Need for Modulation

Why modulation is unavoidable. A message signal such as speech (roughly 20 Hz to 20 kHz in principle, though ordinary conversational speech is dominated by a much narrower band) cannot simply be radiated from an antenna exactly as it is. Three separate, purely physical reasons force it to be shifted onto a much higher-frequency carrier wave first -- a process called modulation, in which some property of the carrier (its amplitude, its frequency, or its phase) is made to vary in step with the instantaneous value of the message signal.

1. Size of the antenna. An antenna radiates a wave efficiently only when its physical length is comparable to the wavelength of that wave (typically a significant fraction of a wavelength, such as one quarter). The wavelength corresponding to an audio frequency of, say, a few kilohertz is many tens of kilometres -- an antenna of that size is simply not buildable. A radio-frequency carrier of a few hundred kilohertz to several gigahertz, by contrast, has a wavelength ranging from hundreds of metres down to centimetres, matching antenna sizes that are entirely practical.

2. Effective power radiated. For an antenna of a given, practical length, the power it can radiate efficiently increases sharply with the frequency of the signal driving it (for a short dipole, the radiated power varies with the square of the ratio of the antenna's length to the wavelength). At audio frequencies, a practical-sized antenna radiates a negligible fraction of the power fed into it; at radio frequencies, the same-sized antenna can radiate efficiently. …