Why modulation is necessary for better communication ?

CONCEPT:

Modulation: 

• The process of varying some characteristics such as amplitude, frequency, or phase of a carrier signal is varied in accordance with the instantaneous value of the low-frequency audio signal(or modulating signal) is known as Modulation.
  • In other words, in modulation, we use a high-frequency carrier wave to carry audio signals(message) to large distances.
  • Audio signals have low frequencies with a bandwidth of about 20 kHz only.
• Such low signals cannot be transmitted directly to large distances. That is why we need modulation of these audio signals because of following reasons:   

1. Practical height of Antenna: The height of the antenna should be comparable to the wavelength of the signal being transmitted.
   • Audio signals have low frequencies (or large wavelengths), so the height of the antenna should be very large(in kilometers) for the effective transmission of signals.
   • So to reduce this height and have a practical height, we need to increase the frequency of message signals, which is achieved by modulation.
2. Effective power radiation by Antenna: The power radiated by an antenna of length L is proportional to  \(\left(\dfrac{L}{\lambda}\right)^2\).  Because of low frequency(or high wavelength), the power radiated, and hence the range of transmission is very small. And in order to increase this power radiation, an increase in frequency is required. 
3. To Prevent intermixing of signals: Audio signals have a very small bandwidth(20Hz to 20kHz). Because of this message signals may get mixed up. 

EXPLANATION:

• Because of the low frequency of signals, the practical height of the antenna becomes very large, effective power radiation is very low and signals may intermix resulting in noises. Because of all these, we need modulation of signals.
• The correct answer is option (4)