Question

(a) Give three reasons why modulation of a message signal is necessary for long distance transmission.
(b) Show graphically an audio signal, a carrier wave and an amplitude modulated wave

Answer 1

(a) (i) Size of the antenna or aerial is about $\frac{\lambda }{4}.$ So if modulation is not done, the required antenna size will go in kilometers which is impractical.

(ii) Effective power radiated by an antenna is proportional to ${\left(\frac{1}{\lambda }\right)}^2.$

(iii) Mixing up of signals from different transmitters.
Audio signal :
Carrier wave:

Amplitude modulated wave: