A crystal radio can be thought of as a radio receiver reduced to its essentials. It consists at a minimum of these components:
• An antenna to pick up the radio waves and convert them to electric currents.
• A tuned circuit to select the signal of the radio station to be received, out of all the signals received by the antenna. This consists of an inductor or tuning coil and a capacitor connected together, one of which is adjustable and used to tune in different stations. The tuned circuit has a natural resonant frequency, and allows radio signals at this frequency to pass while rejecting signals at all other frequencies.
• A semiconductor crystal detector which extracts the audio signal (modulation) from the radio frequency carrier wave. It does this by only allowing current to pass through it in one direction, blocking half of the oscillations of the radio wave. This rectifies the alternating current radio wave to a pulsing direct current, whose strength varies with the audio signal. This current can be converted to sound by the earphone. It was this component that gave crystal sets their name.
• An earphone to convert the audio signal to sound waves so they can be heard. The low power produced by crystal radios is insufficient to power a loudspeaker so earphones are used.
The sound power produced by the earphone of a crystal set comes solely from the radio station being received, via the radio waves picked up by the antenna. The power picked up by a receiving antenna decreases with the square of its distance from the radio transmitter. 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 nano-watts. Early crystal sets could receive signals as weak as 2.5 nano-watts at the antenna. Crystal radios can receive such weak signals without using amplification only due to the great sensitivity of human hearing, which can detect sounds with energy of only 10-16 W/cm2.
Therefore crystal receivers have to be designed to convert the energy from the radio waves into sound as efficiently as possible. Even so, they are usually only able to receive nearby stations, within distances of about a hundred miles for the telegraphy stations of the wireless era and about 25 miles for AM broadcast stations.
