Question

Give reasons for the following: 

Calculate: 

(i) For ground wave transmission, size of antenna should be comparable to the wavelength of the signal, e.g. ~λ/4. 

(ii) Audio signals converted into electromagnetic waves are not transmitted as such directly. 

(iii) The amplitude of modulating signal is kept less than that of the carrier wave. 

Answer 1

Ground wave propagation is possible for radio waves of frequency band 540kHz – 1600kHz (or max. 2MHz) 

If a base band signal of frequency 20kHz, wavelength of the wave must be λ =15km, obviously antenna of size l = 3.75km is not possible to construct and operate. 

If base band signals are translated into high frequency radio wave of frequency n >1MHz, then antenna of few metre can be constructed and can be used in sending the information along the ground. 

(i) Low size of the antenna is required for waves of short wavelength and hence comparable. 

(ii) Audio signals of large wavelength cannot be send directly, because large size antenna is required. 

(iii) Modulation index μ = V_m/V_C, and its value should be less than 1, other distortion will produce in the wave.