire to create a point contact onto a semiconductor material (the crystal). This was a form of diode which had a low turn on voltage. Today, small signal diodes like Schottky diodes or germanium diodes can be used as these have a turn on voltage around 0.2 to 0.3 volts. Even today, crystal radio kits are still available. They provide a good demonstration of how a very basic radio works.