# CBSE Class 12th Physics Notes: Communication Systems (Part – II)

CBSE Class 12th Physics chapter wise Notes based on the chapter Communication Systems are available here. These key notes are important for coming CBSE Class 12 Physics board exam 2017.

Created On: Feb 3, 2017 12:18 IST
Modified On: Feb 3, 2017 14:54 IST

CBSE Class 12th Physics Chapter wise Notes based on the chapter Communication Systems (Chapter 15 of Class 12 Physics NCERT textbook) are available here.  These key notes are important for coming CBSE Class 12 Physics board exam 2017.

These notes are continuation of CBSE Class 12th Physics Notes: Communication Systems (Part ‒ I).

In part I, we have studied about communication system, elements of communication system and basic terminology used in electronic communication systems etc. Now in part II we will study about the topics given below

 Bandwidth of Signals Bandwidth of Transmission Medium Propagation of Electromagnetic Waves Ground wave propagation Sky wave propagation Space wave propagation

CBSE Class 12 Physics Syllabus 2017

The key notes of the chapter are given below

Bandwidth of Signals

Bandwidth refers to the frequency range over which an equipment operates or the portion of the spectrum occupied by the signal. Different types of signals (music, picture or computer data) require different bandwidth.

Speech signal requires a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for telephonic conversation.

To transmit music signal an approximate bandwidth of 20 kHz is required because of the high frequencies produced by the musical instruments. The audible range of frequencies extends from 20 Hz to 20 kHz.

Video signals for transmission of pictures require about 4.2 MHz of bandwidth.

A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission.

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Bandwidth of Transmission Medium

Different types of transmission media offer different bandwidths.

The commonly used transmission media are wire, free space and fiber optic cable.

Coaxial cable offers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz.

Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.

Range of frequencies used for different services are given below

 Some Important Wireless Communication Frequency Bands Service Frequency bands Satellite Communication 5.925-6.425 GHz 3.7-4.2 GHz Uplink Downlink Cellular Mobile Radio 896-901 MHz 840-935 MHz Mobile to base station Base station to mobile Television 54-72 MHz 76-88 MHz 174-216 MHz 420-890 MHz VHF (very high frequencies) TV UHF (ultra high frequencies) TV FM broadcast 88-108 MHz Standard AM broadcast 540-1600 kHz

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Propagation of Electromagnetic Waves

Various propagation modes for em waves are shown in the figure given below

Image Source: NCERT textbooks

Ground wave propagation

Radiowaves traveling through atmosphere and moving along the surface of the earth are termed as ground waves.

Ground wave propagation is suitable for low frequencies (500 kHz to 1500kHz) or for radio broadcast at long wavelength i.e., upto 1 MHz.

To radiate signals with high efficiency, the antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4). At

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Sky wave propagation

Image Source: NCERT textbooks

The frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth.

This mode of propagation is called sky wave propagation and is used by short wave broadcast services.

The ionosphere is further subdivided into several layers as given below in table:

Image Source: NCERT textbooks

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 em waves so that they are diverted towards the earth is similar to total internal reflection in optics.

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Space wave propagation

Television broadcast, microwave links and satellite communication are some examples of communication systems that use space wave mode of propagation.

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.

At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths.

Image Source: NCERT textbooks

If the transmitting antenna is at a height hT, then you can show that the distance to the horizon dT is given as dT is given as, dT = (2RhT)1/2, where R is the radius of the earth (approx. 6400 km).

dT is also know as the radio horizon of the transmitting antenna.

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

dM = (2RhT)1/2 + (2RhR)1/2

Here, hR is the height of receiving antenna.

Space waves are of very high frequency (30 MHz to 300 MHz). Space waves can travel through atmosphere directly from one point to another. Height of the transmission antenna can be calculated from the relation, d = (2hr)1/2

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