6 Unit 6

6.1 Text 6

Radio waves

Radio waves travel long distances through space, sound waves travel only short distances. A radio message may travel around the world, but the actual sound does not go much beyond the microphone. This is somewhat like a telephone. There, sound waves are changed to varying electric currents and these are changed back to sound waves in the receiver.

If you drop a stone into water, circles of waves start to travel in all directions. The farther out they go, the smaller the waves become. If you use a small stone, the waves will be small, but a large stone will produce large waves. The distance from the top of one wave to the top of the next is the wave length. The height of a wave is its amplitude.

Now if you drop a cork in these waves, you can count how many times it bobs up and down in one second. This is wave frequency. The longer the waves, the lower their frequency.

Radio waves have higher frequencies than sound waves. Sound waves range from 16 to 20000 vibrations per second. But the lowest frequency of standard broadcasting radio waves is 550,000 vibrations per second. It is these very short radio waves that travel long distances through space.

When a program is broadcast, the sound vibrations enter the microphone. The microphone is a special kind of telephone transmitter built in much the same way. Here the sound vibrations cause an electric current to vary at the same rates the vibrations enter.

This rate of vibrations is called the audio frequency. It is usually less than 20,000 vibrations per second. The voltage of this fluctuating current is now increased by means of an audio amplifier, which contains a series of vacuum tubes.

After the audio frequency has been amplified it is combined with the carrier frequency. The carrier frequency is the radio wave that will carry the message through space. It has a high frequency and is generated at the sending point. This means that the sound vibrations in the microphone are finally changed into corresponding vibrations of the carrier wave. The carrier waves travel through space from their origin to where they are received. The carrier waves in radio take place of the wires in telephone.

The receiver takes the radio waves and produces sound again. When the radio waves reach the antenna, they cut back and forth across it. This produces a weak alternating current in the antenna. The receiving set is built to catch the weak current, to amplify it, and to tune to the frequency of the incoming wave. Each broadcasting station sends out waves of a certain frequency. The receiving set has a dial which permits tuning to the waves of different frequencies.

Radio waves picked up by the antenna are so weak that vacuum tubes should be used to make them stronger. The vacuum tube acts as a valve to control the flow of current from the circuit. A new device, the transistor, is now being used to replace the vacuum tubes in many radiosets, hearing aids, and other types of electronic equipment.

The current from the circuit or from a battery, now goes into the loud–speaker. The current varies the field of an electromagnet and makes a small disk vibrate. The vibrating disk sets up the sound waves you hear. The radio speaker is similar to the telephone receiver.

Some radio waves do not travel as far as others. We have learned that radio waves used for standard broadcasting can travel long distances. These are the ones used for regular radio called AM radio (amplitude modulating broadcasting). But the waves used for FM radio (frequency modulation broadcasting) and for TV cannot go through space as far. Let us see why.

Radio waves leaving the antenna at the sending point go out in all directions. Some of these waves follow the curve of the earth and we call them ground waves. These give you the best reception on your radio. Others travel up into the sky from the antenna and are called sky waves.

During the day, many of the sky waves that go up into air are lost in outer space. But at night, certain layers of the atmosphere reflect the sky waves back to earth. These are called ionosphere layers.

The ionosphere layers and the earth act like mirrors for the sky waves. The waves hit the ionosphere and are reflected back to earth. Then the earth reflects the waves and sends them back up to be reflected by the ionosphere again. This sometimes continues until the radio waves have gone thousands of miles. Now can you explain why you can hear certain distant radio stations only at night!

To get good reception on FM and TV sets you must live near the station. What we have just learned about sky waves does not hold for certain high–frequency radio waves used for FM and TV. Your family may own one of these sets. If so, you know you cannot get steady reception if you live much beyond 50 miles from the station, unless there is some sort of relay to carry the station signals farther. This is because only the ground waves are useful at higher frequencies. The sky waves go right through the ionosphere without being reflected back to earth. So the station can only send waves to the horizon, which is about 50 miles if no hills or large buildings are in the way. It is important to remember this if you plan on getting an FM or TV set and live far away from stations.