Q.What is meant by 'modulation'? Why must a low-frequency message signal such as speech be superimposed on a high-frequency carrier wave before it can be transmitted efficiently over long distances?
Definition. Modulation is the process of superimposing a low-frequency message signal onto a high-frequency carrier wave, by making some property of the carrier (its amplitude or its frequency, most commonly) vary in step with the message.
Why it is essential (Section 10.3): (i) an antenna radiates efficiently only when its size is comparable to the wavelength; an audio-frequency wavelength needs an antenna many kilometres long, which is not buildable;
(ii) the power a practical-sized antenna can radiate rises sharply with frequency, so at audio frequencies almost no power would actually leave the antenna;
(iii) every unmodulated message would occupy the same narrow audio band, so several transmissions would be superimposed and unrecoverable at a receiver -- modulating each message onto its own distinct carrier frequency gives every transmitter its own non-overlapping slice of spectrum.
Modulation superimposes the message onto a high-frequency carrier (by varying the carrier's amplitude, frequency or phase in step with the message); it is essential because of antenna size, radiated power, and the need to separate multiple transmissions in frequency.
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