# NCERT 12 Physics Communication Chapter 15 Exercise

Q.1. Which of the following frequency/frequencies will be suitable for beyond the horizon communication using sky waves ?
(a) 10 kHz (b) 10 MHz (c) 1 GHz (d) 1000 GHz
Sol. Choice (b) is correct. This is because 10 kHz cannot be radiated due to large antenna size. 1 GHz and 1000 GHz will penetrate.
Q.2. Frequencies in the UHF range normally propagate by means of
(a) ground waves (b) sky waves (c) surface waves (d) space waves
Sol. Frequencies in the UHF range (≈few GHz)normally propagate by means of space waves. Hence option (d) is correct.
Q.3. Digital signals (i) do not provide a continuous set of values, (ii) represent values as discrete steps, (iii) can utilize only binary system, and (iv) can utilize decimal as binary system. Which of the following option is true.
(a) Only (i) and (ii) (b) Only (ii) and (iii) but not (iv) (d) All the above (i) to (iv)
Sol. All the four options (i) to (iv) are true for a digital signal. Hence option (d) is correct.
Q.4. Is it necessary for a transumittinig antenna to be at the same height as that of the receiving antenna for the line of sight communication ? A T. V. transmitting antenna is 81 m tall. How much service area it can cover if the receiving antenna is at the ground level ?
Sol. No, for line of sight communication, the two antenna may not be at the same height.
Q.5. A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75% ?
Q.6. A modulating signal is a square wave as shown in.
The carrier wave is given by
C (t) = 2 sin (8 π t) volt
(i) Sketch the amplitude modulated wave from.
(ii) What is the modulation index ?
Q.7. For an amplitude modulated wave, the maximum amplitude is found to be 10 V while the minimum amplitude is found to be 2 V. Determine the modulation index µ. What would be the value of µ if the minimum amplitude is zero volt ?

Q.8. Due to economic reasons, only the upper side band of an AM wave is transmitted, but at the receiving station, there is a facility for generating the carrier. Show that if device is available which can multiply two signals, then it is possible to recover the modulating signal at the receiver station.

Updated: October 14, 2020 — 1:17 pm