Additional Questions - Chapter 10 Communication Systems 12th Science Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

Updated On May 15, 2024

By SaraNextGen

Additional Questions
Multiple Choice Questions
Question 1.
Which of the following frequencies will be suitable for beyond the horizon communication using sky waves?
(a) $10 \mathrm{kHz}$
(b) $10 \mathrm{MHz}$
(c) $1 \mathrm{GHz}$
(d) $1000 \mathrm{GHz}$
Answer:
(b) $10 \mathrm{MHz}$
Hint:
Frequency of $10 \mathrm{KHz}$ will require very large radiating antenna while frequencies $1 \mathrm{GHz}$ and $1000 \mathrm{GHz}$ will penetrate the ionosphere and cannot be reflected by it.
Question 2.
Frequency in the UHF range normally propagate by means of:
(a) Ground waves
(b) sky waves
(c) surface waves
(d) space waves
Answer:
(d) space waves.
Hint:
Frequency in the UHF ( 0.3 to $3 \mathrm{GHz}$ ) range normally propagate by means of space waves. Their sky wave reflection from ionosphere is not possible.
Question 3.
Antenna is
(a) inductive
(b) capacitive
(c) resistive above its resonant frequency
(d) resistive at resonant frequency
Answer:
(d) resistive at resonant frequency.
Hint:
An antenna is tuned circuit consisting of an inductance and a capacitance. At resonant frequency, it is resistive/.

Question 4.
In frequency modulated wave
(a) frequency varies with time
(b) amplitude varies with time
(c) both frequency and amplitude vary with time
(d) both frequency and amplitude are constant
Answer:
(a) frequency varies with time.
Hint:
In frequency modulated wave, frequency of the carrier wave varies in accordance with the modulating signal.
Question 5.
Laser light is considered to be coherent because it consist of
(a) many wavelengths
(b) uncoordinated wavelengths
(c) coordinated waves of exactly the same wavelength
(d) divergent beam
Answer:
(c) coordinated waves of exactly the same wavelength
Hint:
Laser light consists of waves of save wavelength exactly in same phase. So it is highly coherent.
Question 6.
The waves used by artificial satellites for communication purposes are
(a) microwaves
(b) AM radiowaves
(c) FM radiowaves
(d) X-rays
Answer:
(a) microwaves
Hint:
Microwaves are used in artificial satellites for communication purposes.
Question 7.
An oscillator is producing $\mathrm{FM}$ waves of frequency $2 \mathrm{kHz}$ with a variation of $10 \mathrm{kHz}$. What is the modulation index?
(a) 0.67
(b) 5.00

(c) 0.20
(d) 1.5
Answer:
(b) 5.00
Hint:
$
\mathrm{m}_{\mathrm{f}}=\frac{\Delta f}{f}=\frac{10 k \mathrm{kz}}{2 k \mathrm{~Hz}}=5
$
Question 8.
A laser beam is used for locating distant object because it
(a) has small angular spread
(b) is not absorbed
(c) is coherent
(d) is monochromatic
Answer:
(a) has small angular spread
Hint:
Laser beam has very small angular spread.
Question 9.
In short wave communication, waves of which of the following frequencies will be reflected back by the ionospheric layer having electron density $10^{11} \mathrm{~m}^{-3}$ ?
(a) $2 \mathrm{MHz}$
(b) $10 \mathrm{MHz}$
(c) $12 \mathrm{MHz}$
(d) $18 \mathrm{MHz}$
Answer:
(a) $2 \mathrm{MHz}$
Hint:
Critical frequency, $\mathrm{f}_{\mathrm{c}}=9\left(\mathrm{~N}_{\max }\right)^{1 / 2}$
$
=9\left(10^{11}\right)^{1 / 2}=2.8 \times 10^6 \mathrm{~Hz}=2.8 \mathrm{MHz}
$
$\therefore$ The wave of frequency $2 \mathrm{MHz}$ will be reflected by the ionosphere.
Question 10.
The maximum distance upto which TV transmission from a TV tower of height $h$ can be received is proportional to
(a) $\mathrm{h}^{1 / 2}$
(b) $\mathrm{h}$
(c) ${ }^{3 / 2}$
(d) $\mathrm{h}^2$
Answer:
(a) $h^{1 / 2}$
Hint:

$
\mathrm{d}=\sqrt{2 R h} ; \mathrm{d} \propto \mathrm{h}^{1 / 2}
$
Question 11.
If the highest modulating frequency of the wave is $5 \mathrm{kHz}$, the number of stations that can be accommodated in a $150 \mathrm{kHz}$ band width is
(a) 15
(b) 10
(c) 5
(d) none of these
Answer:
(a) 15
Total Bandwidth
$\overline{\text { Bandwidth/Station }}=\frac{150 \mathrm{khz}}{2 \times 5 \mathrm{kHz}}=15$
Question 12.
In communication with help of antenna if height is doubled, then the range covered with which was initially $r$ would become
(a) $\sqrt{2 r}$
(b) $3 r$
(c) $4 \mathrm{r}$
(d) $5 \mathrm{r}$
Answer:
(a) $\sqrt{2 r}$
Hint:
Initial range, $\mathrm{r}-\sqrt{2 R h}$
When height of antenna is doubled, $\mathrm{r}^{\prime}=\sqrt{2 R \times 2 h}-\sqrt{2 r}$
Question 13.
A laser beam is used for carrying out surgery, because it
(a) is highly monochromatic
(b) is highly coherent
(e) is highly directional
(d) can be sharply focused
Answer:
(d) can be sharply focused
Hint:
A laser beam is highly monochromatic, directional and coherent and hence it can be sharply focused for carrying out surgery.

Question 14.
Ozone layer is present in
(a) troposphere
(b) stratosphere
(c) ionosphere
(d) mesosphere
Answer:
(b) stratosphere
Question 15 .
Ozone layer blocks the radiation of wavelength
(a) less than $3 \times 10^{-7} \mathrm{~m}$
(b) equal to $3 \times 10^{-7} \mathrm{~m}$
(c) more than $3 \times 10^{-7} \mathrm{~m}$
(d) none of these
Answer:
(a) less than $3 \times 10^{-7} \mathrm{~m}$
Hint:
Ozone layer blocks ultraviolet radiation from the sun for this radiation $\lambda<3 \times 10^{-7} \mathrm{~m}$.
Question 16.
What is the cause of Green house effect?
(a) Infrared rays
(b) ultraviolet rays
(c) X-rays
(d) radiowaves
Answer:
(a) Infrared rays
Question 17.
Ozone layer in atmosphere is useful, because it
(a) stops ultraviolet radiation
(b) stops green house effect
(c) stops increase in temperature of atmosphere
(d) absorbs pollutent gases
Answer:
(a) stops ultraviolet radiation

Question 18.
Biological importance of ozone layer is
(a) ozone layer controls $\mathrm{O}_2 / \mathrm{H}_2$ ratio in atmosphere
(b) it stops ultraviolet rays
(c) ozone layer reduces green house
(d) ozone layer reflects radio waves
Answer:
(b) it stops ultraviolet rays
Hint:
Ozone layer stops ultraviolet radiation.
Question 19.
The principle used in the transmission of signals through an optical fibre is
(a) total internal reflection
(b) refraction
(c) dispersion
(d) interference
Answer:
(a) total internal reflection
Hint:
Signals propagate through an optical fibre by suffering repeated total internal reflections.
Question 20.
LANDSAT series of satellites move in near polar orbits at an altitude of
(a) $3600 \mathrm{~km}$
(b) $3000 \mathrm{~km}$
(c) $918 \mathrm{~km}$
(d) $512 \mathrm{~km}$
Answer:
(c) $918 \mathrm{~km}$
Hint:
LANDSAT satellites are polar satellites which move in polar orbit at a height of $918 \mathrm{~km}$ above the surface of the earth.

Question 21.
Which of the following is not a transducer?
(a) loudspeaker
(b) amplifier
(c) microphone
(d) all of these
Answer:
(b) amplifier
Hint:
Any device which converts energy from one form to another is called a transducer. Loudspeaker and microphone are transducers but not an amplifier.
Question 22 .
If a radio receiver amplifies all the signal frequencies equally well, it is said to have high
(a) fidelity
(b) distortion
(c) sensitivity
(d) selectivity
Answer:
(a) fidelity
Hint:
If a radio receiver amplifies all the signal frequencies equally well, it is said to have high fidelity.
Question 23.
The sky wave propagation is suitable for radio waves of frequency
(a) from $2 \mathrm{MHz}$ to 50
(b) upto $2 \mathrm{MHz}$
(c) from $2 \mathrm{MHz}$ to $30 \mathrm{MHz}$
(d) from $2 \mathrm{MHz}$ to $20 \mathrm{MHz}$
Answer:
(c) from $2 \mathrm{MHz}$ to $30 \mathrm{MHz}$
Question 24 .
Refractive index of ionosphere is
(a) zero
(b) more than one
(c) less than one
(d) one
Answer:
(c) less than one
Hint:
Ionosphere is the upper most layer of earth's atmosphere having veiy low density. Its refractive index is less than one.

Question 25.
When radiowaves pass through ionosphere phase difference between space current and capacitive displacement current is
(a) $0 \mathrm{rad}$
(b) $3 \pi / 2 \mathrm{rad}$
(c) $\pi / 2 \mathrm{rad}$
(d) $\pi \mathrm{rad}$
Answer:
(a) $0 \mathrm{rad}$
Hint:
The phase difference between space current and capacitive displacement current is zero.
Question 26.
Advantages of optical fibres are
(a) high bandwidth and EM interference
(b) low bandwidth and EM interference
(c) high bandwidth, low transmission capacity and no EM interference
(d) high bandwidth, high data transmission capacity and no EM interference
Answer:
(d) high bandwidth, high data transmission capacity and no EM interference
Hint:
Optical fibres have high bandwidth, high data transmission capacity and are free from electromagnetic interference.
Question 27.
A TV tower has a height of $100 \mathrm{~m}$. What is the maximum distance upto which the TV transmission can be received? $\mathrm{R}=8 \times 10^6 \mathrm{~m}$
(a) $34.77 \mathrm{~km}$
(b) $32.70 \mathrm{~km}$
(c) $40 \mathrm{~km}$
(d) $40.70 \mathrm{~km}$
Answer:
(c) $40 \mathrm{~km}$
Hint:
$
\mathrm{d}=\sqrt{2 h r}=\sqrt{2 \times 100 \times 8 \times 10^6}=40,000 \mathrm{~m}=40 \mathrm{~km}
$
Question 28.
Modem is a device which performs
(a) modulation
(b) demodulation
(c) rectification
(d) modulation and demodulation
Answer:
(d) modulation and demodulation

Question 29.
Which of the following device is full duplex?
(a) Mobile phone
(b) walky-talky
(c) loud-speaker
(d) radio
Answer:
(a) Mobile phone
Hint:
A mobile phone is a full duplex device by which two persons can talk and hear each other at the same time.
Question 30 .
For a radio signal to travel $150 \mathrm{~km}$ from the transmitter to a receiving antenna, it takes
(a) $5 \times 10^{-4}$ second
(b) $4.5 \times 10^{-3}$ second
(c) $5 \times 10^{-8}$ second
(d) $4.5 \times 10^{-6}$ second
Answer:
(a) $5 \times 10^{-4}$ second
Hint:
$
\mathrm{t}=\frac{s}{v}=\frac{150 \times 10^3 \mathrm{~m}}{3 \times 10^8 \mathrm{~ms}^{-1}}=5 \times 10^{-4} \mathrm{~s} .
$
Short Answer Questions
Question 1.
What is a communication system?
Answer:
The set up used to transmit information from one point to another is called a communication system.
Question 2.
Write down the advantages and limitations of amplitude modulation (AM)?
Answer:
Advantages of AM:
1. Easy transmission and reception
2. Lesser bandwidth requirements
3. Low cost

Limitations of AM:
1. Noise level is high
2. Low efficiency
3. Small operating range
Question 3.
Write down the advantages and limitations of frequency modulation (FM)?
Answer:
Advantages of FM:
1. Large decrease in noise. This leads to an increase in signal-noise ratio.
2. The operating range is quite large.
3. The transmission efficiency is very high as all the transmitted power is useful.
4. FM bandwidth covers the entire frequency range which humans can hear. Due to this, FM radio has better quality compared to AM radio.
Limitations of FM:
1. FM requires a much wider channel.
2. FM transmitters and receivers are more complex and costly.
3. In FM reception, less area is covered compared to AM.
Question 4.
Write down the advantages of phase modulation (PM)?
Answer:
Advantages of PM:
1. FM signal produced from PM signal is very stable.
2. The centre frequency called resting frequency is extremely stable.
Question 5.
Define bandwidth?
Answer:
The frequency range over which the baseband signals or the information signals such as voice, music, picture, etc. is transmitted is known as bandwidth.
Question 6.
Define bandwidth of transmission system?
Answer:
The range of frequencies required to transmit a piece of specified information in a particular channel is called channel bandwidth or the bandwidth of the transmission system.

Question 7.
Define skip distance.
Answer:
The shortest distance between the transmitter and the point of reception of the sky wave along the surface is called as the skip distance.
Question 8.
What is skip zone or skip area.
Answer:
There is a zone in between where there is no reception of electromagnetic waves neither ground nor sky, called as skip zone or skip area.
Question 9.
What is mean by fibre optic communication?
Answer:
The method of transmitting information from one place to another in terms of light pulses through an optical fiber is called fiber optic communication.
Question 10.
Write down the application of ICT in Fisheries?
Answer:
Fisheries:
(a) Satellite vessel monitoring system helps to identify fishing zones.
(b) Use of barcodes helps to identify time and date of catch, species name, quality of fish.
Long Answer Questions
Question 1.
Explain the concept of satellite communication? Write its applications.
Answer:
The satellite communication is a mode of communication of signal between transmitter and receiver via satellite. The message signal from the Earth station is transmitted to the satellite on board via an uplink (frequency band $6 \mathrm{GHz}$ ), amplified by a transponder and then retransmitted to another earth station via a downlink (frequency band $4 \mathrm{GHz}$ ).

The high-frequency radio wave signals travel in a straight line (line of sight) may come across tall buildings or mountains or even encounter the curvature of the earth. A communication satellite relays and amplifies such radio signals via transponder to reach distant and far off places using uplinks and downlinks. It is also called as a radio repeater in sky. The applications are found to be in all fields and are discussed below.
Applications:
Satellites are classified into different types based on their applications. Some satellites are discussed below.
1. Weather Satellites:
They are used to monitor the weather and climate of Earth. By measuring cloud mass, these satellites enable us to predict rain and dangerous storms like hurricanes, cyclones etc.
2. Communication satellites:
They are used to transmit television, radio, internet signals etc. Multiple satellitesare used for long distances.
3. Navigation satellites:
These are employed to determine the geographic location of ships, aircrafts or any other object.
Question 2.
Explain the mobile communication? Write its applications.
Answer:
Mobile communication is used to communicate with others in different locations without the use of any physical connection like wires or cables. It allows the transmission over a wide range of area without the use of the physical link. It enables the people to communicate with each other regardless of a particular location like office, house, etc. It also provides communication access to remote areas.
It provides the facility of roaming:
that is. the user may move from one place to another without the need of compromising on the communication. The maintenance and cost of installation of this communication network are also cheap.
Applications:
1. It is used for personal communication and cellular phones offer voice and data connectivity with high speed.
2. Transmission of news across the globe is done within a few seconds.
3. Using Internet of Things (IoT), it is made possible to control various devices from a single device. Example: home automation using a mobile phone.
4. It enables smart classrooms, online availability of notes, monitoring student activities etc. in the field of education.

Question 3.
What do you know about INTERNET? Write its few applications?
Answer:
Internet is a fast growing technology in the field of communication system with multifaceted tools. It provides new ways and means to interact and connect with people. Internet is the largest computer network recognized globally that connects millions of people through computers. It finds extensive applications in all walks of life.
Applications:
1. Search engine:
The search engine is basically a web-based service tool used to search for information on World Wide Web.
2. Communication:
It helps millions of people to connect with the use of social networking: emails, instant messaging services and social networking tools.
3. E-Commerce:
Buying and selling of goods and services, transfer of funds are done over an electronic network.