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 a carrier wave.

Hint: To understand the actual meaning of modulation let us consider the simple instance which we use in our daily life. If one wants to throw a piece of paper through a very long distance it's not possible if we just throw that paper, we fold that paper to some weight, let it be a stone and throw that paper folded stone. Then that paper can reach very longer distances. Similarly in communication systems modulation plays a key role. Here the message signal which we like to transmit is analogous to paper and the carrier signal is analogous to stone and the person who throws paper is analogous to the transmitter.

Complete answer:
Carrier wave has a constant frequency like sine wave and we vary the characteristics of this carrier wave by overlapping the modulating wave(message signal) on it. This process is called modulation and a modulated wave is produced from the transmitter and received on receiver’s side by receiver and demodulated there to get the original message signal.
Three important factors that justify the need of modulation are
1.Actual antenna/aerial size
2.Effective power which is radiated by antenna
3.Signals from different transmitters getting mixed up

1.Actual antenna size : In case of transmission systems antenna size matters. Generally bigger antennas have big range but bigger antennas might not be always optimal while it depends on transmission frequencies too. Low frequency signal requires bigger antennas but after modulation frequency of message signal gets increased which allows us to use smaller antennas
2.Effective power which is radiated by antenna : when frequency is increased its wavelength decreases which inturn increases the radiated power(power α 1/λ2)
3.Signals from different transmitters getting mixed up: signals getting mixed up causes signal interference which causes deviation In original waveform. The process of nullification of this interference is done by modulation

Carrier wave


Modulator wave


Resultant superposed EM wave


Note:
The common misconception is if we use bigger size antennas we can get bigger ranges and hence maximum optimal output. But this is not true. Bigger is not always better. Size of the antenna depends upon the frequency of the signal to be transmitted too. Moreover due to AM modulation EM noise can be generated. Modulation and demodulation is generally done by modems.