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Monday, 13 March 2017

Communication Systems - II


Bandwidth of Signals:
a) The speech signal requires a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for commercial telephonic communication.
b) For frequencies produced by musical instruments, the audible range of frequencies extends from 20 Hz to 20 kHz.
c) Video signals for transmission of pictures require about 4.2 MHz of bandwidth.
d) A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission.

Bandwidth of Transmission Medium:
The commonly used transmission media are wire, free space and fiber optic cable.

Propagation of Electromagnetic Waves:
For transmission over long distances, signals are radiated into space using devices called antennas. The radiated signals propagate as electromagnetic waves and the mode of propagation is influenced by the presence of the earth and its atmosphere. Near the surface of the earth, electromagnetic waves propagate as surface waves.
Surface wave propagation is useful up to a few MHz frequencies. Long distance communication between two points on the earth is achieved through reflection of electromagnetic waves by ionosphere. Such waves are called sky waves. Sky wave propagation takes place up to frequency of about 30 MHz. Above this frequency, electromagnetic waves essentially propagate as space waves. Space waves are used for line-of-sight communication and satellite communication.

Radio Wave Propagation is by Space Waves:

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