# Question f0b7a

Feb 26, 2017

Need for modulation for transmission of signal:

• Size of antenna would be very large and therefore, impracticable in the absence of modulation.

If we were to transmit information say, sound, without modulation, we see that wavelength for highest frequency would be $\lambda = \frac{c}{n}$$= \frac{3 \times {10}^{8}}{20 \times {10}^{3}} = 15 k m$. For lower frequencies it is still larger. Minimum size of radiating antenna needed is of size $\frac{\lambda}{4}$. Which comes to ~5km.#

• Effective power radiated by the antenna would be small.

Effective radiated power $E$ from an antenna is $\propto \frac{1}{\lambda} ^ 2$. Consequently, un-modulated signals will be radiated in very small powers in comparison with the modulated signals.

• Mixing up of the base band signals from different transmitters will occur if no modulation is adopted. This will lead to loss of intelligent information.