How a Radio Works
Many of you may have drawn some of your own conclusions as to how a radio works. Let's see if those conclusions are right by looking at the place where the whole process starts - A radio station.
An accurate mixture of sound and electricity are the two main ingredients needed to transmit and speak on the radio. The wavy line represents the loud and soft tones of what is said when an announcer is speaking into a microphone.
The microphone turns those loud and soft tones into electrical impulses that correspond accurately with the announcers speech. A diagram of the strength of the electrical impulses would match up with the sound diagram exactly.
At the station, an electrical device called an oscillator is producing another electrical current called an AC -Alternating current. An alternating current is a current that continually changes the direction of its flow in an electrical circuit. This current flows in one direction; then t flows in the opposite direction; then it reverses again, back and forth, many times a second. The AC produced by the oscillator alternates this way about a million times a second. Technically speaking we would say it has a frequency of about a million cycles or a million Hertz (Megahertz). This current could be illustrated like this:
When the electrical impulses from the microphone are combined with the current from the oscillator the stations transmitter would produce something like this:
Notice that the top half of the new current is a mirror image of the top half. In the top half of the diagram, the current is flowing one way; in the bottom half, it is flowing the other way. Also notice that the shape of the alternating current, called the amplitude, now varies in direct proportion with the electrical impulses from the microphone. The pattern of the announcers speech has been superimposed on the alternating current. Because the the amplitude of the alternating current now varies, this method of radio broadcasting is called "Amplitude Modulation," or "AM" for short. (FM radio uses frequency modulation which is a different technique).
A high-frequency current, that is, a current that alternates very rapidly, has a peculiar property: it gives off radio waves which have the same frequency as the current that produced them. Radio waves can be compared to light. For example, the both spread through space at a speed of almost 200 thousand miles per second without needing any wires to carry them. Unlike light, radio waves are not stopped by trees, buildings, or people. However because our eyes are not sensitive to radio signals we cannot see radio waves. The radio waves leave the radio station transmitter tower and travel outward in all directions. In radio broadcasting, this high-frequency current or flow of electricity is called the "carrier frequency" because through amplitude modulation it "carries" the announcers voice away from the station in the form of radio waves.
When the radio waves strike a long piece of wire, such as the antenna in your radio, they cause an electric current. This current is a very weak copy of the transmitter current that sent out the radio waves at the station:
The alternating current set up in the crystal radio antenna travels to the diode. The diode is a one-way electrical switch. In other words, it only lets through electric current moving in one direction, but blocks any current moving in the opposite direction so only the "top half" of the AC current gets through:
The coil works in connection with the rest of the radio to respond only to radio signals which have a carrier frequency corresponding to AM radio stations. In this way other radio signals that may also strike the antenna are filter outed. Just like a musical instrument is tuned the coil is tuned to a particular frequency. This explains why we had to be careful to have the length of the coil close to inches.
From the diode, the modified current goes to the earphone, which turns it into a sound wave having the same shape as the sound impulses in the announcer's speech. Since radio waves travel so fast, we hear the announcer's voice on our radio at almost exactly same time that the announcer speaks.
The alternating electrical current then travels out of the radio through the ground wire The alternating electrical current produced by the radio waves striking the antenna must have a place to which it can flow. The ground wire provides this by connecting the radio to the earth. That is why the wire is called the "ground wire." The current flowing out is replaced by new radio waves striking the antenna. Sometimes your body can substitute for the earth in providing a place for the current to flow.
The diode and the earphone are the only components in the radio that are not commonly found in an average hardware store. However most electronic devices that you own probably have diodes inside. The diode acts as a one-way valve for electricity. This feature helps your radio process the signal broadcast by the radio station. In the early days of radio a small crystal of lead was used as the diode, so this type of radio is often called a "Crystal Radio." Our diode uses a small piece of the metal germanium encased in glass but the name "crystal radio," is often used to describe this type of simple radio.
© 2001 John J. Krupczak, Jr.