# Class 12 Physics Chapter 15 Important Questions Communication System

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Communication System. The best solution of the problem is to practice as many important question of 'Physics Class 12 Chapter 15' as possible to clear the doubts.

Q 1:- What is a communication system?
Ans:- A communication system is the set-up used for communication of Information/message from one place to another.

Q 2:- What are the three basic units of a communication systent?
Ans:- The three basic units of a communication system are

• (a) transmitter,
• (b) communication channel, and

Q 3:- What is a transmission medium?
Ans:- The link which transfers the information from its source to the destination is called the transmission medium or channel.

Q 4:- What is the basic difference between an analog communication system and a digital communication system?
Ans:- In an analog communication system, both message/information signal as well as carrier wave are continuous (sinusoidal) in nature. However, in a digital communication system, the message signal is digitalised and the carrier is also pulse shaped having a lower level and a higher level. Quality of digital communication system is vastly superior than that of an analog system and the retrieved signal is an exact copy of the original information signal without any distortion.

Q 5:- What does the term 'transducer' mean in an electronic communication system?
Ans:- A transducer in an electronic communication system, is a device that transforms some physical variables, e.g., displacement, pressure, force, temperature, etc., into corresponding variations in the electrical signal as the output.

Q 6:- What is the meaning of the term "attenuation" in communication?
Ans:- By the term 'attenuation' we mean gradual loss of the strength of a transmitted signal while propagating through a transmission medium.

Q 7:- What do you mean by the term 'bandwidth' ?
Ans:- The bandwidth refers to the frequency range over which an equipment used for communication purposes operates or the portion of the frequency spectrum occupied by the message signal.

Q 8:- How does the effective power radiated by an antenna vary with wavelength?
Ans:- The effective power radiation by an antenna of the given length is inversely proportional to the square of the wavelength of the carrier wave.

Mathematically,

P∝\frac{1}{Î»^2}

Q 9:- Name the type of communication in which the signal is a discrete binary coded version of the message/information.
Ans:- A digital communication system employs a message signal in the form of a discrete binary coded version.

Q 10:- What is a carrier wave? What is its need?
Ans:- We know that a low frequency baseband message signal cannot be transmitted in wireless communication efficiently. Therefore, there is a need for translating low frequency message signal into high frequency wave, before transmission, such that the translated signal continues to possess the information contained in the original message signal. For this purpose, we make use of a high frequency wave, which is known as the carrier wave. It is called a carrier wave because it carries the message signal over a long distance faithfully.

Q 11:- What is modulation?
Ans:- Modulation is the process of superposing low frequency message/information signal on a high frequency carrier wave such that the superimposed (modulated) wave continues to possess the Information contained in the original message signal.

Q 12:- What do you mean by the term "baseband signal" and "modulated signal" with reference to the modulation process?
Ans:- The original low frequency message/ information signal, which is superimposed on a high frequency carrier wave, is called the "baseband signal". The resulting signal formed as a result of modulation of baseband signal and carrier wave is called the "modulated signal.

Q 13:- What is amplitude modulation?
Ans:- Amplitude modulation (AM) is the process of superimposing low frequency baseband modulating signal on high frequency carrier wave so that the mean frequency of the modulated wave is same as that of carrier wave but its amplitude is altered in accordance with the strength of the modulating signal.

Q 14:- What can be the limiting values of modulation index in AM?
Ans:- The limiting values of modulation index Î¼ for an AM wave are 0 and 1 respectively, i.e., 0 ≤ Î¼ ≤ 1.

Q 15:- What is ground wave propagation?
Ans:- The ground wave propagation is that method of propagation of electromagnetic waves of long wavelengths in which the em wave glides over the surface of the Earth.

Q 16:- What is sky wave propagation?
Ans:- The sky wave propagation is the method of long distance communication achieved by ionospheric reflection of electromagnetic radio waves back towards the Earth.

Q 17:- What mode of communication is employed for transmission of TV signals?
Ans:- For transmission of TV signals, we employ the "space wave" propagation (also known as LOS mode) of em waves.

Q 18:- What is space wave propagation?
Ans:- Space wave propagation is that in which electromagnetic waves (of frequencies more than 40 MHz) travel directly from the transmitting antenna to the receiving antenna through the Earth's troposphere.

Q 19:- Name the type of radio wave propagation involved, when TV signals telecast by a tall transmitting antenna are intercepted directly by the receiver antenna?
Ans:- The type of radio wave propagation is the "space wave propagation".

Q 20:- What is the range of frequencies used for TV transmission? What is common between these waves and light waves?
Ans:- Range of frequencies used for TV transmission directly from transmitting antenna to receiving antenna lies from 54 MHz to 890 MHz in VHF and UHF bands of radio waves.

Q 21:- State two factors by which the range of transmission of signals by a TV tower can be increased.
Ans:- The range of TV transmission can be increased (a) by increasing the height hT of TV transmission tower, and (b) by increasing the height hR of receiving antenna.

Q 22:- Which device is used for transmitting TV signals over long distances of the order of few thousand kilometres?
Ans:- A geostationary satellite is used for transmitting TV signals over long distances of the order of few thousand kilometres. It is an artificial satellite revolving in equatorial plane of the Earth from west to east at a height of nearly 36000 km from the surface of the Earth so as to have a time period of 24 hours (i.e., revolution time of satellite is exactly the same as the time taken by the Earth to complete one rotation about its own axis).

Q 23:- What role does a geostationary satellite play in long range transmission of TV programmes?
Ans:- A geostationary satellite receives the high frequency modulated signal wave transmitted from the Earth station, demodulates it, amplifies the signal, remodulates the signal with a carrier wave of different frequency and transmits it back towards the Earth station.

Q 24:- What is the minimum number of geostationary satellites needed for world wide coverage of the programme of a particular TV channel?
Ans:- We need at least three geostationary satellites located symmetrically in the equatorial plane of the Earth.

Q 25:- "The ionospheric layer in the atmosphere is Known to cause bending of electromagnetic waves so that they get directed towards the Earth ". Name this mode of propagation of electromagnetic waves.
Ans:- In the sky wave propagation mode of em waves, the ionospheric layer of atmosphere causes bending of em waves so that they get directed towards the Earth.

Q 26:- What do you mean by the term 'detection'? What is detector?
Ans:- Detection (or demodulation) is the process of retrieval of information/message signal from the modulated carrier wave at the receiver stage of wireless communication. The device used for detection purpose is called a detector.

Q 27:- What is the purpose of IF stage employed in a radio receiver?
Ans:- In a radio receiver, the carrier frequency is usually changed to a lower frequency by an intermediate frequency (IF) stage preceding the detection. It is done so as to facilitate further processing of signal received from the receiving antenna.

Q 28:- Explain the function of a repeater in a communication system.
Ans:- A repeater in a communication system is used to increase the range of transmission of communication system. The repeater receiver picks up the radio signal from the original transmitter, amplifies it and then retransmits it with or without change in the carrier frequency. In this way, the effect of attenuation is nullified and the strength of the signal is enhanced. As a result, the range of transmission is correspondingly increased.

Q 29:- What is noise in a communication system? What is its cause?
Ans:- The undesired electrical signals, which are creeped up in a communication system, are termed as "noise". The noise may be caused by the following two causes :

(a) The undesired electrical signals may arise due to the random thermal motion of electrons within the system. It is referred to as the internal noise.

(b) The undesired electrical signals may arise due to nearby transmitters, electrical circuits, atmospheric disturbance, etc., leading to the external noise.

Q 30:- Why is ground wave propagation not suitable for high frequencies?
Ans:- The attenuation of ground wave increases very rapidly with increase in frequency. As a result, ground wave propagation is not suitable for high frequencies. Due to this reason, ground wave propagation is limited to only MW amplitude modulated radio broadcast, where maximum frequency involved is about 1600 kHz.

Q 31:- Why are short waves used for long distance radio broadcasts?
Ans:- Short waves of frequency ranging from few MHz to about 30 MHz (or wavelengths ranging from 150 m to 10 m) can be propagated easily through sky wave propagation. In sky wave propagation, the ionospheric layer of the atmosphere plays an important role. Short-radio waves sent towards the ionosphere bend and are directed back towards the Earth. In this way, these waves can reach to long distances without much attenuation. So, for long distance radio broadcasts, we prefer to use short waves and employ sky wave propagation for them.

Q 32:- Why is the transmission of signals using sky waves restricted to maximum frequencies of about 30 MHz only?
Ans:- Electromagnetic waves of frequency greater than a limiting value of about 30 MHz are not reflected back on the Earth by the ionospheric layers of the Earth's atmosphere. In fact, these waves penetrate the ionosphere and are lost in space. Therefore, transmission of signals using sky waves is restricted to a maximum frequency limit of about 30 MHz.

Q 33:- Why are sky waves not used in transmission of TV signals?
Ans:- TV signals have frequencies of 40 MHz or more. Waves of frequency 40 MHz or more cannot be propagated by sky wave mode because these waves cross the ionosphere without being reflected back towards the Earth and are lost in space.

Q 34:- Why is communication using line of sight (LOS) mode limited to frequencies above 40 MHz?
Ans:- For em waves of frequencies above 40 MHz, we cannot use ground wave or sky wave propagation mode. Moreover, at such high frequencies, the size of transmitting and receiving antennas is quite small. Hence, we prefer to use line of sight (LOS) mode of communication in which the high frequency waves travel directly from tansmitting antenna to receiving antenna as the space waves. For waves of lower frequencies, we shall have to use antennas of greater size which is not practicable.

Q 35:- What is the range of frequencies used for satellite communication? What is common between these waves and light waves?
Ans:- For satellite communication, the frequency range used for uplinking is from 5.925 GHz to 6.425 GHz and for downlinking,the frequency ranges from 3.7 GHz to 4.2 GHz. The high frequency radio waves used for satellite communication and the light waves both are electromagnetic waves and travel in free space with a speed of 3×108m/s

Q 36:- For ground wave transmission, size of antenna (l) should be comparable to wavelength (Î») of signal, i.e., l=\frac{Î»}{4}, why?
Ans:- For ground wave transmission, the size of antenna (l) should be comparable to the wavelength of the signal so as to sense the time variation of the signal properly and to radiate signals with high efficiency. As for Î»/4, wave strength changes from zero to the maximum value, the minimum length of antenna must be equal to Î»/4, i.e.,

l_{min}=\frac{Î»}{4}

Q 37:- What is the importance of modulation index in an AM wave?
Ans:- The modulation index in an AM wave determines the strength and quality of the transmitted signal. In fact, in an AM modulated wave, the information signal is carried by the sidebands only and their amplitude depends on modulation index Î¼.

Q 38:- Why is it necessary to use only a geostationary satellite for long distance TV transmission?
Ans:- We use a geostationary satellite because it appears to remain fixed above a certain place on the equatorial line of the Earth and is in contact with the given Earth stations round the clock.

Q 39:- Why do we need a higher bandwidth for transmission of music compared to that for commercial telephonic communication?
Ans:- Frequency of human voice ranges from 300 Hz to 3100 Hz. Thus, a commercial telephonic communication system used for voice communication needs a bandwidth of only 2.8 kHz for transmission. On the other hand, music signals are more complex and their frequency extends throughout the AF range (20 Hz to 20 kHz). Therefore, a music communication system needs a higher bandwidth of at least 20 kHz.

Q 40:- "A communication satellite is essentially a repeater station in space". Justify this statement by analysing the function of a repeater.
Ans:- We know that a repeater in a communication system picks up the signals from the transmitter, amplifies them and retransmits the signals, with or without a change in carrier frequency, so as to enhance the transmission range of communication system. A communication satellite in space does exactly the same work. It receives the signals transmitted by the Earth station, amplifies and retransmits them towards the Earth after changing the frequency of carrier wave. Thus, it is clear that a communication satellite is essentially a repeater station in space.

Q 41:- Why do we need amplification of modulated signal in a transmitter and again need amplification before detection at the receiver stage?
Ans:- The modulated signal is amplified by the use of a power amplifier so that the signal transmitted by the transmitter antenna is strong one. While propagating through the transmission medium, the strength of signal falls on account of attenuation. As a result, the strength of the signal reaching the antenna of receiver is extremely small. Hence, it is again amplified properly before detection.