Crystal Radio - Design

Design

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 a coil of wire called an inductor or tuning coil and a capacitor connected together, one or both of which is adjustable and can be used to tune in different stations. In some circuits a capacitor is not used, because the antenna also serves as the capacitor. 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. Early sets used a cat's whisker detector, a fine wire touching the surface of a pebble of crystalline mineral such as galena. 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 an unamplified 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 nanowatts. In modern crystal sets, signals as weak as 50 picowatts at the antenna can be heard. Crystal radios can receive such weak signals without using amplification only due to the great sensitivity of human hearing, which can detect sounds with a power 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 25 miles for AM broadcast stations, although the radiotelegraphy signals used during the wireless telegraphy era could be received at hundreds of miles, and crystal receivers were even used for transoceanic communication during that period.

Passive receiver development was abandoned with the advent of reliable vacuum tubes around 1920, and subsequent crystal radio research was the work of radio amateurs and hobbyists. Many different circuits have been used. The following sections discuss the parts of a crystal radio in greater detail.