18.

(a) Explain any two factors that justify the need of modulating a low-frequency signal.
 
(b) Write two advantages of frequency modulation over amplitude modulation.

Modulating low-frequency signals are required because: 

i) Size of the antenna: Antenna is required for both transmission and reception of the signal whose size (at least ) should be comparable to the wavelength of the signal so that time variation of the signal can be properly sensed by the antenna. For a low-frequency signal, the wavelength is large. Therefore, a large antenna of such a huge height is practically impossible to construct and operate.

Hence, the need for high-frequency transmission arises. 

ii) Overlapping signals from different transmitters: When an information is transmitted using different transmitters, the signals get overlapped and the information is lost. Hence, high-frequency transmission is required. Each band should be allocated to each frequency range in order to avoid mixing of signals. 

iii) Higher energy: High-frequency signals have high energy and therefore, even after loss due to attenuation, signals can be carried over longer distances. 

b) 

Advantage of frequency modulation over amplitude modulation are: 

i) FM is more efficient than AM because the amplitude of an FM wave is constant, irrespective of its modulation index. Thus, the transmitted power is constant. Also, in AM the transmitted power goes waste in the transmitted carrier.  

ii) FM reception is quite immune to noise in comparison to AM reception. Noise is a form of amplitude variation in the transmitted signal. Thus, using amplitude limiters in the FM receivers, the noise is eliminated.