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COMMERCIAL radio broadcasting in the United States began just fifty years ago, in 1920. By 1925 there were 3 million radios in American homes. By 1940 these had increased to over 45 million, and now there are an estimated 275 million radios in the United States. That is more than one radio per person! Yet, despite their number, most persons understand little about a radio’s operation. Have you ever wondered how it works?

Perhaps you are sitting in an easy chair as you read this and the radio is providing background music. Let us trace the steps that enable this music to travel from the radio station to your home.

Broadcasting the Music

The music is produced by an orchestra or, in most cases today, a recording in the radio studio. The musical strains are transmitted through the air from the record to the microphone. The sound is actually small pressure variations in the air. The ear senses these pressure variations and enables one to hear the music. The microphone also senses these pressure variations, and changes these sound waves into voltage forms or electrical equivalents of the sound waves.

The microphone thus carries out the first basic step in getting the sound into an electrical form. There are many types of microphones, but we will examine the operation of the moving-coil type. The accompanying illustration will help you to visualize its various parts.

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The diaphragm is constructed of paper or other light material that vibrates to the sound of the music. This causes the little coil of wire to be moved back and forth, since it is rigidly attached to the diaphragm. The coil moves back and forth through the strong magnetic field produced by the permanent magnet, and this produces in the coil electrical equivalents of the sound waves. These electrical waves at this point are very weak, and so must be amplified or strengthened. This is done by vacuum tubes or transistors.

From the studio control the electrical waves are carried to the transmitter. In the case of small radio stations, the transmitter may be located right in the studio control room. However, larger stations usually have their powerful transmitters out of town, away from high buildings and other obstacles that might distort their transmissions

The transmitter consists of electrical equipment that produces radio waves, and combines these waves with the electrical waves that have been produced in the microphone. This combination can be done to create either of two kinds of waves, amplitude-modulated (AM) or frequency-modulated (FM) waves.

Amplitude modulation is a changing of the power of the wave, whereas frequency modulation is a changing of the frequency of the wave. An AM wave has the advantage of covering a great distance, since it is a long wave and follows the curve of the earth. On the other hand, FM waves reach for shorter distances, since they do not follow the curve of the earth. An advantage of FM over AM is its relatively noise-free reception.

After the combination in the transmitter of electrical waves and radio waves to form modulated waves, the modulated waves are fed to a broadcasting antenna. The antenna reaches high into the air, sometimes 500 feet or more. From this antenna the waves are sent out into space, spreading like the ripples made by a stone dropped in still water. These modulated waves carry all the variations and tones of the music produced in the studio.

With literally thousands of radio stations broadcasting in the United States, you might wonder how all the waves emitted by these stations are kept from interfering with one another. The Federal Radio Commission was established in 1927 to prevent such interference; it was replaced by the Federal Communications Commission in 1934.

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This Federal commission assigns stations a particular channel or frequency that they may use in broadcasting, and it is a Federal offense for a station to use any other than the frequency assigned to it. This assigned frequency is usually indicated by the number on your radio band at which you pick up that station. The frequency for an AM station may be anywhere between 550 and 1600 kilocycles.

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COMMERCIAL radio broadcasting in the United States began just fifty years ago, in 1920. By 1925 there were 3 million radios in American homes. By 1940 these had increased to over 45 million, and now there are an estimated 275 million radios in the United States. That is more...