The single frequency capability of radio waves has been given the name temporal coherence (or coherence in time) and is illustrated in [Figure 5]. Part a shows a single sine wave, the common way of representing electromagnetic radiation, and particularly temporally coherent radiation. In b we see what temporally incoherent radiation (such as the mixed sounds of the stranded party) would look like.
It was on Christmas Eve 1906 that music and speech came out of a radio receiver for the first time. Today the sight of someone walking, riding, or studying with an earpiece plugged into a transistor radio is common. But the early radio enthusiasts had to wear earphones because it takes considerable power to activate a loudspeaker and the received signal was quite weak. Some means of increasing, or amplifying, the signal was needed if the process was to advance beyond this primitive stage.[7]
The use of vacuum tube or electron tube amplifiers is so widespread that it is unnecessary to explain their operations here in any detail. It is important that the principle of amplification be understood, however. The input or information wave causes the grid to act as a sort of faucet as shown in [Figure 6]. That is, it controls the flow of electrons (the current in the circuit) from cathode to anode. A weak signal can therefore cause a similar, but much stronger, signal to appear in the circuit. The larger signal is subsequently used to power a loudspeaker in the radio set.
Figure 6 Amplification by a three-element vacuum tube.
Power source Cathode Grid Input wave Anode Output wave
The amplification principle can be applied in another equally important way. Once a signal gets started in the circuit, part of it can be fed back into the input of the circuit. Thus the signal is made to go “round and round”, continuously regenerating itself. The device has become an oscillator, that is, a frequency generator that produces a steady and temporally coherent wave. The frequency of the wave can be rigidly controlled by suitable circuitry.
The oscillator plays a vital part in radio transmission, for a transmitter beams energy continuously, not just when sound is being carried. The oscillator generates what is called a “carrier wave”. Information, such as speech or music, is carried in the form of audio (detectable-by-ear) frequencies, which ride “piggyback” on the carrier wave. In other words, the carrier wave is modulated, or varied, in such a way that it can carry meaningful information. The familiar expressions AM and FM, for example, stand for Amplitude Modulation and Frequency Modulation—two different ways of impressing information on the carrier wave. [Figure 7] shows a basic and an amplitude- (or height-) modulated wave.
Figure 7 (a) Unmodulated radio wave. (b) Amplitude-modulated wave carries information.