NEET Important Chapter - Communication Systems

The term communication means the transferring, receiving and processing of information by electronic means. There are basically two communication modes : point to point and broadcast. In point to point mode of communication, the information is transferred from single transmitter to single receiver whereas in broadcast mode of communication, there are a large number of receivers corresponding to single transmitter.

In this chapter, students will learn about the basic communication system and different terms used in communication. Going further in this chapter, students will also learn about the meaning and need of modulation and demodulation.

In this article, we will cover the important concepts and topics that will help students to revise and boost their preparations for NEET.

Important Topics of Communication Systems

• Basic Elements of Communication System

• Important Terms Used in Communication Systems

• Line Communication

• Optical Communication

• Optical Fiber

• Modulation

• Demodulation

• Propagation of Electromagnetic waves

• Satellite Communication

Important Concepts of Communication Systems

List of Important Formulae of Communication Systems

Solved Examples of Communication Systems

1. A TV tower has a height of 60 m. What is the maximum distance and area up to which TV transmission can be received? (Take the radius of earth as $6.4 \times10^6 $m.)

Sol: 

Given, 

Height of the tower (h) = 60 m,

Radius of the earth (R) =  $6.4 \times10^6 $m, 

Maximum distance upto which TV transmission (d) can be received is given by:

$d = \sqrt{2Rh}$

$d = \sqrt{2\times60\times6.4\times10^6}$

$d = 27.7 km$ 

Now, area (A) upto which transmission can be received will be: 

A = ℼd2 = $\pi\times (6.4 \times10^6)^{2}$

A = 2411 km2    

Therefore, distance and area upto which transmission can be received is 27.7 km and 2411 km2, respectively. 

Key Point: If the height of receiving antenna is also given, then maximum line of sight distance (dM), $d_M = \sqrt{2Rh_T}+\sqrt{2Rh_R}$

2. What is the purpose of modulating a signal in transmission?

Sol:

A low frequency signal cannot be transmitted to long distances because of many practical difficulties. On the other hand, effective transmission is possible at high frequencies. So, modulation is always done in communication systems.

Key Point: The phenomenon of superposition of an information signal over a high frequency carrier wave is called modulation.

Previous Year Questions of Communication Systems

1. A TV transmission tower has a height of 140 m and the height of the receiving antenna is 40 m. What is the maximum distance upto which signals can be broadcasted from this tower in LOS (Line of Sight) mode? (Given: radius of earth =$6.4 \times10^6 $m )  

1. 65 km

2. 48 km

3. 40 km

4. 80 km

Sol: 

Given, 

Height of transmission tower (hT) = 140 m, 

Height of receiving tower (hR) = 40 m,

Radius of earth (R) =$6.4 \times10^6 $m, 

In this question, the height of the transmission tower and height of the receiving antenna both are given. So, we will apply the formula: 

$d_M = \sqrt{2Rh_T}+\sqrt{2Rh_R}$

$d_M = \sqrt{2 \times 6.4 \times10^6 \times 140}+\sqrt{2 \times 6.4 \times10^6 \times 40}$

$d_M = 65 km$

So, Option - (a) is the correct answer.

Trick: If the height of receiving antenna is also given, then maximum line of sight distance (dM), $d_M = \sqrt{2R h_T}+\sqrt{2R h_R}$

2. A signal of frequency 20 kHz and peak voltage of 5 volts is used to modulate a carrier wave of frequency 1.2 MHz and peak voltage 25 volts. Choose the correct statement : 

1. Modulation index = 5, side frequency bands are at 1400 kHz and 1000 kHz

2. Modulation index = 0.2, side frequency bands are at 1220 kHz and 1180 kHz

3. Modulation index = 0.8, side frequency bands are at 1180 kHz and 1220 kHz

4. Modulation index = 5, side frequency bands are at 21.2 kHz and 18.8 kHz

Sol:

Given, 

Frequency of signal (fm) = 20 kHz, 

Peak voltage of signal (Vm) = 5 V, 

Frequency of carrier wave (fc) = 1.2 MHz = 1200 kHz, 

Peak voltage of signal (Vm) = 25 V,

Now, Modulation index (μ) is given by, $\frac{V_m}{V_c}$

Therefore, μ = $\frac{5V}{25V}$ = 0.2 

Value of modulation index is 0.2

Side bands are given by:

$f_1 = f_c-f_m$ = 1200 kHz - 20 kHz = 1180 kHz, 

$f_1 = f_c+f_m$ = 1200 kHz + 20 kHz = 1220 kHz,

Therefore, the right answer is option b.

Trick: Questions based on similar formulae will be asked in exams. So, be prepared for it. 

Practice Questions

1. The maximum amplitude of an AM wave is found to be 15 V while its minimum amplitude is found to be 3V. What is the modulation index?

Ans: 0.667

2. A TV transmission tower antenna is at a height of 20 m. How much service area can it cover if the receiving antenna is (i) at ground level, (ii) at a height of 25 m? Calculate the percentage increase in area covered in case (ii) relative to case (i) 

Ans: (i) 804 km2 (ii) 3608 km2 (iii) 349 %

Conclusion

In this article we have provided important information regarding the chapter Electromagnetic Induction such as important concepts, formulae, etc.. Students should work on more solved examples for securing good grades in the NEET exam.