• # question_answer A ground receiver is receiving a signal at (i) 5 MHz and                 (ii) 100 MHz transmitted from a ground transmitter at a height of 300 m located at a distance 100 km. Identify whether it is coming via space wave or sky wave propagation or satellite transponder. Radius of the earth $\simeq 6.4\times {{10}^{6}}m,$ ${{N}_{\max }}$ of the ionosphere $={{10}^{12}}\,\,{{m}^{-3}}.$

Maximum distance covered by space wave communication $=\sqrt{2{{R}_{e}}h}$ $=\sqrt{2\times 6.4\times {{10}^{6}}\times 300}=62\,km$ Since, receiver transmitter distance is 100 km, this is ruled out both for (i) and (ii) further ${{f}_{c}}$ for ionospheric propagation is, ${{f}_{c}}=9{{({{N}_{\max }})}^{1/2}}=9\times {{({{10}^{12}})}^{1/2}}=9MHz$ So, the signal of 5 MHz $(<{{f}_{c}})$ comes via ionospheric mode while signal of 100 MHz comes via satellite mode.