NCERT Physics Class 12 Exemplar Ch 15 Communication Systems Part 3

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Q 23 If the whole earth is to be connected by LOS communication using space waves (no restriction of antenna size or tower height), what is the minimum number of antennas required? Calculate the tower height of these antennas in terms of earth’s radius?


Earth connected by LOS communication pace waves

Earth Connected by LOS Communication Pace Waves

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Since space wave frequency is used, , hence only tower height is taken to consideration.

In three dimensions, 6 antenna towers of would do.

Q 24 The maximum frequency for reflection of sky waves from a certain layer of the ionosphere is found to be, where the maximum electron density at that layer of the ionosphere is. On a certain day it is observed that signals of frequencies higher than 5MHz are not received by reflection from the layer of the ionosphere while signals of frequencies higher than 8MHz are not received by reflection from the layer of the inonosphere. Estimate the maximum electron densities of the and layers on that day.


For layer

For layer

Q 25 On radiating (sending out) an AM modulated signal, the total radiated power is due to energy carried by. Suggest ways to minimize cost of radiation without compromising on information.


  • Of only contains information.

  • Hence cost can be reduced by transmitting both