How Does Your Radio Work?

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.

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.

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.

Receiving the Music

Radio waves coming from broadcasting stations are all around us. We cannot see them, but they are there. How is it, then, that your radio selects the waves carrying the music you want to hear and converts these waves into sound? Let us see.

First, the waves must be received into your radio. The antenna is this receptor. It corresponds to the ears of a person. Most radios today have built-in antennas, although automobile radios frequently have exterior ones. An outside antenna provides better reception for weaker incoming signals.

The radio waves that are sent out or radiated into the surrounding community by the radio station induce a small electrical signal or pressure when they strike your radio antenna. Since the waves from innumerable stations are being fed by means of your antenna into your radio, the waves that carry the music you want to hear must be selected from the rest. How is this done?

This is accomplished by means of the device within your radio called the tuner. The dial, or tuner knob, on your radio can be moved to select the channel or frequency of the station that is broadcasting the desired music. Tuning, therefore, simply involves the selection of one frequency and rejection of all other frequencies. But this has not always been easy to do.

Years ago, when radio was not as developed as it is today, the inability to select desired frequencies precisely was a glaring shortcoming. In fact, when the United States Navy desired to receive SOS signals from a ship in distress, commercial broadcasting stations were obligated to cease broadcasts temporarily so that Navy receiving sets could pick up these radio communications without interference. However, the problem of interference from other stations is rarely encountered today. This is because most radios employ the superheterodyne principle to achieve the precision of selection that avoids the pickup of stations on adjacent channels.

After the tuning circuit picks out the particular station desired, then by means of a vacuum tube (or transistor), called a converter, the modulated waves are changed to a lower frequency. Next the waves are strengthened or amplified by a tube or transistor called the I-F (intermediate-frequency) amplifier.

Now that the modulated wave is amplified sufficiently, it is sent into a detector tube. This tube removes the radio wave, which was produced at the transmitter and has served as a carrier wave all the way to this point, from the electrical wave. Now all that is left is the electrical wave. It is the wave that was created in the microphone at the radio studio from the pressure vibrations produced by the orchestral music.

The output of these waves from the detector is very weak, so they again need strengthening. To achieve this the electrical waves are channeled into the voltage amplifier, which is in the same tube as the detector. Here the voltage is increased, and then the waves are fed into a power-output tube that drives the speaker.

The speaker is the final link between the musical vibrations received at the radio studio microphone and the listener in the home. It transforms the now very strong electrical waves into mechanical vibrations. This is done by just reversing the action of the microphone.

In the speaker a permanent magnet is held stationary, and a coil of wire called the voice coil is connected to the output of the power amplifier. Current is sent through the coil, which is mounted on the cone. This causes the cone, which is usually made of paper, to move in or out, producing mechanical vibrations. These vibrations then produce pressure variations in the air of your room that are like those produced by the music in the studio.

Thus, you hear the pleasant strains of background music as you relax in your easy chair! Truly, radio transmission is a remarkable achievement.

[Diagram on page 21]

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Diagram of a moving-coil microphone

PERMANENT MAGNET

SOFT IRON YOKE

DIAPHRAGM & MOVING COIL

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The inside of a radio, viewed from the top

DETECTOR & VOLTAGE AMP.

I-F AMPLIFIER

TUNER CONDENSER

POWER OUTPUT

CONVERTER

SPEAKER

RECTIFIER

ANTENNA