Radio Broadcast (May-Oct 1922)

156 RADIO BROADCAST wave similar in character to that of figure 4. It has been found possible to do this by varying the amplitude of the radio waves so that this variation in amplitude follows in detail the wave variation produced by the sound. In figure 5, curve A represents a simple sound wave. F1&.6. By means of methods to be described later the amplitude of a continuous radio wave is varied so that the variation in amplitude follows identically the amplitude and frequency of the sound wave. This is shown by the heavy line in B of figure 5. This line, together with the lower inclosing line, is called the envelope of the radio wave. Note that the upper and lower inclosing lines have the same shape. The radio wave is said to have been modulated when it was made to undergo the variations in amplitude. By means of the receiving apparatus, the heavy line shown in figure 5 B affects the receiving telephones. This causes the sound as explained in the previous paragraph. To sum up, then, radio telephony is made possible by the fact that the radio waves are so modulated that their amplitude changes according to the voice waves. Figure 5 B represents very accurately what occurs. One may think of the whole process as a wave carrying a wave. The carrying wave being the radio wave, the carried wave being a wave which by means of the receiving apparatus is transformed into a sound. KIND OF RADIO WAVES RADIO waves can be classified into damped waves, undamped waves, and continuous waves. A damped wave is a wave in which the energy gradually decreases with each succeeding wave until it finally vanishes. In other words, the amplitude of the wave is first large then grows smaller and smaller until the wave disappears. A damped radio wave is represented in figure 6. The heavy line is the wave and the dotted lines follow the diminishing amplitude. A damped wave is the easiest wave to produce and for years was the only kind of a wave used in radio communication. The series of waves shown in Figure 6 is called a wave train. Damped waves can be used in radio telegraphy only; that is they cannot be used in radio telephony. It will be shown later that even a single dot in the Morse code by telegraphy is composed of very many wave trains. It is to be realized, then, that in using damped-wave communication, the signals are composed of a great many wave trains and that between these wave trains there is a space in which there are no waves. An undamped wave is a continuous wave, though a continuous wave is not always an undamped wave. This is like saying that a dog is an animal though an animal is not always a dog. The name continuous wave defines itself. It is a wave that does not die out; in other words, it is unbroken. The amplitude of the wave may vary but it never is zero. A very good example of a continuous wave is shown in figure 5, where it is so labeled. An undamped wave is a continuous wave whose amplitude does not vary. The wave shown in figure i represents an undamped wave, for its amplitude is constant. The terms undamped wave and continuous wave are often used interchangeably. Continuous wave is the broader term. Accurately speaking, undamped waves cannot be used in radio telephony for, as we have shown, the method of radio telephony involves the changing of the amplitude of the radio waves. Undamped waves have certain advantages over damped waves for use in radio communication. They carry much more energy in the same amount of time. For instance, suppose a dot used in radio telegraphy lasts one twentieth of a second. Using a wave length of 1,500 meters, there would be in undampedwave transmission, 10,000 waves in this dot. If this dot was sent out by damped waves there would be, if a wave train occurred 1,000 times a second, 50 wave trains in the dot. If each wave train consists of 40 waves — a reasonable number — the total number of waves in a dot would be 2,000. Thus there are five times as many waves in the undamped-wave dot as in the damped-wave dot. But the damped wave has only one of its waves at maximum ampli