Define the term 'amplitude modulation'. Explain any two factors which justify the need for modulating a low-frequency base band signal.

It is the process of superposition of information/message signal over a carrier wave in such a way that the amplitude of carrier wave is varied according to the information signal/message signal.
Direct transmission, of the low frequency base band information signal, is not possible due to the following reasons;
(i) Size of Antenna: For transmitting a signal, minimum height of the antenna should be λ/4; with the help of modulation wavelength of signal decreases, hence the height of antenna becomes manageable.
(ii) Effective power radiated by an antenna: Effective power radiated by an antenna varies inversely as λ², hence effective power radiated into the space, by the antenna, increases.
(iii)To avoid mixing up of signals from different transmitters.