CBSE Class 12-science Answered

Modulation of message (or information) signal is essential in a transmitter before passing on the signal to antenna on account of the following three reasons:-

(i) Size of antenna: Antenna is needed for both transmission and reception.For transmitting a signal, we need an antenna whose size should be comparable to the wavelength of signal ( λ / 4, where  is the wavelength of signal). For an EM wave of frequency 20 kHz, the wavelength =15 Km and hence the length of antenna should be 3.75 Km which is not feasible. Hence there is a need of translating the information contained in the original signal to a high frequency before transmission.

(ii) Effective power radiated: Effective power radiated by an antenna of length l is found to be directly proportional to square of frequency. For a good transmission we need high power and this point to the need of using high frequency transmission.

(iii) Mixing signals from different transmitters: If a number of transmitters are transmitting base band signals simultaneously, then all these signals cover the same frequency range (20 Hz to 20 KHz) and will get mixed up. If transmission is being done at a high frequency, different users may use adequate bandwidth for that given high frequency.