CBSE Class 12-science Answered

(a)  Sound waves of frequency 20–20,000 Hz cannot be transmitted from a radio transmitter by converting them into electrical waves directly. Such low frequency signals need to be translated into high frequency waves before transmission. Three reasons for this are:

1. For efficient transmission and reception, the transmitting and receiving antennas must have lengths equal to quarter wavelength of the audio signal. Setting up vertical antennas of such size is practically impossible.
2. The energy radiated from an antenna is practically zero.  The power radiated at audio frequency is quite small; hence, transmission occurs in loss.
3. The various information signals transmitted at low frequency get mixed and hence cannot be distinguished.

(b)