# Physics Class 12 NCERT Solutions: Chapter 15 Communication System Part 1

Q: 1: Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves?

(A)

(B)

(C)

(D)

For beyond-the-horizon communication, it is necessary for the signal waves to travel a large distance. Signals cannot be radiated efficiently because of the antenna size. The high energy signal waves penetrate the ionosphere. Frequencies get reflected easily from the ionosphere. Hence, signal waves of such frequencies are suitable for beyond-the-horizon communication.

Q: 2. Frequencies in the UHF range normally propagate by means of:

(A) Ground waves

(B) Sky waves

(C) Surface waves

(D) Space waves

Space waves

Owing to its high frequency, an ultra-high frequency (UHF) wave can neither travel along the trajectory of the ground nor can it get reflected by the ionosphere. The signals having UHF are propagated through line-of-sight communication, which is nothing but space wave propagation.

Q: 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).

A digital signal uses the binary (0 and 1) system for transferring message signals. Such a system cannot utilise the decimal system (which corresponds to analogue signals). Digital signals represent discontinuous values.

Q: 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?

Line-of-sight communication means that there is no physical obstruction between the transmitter and the receiver. In such communications it is not necessary for the transmitting and receiving antennas to be at the same height.

Height of the given antenna,

For range,, the service area of the antenna is given by the relation:

Q: 5. A carrier wave of peak voltage 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?