Question

Write three important factors which justify the need of modulating a message signal. Show diagrammatically how an amplitude modulated wave is obtained when a modulating signal is superimposed on carrier wave.

Answer 1

Modulation of a message signal is needed because-

1.Size of antenna=> Size of antenna should be comparable to the wavelength(at least $\frac{\lambda }{\mu }$ in dimension). So, for an E.M wave of 20kHz, the wavelength $\lambda$ is $15km$. such a long antenna is not possible to construct and operate.We can obtain transmission with reasonable antenna lengths if transmission frequency is high. So, therefore there is a need of translating the information contained i our original low frequency baseband signal into high or radio frequencies before transmission.

2.Effective power radiated by an antenna=>Radiation from a linear antenna shows that power radiated is proportional to $\frac{1}{{\lambda }^2}$. Hence power radiated by a long wavelength baseband would be small. For good transmission we need high powers and hence this also points out the need of using high frequency transmission.

3.Mixing up of signals from different transmitters=>Suppose many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously. All these signals will get mixed up and there is no simple way to distinguish between them. This points out towards a possible solution by using communication at high frequencies and alloting a band of frequencies to each message signal.