Modulation and its Necessity

A message signal spreads over a range of frequencies called signal band width that is why message signal are also known as base band signal.

If we want to transmit an electrical signal of audio frequency over a long range we can’t do it as such due to the following reasons.

1. Size of Antenna :-

l= l/4.

If ν =15KHz.

l=c/ ν=3*10⁸/15*10³

=20000 metres.

l=20000/4=5000 metres.

i.e. to transmit audio frequency signal of frequency 15KHz, the length of antenna should be 5,000 m. which is practically impossible.

If transmitted frequency is raised to 10 MHz.

Then, l =c/ ν=3*10⁸/10*10⁶=30m.

l= l/4=30/4=7.5 m.

i.e. to transmit high frequency  waves of order of 10 MHz ,the length of antenna is 7.5 m which is reasonable . So, there is an urgent need of converting low frequency base band into high frequency signal before transmission and hence, modulation is required.

2)     Effective power radiated by antenna:-

                The power radiated from a linear antenna is p*(l/ l)² , for good transmission high power is needed so l should be small  and frequency should be large. So, to maximize power radiated by antenna there is a need of modulation.

3) Mixing up of signals from different transmitters when many transmitters transmit base band information signals simultaneously they get mixed up and there is no way to distinguish between them. The possible solution in communication at high frequencies allotting a band of frequencies to each transmitter so that there is no mixing and that is why different T.V. broadcast relation have different frequency bands. Hence to avoid mixing of signals from different transmitters there is a need of high frequency that is why modulation is required.