Question

Explain the concept of modulation and its need in long distance communication

Answer 1

Modulation is actually about conveying the messages . It is the addition of information to a carrier signal which are electronic or may be optical, Signal transferred in the waveform.By just varying the frequency ,, amplitude or polarization and phase   the information to be sent can be added.usually modulation is used in electromagnetic signals and can be recognized as the process of varying one or more than one properties of the waveform signal. This happen with the modulation signal which are actually having the information to be transmitted. 
So as it modulation is about transferring the messages so it does not matter you have are sending message to a 50 feet distance or for a long distance communication the modulation is just needed for communication where unmodulated ways can't be used , changing the frequency that matters  like high frequency are more appropriate for far distance transmission .
The popular example of modulation method is Amplitude modulation(AM) and frequency Modulation (FM). 

Answer 2

Modulation is a carrier wave. The method of “superimposing an audio signal over the carrier wave is called modulation” Audio signal becomes weak over long distance.  In order to avoid this, the electrical audio signal is sent with a high frequency wave and that high frequency wave is known as modulation.  In layman’s knowledge, it is like adding some data to a carrier.  Example “Morse code” – it is sent by turning on and off the signals.