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(a) Briefly describe, with the aid of a diagram, the three principal modes of propagation for electromagnetic waves through the Earth's atmosphere at medium frequency [4 marks] (b) A satellite communications system consists of an orbiting antenna transmitting 4 W at a frequency of 4 GHz to an Earth station 36,000 km away. Take the velocity of light, c = 3x108 m/s. (i) Calculate the effective and real areas of the orbiting antenna if the transmitter gain G = 1053 with an aperture efficiency of 60%. (i) Calculate the minimum gain required for the Earth station antenna if the minimum power required for a detectable signal at the ground is 10-10 W. (iii) Find the effective and real areas of the Earth station antenna if its aperture efficiency is 65%. [12 marks] (c) Calculate the boundary, R, between the near field region and the far field region of the above Earth station at 4 GHz. [4 marks] (a) Briefly describe, with the aid of a diagram, the three principal modes of propagation for electromagnetic waves through the Earth's atmosphere at medium frequency [4 marks] (b) A satellite communications system consists of an orbiting antenna transmitting 4 W at a frequency of 4 GHz to an Earth station 36,000 km away. Take the velocity of light, c = 3x108 m/s. (i) Calculate the effective and real areas of the orbiting antenna if the transmitter gain G = 1053 with an aperture efficiency of 60%. (i) Calculate the minimum gain required for the Earth station antenna if the minimum power required for a detectable signal at the ground is 10-10 W. (iii) Find the effective and real areas of the Earth station antenna if its aperture efficiency is 65%. [12 marks] (c) Calculate the boundary, R, between the near field region and the far field region of the above Earth station at 4 GHz. [4 marks]

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