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CBSE NOTES CLASS 12 PHYSICS

CHAPTER 15 COMMUNICATION SYSTEMS

Communication systems

Block diagram of communication system

Modes of communication

Point-to-point communication

Point to multipoint communication-broadcast

Terms related to communication

Transmitter

Receiver

Communication channel

Transducer

Attenuation

Amplification

Repeater

Range of communication

Bandwidth

Bandwidth of some media types

Some wireless frequency bands

Different types of signals

Electrical signal

Noise signal

Analog signal

Digital signal

Coding systems

Rectangular waves

Role of atmosphere in propagation of electromagnetic waves

Different types of waves used in communication

Ground waves

Sky waves

Space waves

Modulation

Demodulation

Baseband signal

Need for modulation

How does size of the antenna related to the frequency of transmitted wave?

How is the effective power radiated by an antenna related to the frequency of transmitted wave?

How does mixing up of signals from different transmitters make modulation necessary?

Mechanism of modulation

Sinusoidal carrier

Pulsed shaped carrier

Amplitude modulation

Modulation index

Side bands

Production and transmission of amplitude modulated wave

Band pass filter

Detection of amplitude modulated wave

Internet

Main applications of Internet

Facsimile (FAX)

Mobile telephony

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 15 COMMUNICATION SYSTEMS

Propagation of electromagnetic waves

The strength of the wave keeps on decreasing with distance travelled. Several factors influence the propagation of e-m waves and the path they follow.

The earth’s atmosphere plays a vital role in the propagation of e-m waves.

Different types of waves used in communication

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Ground waves

A ground wave is a surface wave that propagates close to the surface of the Earth. The mode of propagation is called surface wave propagation and the wave glides over the surface of the earth. The ground has a strong influence on the propagation of the signal.

A wave induces current in the ground over which it passes and it is attenuated as a result of absorption of energy by the earth. The attenuation of surface waves increases very rapidly with increase in frequency. The maximum range of coverage depends on the transmitted power and frequency.

The antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4). At longer wavelengths (i.e., at lower frequencies), the antennas have large physical size and they are located on or very near to the ground. Ground based vertical towers are generally used as transmitting antennas.

Sky waves

Sky wave propagation is a method of radio frequency propagation that uses the area between the surface of the earth and the ionosphere for transmission.

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In this case ionospheric reflection of radio waves is used in long distance communication. This mode of propagation is called sky wave propagation and is used by short wave broadcast services.

The ionosphere has a large number of ions or charged particles. It extends from a height of ~ 65 Km to about 400 Km above the earth’s surface. Ionisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules.

The ionosphere is further subdivided into several layers.

Different layers of atmosphere and their interaction with the propagating electromagnetic waves

Name of the stratum (layer)

Approximate height

Exists during

Frequencies most affected

Troposphere

10 km

Day and night

VHF (up to several GHz)

D (part of stratosphere)

Part of Ionosphere

65-75 km

Day only

Reflects LF, absorbs MF and HF to some degree

E (part of stratosphere)

100 km

Day only

Helps surface waves, reflects HF

F1 (Part of Mesosphere)

170-190 km

Daytime, merges with F2 at night

Partially absorbs HF waves yet allowing them to reach F2

F2(Thermosphere)

Daytime:250-400 km

Night time: 300 km

Day and night

Efficiently reflects HF waves, particularly at night

The degree of ionisation varies with the height. The density of atmosphere decreases with height. At great heights the solar radiation is intense but there are few molecules to be ionised.

Close to the earth, even though the molecular concentration is very high, the radiation intensity received is low so that the ionisation is again low.

At some intermediate height, ionisation density is maximum.

The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.

The phenomenon of bending of e-m waves so that they are diverted back towards the earth is similar to total internal reflection in optics.

Space waves

A space wave travels in a straight line from transmitting antenna to the receiving antenna.

Space waves are used for line-of-sight (LOS) communication as well as satellite communication for frequencies > 40 MHz. The frequencies of these waves are very high.

The antennas are relatively smaller and can be placed at heights of many wavelengths above the ground. Because of line-of-sight nature of propagation, direct waves get blocked at some point by the curvature of the earth. If the signal is to be received beyond the horizon then the receiving antenna must be high enough to intercept the line-of-sight waves.

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If the transmitting antenna is at a height hT, then the distance to the horizon dT is given as dT= 2RhT, where R is the radius of the earth (approximately 6400 km). dT is also called the radio horizon of the transmitting antenna.

The maximum line-of-sight distance dM between the two antennas having heights hT and hR above the earth is given by

dM = 2RhT  + 2RhR

where hR is the height of receiving antenna.

Television broadcast, microwave links and satellite communication use space wave mode of propagation.

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