question_answer

(i) Why is communication using line of sight mode limited to frequencies above \[\mathbf{40}\text{ }\mathbf{MHz}\]? (ii) A transmitting antenna at the top of a tower has a height 36 m and the height of the receiving antenna is 49 m. What is the maximum distance between them for satisfactory communication in line of sight mode?                                

Answer:

                (i) EM waves having frequencies above \[\text{4}0\text{ MHz}\] cannot be reflected by the ionosphere. The sky wave propagation is not possible. Moreover, such high frequency waves are heavily absorbed during their propagation over the surface of the earth. Their ground wave propagation is not possible. Such waves can only propagate by line of sight mode.                 (ii) Here,              \[{{h}_{T}}=36m,\]          \[{{h}_{R}}=49m,\] \[R=6400km=64\times {{10}^{5}}m\]                 Maximum distance for satisfactory communication in LOS mode, \[{{d}_{R}}=\sqrt{2R{{h}_{T}}}+\sqrt{2R{{h}_{R}}}\] \[=\sqrt{2\times 64\times {{10}^{5}}\times 36}+\sqrt{2\times 64\times {{10}^{5}}\times 49m}\] \[=8\times 6\times {{10}^{2}}\sqrt{20}m+8\times 7\times {{10}^{2}}\sqrt{20}m\] \[=10400\sqrt{20m}=10400\times 4.472m\] \[=46510.2m\simeq \mathbf{46}\mathbf{.5km}\]