Explain the need for modulation.

Explanation

Antenna size and need for modulation : In space communication, an antenna is required for broadcasting and receiving message signals.
For efficient radiation and reception, the transmitting and receiving antennas should have lengths comparable to a quarter of the wavelength of the electromagnetic waves. A vertical antenna of the size required at audio frequencies (below 20 kHz) is inmpractical. For example, at 15kHz, the antenna would have to be 5 km long, whereas at 1 MHz (in the broadcastband), it is 75 m.
Even a 5 km long antenna will not be able to efficiently radiate the entire spectrum of music sigmals. Hence, it is required to confine the signals into a narrow frequency band which can be accomplished by modulation.