# NCERT Solutions for class 12th Physics Chapter 15 Communication Systems

#### Exercise

Question 1. Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?
(a) 1O kHz (b) 1O MHz (c) 1 GHz (d) 1000 GHz

Sol. (b) 10 kHz cannot be radiated, size of antenna would be large, 1GHz and 1000 GHz will penetrate the ionosphere.

Question 2. Frequencies in the UHF range normally propagate by means of
(a) Groundwaves (b) Skywaves
(c) Surfacewaves (d) Spacewaves

Sol. (d) Space waves

Question 3. Digital signals
(I) do not provide a continuous set of values
(ii) represent values as discrete steps
(iii) can utilize binary system, and
(iv) can utilize decimal as well as binary systems. Which of the above statements are true?

(a) (i) and (ii) only
(b) (ii) and (iii) only
(c) (i), (ii) and (iii) but not (iv)
(d) All of (i), (ii), (iii) and (iv).

Sol. (c)

Note: Digital signals one those which can take only discreate stepwise values. The binary system employs two levels ‘O’ and ‘I’.

Question 4. Is it necessary for a transmitting antenna to be at the same height as that of the receiving antenna for line-of-sight communication? A TV transmitting antenna is 81m tall. How much service area can it cover if the receiving antenna is at the ground level?

Question 5. A carrier wave of peak voltage 12V 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%?

Question 6. A modulating signal is a square wave, as shown in Fig.

Question 7. For an amplitude modulated wave, the maximum amplitude is found to be lO V while the minimum amplitude is found to be 2V. Determine the modulation index µ.
What would be the value ofµ if the minimum amplitude is zero volt?

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

Related Articles: