Q.Define modulation. Why is it necessary?

Modulation impresses a low-frequency message signal onto a high-frequency carrier wave; it is essential because audio-frequency signals cannot be radiated efficiently, cannot travel far, and would clash with each other if sent directly.

Definition

Modulation is the process in which some characteristic (amplitude, frequency, or phase) of a high-frequency carrier wave is varied in accordance with the instantaneous value of a low-frequency message (information/audio) signal. Depending on which characteristic is varied, we get Amplitude Modulation (AM), Frequency Modulation (FM), or Phase Modulation (PM).

Why modulation is necessary

1. Size of antenna: To transmit a signal efficiently, the antenna length should be comparable to $\lambda/4$ of the signal. Audio signals have frequencies of 20 Hz-20 kHz, corresponding to wavelengths of thousands of kilometres - an impossibly large antenna. Modulating onto a high-frequency carrier (MHz range) reduces the required antenna size to a few metres.
2. Effective power radiated: The power radiated by an antenna is proportional to $(l/\lambda)^2$; a low-frequency signal transmitted directly would need unrealistically high power to radiate effectively. A high-frequency carrier radiates power efficiently even with a practical antenna. …