Write two factors justifying the need of modulation for transmission of a signal.

Need of Modulation: Modulation is needed for (i) Practicable size of antenna. (ii) More effective power radiation by an antenna.

Two factors justifying the need of modulation for transmission of a signal are : 

(a) Managable size of the antenna: Audio signals when converted into emwaves have low frequency and large wavelength λ, so an antenna of large length L =  λ/4 ≈ 3.75 km is required, which is unpractical.

However for modulating audio signals with carrier waves of large frequencies, antenna of small managable size is required.

(b) More effective power radiated by antenna: Power radiated by antenna is p ∝ l²/λ²

This shows that there is a need of higher frequency conversion for effective power transmission by the antenna.