# NCERT Solutions Class 12 Physics Chapter 15

NCERT Solutions for Class 12 Physics Chapter 15 Communication Systems Exercises Solutions to Study Online without downloading or download in PDF form to use it offline. Download CBSE Apps as well as NCERT Books and its Solutions for other subjects also.

 Class: 12 Subject: Physics Chapter 15: Communication Systems

## NCERT Solutions for Class 12 Physics Chapter 15

### 12 Physics Chapter 15 Exercises

NCERT Solutions for Class 12 Physics Chapter 15 Communication Systems exercises are given below to use it online. Click here to go to 12 Physics main page.

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Previous Chapter: Electronic Devices: Semiconductor
Next Chapter: Electric Charges and Fields

##### Practice Questions

1. The em wave radiated from antenna which are transmitted through space along the ground. If a radio wave from the transmitting antenna reaches to the receiving antenna either directly or after reflection from the ground, it is called a ground wave.
2. The maximum range of ground wave propagation depends upon. (i) the frequency of transmitted wave
(ii) the power of the transmitter.
3. Why is the transmission of signals using ground wave communication restricted to a frequency of 1500 kHz?
4. Why are repeaters used in communication? [Answer: Repeater is the combination of a transmitter an amplifier and a receiver which picks up re-transmits it to the receiver sometimes with a change of carrier frequency. Multiple repeaters helps in extending the range of communication system.]
5. Write two differences between point to point communication and broadcast mode of communication. Give one example of each.

###### Try These
• What does ‘LOS communciation¹ mean? Name the types of waves that are used for this communication. Give typical examples, with the help of suitable figure, of communication systems that use space mode propagation.
• An amplitude modulator consist of L–C circuit having a coil of inductance 8mH and capacitance of 5pF. If an audio signal of frequency 10kHz is modulated by the carrier wave generated by the L–C circuit, find the
frequency of upper and lower side bands.
• A communication system is operating at wavelength λ = 750 nm. If only 1% of the frequency is used as channel bandwidth for optical communication then find the number of channels that can be accommodated for transmission of an Video T.V. signal requiring a bandwidth of 4.5 KHz. 9. A message signal of frequency 10 kHz and peak voltage of 10 volts is used to modulate a carrier of frequency 1 MHz and peak voltage of 20 volts. Determine (a) modulation index, (b) the side bands produced.
• 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.
• A transmitting antenna at the top of a tower has a height 32m and the height of the receiving antenna is 50m. What is the maximum distance between them for satisfactory communication in LOS mode?

#### 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, h = 81 m
Radius of earth, R = 6.4 × 106 m
For range, d = 2Rh, the service area of the antenna is given by the relation:
A = πd2 = π (2Rh)
= 3.14 × 2 × 6.4 × 106 × 81
= 3255.55 × 106 m2 = 3255.55 3256 km^2

#### 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%?

Amplitude of the carrier wave, Ac = 12 V
Modulation index, m = 75% = 0.75
Amplitude of the modulating wave = Am Using the relation for modulation index:
m = A_m/A_c
⇒ A_m = m A_m
= 0.75 × 12
= 9 V

#### Frequencies in the UHF range normally propagate by means of: (a) Ground waves. (b) Sky waves. (c) Surface waves. (d) Space waves.

(d) 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.

#### Which of the following frequencies will be suitable for beyond-the-horizon communication using sky waves? (a) 10 kHz (b) 10 MHz (c) 1 GHz (d) 1000 GHz.

(b) 10 MHz
For beyond-the-horizon communication, it is necessary for the signal waves to travel a large distance. 10 KHz signals cannot be radiated efficiently because of the antenna size. The high energy signal waves (1GHz − 1000 GHz) penetrate the ionosphere. 10 MHz frequencies get reflected easily from the ionosphere. Hence, signal waves of such frequencies are suitable for beyond-the-horizon communication.